18 February 2020X-ray binaries: from the lowest luminosities to the ultra-luminous X-ray sources

Mathieu SERVILLAT
CEA Saclay / Harvard-Smithsonian CfA

The population of quiescent X-ray binaries and the ultra-luminous X-ray sources are two classes of X-ray binaries for which the nature is still debated. The latter objects may host a black hole of intermediate mass, and we expect to find in the Galaxy a large population of quiescent black holes or white dwarfs in high mass X-ray binaries that is still elusive. The detection of such objects can help us to better understand how the Galaxy and its central black hole formed. DASCH (Digital Access to a Sky Century at Harvard, i.e. 530 000 photographic plates taken between the 1880s and 1990s) will open a new window in the time domain astronomy which is promising for the detection of quiescent high mass X-ray binaries. I will present the 100-yr light curves of the 4 high mass X-ray binary candidates covered by the current DASCH data (about 8specific fields). In particular, the B0.5IIIVe star SAO 49275 shows significant slow variability of 1 magnitude on time scales 10-50 years. This variability might be linked to the formation and disappearance of the decretion disk of the Be star, or to the presence of a compact object companion, possibly a white dwarf. In a second part, I will focus on the study of the ultra-luminous X-ray source ESO 243-49 HLX-1 with >1042 erg/s, the best intermediate mass black hole candidate currently known. The source was observed in two main, well defined X-ray spectral states that are consistent with the thermal and the hard states, reminiscent of Galactic stellar-mass black hole binaries. Both beamed emission and super-accretion are thus unlikely, leading to a constraint of the mass of >9000 Msun. I will then discuss the spectral energy distribution obtained with HST and Swift XRT which suggests the presence of 10⁷-10⁸ yr stellar population surrounding the black hole.