2018ApJ...864...25G

We present a detailed spectral analysis of the black hole binary XTE J1752-223 in the hard state of its 2009 outburst. Regular monitoring of this source by the Rossi X-ray Timing Explorer mission provided high signal-to-noise spectra along the outburst rise and decay. During one full month this source stalled at ∼30% of its peak count rate at a constant hardness and intensity. By combining all the data in this exceptionally stable hard state, we obtained an aggregate proportional counter array spectrum (3-45 keV) with 100 million counts, and a corresponding high energy X-ray timing experiment spectrum (20-140 keV) with 5.8 million counts. Implementing a version of our reflection code with a physical model for Comptonization, we obtain tight constraints on important physical parameters for this system. In particular, the inner accretion disk is measured very close in, at R_in = 1.7 ± 0.4 R_g. Assuming R_in = R_ISCO, we find a relatively high black hole spin (a_* = 0.92 ± 0.06). Imposing a lamppost geometry, we obtain a low inclination (i = 35° ± 4°), which agrees with the upper limit found in the radio (i < 49°). However, we note that this model cannot be statistically distinguished from a non-lamppost model with a free emissivity index, for which the inclination is markedly higher. Additionally, we find a relatively cool corona (57-70 keV) and large iron abundance (3.3-3.7 solar). We further find that properly accounting for Comptonization of the reflection emission improves the fit significantly and causes an otherwise low reflection fraction (∼0.2-0.3) to increase by an order of magnitude, in line with geometrical expectations for a lamppost corona. We compare these results with similar investigations reported for GX 339-4 in its bright hard state.