High Resolution Chandra HETG spectroscopy of V404 Cygni in Outburst

TL;DR

This study uses high-resolution Chandra HETG spectroscopy to analyze the outburst of V404 Cygni, revealing emission lines, disk wind signatures, and obscuration effects that inform the structure and behavior of the accretion disk and central engine.

Contribution

First detailed high-resolution X-ray spectral analysis of V404 Cygni's 2015 outburst, providing insights into disk structure, wind properties, and obscuration effects.

Findings

Detection of strong emission lines and Fe Kα line indicating obscuration

Observation of P-Cygni profiles suggesting a powerful disk wind

Estimates of emission region distances from ionization states

Abstract

As one of the best-characterized stellar-mass black holes, with good measurements of its mass, distance and inclination, V404 Cyg is the ideal candidate to study Eddington-limited accretion episodes. After a long quiescent period, V404 Cyg underwent a new outburst in June 2015. We obtained two Chandra HETG exposures of 20 ksec and 25 ksec. Many strong emission lines are observed; the ratio of Si He-like triplet lines gives an estimate for the formation region distance of $4\times10^{11}$ cm, while the higher ionization Fe XXV He-like triplet gives an estimate of $7\times10^9$ cm. A narrow Fe K$\alpha$ line is detected with an equivalent width greater than 1 keV in many epochs, signaling that we do not directly observe the central engine. Obscuration of the central engine and strong narrow emission lines signal that the outer disk may be illuminated, and its structure may help to drive the strong variability observed in V404 Cyg. In the highest flux phases, strong P-Cygni profiles consistent with a strong disk wind are observed. The kinetic power of this wind may be extremely high.