# Dramatic Variability of X-ray Absorption Lines in the Black Hole Candidate Cygnus X-1

## TL;DR

This study reports highly variable X-ray absorption lines in Cygnus X-1, revealing rapid changes in the ionized stellar wind's properties over hours, with minimal overall flux variation.

## Contribution

First detection of dramatic, hour-scale variability in X-ray absorption lines in Cygnus X-1, providing new insights into the ionized stellar wind dynamics.

## Key findings

- Absorption lines vary by nearly two orders of magnitude within hours.
- Overall X-ray flux varies only by 20-30%.
- Implications for understanding stellar wind conditions.

## Abstract

We report results from a 30 ks observation of Cygnus X-1 with the High Energy Transmission Grating Spectrometer (HETGS) on board the {\em Chandra X-ray Observatory}. Numerous absorption lines were detected in the HETGS spectrum. The lines are associated with highly ionized Ne, Na, Mg, Al, Si, S, and Fe, some of which have been seen in earlier HETGS observations. Surprisingly, however, we discovered dramatic variability of the lines over the duration of the present observation. For instance, the flux of the Ne X line at 12.14 \AA\ was about $5 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$ in the early part of the observation but became subsequently undetectable, with a 99% upper limit of $0.06 \times 10^{-3}$ photons cm$^{-2}$ s$^{-1}$ on the flux of the line. This implies that the line weakened by nearly two orders of magnitude on a timescale of hours. The overall X-ray flux of the source did also vary during the observation but only by 20--30%. For Cyg X-1, the absorption lines are generally attributed to the absorption of X-rays by ionized stellar wind in the binary system. Therefore, they may provide valuable diagnostics on the physical condition of the wind. We discuss the implications of the results.