2016MNRAS.463...78E

The behaviour of neutron stars in high-mass X-ray binaries (HMXBs) during periods of low mass transfer is of great interest. Indications of spectral softening in systems at low mass-transfer rates suggest that some HMXBs are undergoing fundamental changes in their accretion regime, but the nature of the quiescent X-ray emission is not clear. We performed a 39 ks XMM-Newton observation of the transient HMXB V0332+53, finding it at a very low X-ray luminosity (L_x ∼ 4 x 10³² erg s^–1). A power-law spectral fit requires an unusually soft spectral index (4.4^{+0.9}_{-0.6}), while a magnetized neutron star atmosphere model, with temperature LogT_eff 6.7 ± 0.2 K and inferred emitting radius of ∼0.2-0.3 km, gives a good fit. We suggest that the quiescent X-ray emission from V0332+53 is mainly from a hotspot on the surface of the neutron star. No conclusions on the presence of pulsations could be drawn due to the low count rate. Due to the high absorption column, thermal emission from the rest of the neutron star could be only weakly constrained, to LogT_eff<6.14^{+0.05}_{-6.14} K, or <3 x 10³³ erg s^–1.