XRISM Spectroscopy of the Stellar-Mass Black Hole 4U 1630-472 in Outburst

TL;DR

This study uses XRISM spectroscopy to analyze the complex, variable highly ionized absorption features in the stellar-mass black hole 4U 1630-472 during its 2024 outburst, revealing unprecedented details at low Eddington fractions.

Contribution

It provides the first detailed high-resolution spectral analysis of highly ionized winds in a black hole binary at low Eddington fractions, uncovering complex absorption and possible failed winds.

Findings

Detection of highly ionized absorption at low Eddington fraction (~0.05)

Resolved Fe XXV and Fe XXVI lines with detailed velocity and ionization structure

Evidence for variable obscuration and a tentative very fast outflow (0.026-0.033c)

Abstract

We report on XRISM/Resolve spectroscopy of the recurrent transient and well-known black hole candidate 4U 1630$-$472 during its 2024 outburst. The source was captured at the end of a disk-dominated high/soft state, at an Eddington fraction of $\lambda_\mathrm{Edd} \sim 0.05~(10 M_{\odot}/M_\mathrm{BH})$. A variable absorption spectrum with unprecedented complexity is revealed with the Resolve calorimeter. This marks one of the lowest Eddington fractions at which highly ionized absorption has been detected in an X-ray binary. The strongest lines are fully resolved, with He-like Fe XXV separated into resonance and intercombination components, and H-like Fe XXVI seen as a spin-orbit doublet. The depth of some absorption lines varied by almost an order of magnitude, far more than expected based on a 10% variation in apparent X-ray flux and ionization parameter. The velocity of some absorption components also changed significantly. Jointly modeling two flux segments with a consistent model including four photoionization zones, the spectrum can be described in terms of highly ionized but likely failed winds that sometimes show red-shifts, variable obscuration that may signal asymmetric structures in the middle and outer accretion disk, and a tentative very fast outflow ($v = 0.026-0.033c$). We discuss the impact of these findings on our understanding of accretion and winds in stellar-mass black holes, and potential consequences for future studies.