Kavli Affiliate: Nicola Omodei
| First 5 Authors: Sofia V. Forsblom, Sergey S. Tsygankov, Juri Poutanen, Victor Doroshenko, Alexander A. Mushtukov
| Summary:
Recent observations of X-ray pulsars (XRPs) performed by the Imaging X-ray
Polarimetry Explorer (IXPE) have made it possible to investigate the intricate
details of these objects in a new way, thanks to the added value of X-ray
polarimetry. Here we present the results of the IXPE observations of SMC X-1, a
member of the small group of XRPs displaying super-orbital variability. SMC X-1
was observed by IXPE three separate times during the high state of its
super-orbital period. The observed luminosity in the 2-8 keV energy band of
$L=2times10^{38}$ erg/s makes SMC X-1 the brightest XRP ever observed by IXPE.
We detect significant polarization in all three observations, with values of
the phase-averaged polarization degree (PD) and polarization angle (PA) of
$3.2pm0.8$% and $97degpm8deg$ for Observation 1, $3.0pm0.9$% and
$90degpm8deg$ for Observation 2, and $5.5pm1.1$% and $80degpm6deg$ for
Observation 3, for the spectro-polarimetric analysis. The observed PD shows an
increase over time with decreasing luminosity, while the PA decreases in
decrements of 10deg. The phase-resolved spectro-polarimetric analysis reveals
significant detection of polarization in three out of seven phase bins, with
the PD ranging between 2% and 10%, and a corresponding range in the PA from
$sim$70deg to $sim$100deg. The pulse-phase resolved PD displays an
apparent anti-correlation with the flux. Using the rotating vector model, we
obtain constraints on the pulsar’s geometrical properties for the individual
observations. The position angle of the pulsar displays an evolution over time
supporting the idea that we observe changes related to different super-orbital
phases. Scattering in the wind of the precessing accretion disk may be
responsible for the behavior of the polarimetric properties observed during the
high-state of SMC X-1’s super-orbital period.
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