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

TL;DR

This study uses X-ray polarimetry to determine the geometry of the neutron star GRO J1008$-$57, revealing it as the first nearly orthogonal rotator among X-ray pulsars with polarization properties independent of luminosity.

Contribution

The paper presents the first identification of a nearly orthogonal rotator among X-ray pulsars using phase-resolved spectro-polarimetric data from IXPE.

Findings

Polarization degree varies between nondetection and 15% over pulse phase.

Estimated pulsar inclination is 130 degrees, aligning with orbital inclination.

Magnetic obliquity is approximately 74 degrees.

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.