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A low-luminosity soft state in the short-period black hole X-ray binary Swift J1753.5-0127

A low-luminosity soft state in the short-period black hole X-ray binary Swift J1753.5-0127
A low-luminosity soft state in the short-period black hole X-ray binary Swift J1753.5-0127
We present results from the spectral fitting of the candidate black hole X-ray binary Swift J1753.5-0127 in an accretion state previously unseen in this source. We fit the 0.7–78 keV spectrum with a number of models, however the preferred model is one of a multitemperature disc with an inner disc temperature kTin = 0.252 ± 0.003 keV scattered into a steep power-law with photon index ?=6.39+0.08?0.02 and an additional hard power-law tail (? = 1.79 ± 0.02). We report on the emergence of a strong disc-dominated component in the X-ray spectrum and we conclude that the source has entered the soft state for the first time in its ?10 yr prolonged outburst. Using reasonable estimates for the distance to the source (3 kpc) and black hole mass (5?M?), we find the unabsorbed luminosity (0.1–100 keV) to be ?0.60?per?cent of the Eddington luminosity, making this one of the lowest luminosity soft states recorded in X-ray binaries. We also find that the accretion disc extended towards the compact object during its transition from hard to soft, with the inner radius estimated to be Rin=28.0+0.7?0.4Rg or ?12Rg, dependent on the boundary condition chosen, assuming the above distance and mass, a spectral hardening factor f = 1.7 and a binary inclination i = 55°.
0035-8711
1636-1644
Shaw, A.W.
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Gandhi, P.
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Altamirano, D.
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Uttley, P.
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Tomsick, J.A.
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Charles, P.A.
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Furst, F.
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Rahoui, F.
d98ca9b7-6a98-40ad-8a0f-f278d044ffec
Walton, D.J.
a1a36b18-a77b-48e8-8665-45bbef0c7bd1
Shaw, A.W.
ec30710a-0499-4f4f-9685-4510f75efe79
Gandhi, P.
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9
Altamirano, D.
d5ccdb09-0b71-4303-9538-05b467be075b
Uttley, P.
3772a9a4-219f-4acb-a71b-4402d786a4b4
Tomsick, J.A.
a9cc3ecb-ac1b-4561-8570-edf46d54cd2f
Charles, P.A.
0429b380-0754-4dc1-8def-885c7fa6a086
Furst, F.
b85eb74e-a1c3-4861-ac7b-6e273c052920
Rahoui, F.
d98ca9b7-6a98-40ad-8a0f-f278d044ffec
Walton, D.J.
a1a36b18-a77b-48e8-8665-45bbef0c7bd1

Shaw, A.W., Gandhi, P., Altamirano, D., Uttley, P., Tomsick, J.A., Charles, P.A., Furst, F., Rahoui, F. and Walton, D.J. (2016) A low-luminosity soft state in the short-period black hole X-ray binary Swift J1753.5-0127. Monthly Notices of the Royal Astronomical Society, 458 (2), 1636-1644. (doi:10.1093/mnras/stw417).

Record type: Article

Abstract

We present results from the spectral fitting of the candidate black hole X-ray binary Swift J1753.5-0127 in an accretion state previously unseen in this source. We fit the 0.7–78 keV spectrum with a number of models, however the preferred model is one of a multitemperature disc with an inner disc temperature kTin = 0.252 ± 0.003 keV scattered into a steep power-law with photon index ?=6.39+0.08?0.02 and an additional hard power-law tail (? = 1.79 ± 0.02). We report on the emergence of a strong disc-dominated component in the X-ray spectrum and we conclude that the source has entered the soft state for the first time in its ?10 yr prolonged outburst. Using reasonable estimates for the distance to the source (3 kpc) and black hole mass (5?M?), we find the unabsorbed luminosity (0.1–100 keV) to be ?0.60?per?cent of the Eddington luminosity, making this one of the lowest luminosity soft states recorded in X-ray binaries. We also find that the accretion disc extended towards the compact object during its transition from hard to soft, with the inner radius estimated to be Rin=28.0+0.7?0.4Rg or ?12Rg, dependent on the boundary condition chosen, assuming the above distance and mass, a spectral hardening factor f = 1.7 and a binary inclination i = 55°.

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A low-luminosity soft state in the short period black hole X-ray binary Swift J1753.5-0127.pdf - Accepted Manuscript
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Accepted/In Press date: 18 February 2016
e-pub ahead of print date: 23 February 2016
Published date: 11 May 2016
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 394806
URI: http://eprints.soton.ac.uk/id/eprint/394806
ISSN: 0035-8711
PURE UUID: 3893bdba-4dfb-4bdf-8832-f90b900b6cee
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615
ORCID for D. Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

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Date deposited: 24 May 2016 10:51
Last modified: 17 Dec 2019 01:35

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