A Bayesian analysis of X-ray spectra from the black hole LMXB MAXI J1820+070 using XMM-Newton and NuSTAR.

The X-ray spectra of accreting black holes LMXBs contain information about the geometry of the accretion flow - especially its closest approach to the black hole - and potentially the spacetime near the black hole, determined by its mass and spin. In practice, inferences about the spin and inner flow geometry remain controversial despite major improvements in signal to noise and resolution of the data. Rather than attempt to “measure” the spin or the inner radius from a spectral model with spin or R_in as a free parameter, we attempt to fit two specific models – one with spin a = 0 and R_in = 6r_g, one with spin a = 0.998 and R_in ~ 1r_g – to understand under what conditions these models can be made to reproduce the data for LMXB systems, if they can. We adopt a fully Bayesian approach with the best available prior information on mass, distance and inclination, and the latest and most self-consistent combination of various emission components along with the most accurate models of interstellar absorption, based on XMM-Newton RGS data. Our test case for this project is the recently confirmed black hole LMXB MAXI J1820+070. We fit data over the 0.8 – 70 keV range using 3 XMM-Newton and 2 simultaneous NuSTAR observations.