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

We present results of long Nuclear Spectroscopic Telescope Array (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 Kα 6.7 keV line emission and a nearby molecular cloud, characterized by an extended non-thermal hard X-ray continuum and fluorescent Fe Kα 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 Kα 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 yr. Such strong variations have been observed in several other molecular clouds in the Galactic Centre, including the giant molecular cloud Sgr B2, and point towards a similar propagation of illuminating fronts, presumably induced by the past flaring activity of Sgr A^⋆. We also detect a significant drop of the equivalent width of the fluorescent Fe Ka line, which could mean either that the new clumps have a different position along the line of sight or that the contribution of cosmic ray has become more dominant.