2021PASJ...73.1389U

IGR J00370+6122 is a high-mass X-ray binary with a B1 Ib primary star and a companion suggested to be a neutron star because of the detection of a 346 s pulsation in a one-off 4 ks observation. To better understand the nature of the compact companion, the present work performs timing and spectral studies of the X-ray data of this object, taken with XMM-Newton, Swift, Suzaku, RXTE, and INTEGRAL. In the XMM-Newton data, a sign of coherent 674 s pulsation was detected, for which the previous 346 s period may be the second harmonic. The spectra exhibited the "harder when brighter" trend in the 1-10 keV range, and a flat continuum without clear cutoff in the 10-80 keV range. These properties are both similar to those observed from several low-luminosity accreting pulsars, including X Persei in particular. Thus, the compact object in IGR J00370+6122 is considered to be a magnetized neutron star with a rather low luminosity. The orbital period was refined to 15.6649 ± 0.0014 d. Along the orbit, the luminosity changes by three orders of magnitude, involving a sudden drop from ∼4 x 10³³ to ∼1 x 10³² erg s^–1 at an orbital phase of 0.3 (and probably vice verse at 0.95). Although these phenomena cannot be explained by simple Hoyle-Lyttleton accretion from the primary's stellar winds, they can be explained when incorporating the propeller effect with a strong dipole magnetic field of ∼5 x 10¹³ G. Therefore, the neutron star in IGR J00370+6122 may have a stronger magnetic field compared to ordinary X-ray pulsars.