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

Abstract

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.

11 pages, 7 figures, A&A, in press

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• X-Ray Views of Galactic Accreting Pulsars in High-Mass X-Ray Binaries
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• Neutrino beaming in ultraluminous X-ray pulsars as a result of gravitational lensing by neutron stars
• Energy-resolved pulse profile changes in V 0332+53: Indications of wings in the cyclotron absorption line profile
• Observing Rayleigh-Taylor stable and unstable accretion through a Kalman filter analysis of X-ray pulsars in the Small Magellanic Cloud
• Orbital variability of polarized X-ray radiation reflected from a companion star in X-ray binaries
• Exploring polarization and geometry in the X-ray pulsar 4U 1538-52