Current date: 2026-08-27

Setting default datestamp limit: 0

Datestamp limit: 2026-08-27 (0 days ago)

Created/updated limit: 2026-08-20 (7 days ago)

Suggested sets: physics, physics:astro-ph, physics:gr-qc, physics:physics

Setting default set: physics

OAI-PMH request: http://export.arxiv.org/oai2?verb=ListRecords&from=2026-08-27&until=2026-08-27&set=physics&metadataPrefix=arXiv

Scoring abstracts

Number of records retrieved: 709

Keyword score statistics

score 8 -- 2 abstracts

score 7 -- 2 abstracts

score 6 -- 1 abstracts

score 5 -- 5 abstracts

score 4 -- 1 abstracts

score 3 -- 11 abstracts

score 2 -- 20 abstracts

in total -- 42 abstracts

Articles that appeared on 2026-08-27

[abstract 1 / 42] Wow! (score: 8)
arXiv:2608.25552 [pdf, ps, other]

Title: Energy Partition in AGN-driven Bubbles of NGC 4438: From Nuclear Bubbles to a Galaxy-scale Outflow

Authors: Luan Luan, Jiang-Tao Li, Jianghui Xu, Yang Yang, Guilin Liu, Fulai Guo, Q. Daniel Wang,

Comments: 20 pages, 7 figures, Accepted by ApJ, comments welcome

Subjects: astro-ph.HE astro-ph.GA

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Jets launched by accreting supermassive BLACK HOLEs represent a major mode of ACTIVE GALACTIC NUCLEus (AGN) feedback. However, how their energy is divided among bulk kinetic motion, thermal gas, MAGNETic fields, COSMIC RAYs (CRs), and radiation - and how this distribution changes with spatial scale - remains poorly constrained. NGC 4438 provides a unique laboratory for probing this evolution, hosting two 200-pc-scale nuclear bubbles and a lopsided ~10 kpc galaxy-scale outflow plausibly associated with the same AGN. We present a multi-wavelength analysis to investigate the morphology, radiation mechanisms, and energetics of these structures. Joint radio-X-ray modeling shows that the non-thermal emission in the nuclear bubbles may require two distinct populations of cosmic-ray electrons, suggesting that in addition to shock acceleration at the bubble rim, the highest-energy particles may be linked to acceleration processes closer to the unresolved central engine. A spatially resolved energy inventory reveals that bulk kinetic energy dominates the current energy budget of the nuclear bubbles, while roughly half of the injected energy has already been transformed into thermal, CR, and MAGNETic energy, as well as radiative losses. Across all bubble sizes, the thermal and MAGNETic pressures are consistent within the uncertainties, implying that MAGNETic fields remain dynamically significant on all examined spatial scales. Furthermore, the empirical correlation between radio luminosity and JET power, established for kiloparsec-scale JET bubbles (MerloniHeinz2007), matches the energetics of the galaxy-scale outflow but substantially overestimates the power of the 200-pc-scale nuclear bubbles, underscoring the scale dependence of JET energy dissipation.

[abstract 2 / 42] Wow! (score: 8)
arXiv:2608.25946 [pdf, ps, other]

Title: Evidence for a spectral steepening of the gamma-ray emission from the Galactic Center ridge

Authors: A. Acharyya, F. Aharonian, H. Ashkar, M. Backes, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, F. Brun, C. Burger-Scheidlin, T. Bylund, S. Casanova, D. Cecchin Momesso, M. Cerruti, M. Chakraborty, A. Chen, M. Chernyakova, J. O. Chibueze, O. Chibueze, T. Collins, B. Cornejo, G. Cotter, G. Cozzolongo, J. de Assis Scarpin, M. de Naurois, E. de Ona Wilhelmi, J. Devin, A. Djannati-Ataï, A. Dmytriiev, K. Egberts, K. Egg, C. Escanuela Nieves, P. Fauverge, S. Fegan, K. Feijen, M. D. Filipovic, G. Fontaine, S. Funk, S. Gabici, Y. A. Gallant, M. Genaro, J. F. Glicenstein, J. Glombitza, P. Goswami, M. -H. Grondin, L. Heckmann, B. Heß, W. Hofmann, T. L. Holch, M. Holler, M. Jamrozy, F. Jankowsky, A. Jardin-Blicq, I. Jaroschewski, D. Jimeno, I. Jung-Richardt, K. Katarzynski, D. Kerszberg, B. Khélifi, N. Komin, K. Kosack, D. Kostunin, R. G. Lang, S. Lazarevic, A. Lemière, M. Lemoine-Goumard, J. -P. Lenain, P. Liniewicz, A. Luashvili, J. Mackey, D. Maheso, D. Malyshev, D. Malyshev, V. Marandon, M. G. F. Mayer, A. Mehta, A. M. W. Mitchell, R. Moderski, L. Mohrmann, H. Ndiyavala, J. Niemiec, P. O'Brien, L. Olivera-Nieto, M. O. Moghadam, S. Panny, M. Panter, R. D. Parsons, P. Pichard, T. Preis, G. Pühlhofer, M. Punch, A. Quirrenbach, A. Reimer, O. Reimer, Q. Remy, H. X. Ren, B. Reville, F. Rieger, G. Roellinghoff, G. Rowell, B. Rudak, K. Sabri, V. Sahakian, A. Santangelo, M. Sasaki, F. Schüssler, J. N. S. Shapopi, I. Shebalkova, W. Si Said, H. Sol, L. Stawarz, R. Steenkamp, S. Steinmassl, T. Tanaka, A. M. Taylor, G. L. Taylor, R. Terrier, Y. Tian, M. Tsirou, N. Tsuji, T. Unbehaun, C. van Eldik, C. Venter, J. Vink, V. Voitsekhovskyi, T. Wach, S. J. Wagner, A. Wierzcholska, M. Zacharias, A. Zech, W. Zhong,

Comments: H.E.S.S. Collaboration

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Very-high-energy (VHE) $γ$-ray emission detected from the central molecular zone (CMZ) hints at diffusive propagation of COSMIC RAYs (CRs) injected continuously near the Galactic center (GC). Using H.E.S.S. VHE $γ$-ray observations, we aim to construct a multi-component description of the region in order to derive the spatial and spectral distributions of CRs and discuss their potential origin. We rely on a spectro-morphological analysis, in which spectral and spatial parameters of different components are fit simultaneously, and on a three-dimensional description of the CMZ and CR distributions. The GC ridge emission is well reproduced with the assumed gas distribution model weighted by a $1/r^α$ CR density profile with $α= 1.10 \pm 0.05_{\rm{stat}} \pm 0.10_{\rm{syst}}$. We detect no significant spectral variations across the CMZ, which supports a continuous injection from the GC. We report a significant (> 3 $σ$) curvature in the GC ridge $γ$-ray spectrum, implying a parent proton break (or cutoff energy) of $E_b \sim 15-50$ TeV ($E_{\rm{cut}} \sim 60-160$ TeV). We also derive the best-fit spectral parameters of the other sources in the region and report a significant cutoff at $E \sim 5$ TeV in the spectrum of the pulsar wind nebula G0.9+0.1. Finally we constrain the position of the injection site to be close to the GC (ruling out the Arches and Quintuplet clusters as major contributors to the emission). We discuss possible origins for the GC ridge emission, potential mechanisms for causing the observed curvature, a possible contribution of the gas content to the emission of HESS J1745-290 and derive qualitative constraints on the line-of-sight positions of the clouds shaping the CMZ.

[abstract 3 / 42] Wow! (score: 7)
arXiv:2603.16647 [pdf, ps, other]

Title: Particle acceleration at recollimation shocks in sub-RELATIVISTIC JETs. A model for JETs in Seyfert Galaxies, MicroQUASARs and Protostellar Systems

Authors: Enrico Peretti, Elena Amato, Silvio Sergio Cerri, Giovanni Morlino, Letizia Perfetta Pullano, Sarah Recchia,

Comments: 17 pages, 8 figures, 2 tables. Matching published version. The arXiv abstract has been slightly condensed to comply with the character limit

Subjects: astro-ph.HE astro-ph.GA astro-ph.SR

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Growing observational evidence suggests that subRELATIVISTIC (SR) astrophysical JETs may accelerate particles at slowly evolving standing shocks. Recollimation shocks (RCS) are expected to develop when JETs expand in dense environments; their formation may be mediated by the pressure of the cocoon surrounding the JET, while remaining compatible with a quasi-stationary behavior. Such shocks can be strong and can enable efficient particle acceleration. The aim of this work is to improve the general understanding of particle acceleration via diffusive shock acceleration at RCS by developing a versatile modeling framework applicable to different classes of astrophysical JETs, including Seyfert galaxies (SEY), microQUASARs (MQ), and protostellar systems (PS). We extended an analytic JET hydrodynamics model previously introduced in the literature to the SR regime and used it to identify the expected locations of the RCS and the JET head. Within this framework, we formulated a semi-analytic acceleration and transport model for particles injected at the RCS via diffusive shock acceleration. By solving the space-dependent transport equation, we obtained particle distributions and spectra along the JET, as well as robust predictions for the maximum energies achievable as a function of the intrinsic properties of the system and the source class. Our results indicate that RCS may play a central role in particle acceleration in SR JETs. In SEY such shocks may accelerate particles from PeV up to EeV energies, while in MQ and PS maximum energies of respectively tens of PeV and up to TeV energies are expected. While leptonic emission may be associated with bright knots along the JET, accelerated protons are expected to escape the JET and interact with the surrounding cocoon. Depending on the properties of the system, proton interactions can give rise to extended hadronic emission morphologies.

