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Truncation of the accretion disk at one-third of the Eddington Limit in the neutron star low-mass X-Ray binary Aquila X-1

Truncation of the accretion disk at one-third of the Eddington Limit in the neutron star low-mass X-Ray binary Aquila X-1
Truncation of the accretion disk at one-third of the Eddington Limit in the neutron star low-mass X-Ray binary Aquila X-1
We perform a reflection study on a new observation of the neutron star (NS) low-mass X-ray binary Aquila X-1 taken with NuSTAR during the 2016 August outburst and compare with the 2014 July outburst. The source was captured at ~32% L Edd, which is over four times more luminous than the previous observation during the 2014 outburst. Both observations exhibit a broadened Fe line profile. Through reflection modeling, we determine that the inner disk is truncated ${R}_{\mathrm{in},2016}={11}_{-1}^{+2}\ {R}_{g}$ (where R g = GM/c 2) and ${R}_{\mathrm{in},2014}=14\pm 2\ {R}_{g}$ (errors quoted at the 90% confidence level). Fiducial NS parameters (M NS = 1.4 M ⊙, R NS = 10 km) give a stellar radius of R NS = 4.85 R g ; our measurements rule out a disk extending to that radius at more than the 6σ level of confidence. We are able to place an upper limit on the magnetic field strength of B ≤ 3.0–4.5 × 109 G at the magnetic poles, assuming that the disk is truncated at the magnetospheric radius in each case. This is consistent with previous estimates of the magnetic field strength for Aquila X-1. However, if the magnetosphere is not responsible for truncating the disk prior to the NS surface, we estimate a boundary layer with a maximum extent of ${R}_{\mathrm{BL},2016}\sim 10\ {R}_{g}$ and ${R}_{\mathrm{BL},2014}\sim 6\ {R}_{g}$. Additionally, we compare the magnetic field strength inferred from the Fe line profile of Aquila X-1 and other NS low-mass X-ray binaries to known accreting millisecond X-ray pulsars.
0004-637X
Ludlam, R.
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Miller, J.M.
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Degenaar, N.
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Sanna, A.
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Cackett, E.M.
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Altamirano, D.
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King, A.L.
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Ludlam, R.
14eb530f-6cbb-4886-8f0f-ef7f1bc34c32
Miller, J.M.
078305f0-03c7-4982-95ff-a92bc2f75b54
Degenaar, N.
54149051-d74e-4e43-a7cd-501ec4023511
Sanna, A.
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Cackett, E.M.
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Altamirano, D.
d5ccdb09-0b71-4303-9538-05b467be075b
King, A.L.
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Ludlam, R., Miller, J.M., Degenaar, N., Sanna, A., Cackett, E.M., Altamirano, D. and King, A.L. (2017) Truncation of the accretion disk at one-third of the Eddington Limit in the neutron star low-mass X-Ray binary Aquila X-1. The Astrophysical Journal. (doi:10.3847/1538-4357/aa8b1b).

Record type: Article

Abstract

We perform a reflection study on a new observation of the neutron star (NS) low-mass X-ray binary Aquila X-1 taken with NuSTAR during the 2016 August outburst and compare with the 2014 July outburst. The source was captured at ~32% L Edd, which is over four times more luminous than the previous observation during the 2014 outburst. Both observations exhibit a broadened Fe line profile. Through reflection modeling, we determine that the inner disk is truncated ${R}_{\mathrm{in},2016}={11}_{-1}^{+2}\ {R}_{g}$ (where R g = GM/c 2) and ${R}_{\mathrm{in},2014}=14\pm 2\ {R}_{g}$ (errors quoted at the 90% confidence level). Fiducial NS parameters (M NS = 1.4 M ⊙, R NS = 10 km) give a stellar radius of R NS = 4.85 R g ; our measurements rule out a disk extending to that radius at more than the 6σ level of confidence. We are able to place an upper limit on the magnetic field strength of B ≤ 3.0–4.5 × 109 G at the magnetic poles, assuming that the disk is truncated at the magnetospheric radius in each case. This is consistent with previous estimates of the magnetic field strength for Aquila X-1. However, if the magnetosphere is not responsible for truncating the disk prior to the NS surface, we estimate a boundary layer with a maximum extent of ${R}_{\mathrm{BL},2016}\sim 10\ {R}_{g}$ and ${R}_{\mathrm{BL},2014}\sim 6\ {R}_{g}$. Additionally, we compare the magnetic field strength inferred from the Fe line profile of Aquila X-1 and other NS low-mass X-ray binaries to known accreting millisecond X-ray pulsars.

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Truncation of the Accretion Disk at One-third of the Eddington Limit in the Neutron Star Low-mass X-Ray Binary Aquila X-1 - Accepted Manuscript
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Accepted/In Press date: 5 September 2017
e-pub ahead of print date: 3 October 2017
Additional Information: author Altamirano confirmed Arxiv record 1709.01559v1 is AM copy.

Identifiers

Local EPrints ID: 417632
URI: http://eprints.soton.ac.uk/id/eprint/417632
ISSN: 0004-637X
PURE UUID: 4806ac6e-3404-4017-9df0-8a0e69835f83
ORCID for D. Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

Catalogue record

Date deposited: 07 Feb 2018 17:31
Last modified: 22 Nov 2021 03:06

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Contributors

Author: R. Ludlam
Author: J.M. Miller
Author: N. Degenaar
Author: A. Sanna
Author: E.M. Cackett
Author: D. Altamirano ORCID iD
Author: A.L. King

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