J. M. Miller, J. Raymond, T. R. Kallman, D. Maitra, A. C. Fabian, D. Proga, C. S. Reynolds, M. T. Reynolds, N. Degenaar, E. M. Cackett, J. Kennea, A. Beardmore
We report on a Chandra/HETG X-ray spectrum of the transient X-ray binary MAXI J1305-704. A rich absorption complex is detected in the Fe L band, including density-sensitive lines from Fe XX, XXI, and XXII. Spectral analysis over three wavelength bands with a large grid of XSTAR photoionization models generally requires a gas density of n > 1 E+17 cm^-3. Assuming a luminosity of L = 1 E+37 erg/s, fits to the 10-14 Angstrom band constrain the absorbing gas to lie within r = 3.9 +/- 0.7 E+3 km from the central engine, or about r = 520 +/- 90 (M/5 Msun) r_g, where r_g = GM/c^2. At this small distance from the compact object, gas in stable orbits should have a gravitational red-shift of z = v/c = 3 +/- 1 E-3 (M/5 Msun), and any tenuous inflowing gas should have a free-fall velocity of v/c = 6 +/- 1 E-2 (M/5 Msun)^(1/2). The best-fit single-zone photoionization models measure a red-shift of v/c = 2.6-3.2 E-3. Models with two absorbing zones provide significantly improved fits, and the additional zone is measured to have a red-shift of v/c =4.6-4.9 E-2. Thus, the observed shifts are broadly consistent with those expected at the photoionization radius. The absorption spectrum revealed in MAXI J1305-704 may be best explained in terms of a "failed wind" like those predicted in some recent numerical simulations of black hole accretion flows. The robustness of the velocity shifts was explored through detailed simulations with the Chandra/MARX ray-tracing package, and analysis of the zeroth-order ACIS-S3 spectrum. The simulations and ACIS spectrum suggest that the shifts are not instrumental; however, strong caution is warranted. We discuss our results in the context of accretion flows in stellar-mass black holes and active galactic nuclei, and the potential role of failed winds in emerging connections between disk outflows and black hole state transitions.
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http://arxiv.org/abs/1306.2915
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