X-ray pulsar GRO J1008$-$57 as an orthogonal rotator

Kavli Affiliate: Nicola Omodei

| First 5 Authors: Sergey S. Tsygankov, Victor Doroshenko, Alexander A. Mushtukov, Juri Poutanen, Alessandro Di Marco

| Summary:

X-ray polarimetry is a unique way to probe the geometrical configuration of
highly magnetized accreting neutron stars (X-ray pulsars). GRO J1008$-$57 is
the first transient X-ray pulsar observed at two different flux levels by the
Imaging X-ray Polarimetry Explorer (IXPE) during its outburst in November 2022.
We find the polarization properties of GRO J1008$-$57 to be independent of its
luminosity, with the polarization degree varying between nondetection and about
15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data
with the rotating vector model allowed us to estimate the pulsar inclination
(130 deg, which is in good agreement with the orbital inclination), the
position angle (75 deg) of the pulsar spin axis, and the magnetic obliquity (74
deg). This makes GRO J1008$-$57 the first confidently identified nearly
orthogonal rotator among X-ray pulsars. We discuss our results in the context
of the neutron star atmosphere models and theories of the axis alignment of
accreting pulsars.

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