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Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries

Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries
Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries
Here, we study the rapid X-ray variability (using XMM–Newton observations) of three neutron-star low-mass X-ray binaries (1RXS J180408.9−342058, EXO 1745−248 and IGR J18245−2452) during their recently proposed very hard spectral state. All our systems exhibit a strong to very strong noise component in their power density spectra (rms amplitudes ranging from 34 per cent to 102 per cent) with very low characteristic frequencies (as low as 0.01 Hz). These properties are more extreme than what is commonly observed in the canonical hard state of neutron-star low-mass X-ray binaries observed at X-ray luminosities similar to those we observe from our sources. This suggests that indeed the very hard state is a spectral-timing state distinct from the hard state, although we argue that the variability behaviour of IGR J18245−2452 is very extreme and possibly this source was in a very unusual state. We also compare our results with the rapid X-ray variability of the accreting millisecond X-ray pulsars IGR J00291+5934 and Swift J0911.9−6452 (also using XMM–Newton data) for which previously similar variability phenomena were observed. Although their energy spectra (as observed using the Swift X-ray telescope) were not necessarily as hard (i.e. for Swift J0911.9−6452) as for our other three sources, we conclude that likely both sources were also in very similar state during their XMM–Newton observations. This suggests that different sources that are found in this new state might exhibit different spectral hardness and one has to study both the spectral and the rapid variability to identify this unusual state.
0035-8711
559-576
Wijnands, R.
382893ee-0e03-45e3-9321-db8b9fc74914
Parikh, A.S
21ab1c15-6144-40a1-ac71-618e02bc37d6
Altamirano, D.
d5ccdb09-0b71-4303-9538-05b467be075b
Homan, J.
549baf32-2442-4247-824e-21bb018b05f5
Degenaar, N.
54149051-d74e-4e43-a7cd-501ec4023511
Wijnands, R.
382893ee-0e03-45e3-9321-db8b9fc74914
Parikh, A.S
21ab1c15-6144-40a1-ac71-618e02bc37d6
Altamirano, D.
d5ccdb09-0b71-4303-9538-05b467be075b
Homan, J.
549baf32-2442-4247-824e-21bb018b05f5
Degenaar, N.
54149051-d74e-4e43-a7cd-501ec4023511

Wijnands, R., Parikh, A.S, Altamirano, D., Homan, J. and Degenaar, N. (2017) Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries. Monthly Notices of the Royal Astronomical Society, 472 (1), 559-576. (doi:10.1093/mnras/stx2006).

Record type: Article

Abstract

Here, we study the rapid X-ray variability (using XMM–Newton observations) of three neutron-star low-mass X-ray binaries (1RXS J180408.9−342058, EXO 1745−248 and IGR J18245−2452) during their recently proposed very hard spectral state. All our systems exhibit a strong to very strong noise component in their power density spectra (rms amplitudes ranging from 34 per cent to 102 per cent) with very low characteristic frequencies (as low as 0.01 Hz). These properties are more extreme than what is commonly observed in the canonical hard state of neutron-star low-mass X-ray binaries observed at X-ray luminosities similar to those we observe from our sources. This suggests that indeed the very hard state is a spectral-timing state distinct from the hard state, although we argue that the variability behaviour of IGR J18245−2452 is very extreme and possibly this source was in a very unusual state. We also compare our results with the rapid X-ray variability of the accreting millisecond X-ray pulsars IGR J00291+5934 and Swift J0911.9−6452 (also using XMM–Newton data) for which previously similar variability phenomena were observed. Although their energy spectra (as observed using the Swift X-ray telescope) were not necessarily as hard (i.e. for Swift J0911.9−6452) as for our other three sources, we conclude that likely both sources were also in very similar state during their XMM–Newton observations. This suggests that different sources that are found in this new state might exhibit different spectral hardness and one has to study both the spectral and the rapid variability to identify this unusual state.

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Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries - Accepted Manuscript
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Accepted/In Press date: 2 August 2017
e-pub ahead of print date: 4 August 2017
Published date: November 2017

Identifiers

Local EPrints ID: 417634
URI: http://eprints.soton.ac.uk/id/eprint/417634
ISSN: 0035-8711
PURE UUID: 9e76ba9c-a50d-4e2d-b023-ea8732ba67e1
ORCID for D. Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

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

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Contributors

Author: R. Wijnands
Author: A.S Parikh
Author: D. Altamirano ORCID iD
Author: J. Homan
Author: N. Degenaar

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