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

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{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$$). Intriguingly, we find a two component spectrum, with a hard power-law ($\mathrm{\Gamma }\approx 1.5$) and a soft blackbody-like excess below $\approx 1\mathrm{k}\mathrm{e}\mathrm{V}$. The blackbody component is consistent in $kT$ with a prior quiescent Chandra observation reported by Tsygankov et al. and has an inferred blackbody radius of $\approx 10\mathrm{k}\mathrm{m}$, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at $\approx 1\mathrm{k}\mathrm{e}\mathrm{V}$ and/or $\approx 1.4~$\mathrm{k}\mathrm{e}\mathrm{V}$$\mathrm{.}Thehardcomponentshowspulsationsat$P \approx 31.908~$\mathrm{s}$$(pulsedfraction$0.84 \pm 0.10$), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction $\lesssim 31\mathrm{\%}$). 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.