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

Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114
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 (Lx=1.6x10^33 erg/s). Intriguingly, we find a two-component spectrum, with a hard power-law (Gamma=1.5) and a soft blackbody-like excess below =1 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 =10 km, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at =1 keV and/or =1.4 keV. The hard component shows pulsations at P=31.908 s (pulsed fraction 0.84+/-0.10), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction <~ 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 <~8.4x10^11 G. We compare the quiescent thermal Lx predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-3114 has a high thermal Lx, 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.
0035-8711
4427-4439
Zhao, Yue
bc097758-699d-4c53-83ba-1f75e3efdd3c
Heinke, Craig O.
d7382ed2-cb85-4e15-b2d9-296fc8b6221d
Tsygankov, Sergey S.
057cc7f1-1bb8-463f-9c47-58f6ffda97b9
Ho, Wynn C.G.
d78d4c52-8f92-4846-876f-e04a8f803a45
Potekhin, Alexander Y.
b9551650-f6ed-4c4f-b53c-d019cccc49fa
Shaw, Aarran
ec30710a-0499-4f4f-9685-4510f75efe79
Zhao, Yue
bc097758-699d-4c53-83ba-1f75e3efdd3c
Heinke, Craig O.
d7382ed2-cb85-4e15-b2d9-296fc8b6221d
Tsygankov, Sergey S.
057cc7f1-1bb8-463f-9c47-58f6ffda97b9
Ho, Wynn C.G.
d78d4c52-8f92-4846-876f-e04a8f803a45
Potekhin, Alexander Y.
b9551650-f6ed-4c4f-b53c-d019cccc49fa
Shaw, Aarran
ec30710a-0499-4f4f-9685-4510f75efe79

Zhao, Yue, Heinke, Craig O., Tsygankov, Sergey S., Ho, Wynn C.G., Potekhin, Alexander Y. and Shaw, Aarran (2019) Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114. Monthly Notices of the Royal Astronomical Society, 488 (3), 4427-4439. (doi:10.1093/mnras/stz1946).

Record type: Article

Abstract

We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX J0812.4-3114 in quiescence (Lx=1.6x10^33 erg/s). Intriguingly, we find a two-component spectrum, with a hard power-law (Gamma=1.5) and a soft blackbody-like excess below =1 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 =10 km, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at =1 keV and/or =1.4 keV. The hard component shows pulsations at P=31.908 s (pulsed fraction 0.84+/-0.10), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction <~ 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 <~8.4x10^11 G. We compare the quiescent thermal Lx predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-3114 has a high thermal Lx, 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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1907.04969 - Accepted Manuscript
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stz1946 - Version of Record
Available under License Other.
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More information

Accepted/In Press date: 10 July 2019
e-pub ahead of print date: 12 July 2019
Published date: September 2019

Identifiers

Local EPrints ID: 432505
URI: http://eprints.soton.ac.uk/id/eprint/432505
ISSN: 0035-8711
PURE UUID: 330d5cfd-a860-41b2-8bdc-777ecd86dcbf
ORCID for Wynn C.G. Ho: ORCID iD orcid.org/0000-0002-6089-6836

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Date deposited: 17 Jul 2019 16:30
Last modified: 13 May 2020 00:36

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