Kavli Affiliate: Nicola Omodei
| First 5 Authors: Anna Bobrikova, Sofia V. Forsblom, Alessandro Di Marco, Fabio La Monaca, Juri Poutanen
| Summary:
Weakly magnetized neutron stars in X-ray binaries show complex phenomenology
with several spectral components that can be associated with the accretion
disk, boundary and/or spreading layer, a corona, and a wind. Spectroscopic
information alone is, however, not enough to disentangle these components.
Additional information about the nature of the spectral components and in
particular the geometry of the emission region can be provided by X-ray
polarimetry. One of the objects of the class, a bright, persistent, and rather
peculiar galactic Type I X-ray burster was observed with the Imaging X-ray
Polarimetry Explorer (IXPE) and the X-ray Multi-Mirror Mission Newton
(XMM-Newton). Using the XMM-Newton data we estimated the current state of the
source as well as detected strong absorption lines associated with the
accretion disk wind. IXPE data showed the source to be significantly polarized
in the 2-8 keV energy band with the overall polarization degree (PD) of 1.4% at
a polarization angle (PA) of -2 degrees (errors at 68% confidence level).
During the two-day long observation, we detected rotation of the PA by about 70
degrees with the corresponding changes in the PD from 2% to non-detectable and
then up to 5%. These variations in polarization properties are not accompanied
by visible changes in spectroscopic characteristics. The energy-resolved
polarimetric analysis showed a significant change in polarization, from being
strongly dependent on energy at the beginning of the observation to being
almost constant with energy in the later parts of the observation. As a
possible interpretation, we suggest the presence of a constant component of
polarization, strong wind scattering, or different polarization of the two main
spectral components with individually peculiar behavior. The rotation of the PA
suggests a 30-degree misalignment of the neutron star spin from the orbital
axis.
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