Research Interests
High Mass X-ray Binaries
High Mass X-ray Binaries (HMXB)
X-Ray binaries (XRBs) are accreting systems hosting a star and a compact object. Giacconi (1962) serendipitously discovered the first of these sources, i.e. Sco X-1, during an X-ray observation of the Moon.
There are three types of compact objects that can take part in a binary system: a white dwarf (WD), a neutron star (NS), or a black hole (BH). The figure below shows the different types of XRBs depending on the nature of the compact object. If the compact object is either a NS or a WD, it can feature a strong surface magnetic field B~1012-15 Gauss, or B~106 Gauss, respectively. The magnetic field influences the accretion flow, by driving matter to the accreting column. Strong magnetic field in NS, also, imprints the X-ray spectrum emitted by the compact object, in the form of absorption/emission lines, referred to as cyclotron lines.
A further distinction is based on the mass (and therefore the spectral type) of the companion star. If the mass of the companion is more than 10 Msun the system is referred to as a High Mass X-ray Binary (HMXB). If the mass of the companion star is less than 1 Msun then it is referred to as a Low Mass X-ray binary (LMXB). Finally, if the mass of the companion falls in between 1 Msun and 10 Msun thenit is referred to as an Intermediate Mass X-ray Binary (IMXB).
My research focuses on the class of HMXBs, hosting high mass, early type stellar companion (OB spectral classification). HMXBs can be further divided by the luminosity class of the companion star. If the companion is a dwarf of the main sequence (luminosity class V-III) the system is known as a Be/X-ray Binary (or BeX). If the companion is a supergiant (luminosity class I-II) the system is known as a sgHMXBs.
Super-Giant High Mass X-ray Binaries
Super-Giant High Mass X-ray Binaries (sgHMXB)
Stellar Winds
Stellar Winds
Accretion onto neutron stars
Accretion onto neutron stars
Hydrodynamic Simulations
Hydrodynamic Simulations
Be X-ray Binaries
Be X-ray Binaries (BeXRB)
In Be/X-ray binaries the optical companions are Be stars . Be stars are dwarf, fast-rotating stars, featuring spectral emission lines (explaining the e in their classification). The best studied lines are those of hydrogen (e.g. Balmer series) but Be stars also show He and Fe in emission. Be stars also show an infrared (IR) excess, that is larger than that emitted by an absorption-line B-type star of the same spectral type. The IR excess is related to the presence of a circumstellar disk expelled by the star (equatorial stellar winds).
In Be/X-ray binary systems, the neutron star is orbiting in an eccentric orbit (see the figure below ) crossing the expelled material regularly. This triggers more accretion and therefore more X-ray activity. Be/X-ray binaries can show three different types of activity.
Type I (or normal) outburst: This re-occurring outburst is related to the periastron passage of the neutron star. At its closest proximity to the Be star, the NS is passing through the circumstellar disk. The frequency of the outbursts is related to the orbital period of the source. All Be/X-ray binaries produce type I outburst. The peak luminosity in X-rays is LX~1036-37 erg s-1.
Type II (or giant) outburst: This event is rare and very powerful. It is not related to any orbital modulation, although it occurs close to the periastron. The recurrence of the event is irregular and likely to happen every couple of years. Type II outburst are related to recurrent activity (likely a dramatic expansion of the equatorial disk) of the Be star. Typical peak X-ray luminosity is LX~1038 erg s-1. The latter is close to the Eddington luminosity limit for a NS
X-ray Observatons
X-ray Observations
Optical Observations
Optical Observations