Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Sergey S. Tsygankov, Victor Doroshenko, Juri Poutanen, Jeremy Heyl, Alexander A. Mushtukov
| Summary:
Cen X-3 is the first X-ray pulsar discovered 50 years ago. Radiation from
such objects is expected to be highly polarized due to birefringence of plasma
and vacuum associated with propagation of photons in presence of the strong
magnetic field. Here we present results of the observations of Cen X-3
performed with the Imaging X-ray Polarimetry Explorer. The source exhibited
significant flux variability and was observed in two states different by a
factor of ~20 in flux. In the low-luminosity state no significant polarization
was found either in pulse phase-averaged (with the 3$sigma$ upper limit of
12%) or phase-resolved data (the 3$sigma$ upper limits are 20-30%). In the
bright state the polarization degree of 5.8$pm$0.3% and polarization angle of
$49.6degpm1.5deg$ with significance of about 20$sigma$ was measured from
the spectro-polarimetric analysis of the phase-averaged data. The
phase-resolved analysis showed a significant anti-correlation between the flux
and the polarization degree as well as strong variations of the polarization
angle. The fit with the rotating vector model indicates a position angle of the
pulsar spin axis of about 49$deg$ and a magnetic obliquity of 17$deg$. The
detected relatively low polarization can be explained if the upper layers of
the neutron star surface are overheated by the accreted matter and the
conversion of the polarization modes occurs within the transition region
between the upper hot layer and a cooler underlying atmosphere. A fraction of
polarization signal can also be produced by reflection of radiation from the
neutron star surface and the accretion curtain.
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