[abstract 4 / 42] Wow! (score: 7)
arXiv:2608.25763 [pdf, ps, other]

Title: Dynamics and Spectra-Polarimetric Signatures of GRMHD Simulations with Multiple Magnetic Loops

Authors: Raoul Kinadeter, Christian M. Fromm, Yosuke Mizuno, Antonios Nathanail, Matthias Kadler, Karl Mannheim,

Comments: 15 pages, 14 figures, submitted to A&A

Subjects: astro-ph.HE astro-ph.GA

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Relativistic JETs are a common outcome of accretion onto BLACK HOLEs, yet their presence and variability depend strongly on the MAGNETic and dynamical state of the accretion flow. While some systems, such as Blazars and Quasars, launch powerful persistent JETs, others, including the Galactic Centre BLACK HOLE Sgr~A$^\star$, show only weak or transient outflows. The physical conditions leading to the onset or suppression of JET activity remain poorly understood. We investigate the accretion flow conditions that produce transient JETs or inhibit JET formation, aiming to improve our understanding of BLACK HOLEs that accrete without strong, steady outflows. We further predict observational signatures in total and polarized emission for comparison with recent observations of Sgr~A$^\star$ in the quiescent state from the radio to the $γ$-ray regime. We perform three-dimensional GRMHD simulations of an accreting BLACK HOLE surrounded by a torus threaded by a poloidal multi-loop MAGNETic field of alternating polarities. We follow the evolution of the accretion rate, MAGNETic flux, and JET power, and analyze angular momentum transport. In addition, radiative transfer calculations including Compton scattering are used to derive synthetic total and polarized emission. The simulations show strong variability in JET power while the initial MAGNETic polarity loops accrete, followed by weaker activity at later times as the system approaches a semi-MAD state. The emission from the disk is relatively stable, weakly polarized and consistent with the quiet state values reported for SgrA$^\star$. The resulting JET emission is strongly suppressed, depolarized by Faraday rotation and conversion in the surrounding cold plasma. Upscattering calculations yield near-infrared (NIR) high energy light curves that respect observational constraints of the quiescent NIR and X-ray fluxes in SgrA$^\star$

[abstract 5 / 42] Yes (score: 6)
arXiv:2608.25313 [pdf, ps, other]

Title: Constraining GAMMA-RAY BURST viewing angles with SWIFT/XRT afterglow light curves

Authors: Cheng-Jie Sun, Shuang-Xi Yi, Lin Zhou, Yuan-Chuan Zou, Yu-Peng Yang, Si-Ji Xin, Yan-Kun Qu, Wen-Long Zhang, Fa-Yin Wang,

Comments: 22 pages, 14 figures, 3 tables. Accepted for publication in The Astrophysical Journal

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Gamma-ray bursts (GRBs) are among the most energetic phenomena in the universe, and their afterglow light curves encode information about JET geometry and viewing angle. To constrain GRB viewing angles, we analyzed JET break features in SWIFT X-Ray Telescope afterglow light curves using two top-hat JET models: a simplified geometric model without high-latitude emission (model 1) and a comprehensive model including it (model 2). Both models were applied to a sample of 20 GRBs in an interstellar medium (ISM) and 20 in a wind medium, selected so that JET breaks are attributed to the edge effect with sufficient data coverage, and fitted with Markov Chain Monte Carlo methods. We examined viewing angles and off-axis ratios q = $θ_{\rm obs}/θ_{\rm JET}$ under both density profiles, evaluating the impact of high-latitude emission. Based on reduced chi-squared and Bayesian information criterion comparisons, model 1 fits all GRBs better. Most GRBs have small off-axis ratios (mean q = 0.1851 for model 1), indicating viewing angles generally close to the JET axis; the log-space viewing-angle distribution is approximately Gaussian. A Kolmogorov-Smirnov test shows no significant difference in off-axis ratios between ISM and wind media, nor between bursts with and without an X-ray plateau. While viewing angles decrease significantly with redshift, the off-axis ratio shows no significant evolution, consistent with off-axis alignment being independent of cosmic epoch.

[abstract 6 / 42] Yes (score: 5)
arXiv:2606.09626 [pdf, ps, other]

Title: Peristaltic Flow in Compressible, Ideal Magnetohydrodynamics: A Mechanism For Solar Spicules

Authors: D. Tsiklauri,

Comments: submitted for publication, typos corrected, improvements made

Subjects: astro-ph.SR physics.flu-dyn physics.plasm-ph physics.space-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

We present analytical model for peristaltic transport within compressible, ideal MAGNETohydrodynamics (MHD). By employing small-amplitude perturbation expansion, under thin-tube long-wavelength approximation with a uniform axial background MAGNETic field, we study non-linear coupling between thermodynamic pressure variations and Maxwell's MAGNETic tension stresses. The resulting net time-averaged volumetric flow rate $\langle Q \rangle$ is calculated. When applied to solar chromospheric spicules under equipartition constraints ($β\sim 1$), where sound speed matches the Alfv{é}n speed, we find $\langle Q \rangle = 4ε^2/(M^2-1)$. Because the denominator remains positive across all operational supersonic Mach numbers ($M \approx 2\text{--}10$), upward-propagating mechanical disturbances drive a highly directional, collimated upward flow which we interpret as a spicule. Estimates show that for observationally realistic MAGNETosonic waves with amplitudes of $\approx 10\%$, the peristaltic mechanism generates a localized mass flux $\approx 100$ times that of solar wind. We propose an explicit observational signature of this mechanism, wherein the launch of individual spicular JETs is directly preceded by MAGNETosonic wave trains detectable as localized intensity modulations. Beyond solar chromospheric application, the model may be applicable to traveling MAGNETic field pinches in laboratory plasma devices and astrophysical mass-loading processes in stellar winds and inner regions of MAGNETized accretion disks.

[abstract 7 / 42] Yes (score: 5)
arXiv:2606.16363 [pdf, ps, other]

Title: The Effect of Anomalous Resistivity on Tearing Instability

Authors: D. Tsiklauri,

Comments: submitted for publication, typos corrected, improvements made

Subjects: physics.plasm-ph astro-ph.SR physics.space-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

We study the modification of classical tearing instability due to anomalous resistivity by incorporating a variable, second-order resistivity model into the resistive MAGNETohydrodynamics (MHD) framework. We evaluate the resulting {weakly non-linear boundary-layer scaling laws under a localized current-feedback mechanism}. By extending the {inner-layer analysis to capture localized current feedback}, we resolve localized spatial singularities ($δ$ and $δ'$) at the threshold boundary. These singularities generate unexpected matching jump conditions, demonstrating an early-stage phase-slip layer that forces a hyperbolic, time-dependent growth rate divergence prior to macroscopic saturation. Physical estimates for fusion devices and solar flares prove that this {consistent weakly non-linear matching formulation} triggers an abrupt transition into the explosive RECONNECTion regime, offering an exact analytical resolution to the long-standing solar and tokamak flare/disruption ``trigger problem,'' respectively. Finally, a comparative analysis using a truncated linear expansion of the threshold model regularizes the singular behavior, confirming that the explosive finite-time singularity is uniquely driven by the higher-order non-linear current feedback.

[abstract 8 / 42] Yes (score: 5)
arXiv:2608.25423 [pdf, ps, other]

Title: Is the mysterious {\it Punctum} a primordial BLACK HOLE?

Authors: Man Ho Chan,

Comments:

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

A recent study has discovered a highly polarized millimeter continuum source called {\it Punctum} located at the central region of NGC 4945 which doesn't fit any known category. Apart from the radio detection at $\approx 100$ GHz, there is no identified counterpart to {\it Punctum} in other radio, infra-red, optical, and X-ray bands. All available potential models, including MAGNETar, SUPERNOVA remnant and microQUASAR, cannot provide satisfactory explanation for the exceptionally high POLARIZATION of $\sim 50$\% and large luminosity in mm band $L_{\rm mm} \approx 2\times 10^{35}$ erg/s within a small size $<2$ pc shown in {\it Punctum}. In this article, we discuss the possibility that {\it Punctum} is indeed a primordial BLACK HOLE (PBH) with a DARK MATTER density spike. We show that the SYNCHROTRON radiation due to the electrons and positrons produced from DARK MATTER annihilation can satisfactorily explain the observed properties of {\it Punctum} and we can effectively obtain the annihilation cross section $\langle σv \rangle \sim 10^{-33}$ cm$^3$/s using our developed analytic framework. Furthermore, if the {\it Punctum} PBH mass is $\sim 10-100M_{\odot}$, the preferred DARK MATTER mass range is $m_{\rm DM} \sim 10-1000$ MeV.

[abstract 9 / 42] Yes (score: 5)
arXiv:2608.25441 [pdf, ps, other]

Title: A generalized energy-consistent finite difference scheme for 10-moment MAGNETohydrodynamics

Authors: Keita Akutagawa, Shinsuke Imada, Munehito Shoda,

Comments: 26 pages, 16 figures, submitted to Journal of Computational Physics

Subjects: physics.plasm-ph physics.comp-ph physics.flu-dyn

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Pressure anisotropy and off-diagonal pressure stresses are ubiquitous and play important roles in collisionless/weakly collisional plasmas. The Chew-Goldberger-Low (CGL) MHD model is often used; however, it can lose hyperbolicity when the pressure anisotropy or plasma beta becomes large, making it hard to develop approximate Riemann solvers. An alternative approach is to use the 10-moment MHD equations, but their eigenmode analysis is also difficult, which similarly hinders the development of less-diffusive Riemann solvers. This paper presents a new energy-consistent finite difference scheme for 10-moment MHD designed to operate over a broad range of plasma beta. The proposed scheme extends the 10-moment MHD model using the energy-consistent finite-difference approach developed for conventional MHD. Nonlinear filtering is applied to all six independent components of the pressure tensor, and the kinetic and MAGNETic energies dissipated by the filtering are explicitly transferred to the diagonal pressure components under an equipartition assumption to maintain consistency with the total energy balance. The proposed scheme is validated against seven test problems in the isotropic limit, the gyrotropic limit, and without isotropization/gyrotropization. The results demonstrate the expected spatial convergence and total energy behavior, reproduce the linear growth rate, and yield pressure tensor structures qualitatively consistent with theoretical expectations and previous simulations, spanning plasma beta values from $10^{-10}$ to $10^{10}$. The proposed scheme provides a promising framework for large-scale simulations of collisionless plasmas across widely separated plasma beta regimes and opens a path toward applications such as solar wind turbulence and plasmoid-mediated RECONNECTion.

