Observational evidence for stellar-mass black holes

Abstract

Abstract. Radial velocity studies of X-ray binaries provide the most solid evidence for the existence of stellar-mass black holes. We currently have 20 confirmed cases, with dynamical masses in excess of 3 M⊙. Accurate masses have been obtained for a subset of systems which gives us a hint at the mass spectrum of the black hole population. This review summarizes the history of black hole discoveries and presents the latest results in the field.

The Historical X-ray Transient KY TrA in quiescence

“…A single spectrum obtained in 2004 May shows the Hα emission characteristic of X-ray transients in quiescence with a full-width-half-maximum FWHM 2700± 280 km s −1 . If the system follows the FWHM -K 2 correlation found by Casares (2015), this would correspond to a velocity semi-amplitude of the donor star of K 2 =630±74 km s −1 . Based on the outburst amplitude and colours of the optical counterpart in quiescence we derive a crude estimate of the orbital period of 8 h and an upper limit of 15 h which would lead to mass function estimates of ≈ 9 M and <16 M respectively.…”

“…After analyzing the Hα emission profiles of 12 dynamically confirmed black holes and 2 neutron star X-ray transients (XTs) in quiescence, Casares (2015) has found a tight correlation between the FWHM of the Hα line and the velocity semi-amplitude of the donor star, where K 2 =0.233(13)×FWHM. We have applied this relation to KY TrA and predict K 2 =630±74 km/s.…”

“…However, given the contaminating flux from an interloper, the variability would be diluted to the extent that any intrinsic variability above ∼0.15 mag would not be detectable given our error levels, and so the inclination might not be as low. Interestingly, KY TrA has one of the broadest Hα lines among SXTs (a summary of the parameters of the XTs can be found in Casares (2015) Table 3), suggesting that, if KY TrA is viewed at low inclination, it must have a very short orbital period. Some clues about the inclination are provided by the EW of the Hα line, since it depends on the binary geometry.…”

“…After subtracting quadratically the instrumental resolution we find FWHM=2700±280 km/s where the error is the formal 1-σ on the fitted parameter as derived through χ 2 minimization. We note that the quoted error is within the typical 10% standard deviation caused by intrinsic line variability and therefore we take this as realistic (Casares 2015). We also extracted the equivalent width (EW) by in- tegrating the Hα flux after continuum normalization and find EW 72 ± 7Å.…”

“…The distribution of black hole masses can only be determined from the study of X-ray binaries (eg., Casares 2007) and it is intricately related to the population and evolution of massive stars, the energetics and dynamics of supernova explosions, and the critical mass dividing neutron stars and black holes. Several attempts to extract statistical information from the observed mass distribution have been made (Bailyn et al 1998;Özel et al 2010;Farr et al 2011;Kreidberg 2012), however, the small number of black holes prevents us from extracting very compelling conclusions.…”