SRG/eROSITA view of X-ray reflection in the Central Molecular Zone: a snapshot in September–October 2019

Abstract

ABSTRACT X-ray reflection off dense molecular clouds in the Galactic Centre region has established itself as a powerful probe for the past activity record of the supermassive black hole Sgr A* on a time-scale of a few hundred years. Detailed studies of the reflection signal from individual clouds allow us to estimate parameters of the brightest flare(s) and explore properties of the dense gas distribution inside and around them. On the other hand, wide-area surveys covering the full Central Molecular Zone (CMZ) are crucial to spot the brightening of the new clouds and long-term decay of the flux from the once bright ones. Here, we present and discuss the data obtained by the SRG/eROSITA telescope in the course of its commissioning observations in late 2019 in regard to the X-ray reflection off CMZ clouds located to the East of Sgr A*. We measure the hard X-ray (4–8 keV) flux from the currently brightest (in reflected emission) molecular complex, Sgr A, and derive upper limits for other molecular complexes. We confirm that the Sgr A complex keeps being bright at the level of $4\times 10^{-13}\,{\rm erg\, s^{-1}\,cm^{-2}\,arcmin^{-2}}$, making it an excellent candidate for the deep forthcoming high-resolution imaging and polarimetric observations. We also discuss distinct features of the reflected emission from the clouds for which the primary illumination front has already passed away and the signal is dominated by multiply scattered radiation.