[abstract 10 / 42] Yes (score: 5)
arXiv:2608.25571 [pdf, ps, other]

Title: Energetics of AGN Feedback

Authors: Ross J. Turner, Andrew Sullivan, William R. Q. Gaffney,

Comments: 15 pages, 9 figures, 2 tables; accepted in MNRAS

Subjects: astro-ph.GA astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Integrating realistic ACTIVE GALACTIC NUCLEus (AGN) feedback into cosmological hydrodynamical simulations remains a major challenge as resolving the spatial coupling of JET energy over cosmic timescales is computationally prohibitive. We present an analytic framework that predicts the radial and polar-angle dependence of feedback energy from lobed AGNs, enabling a physically motivated, computationally efficient prescription for JET feedback. Built upon the Radio AGN in Semi-analytic Environments (RAiSE) dynamical model, our approach tracks the post-AGN JET phase evolution of radio sources through two distinct mechanisms: buoyantly rising bubbles subject to ablation, and the gravitational collapse of swept-up gas in the shocked shell. We find that these two feedback mechanisms produce strongly contrasting spatial distributions across ten representative cluster environments. Buoyant bubbles preferentially deposit energy in steep density gradients near the core radius, while the collapsing shocked gas shell drives heating within flatter cluster cores. Weak, short-lived AGN outbursts ($Q<10^{37}$ W and $t_{\rm on}<10$ Myr) confine their energy deposition to the inner 10 kpc of the cluster; longer-lived events ($t_{\rm on} \geqslant 100$ Myr) deposit less than 1% of their injected energy within 30 kpc. We find that time-averaged heating rates across multiple outbursts are sufficient to offset radiative cooling in all but the highest density cluster cores (within 100 kpc) for duty cycles of $0.08< δ\leqslant 1$. This framework provides a scalable basis for modelling anisotropic, physically motivated JET feedback that can be incorporated into next-generation cosmological simulations.

[abstract 11 / 42] Yes (score: 4)
arXiv:2608.25507 [pdf, ps, other]

Title: A Population Study for Searching Supermassive Binary Black Holes in Active Galactic Nuclei: Continuum Spectral Features and Periodic Variabilities

Authors: Zekun Li, Changshuo Yan, Youjun Lu,

Comments: 22 pages, 10 figures

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Active sub-pc supermassive binary BLACK HOLEs (SMBBHs) are expected to exhibit various electroMAGNETic signatures due to unique dynamical and geometric structures of their accretion, but observational identification of them remains challenging. In this paper, we adopt semianalytic models to investigate both deficits in spectral-energy-distributions (SEDs) of these systems induced by gaps/holes in their accretion disks and periodic variations in their light curves induced by either orbital-modulated Doppler boosting or accretion rate variation of each SMBH component. We construct a population model to generate SMBBHs across cosmic time by considering their orbital evolution and associated accretion and radiation processes. By estimating the continuum emission from each mock system and its variation, we investigate the detection of SMBBHs via either SED-deficit signature or light curve periodicity under reasonably given criteria. We find that all-sky surveys with filters similar to those of the China Space Station Telescope or Rubin/LSST could identify up to approximately $3\times 10^2$ SMBBHs via SED-deficit features and/or $10^3$ SMBBHs via periodicity ($\lesssim5$\,yr), although both selections may suffer from a high rate of false positives. A few to a dozen SED-deficit SMBBHs may be detected by future pulsar timing arrays with signal-to-noise ratio $\gtrsim3$, enabling multi-messenger observations. Only $\sim20\%-53\%$ of SED-deficit selected SMBBHs may also be detected via periodic variations, and $\sim7\%-9\%$ of periodic variation selected SMBBHs may be detected via SED-deficit signatures. The false positives for those SMBBHs selected jointly by both methods are negligible, highlighting the importance of searching for active SMBBH systems using joint methods.

[abstract 12 / 42] (score: 3)
arXiv:2601.18859 [pdf, ps, other]

Title: Energetic Ceilings and Benchmarks of Astrophysical Gravitational-Wave Backgrounds

Authors: Chiara M. F. Mingarelli,

Comments: 10 pages, 3 figures, 3 tables, updated from v1

Subjects: astro-ph.HE gr-qc

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Every astrophysical gravitational-wave background (GWB) is limited by the rest mass its sources can convert into gravitational radiation. We combine available source mass, merger factor, radiative efficiency, and merger epochs for six populations: supermassive BLACK HOLE binaries (SMBHBs), extreme-mass-ratio inspirals (EMRIs), IMBHs inspiralling into SMBHs, stellar mass binary BLACK HOLEs (sBBHs), binary neutron stars (BNSs), and Pop~III binary BLACK HOLEs. For each, we calculate a GWB benchmark and an upper limit. For SMBHBs, the local mass census of Liepold \& Ma (2024), together with the repeated-merger boost of Sato-Polito et al. (2024), allows us to compute a ceiling on the nanoHertz GWB. Anchoring the GWB to this census, and assuming one merger per SMBH, gives a benchmark of $A_{\rm bench} = 2.11^{+0.43}_{-0.32}\times10^{-15}$ at $f=1\,{\rm yr}^{-1}$. For the ceiling, we assume an infinite number of previous mergers, yielding $A_{\rm ceil}=3.76^{+0.76}_{-0.56}\times10^{-15}$, consistent with published PTA amplitudes. Under this mass function and merger model, the measured GWB amplitude therefore does not require additional SMBH mass. A measured GWB can instead constrain its source population. Using the revised NANOGrav 15-yr GWB amplitude and this SMBH census gives $f_{\rm merge}=1.11^{+1.35}_{-0.63}$, where 1 means one merger per present-day remnant. For the other five populations, comparable censuses do not yet exist, so we report upper limits from the best available source constraints, including compact-binary merger rates. Finally, the six adopted upper limit cases yield a present-day gravitational-wave energy budget of $ρ_{\rm gw}^{(6)}=3.33^{+1.32}_{-0.83}\times10^{-7}ρ_c$. A larger energy density would require more source mass, mergers, radiative efficiency, or later epochs, or point to additional source populations or new physics.

[abstract 13 / 42] (score: 3)
arXiv:2603.07374 [pdf, ps, other]

Title: A Reproducible Black Hole-Neutron Star Merger Gallery Example for the Einstein Toolkit

Authors: Rahime Matur, Beyhan Karakaş, Roland Haas, Ian Hawke, Nils Andersson, Steven R. Brandt,

Comments: 13 pages, 8 figures, 1 table. Accepted for publication in Physica Scripta. Link to the gallery: https://einsteintoolkit.org/gallery/bhns/index.html

Subjects: astro-ph.HE gr-qc

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Black hole-neutron star mergers are important sources of gravitational waves and potential electroMAGNETic counterparts, but publicly available numerical relativity configurations for these systems remain limited. In this work, we present a fully reproducible BLACK HOLE-neutron star merger simulation performed exclusively with official Einstein Toolkit thorns and configured to target the detected event \texttt{GW230529}. We evolve the system at three resolutions, with finest grid spacings of $162$, $222$, and $310$~m, and assess the numerical robustness of the resulting dynamics and gravitational wave signal. The entire setup, from initial data to a parameter file with some of the analysis scripts, is publicly released as a new Einstein Toolkit gallery example and is distributed as part of the Hypatia release, establishing a reference BLACK HOLE-neutron star merger configuration within the Einstein Toolkit.

[abstract 14 / 42] (score: 3)
arXiv:2606.10823 [pdf, ps, other]

Title: AGN-driven BBH mergers: Black hole populations and hierarchical growth across the AGN parameter space

Authors: Maria Paola Vaccaro, Michela Mapelli, Alessandro Alberto Trani, Boyuan Liu,

Comments: 21 pages, 15 figures

Subjects: astro-ph.GA astro-ph.CO astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Active galactic nuclei (AGNs) have been proposed as efficient environments for the formation of binary BLACK HOLEs (BBHs). We present an updated semi-analytical framework for BBH formation and evolution in AGN disks, following the capture, migration, pair-up, gas-driven hardening, binary--single encounters, and merger of stellar-origin BLACK HOLEs. We systematically explore the dependence of the resulting BBH merger population on the main AGN parameters, namely the supermassive BLACK HOLE mass $M_\bullet$, the Eddington ratio $λ_\bullet$, and the disk viscosity parameter $α$, and construct an intrinsic BBH population by weighting the simulations according to observed low-redshift AGN properties. We find that AGN disks can produce repeated mergers and build a high-mass tail extending beyond the pair-instability mass gap and into the intermediate-mass range. Hierarchical growth is more efficient in lower-viscosity disks, with $α=0.01$, while higher-viscosity disks suppress the formation of massive remnants. The merger efficiency generally increases with $λ_\bullet$, but its dependence on $M_\bullet$ is non-trivial. The AGN-assisted BBH population is characterized by increasingly unequal mass ratios at high primary mass, a correlation between primary mass and $|χ_{\rm eff}|$, and an effective-spin distribution that depends strongly on the fraction of binaries born in prograde or retrograde configurations. We find that the AGN channel can reproduce systems broadly consistent with the massive BBH events GW190521 and GW231123. We test several variations of the physical model, including different formalisms for migration torques, gas hardening, and three-body encounters. The general properties of the population are robust across these variations, with the high-mass tail and spin signatures persisting in all cases except when gas hardening is switched off.

