Ignacio González Martínez-País, Pablo Rodríguez Gil, Montserrat Armas Padilla, Jorge Casares Velázquez, Manuel Linares Alegret, Cristina Zurita Espinosa, Pedro José Blay Serrano, Artemio Herrero Davó, Daniel Mata Sánchez, Manuel Pérez Torres, Vikram Dhillon, Garik Israelian, Tariq Shahbaz, Felipe Jiménez Ibarra, Sergio Palomo Nevado, Jonay González Hernández
P. Charles (Univ. Southampton); P. Jonker (Univ. Utrech); T. Marsh, D. Steeghs, B. Gaensicke (Univ. Warwick); R. Fender, S. Motta, K. O'Brien (Univ. Oxford); J.M. Paredes, M. Ribó (Univ. Barcelona, Spain); J. Marti (Univ. Jaén, Spain); T. Belloni, S. Campana, P. D’Avanzo (Obs. Brera); I. Negueruela, (Univ. Alicante, Spain); J.M. Corral-Santana (ESO, Chile); D. Russell (Univ. Abu Dhabi); G. Ponti, B. de Marco (MPE); Rudy Wijnands, N. Degenaar (Univ. Amsterdam); A. de Ugarte (IAA, Spain); Y. Ueda (Univ. Kyoto); R.I. Hynes (Univ. Louisiana).
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 actual number of BHs is still too low to perform statistical analysis of the BH binary population with respect to the NS binary population.
This project has the following scientific goals:
1- 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 or BH). Several spectrophotometric techniques will be exploited in the optical and IR in order to do so (including the analysis of fluorescence Bowen emission from the companion star, a new avenue opened by our group). We are also exploring new strategies to enlarge significantly the number of quiescent BHs known. When the sample is large enough we will perform statistical analysis of BH binaries versus NS binaries (e.g. 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 to the equation of state of nuclear matter (e.g. Mmax and Mmin of NS and BHs), age and evolutionary history of these binaries.
2- Study the structure of accretion discs in different energy bands (optical-X rays). The high energy spectral distribution and time variability during outburst is important to constrain the eruption models and accretion disc properties (e.g. radius of adjective disc or ADAF). In the optical range, we will study the spectral evolution through the outburst/quiescent phases and 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 constrains to disc radius, mass transfer rate and evolutionary state of the companion stars. In addition, we have opened a new window with the discovery of fast optical variability (mins-segs) from quiescent accretion discs in several BHs and NS. It is important to expand the sample and study the spectrum of the variability to set constraints to the mechanism responsible for this 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 inclination (through eclipses and irradiation effects) and the binary mass ratio through the “superhump” period (beat period between the disc precession period and Porb).
3- Study gamma-ray binaries, formed by a young pulsar orbiting a massive star. The interaction of the stellar and pulsar winds triggers very high energy emission (MeV-TeV), which is strongly modulated with the orbital period and is detected by Cherenkov telescopes such as MAGIC and gamma-ray satellites like Fermi. Intensive optical monitoring allows to determine the fundamental binary parameters. We have recently discovered the first BH in a gamma-ray binary.
Furthermore, we plan to study the chemical composition of companion stars and, in particular, prove the origin of the large Li and α-element's abundances discovered by our group. Therefore:
1. We will 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 both in the accretion discs and the atmosphere of the companion stars. The isotopic ratio Li7/Li6 is a good tracer of spallation mechanisms which are thought to produce these elements in the environment of BH and NS.
- Our team has led the multi-wavelength study of the 2015 outburst of the BH transient V404 Cyg, arguably one of most interesting events of this class ever observed. This led to a very relevant publication (Muñoz-Darias et al. 2016, Nature) where the discovery of an accretion disc wind outflow – simultaneous with the radio jet and variable X-ray emission – is reported.
- BlackCAT: A catalogue of stellar-mass black holes in X-ray transients. In 2016 our team led a paper presenting an ambitious stellar-mass black hole catalogue. It includes both dynamically confirmed black holes and strong candidates, together with a population study. The fast growing number of citations shows the potential impact of this work in the research community (Corral-Santana, Casares, Muñoz-Darias et al., 2016, A&A).