Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114

Abstract

We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX J0812.4-3114 in quiescence ($L_X\approx 1.6\times 10^{33}\mathrm{erg}{\mathrm{s}}^{-1}$). Intriguingly, we find a two component spectrum, with a hard power-law ($\Gamma \approx 1.5$) and a soft blackbody-like excess below $\approx 1\mathrm{keV}$. The blackbody component is consistent in $kT$ with a prior quiescent Chandra observation reported by Tsygankov et al. and has an inferred blackbody radius of $\approx 10\mathrm{km}$, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at $\approx 1\mathrm{keV}$ and/or $\approx 1.4\mathrm{keV}$. The hard component shows pulsations at $P\approx 31.908\mathrm{s}$ (pulsed fraction $0.84\pm 0.10$), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction $\lesssim 31\%$). We conclude that the pulsed hard component suggests low-level accretion onto the neutron star poles, while the soft excess seems to originate from the entire NS surface. We speculate that, in quiescence, the source switches between a soft thermal-dominated state (when the propeller effect is at work) and a relatively hard state with low-level accretion, and use the propeller cutoff to estimate the magnetic field of the system to be $\lesssim 8.4\times 10^{11}\mathrm{G}$. We compare the quiescent thermal $L_X$ predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-3114 has a high thermal $L_X$, at or above the prediction for minimum cooling mechanisms. This suggests that RX J0812.4-3114 either contains a relatively low-mass NS with minimum cooling, or that the system may be young enough that the NS has not fully cooled from the supernova explosion.

13 pages, 10 figures, accepted for publication in MNRAS

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