The Soft X-ray Spectrum of the High Mass X-Ray Binary V0332+53 in Quiescence

Abstract

The propeller effect should cut off accretion in fast-spinning neutron star high mass X-ray binaries (HMXBs) at low mass transfer rates. However, accretion continues in some HMXBs at $L_x<10^{34}$ erg s${}^{-1}$, as evidenced by continuing pulsations. Indications of spectral softening in systems in the propeller regime suggest that some HMXBs are undergoing fundamental changes in their accretion regime. A 39 ks \textit{XMM-Newton} observation of the transient HMXB V0332+53 found it at a very low X-ray luminosity ($L_x\sim 4\times 10^{32}$ erg s${}^{-1}$). A power-law spectral fit requires an unusually soft spectral index ($4.4_{-0.6}^{+0.9}$), while a magnetized neutron star atmosphere model, with temperature \lt\ 6.7$\pm 0.2$ K and inferred emitting radius of $\sim 0.2-0.3$ km, gives a good fit. We suggest that the quiescent X-ray emission from V0332+53 is mainly from a hot spot on the surface of the neutron star. We could not detect pulsations from V0332+53, due to the low count rate. Due to the high $N_H$, thermal emission from the rest of the neutron star could be only weakly constrained, to \lt\ $<$6.14${}_{-6.14}^{+0.05}$ K, or $<3\times 10^{33}$ erg s${}^{-1}$.

6 pages, 3 figures, 1 table, submitted to MNRAS