Kavli Affiliate: Nicola Omodei
| First 5 Authors: Ildar Khabibullin, Ildar Khabibullin, , ,
| Summary:
Extended X-ray emission observed in the direction of several molecular clouds
in the Central Molecular Zone (CMZ) of our Galaxy exhibits spectral and
temporal properties consistent with the `X-ray echo’ scenario. It postulates
that the observed signal is a light-travel-time delayed reflection of a short
($delta t<$1.5 yr) and bright ($L_rm X>10^39~rm erg~s^-1$) flare,
most probably produced a few hundred years ago by Sgr A*. This scenario also
predicts a distinct polarization signature for the reflected X-ray continuum,
with the polarization vector being perpendicular to the direction towards the
primary source and polarization degree (PD) being determined by the scattering
angle. We report the results of two deep observations of the currently
brightest (in reflected emission) molecular complex Sgr A taken with the
Imaging X-ray Polarimetry Explorer (IXPE) in 2022 and 2023. We confirm the
previous polarization measurement for a large region encompassing Sgr A complex
with higher significance, but also reveal an inconsistent polarization pattern
for the brightest reflection region in its center. X-ray polarization from this
region is almost perpendicular to the expected direction in the case of Sgr A*
illumination and shows a smaller PD compared to the large region. This could
indicate the simultaneous propagation of several illumination fronts throughout
the CMZ, with the origin of one of them not being Sgr A*. The primary source
could be associated with the Arches stellar cluster or a currently unknown
source located in the closer vicinity of the illuminated cloud, potentially
lowering the required luminosity of the primary source. Although significantly
deeper observations with IXPE would be required to unequivocally distinguish
between the scenarios, a combination of high-resolution imaging and
micro-calorimetric spectroscopy offers an additional promising path forward.
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