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Extreme quiescent variability of the transient neutron star low-mass X-ray binary EXO 1745−248 in Terzan 5

Extreme quiescent variability of the transient neutron star low-mass X-ray binary EXO 1745−248 in Terzan 5
Extreme quiescent variability of the transient neutron star low-mass X-ray binary EXO 1745−248 in Terzan 5
EXO 1745−248 is a transient neutron star low-mass X-ray binary that resides in the globular cluster Terzan 5. We studied the transient during its quiescent state using 18 Chandra observations of the cluster acquired between 2003 and 2016. We found an extremely variable source, with a luminosity variation in the 0.5–10 keV energy range of approximately three orders of magnitude (between 3 × 1031 and 2 × 1034 erg s−1) on time-scales from years down to only a few days. Using an absorbed power-law model to fit its quiescent spectra, we obtained a typical photon index of ∼1.4, indicating that the source is even harder than during outburst and much harder than typical quiescent neutron stars if their quiescent X-ray spectra are also described by a single power-law model. This indicates that EXO 1745−248 is very hard throughout the entire observed X-ray luminosity range. At the highest luminosity, the spectrum fits better when an additional (soft) component is added to the model. All these quiescent properties are likely related to strong variability in the low-level accretion rate in the system. However, its extreme variable behaviour is strikingly different from the one observed for other neutron star transients that are thought to still accrete in quiescence. We compare our results to these systems. We also discuss similarities and differences between our target and the transitional millisecond pulsar IGR J18245−2452, which also has hard spectra and strong variability during quiescence.
0035-8711
2777-2788
Rivera Sandoval, L.E.
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Wijnands, R.
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Degenaar, N.
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Cavecchi, Y.
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Heinke, C.O.
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Cackett, E.M.
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Homan, J.
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Altamirano, D.
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Bahramian, A.
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Sivakoff, G.R.
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Miller, J.M.
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Parikh, A.S.
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Rivera Sandoval, L.E.
c2e0d440-1096-4ced-bee5-7670b9a05802
Wijnands, R.
382893ee-0e03-45e3-9321-db8b9fc74914
Degenaar, N.
54149051-d74e-4e43-a7cd-501ec4023511
Cavecchi, Y.
939cba7d-c099-4d5a-a962-1fee916ea176
Heinke, C.O.
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Cackett, E.M.
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Homan, J.
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Altamirano, D.
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Bahramian, A.
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Sivakoff, G.R.
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Miller, J.M.
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Parikh, A.S.
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Rivera Sandoval, L.E., Wijnands, R., Degenaar, N., Cavecchi, Y., Heinke, C.O., Cackett, E.M., Homan, J., Altamirano, D., Bahramian, A., Sivakoff, G.R., Miller, J.M. and Parikh, A.S. (2018) Extreme quiescent variability of the transient neutron star low-mass X-ray binary EXO 1745−248 in Terzan 5. Monthly Notices of the Royal Astronomical Society, 479 (2), 2777-2788. (doi:10.1093/mnras/sty1535).

Record type: Article

Abstract

EXO 1745−248 is a transient neutron star low-mass X-ray binary that resides in the globular cluster Terzan 5. We studied the transient during its quiescent state using 18 Chandra observations of the cluster acquired between 2003 and 2016. We found an extremely variable source, with a luminosity variation in the 0.5–10 keV energy range of approximately three orders of magnitude (between 3 × 1031 and 2 × 1034 erg s−1) on time-scales from years down to only a few days. Using an absorbed power-law model to fit its quiescent spectra, we obtained a typical photon index of ∼1.4, indicating that the source is even harder than during outburst and much harder than typical quiescent neutron stars if their quiescent X-ray spectra are also described by a single power-law model. This indicates that EXO 1745−248 is very hard throughout the entire observed X-ray luminosity range. At the highest luminosity, the spectrum fits better when an additional (soft) component is added to the model. All these quiescent properties are likely related to strong variability in the low-level accretion rate in the system. However, its extreme variable behaviour is strikingly different from the one observed for other neutron star transients that are thought to still accrete in quiescence. We compare our results to these systems. We also discuss similarities and differences between our target and the transitional millisecond pulsar IGR J18245−2452, which also has hard spectra and strong variability during quiescence.

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Accepted/In Press date: 7 June 2018
e-pub ahead of print date: 11 June 2018
Published date: 11 September 2018
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2018 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Identifiers

Local EPrints ID: 424360
URI: http://eprints.soton.ac.uk/id/eprint/424360
ISSN: 0035-8711
PURE UUID: dfdd2f9f-e92f-4ebf-89a9-b375728fe467
ORCID for Y. Cavecchi: ORCID iD orcid.org/0000-0002-6447-3603
ORCID for D. Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

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Date deposited: 05 Oct 2018 11:36
Last modified: 21 Sep 2019 00:26

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Contributors

Author: L.E. Rivera Sandoval
Author: R. Wijnands
Author: N. Degenaar
Author: Y. Cavecchi ORCID iD
Author: C.O. Heinke
Author: E.M. Cackett
Author: J. Homan
Author: D. Altamirano ORCID iD
Author: A. Bahramian
Author: G.R. Sivakoff
Author: J.M. Miller
Author: A.S. Parikh

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