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Limits on thermal variations in a dozen quiescent neutron stars over a decade

Limits on thermal variations in a dozen quiescent neutron stars over a decade
Limits on thermal variations in a dozen quiescent neutron stars over a decade
In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the origin of the thermal X-ray component may be either release of heat from the core of the neutron star, or continuing low-level accretion. In general, heat from the core should be stable on timescales < 10^4 years, while continuing accretion may produce variations on a range of timescales. While some quiescent neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal components on a range of timescales, several others, particularly those in globular clusters with no detectable nonthermal hard X-rays (fit with a powerlaw), have shown no measurable variations. Here, we constrain the spectral variations of 12 low mass X-ray binaries in 3 globular clusters over ~ 10 years. We find no evidence of variations in 10 cases, with limits on temperature variations below 11% for the 7 qLMXBs without powerlaw components, and limits on variations below 20% for 3 other qLMXBs that do show non-thermal emission. However, in 2 qLMXBs showing powerlaw components in their spectra (NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline in temperature, suggesting the presence of continuing low-level accretion. This work adds to the evidence that the thermal X-ray component in quiescent neutron stars without powerlaw components can be explained by heat deposited in the core during outbursts. Finally, we also investigate the correlation between hydrogen column density (NH) and optical extinction (AV) using our sample and current models of interstellar X-ray absorption, finding NH(cm^-2) = (2.81+/-0.13)x10^21 AV.
0035-8711
3475-3488
Bahramian, Arash
390154f6-92b9-4e9f-9c5e-2eeaec97460a
Heinke, Craig O.
d7382ed2-cb85-4e15-b2d9-296fc8b6221d
Degenaar, Nathalie
347a2379-6b78-482e-976c-52f08a96114d
Chomiuk, Laura
e1deb1c4-75c7-4f74-8e33-690a50b93cdb
Wijnands, Rudy
4ac16ff6-b564-4e5d-89d4-36ff92c6030c
Strader, Jay
2b225e8e-ac84-429d-9f91-dc340d75e40e
Ho, Wynn C.G.
d78d4c52-8f92-4846-876f-e04a8f803a45
Pooley, David
72898f52-e7ed-441b-910c-95cd1e57db0d
Bahramian, Arash
390154f6-92b9-4e9f-9c5e-2eeaec97460a
Heinke, Craig O.
d7382ed2-cb85-4e15-b2d9-296fc8b6221d
Degenaar, Nathalie
347a2379-6b78-482e-976c-52f08a96114d
Chomiuk, Laura
e1deb1c4-75c7-4f74-8e33-690a50b93cdb
Wijnands, Rudy
4ac16ff6-b564-4e5d-89d4-36ff92c6030c
Strader, Jay
2b225e8e-ac84-429d-9f91-dc340d75e40e
Ho, Wynn C.G.
d78d4c52-8f92-4846-876f-e04a8f803a45
Pooley, David
72898f52-e7ed-441b-910c-95cd1e57db0d

Bahramian, Arash, Heinke, Craig O., Degenaar, Nathalie, Chomiuk, Laura, Wijnands, Rudy, Strader, Jay, Ho, Wynn C.G. and Pooley, David (2015) Limits on thermal variations in a dozen quiescent neutron stars over a decade. Monthly Notices of the Royal Astronomical Society, 452 (4), 3475-3488. (doi:10.1093/mnras/stv1585).

Record type: Article

Abstract

In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the origin of the thermal X-ray component may be either release of heat from the core of the neutron star, or continuing low-level accretion. In general, heat from the core should be stable on timescales < 10^4 years, while continuing accretion may produce variations on a range of timescales. While some quiescent neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal components on a range of timescales, several others, particularly those in globular clusters with no detectable nonthermal hard X-rays (fit with a powerlaw), have shown no measurable variations. Here, we constrain the spectral variations of 12 low mass X-ray binaries in 3 globular clusters over ~ 10 years. We find no evidence of variations in 10 cases, with limits on temperature variations below 11% for the 7 qLMXBs without powerlaw components, and limits on variations below 20% for 3 other qLMXBs that do show non-thermal emission. However, in 2 qLMXBs showing powerlaw components in their spectra (NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline in temperature, suggesting the presence of continuing low-level accretion. This work adds to the evidence that the thermal X-ray component in quiescent neutron stars without powerlaw components can be explained by heat deposited in the core during outbursts. Finally, we also investigate the correlation between hydrogen column density (NH) and optical extinction (AV) using our sample and current models of interstellar X-ray absorption, finding NH(cm^-2) = (2.81+/-0.13)x10^21 AV.

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Published date: 4 August 2015
Organisations: Applied Mathematics

Identifiers

Local EPrints ID: 378940
URI: https://eprints.soton.ac.uk/id/eprint/378940
ISSN: 0035-8711
PURE UUID: 78ed98e7-a317-4910-9591-03b5512c03a4
ORCID for Wynn C.G. Ho: ORCID iD orcid.org/0000-0002-6089-6836

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Date deposited: 21 Jul 2015 12:59
Last modified: 15 Aug 2019 00:40

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