Variation of the X-ray non-thermal emission in the Arches cloud

TL;DR

This study analyzes X-ray observations of the Arches cloud from 2000 to 2013, revealing a significant flux drop in 2012 that supports the reflection of a transient X-ray source as the origin of the non-thermal emission.

Contribution

It provides the first evidence of variability in the Arches cloud's non-thermal X-ray emission, favoring the X-ray reflection scenario over cosmic-ray origin.

Findings

30% flux decrease in 2012 with >4 sigma significance

Supports X-ray reflection from a transient source as the emission origin

Discriminates between cosmic-ray and X-ray irradiation scenarios

Abstract

The origin of the iron fluorescent line at 6.4 keV from an extended region surrounding the Arches cluster is debated and the non-variability of this emission up to 2009 has favored the low-energy cosmic-ray origin over a possible irradiation by hard X-rays. By probing the variability of the Arches cloud non-thermal emission in the most recent years, including a deep observation in 2012, we intend to discriminate between the two competing scenarios. We perform a spectral fit of XMM-Newton observations collected from 2000 to 2013 in order to build the Arches cloud lightcurve corresponding to both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a 30% flux drop in 2012, detected with more than 4 sigma significance for both components. This implies that a large fraction of the studied non-thermal emission is due to the reflection of an X-ray transient source.