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

Abstract

Molecular cloud Sgr B2 is a natural Compton mirror in the Central Molecular Zone. An observed fading of the Sgr B2 X-ray emission in continuum and Fe 6.4 keV line indicates, as believed, a past X-ray flare activity of the supermassive black hole Sgr A ★ . The Sgr B2 was investigated by the INTEGRAL observatory at hard X-rays in 2003-2009, showing a 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 the 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.

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

“…Since the multiply scattered emission is expected to stay present for relatively longer periods of time, accumulation of the signal over years by such missions as INTEGRAL and NuSTAR might offer an opportunity to check this prediction very soon (e.g. Kuznetsova et al 2021).…”