[abstract 15 / 42] (score: 3)
arXiv:2607.06458 [pdf, ps, other]

Title: Exact 1D Nonlinear Solutions for Proton-Driven Plasma Wakefields: Benchmarking Against AWAKE Data Envelopes

Authors: D. Tsiklauri,

Comments: submitted for publication, typos corrected, improvements made

Subjects: physics.plasm-ph astro-ph.HE astro-ph.SR physics.acc-ph physics.space-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

The analytical modeling of a plasma wakefield driven by a RELATIVISTIC proton beam is an element in optimizing advanced plasma-based acceleration schemes. In this work, we present a 1D nonlinear fluid framework under the quasi-static approximation to describe the wake potential excited by a positively charged proton driver. We examine our model using a two-bunch pump-probe configuration, demonstrating close agreement between the analytical invariants and adaptive numerical integrations. The distinct geometric curvature changes observed at the micro-bunch boundaries are shown to be physical consequences of step-discontinuities in the second derivative of the wake potential across the beam interfaces. Furthermore, by scaling this numerical framework to a train of $N=100$ micro-bunches undergoing seeded self-modulation (SSM), we model the physical parameters of the CERN AWAKE facility ($n_0 = 7.0 \times 10^{14}\text{ cm}^{-3}$). Our model replicates the characteristic linear growth envelope and matches the calibrated field envelope boundaries of approximately $\pm 0.75\text{ GV/m}$ inferred from the experiment. This piece-wise framework provides a computationally efficient foundation for investigating customized, asymmetric micro-bunch profiles designed to optimize the transformer ratio beyond the fundamental symmetric limit of 2.

[abstract 16 / 42] (score: 3)
arXiv:2608.23673 [pdf, ps, other]

Title: A simulation-based quality-control framework for the broad-line region radius-luminosity relation

Authors: Juri Wladimir Seib, Francisco Pozo Nuñez, Sarah Elena Ivana Bosman,

Comments: Accepted for publication in A&A, Updated the version to match language edited and final submitted version

Subjects: astro-ph.GA

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

The broad-line region (BLR) radius-luminosity (R-L) relation underpins single-epoch BLACK HOLE mass estimates in ACTIVE GALACTIC NUCLEi (AGN). The published Hβsample is heterogeneous in observational quality, and it remains unclear how much of its scatter is intrinsic rather than caused by systematics in lag recovery. We quantify the contribution of unreliable lag recovery to the observed scatter of the HβR-L relation and provide tools for quality control. We compile a publicly available database of reverberation mapping measurements for ~1200 AGN from 32 campaigns spanning more than three decades, including lags, luminosities, line widths, BLACK HOLE masses, and observational metadata. We develop a simulation-based consistency framework in which damped random walk light curves are sampled according to each campaign's baseline, cadence, and signal-to-noise ratio, and lags are recovered with the interpolated cross-correlation function (ICCF). Comparing expected, reported, and simulation-retrieved lags defines a four-tier flagging scheme. We refit the R-L relation with UltraNest for progressively cleaner samples and construct consensus samples across three reference slopes to reduce model dependence. Of 248 Hβsources, ~40% show discrepancies between reported and simulation-retrieved lags, while ~5% show direct inconsistencies between expected and retrieved lags. Excluding flagged sources and correcting for model bias reduces the inferred intrinsic scatter from σ= 0.26^{+0.02}_{-0.01} dex to 0.11\pm0.01 dex, with a corrected slope of α= 0.48\pm0.02. Our results indicate that a substantial fraction of the observed HβR-L scatter arises from observational limitations and lag-recovery biases rather than intrinsic AGN diversity. The database, simulation code, and RM-Scout campaign-planning tool are publicly available.

[abstract 17 / 42] (score: 3)
arXiv:2608.25044 [pdf, ps, other]

Title: To be lensed or not to be lensed: on the nature of the alleged high-z lensed QUASARs J0109-5424 and P170+20

Authors: Aurora Mata-Sanchez, Emanuele P. Farina, Anniek J. Gloudemans, Debora Pelliccia, Roberto Decarli, Eduardo Banados, Silvia Belladitta, Manuela Bischetti, Sarah E. Bosman, Xander Byrne, Thomas Connor, Frederick B. Davies, Thales A. Gutcke, Weizhe Liu, Chiara Mazzucchelli, Romain A. Meyer, Silvia Onorato, Fabian Walter, Feige Wang, Jinyi Yang,

Comments: Accepted for publication in the Astrophysical Journal, 10 pages, 4 figures

Subjects: astro-ph.GA

Created: 2026-08-25; Updated: 2026-08-27; Datestamp: 2026-08-27

We present a detailed multi-wavelength analysis of two gravitationally lensed z>6 QUASAR candidates: J0109-5424 and P170+20. Using JWST/NIRSpec near-infrared data from the Aether survey, we demonstrate that J0109-5424 is in fact a FeLoBAL QUASAR at z=2.07, exhibiting strong iron broad absorption lines that mimic the Lyman break. From the Pabeta emission line, we estimate a BLACK HOLE mass of 3.9 x 10^8 Msun and a bolometric luminosity of Lbol = 7.7 x 10^45 erg/s for this QUASAR. Follow-up HST/ACS WFC imaging of P170+20 with the F555W filter, probing emission below the Lyman limit of the QUASAR, reveals a compact source located 0.09+/-0.04 arcsec from the QUASAR position. PSF modelling and subtraction yield no evidence for extended emission. This is inconsistent with a foreground galaxy acting as a lens. Furthermore, we obtain Gemini-South/Flamingos 2 near-infrared observations to complement the existing optical Gemini-North/GMOS spectrum of P170+20. Our spectral analysis shows that the observed spectral shape of P170+20 is consistent with a composite QUASAR and M-dwarf spectrum. However, this interpretation remains uncertain, as a chance alignment between an ultracool dwarf and a QUASAR within 0.1 arcsec would be unlikely. Overall, our findings establish J0109-5424 as an intermediate redshift source and do not favor the lensed nature of P170+20, although this possibility cannot be conclusively ruled out, highlighting the challenges of selecting lensed QUASARs at high redshift.

[abstract 18 / 42] (score: 3)
arXiv:2608.25253 [pdf, ps, other]

Title: What is the reason for the lack of cool evolved stars near the center of the Galaxy?

Authors: Petr Kurfürst, Michal Zajaček, Norbert Werner, Jiří Krtička, Matúš Labaj,

Comments: Proceedings IAU Symposium No. 405: Traversing the Galactic Center in Space and Time

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

The observed deficit of bright late-type giants in the central parsec of the Milky Way remains an open problem. We investigate whether repeated passages of red giants (RGs) through a past JET from Sgr$~$A$^*$ can modify their envelopes and apparent spectral types. Three-dimensional hydrodynamical simulations follow a $1\,M_\odot$, $100\,R_\odot$ RG through up to ten JET crossings at $10^{-3}\,\text{pc}$, using JET kinetic luminosities of $10^{42}$, $10^{44}$, and $10^{48}\,\text{erg}\,\text{s}^{-1}$. Each passage produces shocks, envelope ablation, and an asymmetric downstream tail. For JET luminosities up to $10^{44}\,\text{erg}\,\text{s}^{-1}$, the cumulative ablated mass evolves approximately as $ΔM\propto t^{1/2}$ and reaches about $10^{-4}\,M_\odot$ over a $10^5\,\text{yr}$ active phase. Repeated heating also raises the surface temperature from about $3600$ to $8500\,\text{K}$ during the first ten passages, potentially making an M-type giant appear as an A-type source. Jet feedback may therefore contribute to the apparent depletion of cool giants and the excess of hot stars in galactic nuclei (GN).

[abstract 19 / 42] (score: 3)
arXiv:2608.25556 [pdf, ps, other]

Title: Sequential Magnetic Reconnections in a Fishbone-like Structure Leading to Recurrent Brightenings

Authors: Bo Yang, Jiayan Yang, Junchao Hong, Yi Bi,

Comments: 21 pages, 7 figures

Subjects: astro-ph.SR

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

The fine-scale release of MAGNETic free energy in the solar atmosphere is a fundamental open question in solar physics. Multi-wavelength observations at high spatiotemporal resolution now offer a direct window into this process. Using data from NVST, SDO, IRIS, and Hinode, we reveal the energy release process in a fishbone-like MAGNETic structure within active region 12297. The fishbone-like structure consists of a spine along a narrow, elongated positive-polarity field, with herringbone branches rooted in negative-polarity sunspots. Persistent photospheric MAGNETic flux emergence and shearing motions are observed beneath the fishbone-like structure, which may play a key role in maintaining its topology and producing the recurrent brightenings. During brightenings, compact bright features propagate sequentially from west to east along the spine, accompanied by bidirectional flows along the branches and plasma blobs ejected toward the distant positive sunspot. These propagating features could be interpreted as signatures of sequential MAGNETic RECONNECTion events occurring in chronological order at nodes along the spine, predominantly in the chromosphere and transition region. Our observations may provide evidence that recurrent brightenings could arise from repeated sequential RECONNECTion events organized by a coherent MAGNETic structure, deepening our understanding of how such recurrent brightenings are generated and how MAGNETic free energy is dissipated at fine scales in solar active regions.

