Investigating the origin of the faint non-thermal emission of the Arches cluster using the 2015–2016 NuSTAR and XMM–Newton X-ray observations

Monthly Notices of the Royal Astronomical Society. 2019. Vol. 484. No. 2. P. 1627–1636.

Lutovinov A.

Recent NuSTAR and XMM–Newton observations of the molecular cloud around the Arches
stellar cluster demonstrate a dramatic change both in morphology and intensity of its nonthermal
X-ray emission, similar to that observed in many molecular clouds of the Central
Molecular Zone at the Galactic Center. These variations trace the propagation of illuminating
fronts, presumably induced by past flaring activities of SgrA. In this paper, we present results
of a long NuSTAR observation of the Arches complex in 2016, taken a year after the previous
XMM+NuSTAR observations which revealed a strong decline in the cloud emission. The 2016
NuSTAR observation shows that both the non-thermal continuum emission and the Fe Kα
6.4 keV line flux are consistent with the level measured in 2015. No significant variation has
been detected in both spectral shape and Fe Kα equivalent width EW6.4 keV, which may be
interpreted as the intensity of the Arches non-thermal emission reaching its stationary level.
At the same time, the measured 2016 non-thermal flux is not formally in disagreement with
the declining trend observed in 2007–2015. Thus, we cannot assess whether the non-thermal
emission has reached a stationary level in 2016, and new observations, separated by a longer
time period, are needed to draw stringent conclusions. Detailed spectral analysis of three bright
clumps of the Arches molecular cloud performed for the first time showed different EW6.4 keV
and absorption. This is a strong hint that the X-ray emission from the molecular cloud is a mix
of two components with different origins.

Keywords: X-rays, clouds, interstellar medium