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

TL;DR

This study investigates the long-term evolution of Sgr B2's hard X-ray emission after 2009 using INTEGRAL data, revealing a persistent residual emission likely due to past irradiation or cosmic rays, despite the decay of the primary source.

Contribution

It provides the first detailed analysis of Sgr B2's hard X-ray emission evolution post-2009, highlighting residual emission mechanisms beyond the primary flare activity.

Findings

Sgr B2's hard X-ray emission decreased until 2011 and then stabilized.

The residual emission aligns with models of molecular gas irradiation by X-rays and cosmic rays.

The source IGR J17475-2822 showed a linear decay followed by a constant level.

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 K$\alpha$ 6.4 keV line indicates, as believed, a past X-ray flare activity of the supermassive black hole Sgr A$^{\star}$. 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 $\sim2$ until 2011, after which it reaches a constant level of $\sim1$ mCrab. The source spectrum above 17 keV is consistent with a power-law model with $\Gamma=1.4$ and a high-energy cut-off at $\sim43$ keV. The Sgr B2 residual emission after $\sim2011$ 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.