[abstract 20 / 42] (score: 3)
arXiv:2608.25620 [pdf, ps, other]

Title: Ultraluminous X-ray sources in the first eROSITA survey I. Candidate catalogs

Authors: P. Weber, T. P. Roberts, S. Hämmerich, E. Kyritsis, A. Zezas, M. G. F. Mayer, A. Zainab, I. Kreykenbohm, A. Merloni, M. Sasaki, A. Schwope, M. Middleton, A. Basu-Zych, A. Hornschemeier, N. Vulic, M. Salvato, N. Webb, H. Tranin, N. Schettino, C. Kirsch, S. Saeedi, F. Zangrandi, M. Lorenz, L. Dauner, T. Dauser, F. Haberl, C. Maitra, Z. Igo, A. Santangelo, L. Ducci, J. Wilms,

Comments: 15 pages, 10 figures, in press in Astronomy and Astrophysics, volume 712

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Ultraluminous X-ray sources (ULXs) are luminous non-nuclear X-ray point sources embedded in galaxies. Their luminosities exceeding 10^39 erg/s potentially require sub-Eddington accretion by objects with masses from ~10^2$M_{\odot}$ to ~10^4$M_{\odot}$ or supercritical accretion beyond the Eddington limit by stellar mass BLACK HOLEs or neutron stars. We identified ULXs by searching for X-ray counterparts in the ~10^5 galaxies of the HECATE catalog in the footprint of the first eROSITA all-sky survey, while excluding nuclear regions to avoid the selection of ACTIVE GALACTIC NUCLEi (AGN). We characterized the completeness of the sample and its contamination by unidentified AGN and determined the influence of source confusion. The catalog is unaffected by X-ray selection biases. We removed known contaminants such as SUPERNOVAe, stars, and AGN, and manually vetted all candidates to further remove contaminants. We derived the X-ray selection function and fraction of unidentified AGN interlopers using a simulation with subsequent source detection. We present two samples of ULX candidates. The main sample consists of 90 sources with highly confident X-ray detections, 53 of which are identified for the first time. This sample is complete to a distance of ~7Mpc, contains at most 29% of unknown background AGN, and is mostly unaffected by source confusion. Several candidates are newly identified even though their locations have been observed by other instruments before, and some previously reported candidates we expected to identify are absent from our sample, demonstrating the transient behavior of ULXs. Based on our list of less confident detections, we also provide an extended catalog of 260 sources identified as potential candidates for further ULX identifications. Of these, 245 are identified as potential candidates for the first time. (Abridged)

[abstract 21 / 42] (score: 3)
arXiv:2608.25715 [pdf, ps, other]

Title: The properties of the first continuous gravitational waves: Optimizing pulsar timing observations for supermassive BLACK HOLE binary detection

Authors: Blaze Houlden, Eric Thrane, Katie Auchettl,

Comments: Submitted to ApJ. 23 pages, 14 figures

Subjects: astro-ph.HE astro-ph.CO gr-qc

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Several pulsar timing arrays have reported evidence for a nanohertz gravitational-wave background. This background is thought to arise from the superposition of signals from a population of inspiraling supermassive BLACK HOLE binaries. A key test of this interpretation is the search for individually resolvable binaries whose signals emerge above this background. Here we investigate this possibility by addressing two related questions: (1) what are the most probable properties of the first resolvable binary; and (2) how can pulsar timing observations be optimized to maximize its prospects for electroMAGNETic identification? By simulating populations of supermassive BLACK HOLE binaries constrained by evidence for the gravitational-wave background and studying the loudest binary in each simulation, we infer the expected properties of the first individually resolvable systems. If MeerKAT continues its current observing strategy for a total of 9 years, the probability of detecting an individual binary is $\approx10-27\%$. Improving the timing precision by a factor of two changes this probability to $\approx11-28\%$, whereas increasing the cadence by a factor of four gives $\approx12-30\%$. Combining both improvements yields a detection probability of $\approx12-33\%$. Furthermore, implementing a high-cadence observing campaign preferentially resolves higher-frequency systems ($\gtrsim\unit[10]{nHz}$), whose shorter orbital periods make them considerably easier to identify through electroMAGNETic observations than lower-frequency binaries that repeat infrequently. We discuss prospects of electroMAGNETic follow-up of gravitational-wave resolved binaries with current high-cadence all-sky optical surveys.

[abstract 22 / 42] (score: 3)
arXiv:2608.25978 [pdf, ps, other]

Title: Winds Against Alignment: AGN Feedback and the Spin Evolution of Massive Black Hole Binaries

Authors: Francesco Bollati, Marta Volonteri, Alessandro Lupi, Massimo Dotti, Francesco Haardt,

Comments:

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

The interaction between massive BLACK HOLE (MBH) binaries and circumbinary discs (CBDs) affects the spin orientations of merging binaries - a key observable for the future LISA mission. While gas accretion is known to align BH spins with the orbital angular momentum via the Bardeen-Petterson effect, the impact of AGN feedback on this process has remained largely unexplored. We present hydrodynamical simulations of an equal-mass MBH binary embedded in a CBD, using the GIZMO code with a subgrid model that self-consistently evolves BH mass, spin, and anisotropic AGN feedback. We explore different BH spin magnitudes and orientations to identify the configurations producing the strongest feedback effect on the disc structure and binary evolution. AGN winds substantially modify the CBD and minidisc structure, enlarging the central cavity and, after 20-25 binary orbits, destroying the minidiscs in all feedback simulations. Feedback-driven cavity excavation suppresses the gravitational torques driving orbital evolution, causing the binary to stall. AGN feedback dramatically affects MBH accretion, reducing the Eddington ratio by one to two orders of magnitude before ultimately suppressing it entirely. This strongly inhibits Bardeen-Petterson spin alignment, delaying it well beyond the feedback-free timescale. Extending one simulation to 70 binary orbits reveals a feedback-regulated duty cycle of alternating active and quiescent phases, with the binary spending most of its time in low-density, quiescent conditions that further suppress alignment. These results indicate that AGN feedback is an important, previously overlooked channel for preserving spin misalignment in MBH binaries prior to coalescence, with direct implications for interpreting LISA gravitational-wave observations.

[abstract 23 / 42] (score: 2)
arXiv:2503.13118 [pdf, ps, other]

Title: Gravitational Wave Effects on Radio Spectral Lines of Atomic Hydrogen: Hyperfine Splitting and Broadening Mechanisms

Authors: Nontapat Wanwieng, Nithiwadee Thaicharoen, Narupon Chattrapiban, Apimook Watcharangkool,

Comments:

Subjects: gr-qc astro-ph.HE quant-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

We explore the effects of gravitational waves (GWs) on hydrogen's radio spectral lines, focusing on the ground-state hyperfine transition and radiative transitions in highly excited Rydberg states. To analyze GW impacts on hyperfine structure, we derive Maxwell's equations in a gravitational-wave background using linearized gravity and the $3+1$ formalism. Our findings reveal that GWs induce energy shifts in hyperfine MAGNETic substates, modifying the 21 cm line. However, these energy shifts fall well below the detection limits of current radio astronomical instruments. For transitions in highly excited states, which produce radio recombination lines (RRL), the influence of GW manifests itself as spectral broadening, with the fractional linewidth for $\mathrm{H}nα$ scaling as $Δν/ν_0 \sim n^7ω^2_{\mathrm{gw}}h(t)$. This suggests that RRLs could serve as probes for ultra-high-frequency GWs, particularly given that Rydberg atoms in the interstellar medium can reach quantum numbers above $n=100$. As an example of possibly detectable high frequency GW source, We investigate GWs emitted during the inspiral of planetary-mass primordial BLACK HOLE binaries, where GW-induced broadening in RRLs could exceed natural broadening effects. Additionally, we examine the influence of the recently detected stochastic gravitational-wave background on hydrogen spectral lines.

[abstract 24 / 42] (score: 2)
arXiv:2512.12985 [pdf, ps, other]

Title: X-ray Variability and Photosphere Evolution during Accretion Disk Formation in Tidal Disruption Events

Authors: Xiaoshan Huang, Maria Renee Meza, Sol Bin Yun, Brenna Mockler, Shane W. Davis, Yan-fei Jiang,

Comments: 18 figures, 27 pages

Subjects: astro-ph.HE

Created: 2026-08-25; Updated: 2026-08-27; Datestamp: 2026-08-27

The early time emission in tidal disruption events (TDEs) originates from both accretion and shocks, producing photons that eventually emerge from an inhomogeneous photosphere. We model disk formation following debris stream self-intersection in a TDE using three-dimensional, frequency-integrated and multi-group radiation hydrodynamic simulations. We find a more circularized disk forms about 24 days following the stream-stream collision, once the mass fallback rate passes its peak and the debris stream density decreases. Despite the absence of a circularized disk at early times, various shocks and the asymmetric photosphere are sufficient to drive a wide range of x-ray-to-optical ratios and soft-X-ray variability. We find that with strong apsidal precession, the first light is from the stream-stream collision. It launches an optically-thick outflow, but only produces modest prompt emission. The subsequent optical and ultraviolet (UV) light curve rise is mainly powered by shocks in the turbulent accretion flow close to the BLACK HOLE. The optical-UV luminosity peaks roughly when the disk forms and shock-driven outflows subside. Radiation pressure clears the polar region and creates optically-thin channels. We obtain the broadband spectral energy distribution (SED) by post-processing multi-group simulations with 16-20 frequency groups. The SED has a black body component that peaks in the extreme UV. The soft X-ray component either resembles a thermal tail, or can be described by a power law associated with bulk Compton scattering. The blackbody parameters are broadly consistent with observed optical TDEs and vary weakly with viewing angle, but the soft X-ray emission is highly angle-dependent.

[abstract 25 / 42] (score: 2)
arXiv:2604.00781 [pdf, ps, other]

Title: Real-time virtual circuits for plasma shape control via neural network emulators: dynamic validation in closed-loop simulations

Authors: K. Pentland, A. Ross, N. C. Amorisco, P. Cavestany, T. Nunn, A. Agnello, G. K. Holt, C. Vincent,

Comments:

Subjects: physics.plasm-ph physics.comp-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Reliable confinement and stable performance of tokamak fusion plasmas require accurate real-time MAGNETic shape control. A promising route to reduced latency and increased flexibility in plasma control systems (PCS) is to emulate physics-based controllers using neural networks (NNs). In prior work, we have demonstrated that virtual circuits (VCs), which define the poloidal field coil current vectors able to modify each plasma shape parameter independently, can be accurately emulated with NN models trained on a large library of simulated Grad-Shafranov equilibria. This enables MAGNETic controllers to accurately adapt to evolving plasma equilibria using real-time VCs (RTVCs), in contrast to pre-set VC schedules whose performance degrades upon departure from their reference equilibria. Here, we investigate the performance and robustness of these RTVCs in closed-loop simulations using the FreeGSNKE Pulse Design Tool (FPDT): a framework that couples the FreeGSNKE evolutive equilibrium solver with a virtual PCS. The FPDT models the coupling between controllers, plasma current and shape response, and actuator constraints. Using the RTVCs within the FPDT, we demonstrate effective in-silico control of MAST Upgrade (MAST-U) plasma scenarios and show that the emulators are robust in the presence of input measurement uncertainty and under different update frequencies. These results establish the viability of RTVCs for closed-loop plasma shape control, representing a key step toward real-time deployment in the MAST-U PCS.

