Proof of principle X-ray reflection mass measurement of the black hole in H1743-322

The black hole X-ray binary H1743-322 lies in a region of the Galaxy with high extinction, and therefore it has not been possible to make a dynamical mass measurement. In this paper, we make use of a recent model which uses the X-ray reflection spectrum to constrain the ratio of the black hole mass to the source distance. By folding in a reported distance measurement, we are able to estimate the mass of the black hole to be 12 ± 2 M_☉ (1σ credible interval). We are then able to revise a previous disc continuum fitting estimate of black hole spin a_∗ (previously relying on a population mass distribution) using our new mass constraint, finding a_∗ = 0.47 ± 0.10. This work is a proof of principle demonstration of the method, showing it can be used to find the mass of black holes in X-ray binaries.