Institute professor at the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences in Warsaw, Poland.
CV Publications arXiv/astro-ph filter (old version)
CAMK PAN, ul. Bartycka 18, 00-716 Warszawa, POLAND
tel: (+48 22) 3296-128
Observations of the Universe in energetic gamma-rays by Fermi and AGILE space observatories, as well as by ground-based Cherenkov telescopes H.E.S.S., MAGIC, VERITAS reveal strongly variable luminous gamma-ray sources, in particular blazars and pulsars. The production of such gamma rays must involve efficient particle acceleration in very compact regions, which makes one of the most interesting challenges in modern astrophysics. The leading theoretical scenario involves dissipation of magnetic energy, microscopically by means of magnetic reconnection, macroscopically by means of plasma instabilities and magnetized turbulence. These processes are still poorly understood, however, a rapid progress is being made by means of direct plasma simulations, in particular by the fully kinetic particle-in-cell algorithm. Such investigations are now also performed in Poland.
This timely project is supported by the National Science Centre with a 5-year research grant SONATA BIS. I have an active international collaboration with two research groups centered at the University of Colorado Boulder and Stanford University. Numerical simulations have been pefrormed on large supercomputers: Mira at ALCF (USA), and Prometheus at Cyfronet, AGH (Poland).
The research group includes two Ph.D. students: Qiang Chen and Jose Ortuno-Macias. Former postdocs: Varadarajan Parthasarathy, Krzysztof Hryniewicz.
Magnetic reconnection is the primary mechanism of energy dissipation in highly magnetized plasmas, including those in relativistic jets of blazars. With Ph.D. student Jose Ortuno-Macias, we performed kinetic numerical simulations of relativistic magnetic reconnection with open boundaries on the left and right sides of the simulation box. This allowed free escape of structures forming in magnetic reconnection layer, in particular the quasi-circular plasmoids. In the above figure, the green lines are the magnetic field lines and the color map indicates horizontal velocity: the red regions move to the right and the blue regions move to the left. In the right half of the figure one can see several red plasmoids, with the two largest ones about to merge: even though they both move to the right, the largest one is slower than the one immediately to the left. We argue that such tail-on mergers are important for production of rapid radiation flares. On the left side of the figure there is a conical blue region called a minijets. We demonstrate that minijets co-exist with the plasmoids, they can accelerate particles to higher energies, but they are not so important for radiation signatures because of their relatively low particle densities. (arXiv:1911.06830, Nov 2019)
M87* is the name of the supermassive black hole located at the heart of the giant elliptical galaxy M87. Besides Sgr A* located at the center of our Milky Way galaxy, it is one of the largest black holes in terms of angular size. It was the target of observations by the Event Horizon Telescope (EHT) project, which produced the first resolved image of immediate black hole environment, filled by energetic plasma that shines in radio, millimeter, infrared, X-ray and gamma-ray bands. The image has a characteristic shape of a crescent with a dark central shadow. According to General Relativity, such crescent results from relativistic bending of light rays that form a characteristic structure called photon ring. The angular size of the crescent is 42 micro-arcseconds, which can be used to estimate the black hole mass as 6 billion solar masses. The crescent is generally oriented along the southern side of the photon ring, the brightness contrast results from Doppler beaming due to relativistic motions of plasma around the black hole. The EHT image has been compared with images obtained from computer simulations of plasma accreting onto rotating black holes. Simulated images need to be properly scaled, centered and rotated. Every simulation has a well defined direction of the black hole spin vector. Simulations also produce a pair of relativistic jets - these are fast magnetized outflows directed along the black hole spin far away from the black hole. A relativistic jet is also observed in galaxy M87, not by the EHT, but by various radio telescopes observing at longer wavelengths and probing larger distances from the black hole. It is in fact a very famous jet, discovered in 1918 and observed in great detail. Based on established theory for the origin of jets, the spin of black hole M87* should be directed along the M87 jet. However, when the black hole spin is aligned with the M87 jet, the simulated images do not fit to the EHT image of M87*. The best fit is obtained for the projected black hole spin oriented 80 degrees from the projected jet direction. It appears that the crescent image of M87* has a wrong orientation. However, numerical simulations of black hole accretion produce images that are highly variable, with fluctuations that can appear in different sectors of the innermost accretion flow that is magnified into different sections of the photon ring. We argue the SEE section of the M87* image cannot be explained by intrinsically symmetric models with proper orientation of the black hole spin vector. This part of the image should disappear in future observations of M87*. (arXiv:1908.10376, Aug 2019)
Numerical investigation of the "ABC" magnetic fields was extended into the third dimension with the first kinetic simulations of the lowest-order unstable isotropic magnetic configurations of this kind. Most results of the 2D investigations are confirmed: linear growth rate of the coalescence instability, formation of kinetically thin current layers, non-thermal particle acceleration, conservation of total magnetic helicity, and slowly damped oscillations around the asymptotic final state. Dissipation of magnetic energy is enabled by localized magnetic reconnection, which ends with dramatic mergers of current layer pairs connected by common magnetic flux. Particle acceleration proceeds efficiently due to electric fields roughly perpendicular to the local magnetic fields, with reduced role of the non-zero E.B scalar. This article was published in the Monthly Notices of the Royal Astronomical Society. (arXiv:1809.07773, Sep 2018)publication and supplementary movies
