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Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1
Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1
Aql X-1 is a prolific transient neutron star low-mass X-ray binary that exhibits an accretion outburst approximately once every year. Whether the thermal X-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. In this work, we use Swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that the X-ray light curves and measured neutron star surface temperatures are consistent with the expectations of the crust cooling paradigm. By using a thermal evolution code, we find that ?1.2–3.2 MeV nucleon?1 of shallow heat release describes the observational data well, depending on the assumed mass-accretion rate and temperature of the stellar core. We find no evidence for varying strengths of this shallow heating after different outbursts, but this could be due to limitations of the data. We argue that monitoring Aql X-1 for up to ?1 yr after future outbursts can be a powerful tool to break model degeneracies and solve open questions about the magnitude, depth, and origin of shallow heating in neutron star crusts.
0035-8711
4001-4014
Waterhouse, A.C.
1450afd2-a51f-494d-ba9e-65b35f399380
Degenaar, N.
54149051-d74e-4e43-a7cd-501ec4023511
Wijnands, R.
382893ee-0e03-45e3-9321-db8b9fc74914
Brown, E.F.
357f29dd-cc4e-4a6e-9fea-4aad75253aa4
Miller, J.M.
078305f0-03c7-4982-95ff-a92bc2f75b54
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Linares, M.
cea3a79a-1831-4a7f-be41-3bf3b7b25383
Waterhouse, A.C.
1450afd2-a51f-494d-ba9e-65b35f399380
Degenaar, N.
54149051-d74e-4e43-a7cd-501ec4023511
Wijnands, R.
382893ee-0e03-45e3-9321-db8b9fc74914
Brown, E.F.
357f29dd-cc4e-4a6e-9fea-4aad75253aa4
Miller, J.M.
078305f0-03c7-4982-95ff-a92bc2f75b54
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Linares, M.
cea3a79a-1831-4a7f-be41-3bf3b7b25383

Waterhouse, A.C., Degenaar, N., Wijnands, R., Brown, E.F., Miller, J.M., Altamirano, Diego and Linares, M. (2016) Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1. Monthly Notices of the Royal Astronomical Society, 456 (4), 4001-4014. (doi:10.1093/mnras/stv2959).

Record type: Article

Abstract

Aql X-1 is a prolific transient neutron star low-mass X-ray binary that exhibits an accretion outburst approximately once every year. Whether the thermal X-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. In this work, we use Swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that the X-ray light curves and measured neutron star surface temperatures are consistent with the expectations of the crust cooling paradigm. By using a thermal evolution code, we find that ?1.2–3.2 MeV nucleon?1 of shallow heat release describes the observational data well, depending on the assumed mass-accretion rate and temperature of the stellar core. We find no evidence for varying strengths of this shallow heating after different outbursts, but this could be due to limitations of the data. We argue that monitoring Aql X-1 for up to ?1 yr after future outbursts can be a powerful tool to break model degeneracies and solve open questions about the magnitude, depth, and origin of shallow heating in neutron star crusts.

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Accepted/In Press date: 16 December 2015
e-pub ahead of print date: 14 January 2016
Published date: 11 March 2016
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 394845
URI: http://eprints.soton.ac.uk/id/eprint/394845
ISSN: 0035-8711
PURE UUID: 647dd737-eadf-46aa-af7a-64e36d73e35b
ORCID for Diego Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

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Date deposited: 24 May 2016 15:24
Last modified: 17 Dec 2019 01:35

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