Research

 X-ray binaries are compact binaries dominated by accretion processes onto a neutron star (NS) or black hole (BH). A subgroup of these (the so-called X-ray transients) are characterized by recurrent outbursts (with a timescale of decades) when luminosity increases by a factor of 103–106 in the optical and X-ray domain, respectively. These systems are particularly important since they have provided the most solid BH candidates via the determination of companion star's mass function.

 The analysis of these compact remnants is fundamental to our knowledge of the late stages of massive star evolution. Unfortunately, the current number of detected BHs is still too low to perform a statistical analysis of the BH binary population with respect to the NS binary population.

 This project has the following scientific goals:

  1. To increase the sample of BHs by measuring mass functions of newly discovered X-ray transients. Also, to determine mass ratios and inclinations in order to derive the masses of the two stars and hence the nature of the compact objects (NS and BH). Several spectrophotometric techniques will be exploited in the optical and IR in order to do so.
  2. To perform a statistical analysis of the sample of BH binaries with respect to NS binaries (e.g. the distribution of masses, mass ratios, galactic distribution) in order to characterize the two populations of compact objects. We aim to derive bounds to the mass distribution and set constraints on the equation of state of nuclear matter (e.g. Mmax and Mmin of NSs and BHs) and the mass loss of the progenitors. On the other hand, we expect to constrain the age and evolutionary state of these binaries.
  3. To study the structure of accretion discs in different energy bands (optical–X-ray). The high energy spectral distribution and time variability during outburst is important to constrain the eruption models and accretion disc properties (e.g. the radius of the advective disc of ADAF). We may also constrain the nature of the compact object through the analysis of X-ray emission line profiles (e.f. 6.4 keV). In the optical range, we will study the orbital modulation of emission-line profiles using Doppler tomography techniques. These will yield the radial distribution of emissivity in the disc and will set constraints on the disc radius, mass transfer rate and evolutionary state of the companion stars.

 In addition, we have opened up a new window with the discovery of fast optical variability on the scale of minutes and seconde) from quiescent accretion discs in several BHs and NSs. It is important to expand the sample and study the spectrum of the variability to set constraints on the mechanism responsible for the variability. For instance, the analysis of quasi-periodic oscillations (QPOs) and noise properties will enable us to distinguish between possible irradiated warped disc models and learn about disc instabilities.

 On the other hand, the study of photometric variability during outburst episodes and quiescence will enable us to determine fundamental parameters, such as Porb and inclinations (through eclipses and irradiation effects) and the binary mass ratio through the 'superhump' period (beat period between the disc precession period and Porb).

 Furthermore, we plan to study the chemical composition of companion stars and, in particular, prove the origin of the large Li and a-element abundances discovered by our group. Therefore, we will:

  1. Perform metallicity analysis of the companion stars to find evidence of the SN explosion which formed the BH/NS. Abundance anomalies will enable us to trace the evolutionary history of the progenitor stars.
  2. Search for evidence of Li in both accretion discs and the atmosphere of the companion stars. The isotropic Li7/Li6 ratio is a good tracer of spallation mechanisms, which are thought to produce these elements in the environment of BHs and NSs.