paper

Evidence for a black hole in the historical X-ray transient A 1524-61 (=KY TrA)

arXiv:2311.08480

Abstract

We present VLT spectroscopy, high-resolution imaging and time-resolved photometry of KY TrA, the optical counterpart to the X-ray binary A 1524-61. We perform a refined astrometry of the field, yielding improved coordinates for KY TrA and the field star interloper of similar optical brightness that we locate $0.64 \pm 0.04$ arcsec SE. From the spectroscopy, we refine the radial velocity semi-amplitude of the donor star to $K_{2} = 501 \pm 52$ km s $^{- 1}$ by employing the correlation between this parameter and the full-width at half-maximum of the H $I^\pm$ emission line. The $r$ -band light curve shows an ellipsoidal-like modulation with a likely orbital period of $0.26 \pm 0.01$ d ( $6.24 \pm 0.24$ h). These numbers imply a mass function $f (M_{1}) = 3.2 \pm 1.0$ M $_{⊙}$ . The KY TrA de-reddened quiescent colour $(r - i)_{0} = 0.27 \pm 0.08$ is consistent with a donor star of spectral type K2 or later, in case of significant accretion disc light contribution to the optical continuum. The colour allows us to place a very conservative upper limit on the companion star mass, $M_{2} \leq 0.94$ M $_{⊙}$ , and, in turn, on the binary mass ratio, $q = M_{2} / M_{1} \leq 0.31$ . By exploiting the correlation between the binary inclination and the depth of the H $I^\pm$ line trough, we establish $i = 57 \pm 13$ deg. All these values lead to a compact object and donor mass of $M_{1} = 5. 8_{- 2.4}^{+ 3.0}$ M $_{⊙}$ and $M_{2} = 0.5 \pm 0.3$ M $_{⊙}$ , respectively, thus confirming the black hole nature of the accreting object. In addition, we estimate a distance toward the system of $8.0 \pm 0.9$ kpc.

7 pages, 5 figures