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Crust cooling of the neutron star in Aql X-1: different depth and magnitude of shallow heating during similar accretion outbursts

Crust cooling of the neutron star in Aql X-1: different depth and magnitude of shallow heating during similar accretion outbursts
Crust cooling of the neutron star in Aql X-1: different depth and magnitude of shallow heating during similar accretion outbursts
The structure and composition of the crust of neutron stars plays an important role in their thermal and magnetic evolution, hence in setting their observational properties. One way to study the properties of the crust of a neutron star, is to measure how it cools after it has been heated during an accretion outburst in a low-mass X-ray binary (LMXB). Such studies have shown that there is a tantalizing source of heat, of currently unknown origin, that is located in the outer layers of the crust and has a strength that varies between different sources and different outbursts. With the aim of understanding the mechanism behind this ‘shallow heating’, we present Chandra and Swift observations of the neutron star LMXB Aql X-1, obtained after its bright 2016 outburst. We find that the neutron star temperature was initially much lower, and started to decrease at much later time, than observed after the 2013 outburst of the source, despite the fact that the properties of the two outbursts were very similar. Comparing our data to thermal evolution simulations, we infer that the depth and magnitude of shallow heating must have been much larger during the 2016 outburst than during the 2013 one. This implies that basic neutron star parameters that remain unchanged between outbursts do not play a strong role in shallow heating. Furthermore, it suggests that outbursts with a similar accretion morphology can give rise to very different shallow heating. We also discuss alternative explanations for the observed difference in quiescent evolution after the 2016 outburst.
0035-8711
4477-4486
Degenaar, N.
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Ootes, L.S.
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Page, D.
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Wijnands, R.
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Parikh, A.S.
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Homan, J.
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Cackett, E.M.
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Miller, J.M.
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Altamirano, Diego
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Linares, M.
cea3a79a-1831-4a7f-be41-3bf3b7b25383
Degenaar, N.
54149051-d74e-4e43-a7cd-501ec4023511
Ootes, L.S.
ad187f1b-e024-4fa3-8f53-afac44c52b8f
Page, D.
b47b77ff-942c-478f-9002-6d0b588e769b
Wijnands, R.
382893ee-0e03-45e3-9321-db8b9fc74914
Parikh, A.S.
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Homan, J.
549baf32-2442-4247-824e-21bb018b05f5
Cackett, E.M.
2cd7b6af-a0e0-4265-94f6-092d68d1ba74
Miller, J.M.
078305f0-03c7-4982-95ff-a92bc2f75b54
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Linares, M.
cea3a79a-1831-4a7f-be41-3bf3b7b25383

Degenaar, N., Ootes, L.S., Page, D., Wijnands, R., Parikh, A.S., Homan, J., Cackett, E.M., Miller, J.M., Altamirano, Diego and Linares, M. (2019) Crust cooling of the neutron star in Aql X-1: different depth and magnitude of shallow heating during similar accretion outbursts. Monthly Notices of the Royal Astronomical Society, 488 (4), 4477-4486. (doi:10.1093/mnras/stz1963).

Record type: Article

Abstract

The structure and composition of the crust of neutron stars plays an important role in their thermal and magnetic evolution, hence in setting their observational properties. One way to study the properties of the crust of a neutron star, is to measure how it cools after it has been heated during an accretion outburst in a low-mass X-ray binary (LMXB). Such studies have shown that there is a tantalizing source of heat, of currently unknown origin, that is located in the outer layers of the crust and has a strength that varies between different sources and different outbursts. With the aim of understanding the mechanism behind this ‘shallow heating’, we present Chandra and Swift observations of the neutron star LMXB Aql X-1, obtained after its bright 2016 outburst. We find that the neutron star temperature was initially much lower, and started to decrease at much later time, than observed after the 2013 outburst of the source, despite the fact that the properties of the two outbursts were very similar. Comparing our data to thermal evolution simulations, we infer that the depth and magnitude of shallow heating must have been much larger during the 2016 outburst than during the 2013 one. This implies that basic neutron star parameters that remain unchanged between outbursts do not play a strong role in shallow heating. Furthermore, it suggests that outbursts with a similar accretion morphology can give rise to very different shallow heating. We also discuss alternative explanations for the observed difference in quiescent evolution after the 2016 outburst.

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Crust cooling of the neutron star in Aql X-1_different depth and magnitude of shallow heating during similar accretion outbursts - Accepted Manuscript
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Accepted/In Press date: 12 July 2019
e-pub ahead of print date: 16 July 2019
Published date: October 2019
Additional Information: arXiv is AM

Identifiers

Local EPrints ID: 434777
URI: http://eprints.soton.ac.uk/id/eprint/434777
ISSN: 0035-8711
PURE UUID: 0570dd03-d8ef-409c-8fc1-1ef441a15535
ORCID for Diego Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

Catalogue record

Date deposited: 09 Oct 2019 16:30
Last modified: 26 Nov 2021 03:02

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Contributors

Author: N. Degenaar
Author: L.S. Ootes
Author: D. Page
Author: R. Wijnands
Author: A.S. Parikh
Author: J. Homan
Author: E.M. Cackett
Author: J.M. Miller
Author: M. Linares

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