Sgr B2 hard X-ray emission with INTEGRAL after 2009: still detectable?

Abstract

The molecular cloud Sgr B2 is a natural Compton mirror in the Central Molecular Zone. It is believed that the observed fading of the Sgr B2 X-ray emission in continuum and the Fe Kα 6.4 keV line indicates past X-ray flare activity of the supermassive black hole Sgr A^⋆. Sgr B2 was investigated by the INTEGRAL observatory in the hard X-ray in 2003-2009, showing clear decay of its hard X-ray emission. In this work, we present a long-term time evolution of the Sgr B2 hard X-ray continuum after 2009, associated with the hard X-ray source IGR J17475-2822 as observed by INTEGRAL. The 30-80 keV sky maps, obtained in 2009-2019, demonstrate a significant excess spatially consistent with IGR J17475-2822. The observed 2003-2019 light curve of IGR J17475-2822 is characterized by a linear decrease by a factor of ~2 until 2011, after which it reaches a constant level of ~1 mCrab. The source spectrum above 17 keV is consistent with a power-law model with Γ = 1.4 and a high-energy cut-off at ~43 keV. The Sgr B2 residual emission after ~2011 shows a good correspondence with models of X-ray emission due to the irradiation of the molecular gas by hard X-rays and low-energy cosmic ray ions. We discuss the possible origin of the residual Sgr B2 emission after 2011 within these models, including theoretically predicted multiply scattered emission.