A complex environment around Cir X-1

TL;DR

This paper analyzes Chandra X-ray observations of Cir X-1, revealing state-dependent spectral features, including emission and absorption lines, and suggests changes in accretion geometry and ionized plasma conditions during flaring activity.

Contribution

It provides the first detailed spectral analysis of Cir X-1 across different flux states, highlighting the presence of broadened absorption lines and ionized plasma characteristics.

Findings

Hard state shows emission lines of highly ionized elements.

Flaring state exhibits strong resonant absorption lines, including a broadened Fe K alpha feature.

Spectral features indicate changes in accretion and ionized plasma conditions.

Abstract

We present the results of an archival 54 ks long Chandra observation of the peculiar source Cir X--1 during the phase passage 0.223-0.261. A comparative analysis of X-ray spectra, selected at different flux levels of the source, allows us to distinguish between a very hard state, at a low countrate, and a brighter, softer, highly absorbed spectrum during episodes of flaring activity, when the unabsorbed source luminosity is about three times the value in the hard state. The spectrum of the hard state clearly shows emission lines of highly ionized elements, while, during the flaring state, the spectrum also shows strong resonant absorption lines. The most intense and interesting feature in this latter state is present in the Fe K alpha region: a very broadened absorption line at energies ~ 6.5 keV that could result from a smeared blending of resonant absorption lines of moderately ionized iron ions (Fe XX - Fe XXIV). We also observe strong resonant absorption lines of Fe XXV and Fe XXVI, together with a smeared absorption edge above 7 keV. We argue that the emitting region during the quiescent/hard state is constituted of a purely photo-ionized medium, possibly present above an accretion disk, or of a photo-ionized plasma present in a beamed outflow. During the flaring states the source undergoes enhanced turbulent accretion that modifies both the accretion geometry and the optical depth of the gas surrounding the primary X-ray source.