X-ray reverberation black hole mass and distance estimates of Cygnus X-1

Abstract

We fit X-ray reverberation models to Rossi X-ray Timing Explorer data from the X-ray binary Cygnus X-1 in its hard state to yield estimates for the black hole mass and the distance to the system. The rapid variability observed in the X-ray signal from accreting black holes provides a powerful diagnostic to indirectly map the ultra-compact region in the vicinity of the black hole horizon. X-ray reverberation mapping exploits the light crossing delay between X-rays that reach us directly from the hard X-ray emitting 'corona', and those that first reflect off the accretion disc. Here we build upon a previous reverberation mass measurement of Cygnus X-1 that used the RELTRANS software package. Our new analysis enhances signal to noise with an improved treatment of the statistics, and implements new RELTRANS models that are sensitive to distance. The reduced uncertainties uncover evidence of mass accretion rate variability in the inner region of the disc that propagates towards the corona, which we approximate as a point-like 'lamppost' source. Our best fitting model returns a mass of $M=15\pm 4 ~M_\odot$ and a distance of $D=3.4_{-1.2}^{+1.6}$ kpc (90 per cent uncertainties), which are consistent with the most recent dynamical and parallax measurements respectively.