[abstract 26 / 42] (score: 2)
arXiv:2604.11684 [pdf, ps, other]

Title: Gyrokinetic equilibria of high temperature superconducting MAGNETic mirrors

Authors: Maxwell H. Rosen, Manaure Francisquez, James Juno, Benicio Terry-Mendoza, Ammar Hakim, Gregory W. Hammett,

Comments: 9 pages, 4 figures

Subjects: physics.plasm-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

High-temperature superconducting (HTS) MAGNETs and other advances have led to renewed interest in MAGNETic mirrors for fusion energy. The non-Maxwellian nature of mirror plasmas necessitates kinetic modeling to predict, optimize and design mirrors. Explicit gyrokinetic full-f codes can be used to study instabilities and turbulent transport in tokamaks and mirrors, but they have been prohibitively expensive to integrate directly over the very long time scales required to compute kinetic plasma equilibrium. We demonstrate that these studies are now feasible thanks to novel multiscale methods delivering a 30,000X speed-up. The resulting kinetic equilibrium, electrostatic potential, and ion confinement time are consistent with analytic theory. This transformative capability opens the door to a new way of obtaining equilibria for mirrors, and we discuss how this technique may also accelerate calculations for tokamaks and stellarators. The models presented in this article address critical multiscale problems in modeling MAGNETic mirrors, opening a new research avenue for equilibrium studies using an explicit continuum gyrokinetic code.

[abstract 27 / 42] (score: 2)
arXiv:2606.08838 [pdf, ps, other]

Title: Line-of-sight acceleration in compact binaries with higher harmonics and eccentricity

Authors: Soumen Roy, Justin Janquart,

Comments: 18 pages, 8 figures. Updated to match the published version

Subjects: gr-qc astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Direct detections of gravitational waves provide a unique opportunity to probe the astrophysical origin of compact binary mergers. The formation channels of these systems remain highly debated, and a fraction may originate in dynamical environments or ACTIVE GALACTIC NUCLEi. Binaries formed in such environments are expected to experience line-of-sight acceleration from their surroundings, which can imprint characteristic signatures on the observed gravitational-wave signal. Here, we re-derive the line-of-sight acceleration effects and implement them in state-of-the-art quasi-circular waveform models with precession and higher-order modes. We also implement the corrections in eccentric waveform models, applying them consistently to all contributing harmonics. Using this model, we investigate the impact of these effects on the inference of line-of-sight acceleration and analyze a few interesting GWTC events observed during the third observing run of LIGO and Virgo. We find no substantial evidence for line-of-sight acceleration in these events. We also show that an inconsistent treatment of line-of-sight acceleration between higher harmonics can lead to biased conclusions. Our model provides a robust framework for uncovering line-of-sight acceleration in current and future gravitational-wave observations, enabling more accurate probes of environmental signatures in compact-binary formation.

[abstract 28 / 42] (score: 2)
arXiv:2606.23460 [pdf, ps, other]

Title: Phase-mixing of acoustic waves: From the solar tachocline to marine vessel hydroacoustics

Authors: D. Tsiklauri,

Comments: submitted for publication, typos corrected, improvements made

Subjects: astro-ph.SR physics.ao-ph physics.plasm-ph physics.space-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

We adapt the \textit{MAGNETohydrodynamic} wave phase-mixing paradigm [Tsiklauri et al. (2003)] to investigate harmonic wave, Gaussian pulse, and hyperbolic pulse propagation and damping in media governed by transverse sound speed gradients. This universal mechanism is applied to two distinct problems using the harmonic wave phase-mixed solutions. First, we resolve the 26-year-old helioseismic mystery of low-degree global $p$-mode linewidth anomalies observed by BiSON above $ν\approx 3000\,μ\text{Hz}$, demonstrating that a localised, non-turbulent sound speed gradient in the solar tachocline shear zone acts as the necessary high-efficiency energy sink. Second, we translate this formalism into a terrestrial metamaterial fluid engineering design. We show that an engineered microstructured mesh or stern cowl generating a controlled radial sound speed gradient induces severe near-field phase-mixing. This collapses the traditional bulk viscous damping length of $100\text{-kHz}$ propeller acoustic signatures from several kilometres down to a practical design envelope of approximately $10\text{ metres}$, providing an actionable paradigm for compact stealth shielding. Finally, as an independent theoretical extension, we establish a new power-law scaling governing a hyperbolic secant-squared pulse evolution: under developed-stage phase-mixing, its peak wave envelope decays as $\max(P_1) \propto x^{-9/2}$.

[abstract 29 / 42] (score: 2)
arXiv:2606.27661 [pdf, ps, other]

Title: Extreme Mass Ratio Inspirals in Light of Quasi-periodic Eruptions: Milli-Hertz Gravitational Wave Background

Authors: Brennen Black, Xian Chen, Zhen Pan,

Comments: 9 pages, 2 figures, and comments are welcome! This is version 2, revised for clarity and has added discussion. Accepted for publication in PRD

Subjects: astro-ph.HE gr-qc

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Quasi-periodic eruptions (QPEs) are repeated X-ray bursts originating in galactic nuclei. Of the many proposed models, the favored model is the disk-collision model in which a stellar mass orbiter collides with a disk formed from a tidal disruption event, generating flares twice per orbit. In this model QPEs are tracers of circular extreme mass ratio inspirals (EMRIs) and can be used to infer the EMRI formation rate and estimate their contribution to the stochastic gravitational wave background (SGWB) in the Laser Interferometer Space Antenna (LISA) band. Whether the secondary is a stellar-mass BLACK HOLE or a main sequence star is still debated and leads to different results for the EMRI rate and SGWB. We obtain fiducial rates -- subject to systematic uncertainties -- of $R_{\rm SE} = 2.88\times10^{-6}$ per galaxy per year for stellar EMRIs and $R_{\rm BHE} = 6.07\times10^{-6}$ per galaxy per year for BLACK HOLE EMRIs, then estimate their contribution to the SGWB. We find that only BLACK HOLE EMRIs contribute to the 1 - 10 milliHertz band resolvable by LISA, and depending on the secondary mass and formation radius can contribute from just below the LISA sensitivity curve to roughly two orders of magnitude above it. Stellar EMRIs, being tidally disrupted before reaching the 1 - 10 milliHertz band, only contribute to sub-milliHertz frequencies and remain below the LISA sensitivity curve.

[abstract 30 / 42] (score: 2)
arXiv:2608.03593 [pdf, ps, other]

Title: General-RELATIVISTIC structure of two-component quantum dark fermion stars

Authors: Ilídio Lopes,

Comments: 19 pages, 10 figures and 3 tables. Accepted for publication in Phys. Rev. D 114, 035032, Published 25 August, 2026

Subjects: astro-ph.HE astro-ph.GA gr-qc hep-ph hep-th

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

We develop a general-RELATIVISTIC framework for two-component quantum dark fermion stars: equilibrium configurations of two degenerate fermion species governed by gravity, a Yukawa-mediated dark fifth force and a globally relevant Bohm quantum-pressure correction. The treatment retains the full covariant form of the nonlinear Klein--Gordon equation in the Schwarzschild interior, with closure relations valid at arbitrary compactness, from the ultralight baseline up to densities of order $0.16\,\mathrm{fm}^{-3}$ at which RELATIVISTIC scalar densities and self-consistent effective fermion masses become unavoidable. Two Lagrangian parameters, the dark-fermion mass and the dimensionless ratio between the Yukawa channel and gravity, together fix the equilibrium structure; a joint measurement of mass and radius therefore constrains the dark-sector microphysics directly from gravitational-wave observables. For particle masses in the band $10^{-11}$--$10^{-10}$ eV, the configurations exhibit radii of $3$-$24\,\mathrm{km}$ and compactness in the range $0.14$-$0.34$, behaving as dual mimickers: at moderate compactness they overlap with neutron stars in mass, radius, and inspiral frequency; at the most compact end ($κ\sim 0.34$, $R_T/R_S\simeq 1.5$) they cross the photon sphere and could masquerade as low-mass BLACK HOLEs in the mass-gap region. Tidal-deformability measurements discriminate against both populations: the dimensionless tidal deformability is measurably smaller than the neutron-star value yet remains non-zero, unlike that of a genuine BLACK HOLE. These objects populate the sensitivity bands of LISA, the Einstein Telescope, and Cosmic Explorer, producing astrometric microlensing signatures individually resolvable by \textit{Gaia}. The framework thus provides a reproducible and falsifiable template for constraining dark-sector properties through multi-messenger observations.

