Study of the many fluorescent lines and the absorption variability in GX 301-2 with XMM-Newton

TL;DR

This study analyzes the X-ray spectral variability and fluorescent emission lines, including the first detection of Cr K-alpha, in the HMXB GX 301-2 during a pre-periastron flare, revealing complex accretion and emission processes.

Contribution

First detection of Cr K-alpha fluorescence in a HMXB and detailed analysis of spectral variability and absorption during flares.

Findings

Fluorescent lines are highly variable and follow overall flux.

The absorption column density varies independently of continuum flux.

A dip in flux suggests temporary cessation of accretion with iron afterglow emission.

Abstract

We present an in-depth study of the High Mass X-ray Binary (HMXB) GX 301-2 during its pre-periastron flare using data from the XMM-Newton satellite. The energy spectrum shows a power law continuum absorbed by a large equivalent hydrogen column on the order of 10^24 cm^2 and a prominent Fe K-alpha fluorescent emission line. Besides the Fe K-alpha line, evidence for Fe K-beta, Ni K-alpha, Ni K-beta, S K-alpha, Ar K-alpha, Ca K-alpha, and Cr K-alpha fluorescent lines is found. The observed line strengths are consistent with fluorescence in a cold absorber. This is the first time that a Cr K-alpha line is seen in emission in the X-ray spectrum of a HMXB. In addition to the modulation by the strong pulse period of ~685 sec the source is highly variable and shows different states of activity. We perform time-resolved as well as pulse-to-pulse resolved spectroscopy to investigate differences between these states of activity. We find that the fluorescent line fluxes are strongly variable and generally follow the overall flux. The N_H value is variable by a factor of 2, but not correlated to the continuum normalization. We find an interval of low flux in the light curve in which the pulsations cease almost completely, without any indication of an increasing absorption column. We investigate this dip in detail and argue that it is most likely that during the dip the accretion ceased and the afterglow of the fluorescent iron accounted for the main portion of the X-ray flux. A similar dip was found earlier in RXTE data, and we compare our findings to these results