Revealing the state transition of Cen X-3 at high spectral resolution with Chandra

TL;DR

This study uses high-resolution Chandra X-ray spectra to analyze the rapid state transition of the high-mass X-ray binary Cen X-3, revealing plasma cooling, increased absorption, and wind clump interactions during the episode.

Contribution

It provides the first detailed phase-resolved spectral analysis of Cen X-3's flux transition, highlighting plasma cooling and wind clump effects with high spectral resolution.

Findings

Large local column density during transition causes Fe Kα Compton shoulder.

Rapid plasma cooling occurs within 10 ks, enabling matter entry into the magnetosphere.

Detection of Doppler modulation in Fe XXV line indicating wind clumps.

Abstract

Cen X-3 is a compact, high-mass X-ray binary (HMXRB), likely powered by Roche lobe overflow. We present a phase-resolved X-ray spectral and timing analysis of a target of opportunity \textit{Chandra} observation made during a low-flux to high-flux transition. The high-resolution spectra allow us to delve into the events that occurred during this episode. The spectrum is described by a single black body absorbed by a local column density of the order of $10^{23-24}$ cm$^{-2}$, which is one to two orders of magnitude higher than found for previous analyses of data taken at similar orbital phases. Such a large column produces a Compton shoulder in the Fe K$\alpha$ line. The transition appears to be caused by the onset of efficient cooling, which cools the plasma by 10 million degrees in just 10 ks, allowing matter to enter the magnetosphere. This happens after a major disturbance, probably the arrival of a train of wind clumps with individual masses in the range $10^{19-20}$ g. This train moves ballistically in an eccentric orbit around the NS, producing a distinctive Doppler modulation in the \ion{Fe}{xxv} line.