Chandra Spectroscopy of MAXI J1305-704: Detection of an Infalling Black Hole Disk Wind?. (arXiv:1306.2915v1 [astro-ph.HE])

Chandra Spectroscopy of MAXI J1305-704: Detection of an Infalling Black Hole Disk Wind?. (arXiv:1306.2915v1 [astro-ph.HE]):
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.