Reflection Spectroscopy of the Black Hole Binary XTE J1752−223 in Its Long-stable Hard State

Abstract

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.

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