2014ApJ...788...53M

We report on a high-resolution 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, Fe XXI, and Fe XXII. Spectral analysis over three wavelength bands with a large grid of XSTAR photoionization models generally requires a gas density of n ≥ 10¹⁷/cm3. Assuming a luminosity of L = 10³⁷ erg/s, fits to the 10-14 Å band constrain the absorbing gas to lie within r = (3.9±0.7)x10³ km from the central engine, or about r = 520±90 (M/5 M_☉) r_g, where r_g= GM/c ². At this small distance from the compact object, gas in stable orbits should have a gravitational redshift of z = v/c ≃ (3±1)x10^–3 (M/5 M_☉), and any tenuous inflowing gas should have a free-fall velocity of v/c ≃ (6±1)x10^–2 (M/5 M_☉)^1/2. The best-fit single-zone photoionization models measure a redshift of v/c = (2.6-3.2)x10^–3. Models with two absorbing zones provide significantly improved fits, and the additional zone is measured to have a redshift of v/c = (4.6-4.9)x10^–2 (models including two zones suggest slightly different radii and may point to lower densities). 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. These tests are particularly important given the anomalously large angle between the source and the optical axis in this observation. 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, as well as the potential role of failed winds in emerging connections between disk outflows and black hole state transitions.