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The neutron star transient and millisecond pulsar in M28: from sub-luminous accretion to rotation-powered quiescence

The neutron star transient and millisecond pulsar in M28: from sub-luminous accretion to rotation-powered quiescence
The neutron star transient and millisecond pulsar in M28: from sub-luminous accretion to rotation-powered quiescence
The X-ray transient IGR J18245–2452 in the globular cluster M28 contains the
first neutron star (NS) seen to switch between rotation-powered and accretion-powered pulsations. We analyse its 2013 March-April 25 d-long outburst as observed by Swift, which had a peak bolometric luminosity of ∼6% of the Eddington limit (LEdd), and give detailed properties of the thermonuclear burst observed on 2013 April 7. We also present a detailed analysis of new and archival Chandra data, which we use to study quiescent emission from IGR J18245–2452 between 2002 and 2013. Together, these observations cover almost five orders of magnitude in X-ray luminosity (LX, 0.5–10 keV). The Swift spectrum softens during the outburst decay (photon index Γ from 1.3 above LX/LEdd=10−2 to ∼2.5 at LX/LEdd=10−4 ), similar to other NS and black hole (BH) transients. At even lower luminosities, LX/LEdd=[10−4–10−6], deep
Chandra observations reveal hard (Γ=1–1.5), purely non-thermal and highly variable X-ray emission in quiescence. We therefore find evidence for a spectral transition at LX/LEdd∼10−4, where the X-ray spectral softening observed during the outburst decline turns into hardening as the source goes to quiescence. Furthermore, we find a striking variability pattern in the 2008 Chandra light curves: rapid switches between a high-luminosity “active” state (LX≃3.9×1033 erg s−1) and a low-luminosity “passive” state (LX ≃5.6×1032 erg s−1), with no detectable spectral change. We put our results in the context of low luminosity accretion flows around compact objects and X-ray emission from millisecond radio pulsars. Finally, we discuss possible origins for the observed mode switches in quiescence, and explore a scenario where they are caused by
fast transitions between the magnetospheric accretion and pulsar wind shock emission regimes.
0035-8711
251-261
Linares, Manuel
d501c137-c3eb-45ca-b05c-4d4040b42b7b
Bahramian, Arash
390154f6-92b9-4e9f-9c5e-2eeaec97460a
Heinke, Craig O.
bd623ccb-27c1-4b45-8c3e-0609c69f7f2f
Wijnands, Rudy
4ac16ff6-b564-4e5d-89d4-36ff92c6030c
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Homan, Jeroen
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Bogdanov, Slavko
39f20582-4d71-4b1a-ae2a-b266367341ef
Pooley, David
72898f52-e7ed-441b-910c-95cd1e57db0d
Linares, Manuel
d501c137-c3eb-45ca-b05c-4d4040b42b7b
Bahramian, Arash
390154f6-92b9-4e9f-9c5e-2eeaec97460a
Heinke, Craig O.
bd623ccb-27c1-4b45-8c3e-0609c69f7f2f
Wijnands, Rudy
4ac16ff6-b564-4e5d-89d4-36ff92c6030c
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Homan, Jeroen
67eb2764-46f0-455b-b1f0-f4824c129e87
Bogdanov, Slavko
39f20582-4d71-4b1a-ae2a-b266367341ef
Pooley, David
72898f52-e7ed-441b-910c-95cd1e57db0d

Linares, Manuel, Bahramian, Arash, Heinke, Craig O., Wijnands, Rudy, Altamirano, Diego, Altamirano, Diego, Homan, Jeroen, Bogdanov, Slavko and Pooley, David (2014) The neutron star transient and millisecond pulsar in M28: from sub-luminous accretion to rotation-powered quiescence. Monthly Notices of the Royal Astronomical Society, 438 (1), 251-261. (doi:10.1093/mnras/stt2167).

Record type: Article

Abstract

The X-ray transient IGR J18245–2452 in the globular cluster M28 contains the
first neutron star (NS) seen to switch between rotation-powered and accretion-powered pulsations. We analyse its 2013 March-April 25 d-long outburst as observed by Swift, which had a peak bolometric luminosity of ∼6% of the Eddington limit (LEdd), and give detailed properties of the thermonuclear burst observed on 2013 April 7. We also present a detailed analysis of new and archival Chandra data, which we use to study quiescent emission from IGR J18245–2452 between 2002 and 2013. Together, these observations cover almost five orders of magnitude in X-ray luminosity (LX, 0.5–10 keV). The Swift spectrum softens during the outburst decay (photon index Γ from 1.3 above LX/LEdd=10−2 to ∼2.5 at LX/LEdd=10−4 ), similar to other NS and black hole (BH) transients. At even lower luminosities, LX/LEdd=[10−4–10−6], deep
Chandra observations reveal hard (Γ=1–1.5), purely non-thermal and highly variable X-ray emission in quiescence. We therefore find evidence for a spectral transition at LX/LEdd∼10−4, where the X-ray spectral softening observed during the outburst decline turns into hardening as the source goes to quiescence. Furthermore, we find a striking variability pattern in the 2008 Chandra light curves: rapid switches between a high-luminosity “active” state (LX≃3.9×1033 erg s−1) and a low-luminosity “passive” state (LX ≃5.6×1032 erg s−1), with no detectable spectral change. We put our results in the context of low luminosity accretion flows around compact objects and X-ray emission from millisecond radio pulsars. Finally, we discuss possible origins for the observed mode switches in quiescence, and explore a scenario where they are caused by
fast transitions between the magnetospheric accretion and pulsar wind shock emission regimes.

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The neutron star transient and millisecond pulsar in M28: - Accepted Manuscript
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Accepted/In Press date: 6 November 2013
e-pub ahead of print date: 13 December 2013
Published date: 11 February 2014
Additional Information: Author has confirmed Arxiv record 1310.7937 is AM.

Identifiers

Local EPrints ID: 418128
URI: http://eprints.soton.ac.uk/id/eprint/418128
ISSN: 0035-8711
PURE UUID: e8e109ea-52d1-4691-a10f-7078f774eb34
ORCID for Diego Altamirano: ORCID iD orcid.org/0000-0002-3422-0074
ORCID for Diego Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

Catalogue record

Date deposited: 22 Feb 2018 17:30
Last modified: 26 Nov 2021 03:02

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Contributors

Author: Manuel Linares
Author: Arash Bahramian
Author: Craig O. Heinke
Author: Rudy Wijnands
Author: Jeroen Homan
Author: Slavko Bogdanov
Author: David Pooley

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