Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114. (arXiv:1907.04969v1 [astro-ph.HE])

Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114. (arXiv:1907.04969v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhao_Y/0/1/0/all/0/1">Yue Zhao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Heinke_C/0/1/0/all/0/1">Craig O. Heinke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tsygankov_S/0/1/0/all/0/1">Sergey S. Tsygankov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ho_W/0/1/0/all/0/1">Wynn C. G. Ho</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Potekhin_A/0/1/0/all/0/1">Alexander Y. Potekhin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shaw_A/0/1/0/all/0/1">Aarran W. Shaw</a>

We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX
J0812.4-3114 in quiescence ($L_X approx 1.6 times
10^{33}~mathrm{erg~s^{-1}}$). 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.

We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX
J0812.4-3114 in quiescence ($L_X approx 1.6 times
10^{33}~mathrm{erg~s^{-1}}$). 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.

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