NuSTAR and XMM-Newton observations of the Arches cluster in 2015: fading hard X-ray emission from the molecular cloud

TL;DR

This study reports the fading of non-thermal hard X-ray emission from the molecular cloud near the Arches cluster, indicating a decline consistent with a half-life decay, likely caused by past activity of Sgr A*.

Contribution

First detailed long-term X-ray observations showing the temporal evolution of the Arches cloud's non-thermal emission and its implications for past Galactic Centre activity.

Findings

Non-thermal emission of the Arches cloud decreased significantly after 2012.

The decay of emission follows an approximate half-life of 8 years.

Variations are consistent with propagation of illuminating fronts from Sgr A* flaring activity.

Abstract

We present results of long NuSTAR (200 ks) and XMM-Newton (100 ks) observations of the Arches stellar cluster, a source of bright thermal (kT~2 keV) X-rays with prominent Fe XXV K_alpha 6.7 keV line emission and a nearby molecular cloud, characterized by an extended non-thermal hard X-ray continuum and fluorescent Fe K_alpha 6.4 keV line of a neutral or low ionization state material around the cluster. Our analysis demonstrates that the non-thermal emission of the Arches cloud underwent a dramatic change, with its homogeneous morphology, traced by fluorescent Fe K_alpha line emission, vanishing after 2012, revealing three bright clumps. The declining trend of the cloud emission, if linearly fitted, is consistent with half-life decay time of ~8 years. Such strong variations have been observed in several other molecular clouds in the Galactic Centre, including the giant molecular cloud Sgr B2, and point toward a similar propagation of illuminating fronts, presumably induced by the past flaring activity of Sgr A*.