Yet another dramatic gamma-ray flare in blazar 3C 279 was observed by the Fermi Large Area Telescope with unprecedented detail, thanks to a timely pointing of the telescope at the source. Fermi operates almost exclusively in the survey mode, sweeping most of the sky every two orbits (3h), pointed observations are very rare and risky, as it is impossible to predict in advance the exact moment of the flare peak. This time it worked perfectly, and it allowed for significantly deeper exposure and flux measurements on time scales as short as 2 min. For the first time, a blazar was found to be variable at such short time scales in the GeV photon energy range (similar variations were previously observed by ground-based Cherenkov telescopes in TeV blazars). This simple observational fact poses extreme requirements on the physical parameters of the gamma-ray emitting region. In fact, we don't have a good explanation for such extremely short gamma-ray variability time scale. This official article of the Fermi LAT Collaboration, led by Masaaki Hayashida and Grzegorz Madejski, was published in the Astrophysical Journal Letters. (arXiv:1605.05324, May 2016)
Numerical simulations of magnetic reconnection are typically initiated from the Harris equilibrium involving a predefined current layer of microphysical thickness scale. "ABC" fields are simple magnetostatic waves supported by smooth currents, and they were found to be generally unstable to coalescescence instability, in which magnetic domains of the same sign attract each other and tend to merge, increasing the typical domain size (inverse cascade). These mergers produce temporary evolving current layers that allow for efficient reconnection and particle acceleration. In collaboration with Jonathan Zrake, Yajie Yuan, William East and Roger Blandford (Stanford University), we performed one of the first particle-in-cell simulations of mildly relativistic "ABC" fields. We described in detail the complex evolution of the current layers and the resulting particle acceleration. This article was published in the Astrophysical Journal. (arXiv:1603.04850, Mar 2016)
Magnetic reconnection allows for efficient conversion of magnetic energy into particle energy. It is considered as a plausible energy dissipation mechanism in highly magnetized relativistic plasma environments, e.g. in relativistic jets or pulsar wind nebulae. It is a highly complex, non-linear process, the understanding of which requires computationally expensive numerical simulations. In this work, we analyze the results of a single kinetic "particle-in-cell" simulation of relativistic reconnection in order to identify the most important particle acceleration sites. We consider the following three types of structures: magnetic X-points, plasmoids, and regions between merging plasmoids. This article, written in collaboration with Dmitri Uzdensky, Gregory Werner, Mitchell Begelman (University of Colorado) and Benoit Cerutti (Princeton University), was published in the Astrophysical Journal. The figure illustrates the time evolution of the plasma temperature profile along the main reconnection plane (see a high-resolution version; 3.7 MB). (arXiv:1508.02392, Aug 2015)
Fermi Large Area Telescope detected an unusual gamma-ray flare in well known blazar 3C 279 in December 2013. The main characterisics are: a very short variability time scale estimated at 2 hours, and very hard gamma-ray spectrum with photon index about 1.7, extending up to the photon energy of 5 GeV. The lack of simultaneous optical activity gives a very strong constraint on the so-called Compton dominance parameter, which in turn indicates a very strongly matter dominated emitting region with magnetic power of order 0.0001 of the total power. This result is a serious challenge to the models of dissipation in relativistic jets that rely on the process of magnetic reconnection. This article, led by Masaaki Hayashida and including the Fermi LAT Collaboration, was published in the Astrophysical Journal. (arXiv:1502.04699, Jul 2015)
In the most luminous blazars, the observed emission is strongly dominated by non-thermal component peaking in the gamma-ray band. It is thought to be produced in a relativistic jet by inverse Compton scattering of external radiation fields. There is an old debate about the exact location of the main gamma-ray emitting regions, and their physical parameters. We performed a systematic study of the constraints on the parameter space of emitting regions in luminous blazars, and we applied it to 7 actual flares observed in blazars by the Fermi Large Area Telescope supported by numerous multiwavelength observatories. In every case, our constraints define a relatively narrow region of allowed values of the distance scale and the Lorentz factor. For Lorentz factors of about 20, the preferred distance scale is about 0.1-1 pc. We propose this method to be used in multiwavelength studies of individual blazars in addition to the standard practice of modeling the observed spectral energy distribution. This article, written in collaboration with Mitch Begelman and Marek Sikora, was published in the Astrophysical Journal. (arXiv:1405.7694, May 2014)
I work on relativistic jets in active galactic nuclei. Our study focuses on their physical structure, energy dissipation mechanisms and non-thermal radiative processes.
Recently, I also began kinetic numerical simulations of relativistic magnetic reconnection, a dissipation mechanism that can be important in highly magnetized plasmas including the relativistic jets.
Modeling of nonthermal leptonic processes in the context of blazars, including synchrotron radiation with polarization and self-absorption, Inverse Compton scattering with full cross section, evolution of electron distribution, pair production absorption, special-relativistic effects.
Observational properties of blazars and radio galaxies, the structure of relativistic jets and dissipation mechanisms responsible for non-thermal emission: shock waves and magnetic reconnection.
Analysis of gamma-ray data from the Fermi Large Area Telescope. Coordination of multiwavelength campaigns on blazars.
Performing kinetic (particle-in-cell) parallelized numerical simulations with the Zeltron code on various supercomputers to investigate the dynamics, particle acceleration and radiation distribution in relativistic magnetic reconnection.
Last update: Mar 17th, 2022