[abstract 31 / 42] (score: 2)
arXiv:2608.23976 [pdf, ps, other]

Title: An Inverse Grad-Shafranov Neural Network Approach to Tokamak Magnetic Control

Authors: Allen M. Wang, Adriano Mele, Cosmas Heiß, Cristian Galperti, Zander Keith, Alessandro Pau, Antoine Merle, Olivier Sauter, Daniel Gonzalez Castiñeiras, Francesco Carpanese, Federico Felici, Mark Dan Boyer, Cristina Rea, TCV Team, EUROfusion Tokamak Exploitation Team,

Comments:

Subjects: physics.plasm-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

A new approach to tokamak MAGNETic control enabling high-precision plasma shaping and novel real-time adaptability is experimentally demonstrated on the Tokamak a Configuration Variable (TCV). The method is motivated by the insight that, under appropriate assumptions, a real-time inverse Grad-Shafranov solver approximates an optimal control policy for plasma boundary regulation. Building on this, a control architecture is developed in which classical controllers enforce operational constraints while a fast surrogate model provides a real-time inverse mapping from the desired plasma boundary to Poloidal Field Coil currents. Experimental results on TCV demonstrate improved plasma shaping with respect to the standard discharge preparation procedure --- albeit without explicit real-time shape feedback --- while enabling flexible response to asynchronous events. It is shown that a single network provides satisfactory performance across a range of plasma MAGNETic configurations. Real-time adaptivity is demonstrated in simulation, and partially in experiment, through adaptive strike point motion and early termination in response to a real-time trigger. These results suggest a viable path toward MAGNETic control architectures that reduce reliance on dense diagnostic coverage while maintaining high-accuracy plasma shaping, with potential relevance for future fusion power plant operation.

[abstract 32 / 42] (score: 2)
arXiv:2608.25150 [pdf, ps, other]

Title: Wing-Rotor Aerodynamic Interactions in Small UAVs During Hover and Cruise

Authors: Dev Pradeepkumar Nayak, Seungmin Choi, Muhammad Saif Ullah Khalid,

Comments:

Subjects: physics.flu-dyn

Created: 2026-08-25; Updated: 2026-08-27; Datestamp: 2026-08-27

A compact vertical take-off and landing aircraft requires the same tilt-rotor configuration to perform two fundamentally different aerodynamic tasks: sustain hover and deliver efficient cruise. This work investigates the underlying wing-rotor interactions in both operating regimes using a validated unsteady Reynolds-averaged Navier-Stokes equations-based computational framework. For cruise, the advance ratio governs the balance between thrust production, propulsive efficiency, and wake coherence. Lower advance ratios produce a tightly wound slipstream that undergoes strong vortex interactions, leapfrogging, and early wake bifurcation. At higher advance ratios, the slipstream retains a narrower and more coherent JET-like structure, improving propulsive efficiency while reducing both thrust of the propellor and lift of the wing. The flow impingement at the wing is characterized through the approaching, interaction, and convection phases, revealing the combined influence of vortex stretching, wake bifurcation, blockage, image-induced velocity, and streamwise momentum convection on the downstream wake. In hover, the propeller's rotational speed governs the overall aerodynamic performance more strongly than the wing's placement. Although the position of the wing with respect to the propellor modifies the local wake interactions and flow impingement on its leading edge, its influence on the integrated thrust coefficient and figure of merit remains limited. We also explain the vortex and wake dynamics around the propellor and the wing responsible for governing these aerodynamic performance.

[abstract 33 / 42] (score: 2)
arXiv:2608.25341 [pdf, ps, other]

Title: A systematic study of AGN feedback in a disk galaxy using MACER. III. High Gas Fractions in AGN Hosts

Authors: Yuxuan Zou, Feng Yuan, Suoqing Ji, Jinyi Shangguan, Hassen M. Yesuf, Lu Shen, Luis C. Ho,

Comments: 14 pages, 9 figures. To be submitted to ApJ

Subjects: astro-ph.GA

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

We use high-resolution hydrodynamic simulations in the MACER framework to explain why low-redshift PG QUASAR hosts can retain substantial cold-gas reservoirs, with gas fractions and gas-to-stellar mass ratios showing little dependence on instantaneous AGN luminosity. This paper is the third in a series systematically studying AGN feedback in a disk galaxy subject to cosmological gas inflow. The simulations include multiphase gas, STAR FORMATION, stellar feedback, and self-consistent radiative and mechanical AGN feedback. We reproduce the observed weak connection between host-galaxy gas content and AGN luminosity over L_AGN/L_Edd ~ 10^{-5}-10, while the galaxy nevertheless undergoes pronounced gas depletion and star-formation quenching. During the quenching phase, the cold-gas mass declines by nearly three orders of magnitude, and the evolution of the gas distribution shows that AGN feedback progressively removes both cold and hot gas from the galaxy. The STAR FORMATION rate is more closely linked to the cold-gas mass than to AGN luminosity. This behavior arises from a timescale mismatch: AGN luminosity varies on ~ 10^5-10^6 yr timescales, whereas repeated AGN-driven outflows cumulatively deplete the galaxy-scale gas reservoir over ~ 1 Gyr. Our simulations therefore provide a physical explanation for the gas-rich PG QUASAR hosts and show that their observed gas properties are fully consistent with effective, long-term ejective AGN feedback.

[abstract 34 / 42] (score: 2)
arXiv:2608.25400 [pdf, ps, other]

Title: Broadband 12-26 GHz Radio Radiation Reveals Evidence for Micro-flares on AU Mic

Authors: Isaiah I. Tristan, Rachel A. Osten, Yuta Notsu, Adina D. Feinstein, Adam F. Kowalski,

Comments: 11 pages, 4 figures, 4 tables. Accepted to ApJ

Subjects: astro-ph.SR

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

We present sequential 12-18 and 18-26 GHz radio-band ($Ku$, $K$) VLA observations of the 22 Myr dM1e star AU Mic. We detect two flares and two marginal events over a total of 3 contiguous hours on source, resulting in a radio flare rate of $\sim$1 flare hour$^{-1}$. While this rate is consistent with previous $Ku$-band observations, both flaring ($<$1 mJy) and quiescent ($\sim$0.4 mJy) flux densities are significantly lower. Furthermore, the quiescent spectral shape here is distinct, allowing for unique constraints on the radio-emitting sources of AU Mic. The time-averaged quiescent spectrum is best described by gyroSYNCHROTRON radiation with a peak around 17 GHz and an optically thin spectral index of $α\approx -0.6$. We estimate that the source regions have MAGNETic field strengths of $\sim$1 kG and cover a fraction of $<$0.5% of the stellar surface, yet the instantaneous total electron kinetic energies are $\sim$10$^{28}$ erg. The power-law index describing the distribution of electrons with energy derived from the spectral index, $δ\approx 2$, implies a near-continuous injection of electrons. This could arise from micro-flares that occur over the surface of AU Mic that sustain the radio radiation. One clear flare per band occurs, with decay-to-rise $e$-folding time ratios of $3 - 4$, indicating MAGNETic trapping of the electrons. The $Ku$-band flare is optically thick during the rise and peak times, indicating a peak frequency above 18 GHz. Together, these quiescent and flaring characteristics suggest that continuous, unresolved micro-flaring and MAGNETic trapping dominate the non-thermal radio emission of active M-dwarf coronae.

[abstract 35 / 42] (score: 2)
arXiv:2608.25497 [pdf, ps, other]

Title: Energy-dependent extension of the Crab Nebula as a diagnostic of ultrahigh-energy hadronic emission

Authors: Fang-Wu Lu, Bo-Tao Zhu, Ji-Yang Ren, Li Zhang,

Comments: 5 pages, 3 figures. Published in Physical Review D

Subjects: astro-ph.HE

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

The energy-dependent apparent extension of the Crab Nebula, inconsistent with leptonic expectations between X-ray and TeV bands, can be naturally accounted for in a spatially resolved lepto-hadronic scenario. A transition to hadronic dominance occurs above $\sim400~\rm{TeV}$, with nearly pure hadronic emission at PeV energies. The predicted $68\%$ containment radius at $1.1~\rm{PeV}$ is $\sim1.4'$, about six times larger than in purely leptonic scenarios. This energy-dependent extension therefore provides an observable discriminator of the origin of ultra-high-energy gamma-ray emission in pulsar wind nebulae, offering a diagnostic of PeV hadronic acceleration in the Crab Nebula.

[abstract 36 / 42] (score: 2)
arXiv:2608.25526 [pdf, ps, other]

Title: Ion-acoustic eigenmodes in a helical MAGNETic mirror

Authors: Ivan Chernoshtanov,

Comments: 11 pages, 5 figures

Subjects: physics.plasm-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Plasma rotation together with corrugation of MAGNETic field can result in coupling acoustic waves with different azimuthal wavenumbers and formation of eigenmodes with discrete frequency spectrum and zero longitudinal group velocity in a helical MAGNETic mirror. Such eigenmodes can be destabilized by plasma rotation as well as resonant interaction with trapped ions. Equations describing these waves in the linear approximation are derived and results of numerical solving the equations are discussed. Probably these eigenmodes may drive anomalous ions scattering which is needed for effective suppressing flow of rarefied plasma through a helical mirror.

[abstract 37 / 42] (score: 2)
arXiv:2608.25650 [pdf, ps, other]

Title: A unified gas-kinetic wave-particle method for multiscale gas-mixture flow with an elementary chemical reaction

Authors: Cao Junzhe, Wei Yufeng, Long Wenpei, Zhong Chengwen, Xu Kun,

Comments:

Subjects: physics.comp-ph physics.flu-dyn

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Hypersonic flows in the near space often couple continuum-rarefied multiscale effect with finite-rate chemistry. This paper extends the UGKWP method to multiscale gas mixture flows with a single elementary reaction. In the UGKWP method, hydrodynamic waves are employed to describe near-equilibrium distribution functions, and numerical particles are used for the evolution of nonequilibrium ones. The adaptive conversion between waves and particles, guided by the characteristic integral solution, together with the introduction of dt into the flux as an observation scale, has enabled the UGKWP method to succeed in many multiscale problems involving complex physics. In this work, rather than relying on a comprehensive reactive kinetic model for the entire distribution function, chemical source terms are first evaluated at the macroscopic level and then incorporated into the wave-particle update, while free-transport particles are kept chemically inactive in the monatomic setting considered here. This approach leverages the modeling advantages of wave-particle decoupling, facilitating extension to more complex chemical reactions. Moreover, an approximate extension of an advanced multispecies kinetic model is developed in this work for multispecies effect with species number larger than two. The present UGKWP method is assessed for the Zeldovich-type reaction O2+N=NO+O through hypersonic cylinder flows over a wide Knudsen number range, covering chemically inert, forward exothermic, forward endothermic and dE=0 conditions, and through shock structures with hot upstream/downstream equilibrium states. Agreement with DSMC is obtained for gas mixture flow fields, species mole fractions and wall quantities. A three-dimensional side JET flow over a blunt cone is further simulated to demonstrate the three-dimensional capability of the present code.

