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

TL;DR

This study analyzes the soft X-ray spectrum of the high mass X-ray binary V0332+53 during quiescence, revealing a very soft spectrum likely originating from a hot spot on the neutron star surface, challenging existing accretion models.

Contribution

It provides the first detailed spectral analysis of V0332+53 in quiescence, suggesting surface hot spot emission and examining the implications for accretion regimes in HMXBs.

Findings

Spectral index of 4.4 indicates very soft emission

Thermal emission consistent with a hot spot on neutron star surface

No pulsations detected due to low count rate

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 ($4.4^{+0.9}_{-0.6}$), while a magnetized neutron star atmosphere model, with temperature \lt\ 6.7$\pm 0.2$ K and inferred emitting radius of $\sim0.2-0.3$ 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\ $<$6.14$^{+0.05}_{-6.14}$ K, or $<3\times10^{33}$ erg s${^{-1}}$.