Does INTEGRAL detect Sgr B2 since its fading in 2009?
The observed variability of X-ray emission of the molecular clouds of
the Central Molecular Zone (CMZ) indicates a possible flare activity in
the past of the supermassive black hole Sgr A*. The molecular cloud Sgr
B2 is the densest and most massive Compton mirror in the CMZ. It is
usually associated with the hard X-ray source IGR J17475-2822, detected
by INTEGRAL (see INTEGRAL POM
October 2004,
September 2005). The
INTEGRAL observations of the Galactic Center region in 2003-2009
demonstrated a clear decay of the Sgr B2 hard X-ray emission (see
INTEGRAL POM
August 2010),
supporting the mechanism of reflection of Sgr
A* X-ray flares from molecular clouds matter.
Thanks to the on-going INTEGRAL operations, we can extend the baseline
of Sgr B2 observations over another decade to further check the
conditions of this massive molecular cloud in hard X-rays. The
IBIS/ISGRI data show a significant detection of Sgr B2 after 2009. The
17-year light curve is characterized by an overall drop with a factor of
2 before 2011 and a following constant level at the 1 mCrab flux level.
The characteristic time of the linear decay during 2003-2011 was
estimated at τ = 6 ±2 years, which is consistent with the
value τ = 8.2 ±1.5 years, as observed by Terrier et al. (2010,
ApJ 719, 143). The INTEGRAL maps of the Sgr B2 region, obtained before
and after 2011 (see top left and top right graphs of the image,
respectively), demonstrate the detection of IGR J17475–2822 at 24σ
and 11σ significance, correspondingly. The spectral shape of the
emission before and after 2011 is well described by a power-law with
Γ about 1.4 and a high-energy cut-off at 43 keV. The Sgr B2
emission before 2011 strongly supports the reflection scenario, while
the nature of the residual emission seen as the constant level is still
unclear.