[abstract 38 / 42] (score: 2)
arXiv:2608.25919 [pdf, ps, other]

Title: HII-CHI-mistry for ACTIVE GALACTIC NUCLEi: optical sulphur abundances without empirical ionisation correction factors

Authors: E. Pérez-Montero, O. L. Dors, I. A. Zinchenko, B. Pérez-D\'ıaz, J. M. V\'ılchez,

Comments: 13 pages, 9 figures. To be submitted to the Open Journal of Astrophysics

Subjects: astro-ph.GA

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

> The determination of sulphur abundances in ACTIVE GALACTIC NUCLEi (AGN) is uncertain because ionisation correction factors (ICFs) are commonly calibrated for HII regions. We extend the HII-CHI-mistry methodology to derive total S/H in AGN from optical emission lines through comparison with photoionisation models. Tests against Seyfert galaxies with direct-method ionic abundances favour models with alpha_ox=-1.2 and a 98% free-electron stopping criterion. While O/H agrees with previous determinations, HCm yields systematically larger S/H than direct-method analyses using classical ICFs, suggesting that these prescriptions underestimate higher sulphur ionisation stages in AGN. Applied to MaNGA Seyfert and LINER-like regions, the method reveals a common positive S/O-metallicity relation. Seyferts show slightly larger median S/O than star-forming regions at high O/H, while LINERs occupy an intermediate position. These differences become weaker when S/H rather than O/H is used as the metallicity tracer, although Seyferts still tend to populate the upper envelope. Sulphur may therefore provide a robust tracer of gas-phase metallicity, while the differences between Seyferts and LINERs may reflect differences in their ionisation conditions and ionising SEDs.

[abstract 39 / 42] (score: 2)
arXiv:2608.25985 [pdf, ps, other]

Title: Particle acceleration in Alfvénic turbulence with a strong guide field

Authors: Daniel Humphrey, Stanislav Boldyrev, Vadim Roytershteyn,

Comments: 8 pages, 5 figures

Subjects: astro-ph.HE astro-ph.GA

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Magnetically dominated Alfvénic turbulence creates an effective environment for particle acceleration. However, when a strong mean field is present, traditional mechanisms like mirror and curvature acceleration become inefficient at explaining non-thermal particle energy distributions. Based on numerical and phenomenological study, we propose that in such turbulence, particles are accelerated in charge-starved current sheets, corresponding to current velocities approaching the speed of light. The distributions of the electric currents, plasma density, fluctuations of electric charge, as well as the energy distributions of accelerated particles, approximately follow log-normal statistics. Non-thermal particle distributions thus arise from particle acceleration in these charge-starved current sheets rather than from conventional FERMI-type particle acceleration by turbulent eddies.

[abstract 40 / 42] (score: 2)
arXiv:2608.26024 [pdf, ps, other]

Title: First Plasma Commissioning and Operational Highlights from India's First Spherical Tokamak at IPR

Authors: Kishore Mishra, Aditya Verma, N. Mansoori, Saurabh Verma, Y. Paravastu, M. S. Khan, Arvind Kumar, S. G. Thatipamula, Vishal Verma, M Sheetal, Mohit, U. Thaker, Ayush, Jignesh Patel, Praveenlal, Jagabandhu Kumar, F. S. Pathan, S. Ranjithkumar, S. Sam, Prasada Rao P., A. Jaiswal, S. Jha, Neelam Ramaiya, Utsav Rajvanshi, Santosh Pandya, K. Tahaliyani, S. Purohit, Suraj Gupta, Kiran Patel, Suman Aich, D. Kumawat, Deepak Kumar, Saurav Kumar, Rahul Kumar, Vismaysinh Raulji, Pramila, Praveena Kumari, Minsha Shah, Arun Prakash, Narendra Bhupesh, Amit K. Singh, V. Menon, Deepti Sharma, A. Kundu, G. K. Rajan, Prakash Parmar, Ankit Kumar, Vishnu Patel, Imran Mansuri, M Bhandarkar, H. Chudasma, Atish Sharma, Kirit Patel, H Masand, Prem Kumar, Parmesh Kumar, S Kannaujiya, Pramod Parmar, Arvind L. Thakur, C. G. Virani, Ankit Kumar, K. K. Ambulkar, Vilas Chaudhari, C. Danani, Y. S. S. Srinivas, S. Nair, K. Mahajan, R. Rajpal, M. B. Chowdhury, Manoj Kumar, S. K. Pathak, P. K. Sharma, Z. Khan, E. Rajendrakumar, J. Ghosh, Team ST,

Comments: 29 pages, 15 figures

Subjects: physics.plasm-ph

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

A compact Spherical Tokamak(ST) is commissioned at Institute for Plasma Research (IPR) to explore low aspect ratio tokamak physics and technologies that complement to the existing high aspect ratio tokamaks namely ADITYA-U and SST-1 by enabling studies on non-inductive startup, current drive in over dense plasmas, and shaped plasma physics on a low cost platform. The device, India's first spherical tokamak has completed major mechanical, MAGNETic, and electrical integration, and the coil system has been successfully tested with series of integrated commissioning. First plasma experiments have been carried out with a modest Ohmic system assisted by a 2.45GHz microwave system, supported by a centralized control and data acquisition system. An initial diagnostic set comprising visible imaging, spectroscopy, MAGNETics, and radiation monitors required for machine operation has been installed. This paper presents the integrated commissioning experiences and first plasma experiments of the newly installed machine.

[abstract 41 / 42] (score: 2)
arXiv:2608.26039 [pdf, ps, other]

Title: Accretion Disk Sizes and Temperature Profiles in Lensed Quasars: NIR Microlensing Challenges Thin Disk Theory

Authors: V. Motta, E. Mediavilla,

Comments: This is the Author Accepted Manuscript of an article published in Astronomy & Astrophysics. The original publication will be available at A&A

Subjects: astro-ph.GA

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Microlensing and reverberation mapping measurements of QUASAR accretion disk sizes and temperature gradients disagree with thin disk theory predictions. Previous microlensing results rely on heterogeneous wavelength coverage -primarily UV broad emission lines (BELs) from small samples -probing the disk only out to a typical radius of $\lesssim$5 light days on average. We use microlensing estimates from an homogeneous sample of near-infrared (NIR) observations of lensed QUASARs (21 image-pairs from 7 lens systems) to extend disk size measurements out to 14 light days. This analysis leverages narrow emission lines (NELs), which provide a more reliable microlensing-free baseline than BEL cores. We derive Bayesian accretion disk size estimates that reproduce the observed microlensing magnifications, as simulated from magnification maps. NEL-based sizes yield a logarithmic slope of $p=0.68\pm0.23$, consistent with prior estimates corresponding to inner disk regions ($r \lesssim$5 light days). Using a new homogeneous NIR dataset that allows us to reach radial distances of up to 14 light days, we find that accretion disks in these previously unexplored regions are also larger and exhibit steeper temperature gradients than thin disk theory predicts. The increased precision allows us to reject the theoretical logarithmic slope $p=4/3$ at the 98\% confidence level. Any hypothesis invoking BLR contamination to explain this discrepancy must account for how such contamination modulates the underlying accretion disk such that the combination of both results in a power law with logarithmic slope $p=0.68\pm0.23$ across a broad wavelength baseline spanning from $\sim$X-Ray to $\sim 5000$Å.

[abstract 42 / 42] (score: 2)
arXiv:2608.26064 [pdf, ps, other]

Title: Neural-Network and Reduced-order Modeling Workflows for AI-Driven CFD: Fast Response Surfaces, Reduced Dynamics and Jet in Cross-flow Examples

Authors: Kaku E. Eduku, Pavel P. Popov, Gustaaf Jacobs,

Comments: 18 pages, 4 figures. Book chapter manuscript. Companion Python/JAX software available at https://github.com/KjayJunIOR/parametric-sindy-jax-gridsearch

Subjects: physics.flu-dyn

Created: 2026-08-26; Updated: 2026-08-27; Datestamp: 2026-08-27

Highly resolved computational fluid dynamics (CFD) simulations are essential for design but too expensive for dense design-space sampling. This chapter presents an AI-driven CFD workflow that combines scalar-response modeling and reduced-order dynamics using JET-in-cross-flow examples. A reacting hydrogen JET-in-cross-flow study is first used to train a multilayer perceptron (MLP) mapping injector spacing to unburnt hydrogen throughput, wall heat transfer, and bulk temperature concentration, with shape-preserving interpolation as a baseline. The CFD samples show a non-monotonic spacing response, and the MLP identifies an intermediate-to-wide favorable region near eight JET diameters. Leave-one-sample-out validation shows strong dependence on the predicted quantity: the bulk temperature concentration is robust, while heat transfer and unburnt hydrogen throughput are substantially harder to predict. Sparse Identification of Nonlinear Dynamics (SINDy) is then used as a reduced-order modeling framework for field-derived Reynolds-stress statistics. The parametric SINDy model provides compact field-level predictions at simulated and out-of-sample spacings, though its aggregate spacing-mean Reynolds-stress error is $6.7\%$ higher than the POD-basis reconstruction because of degradation at selected cases. The broader conclusion is that AI-driven CFD is not a single-model prescription: MLPs are effective for fast scalar responses, while POD--SINDy is better suited when transient reduced dynamics and field-derived statistics are central to the question.