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 \sim 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 \sim 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 ^ { \star } .