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Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2

Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2
Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2
Context: it is thought that ultraluminous X-ray sources (ULXs) are mainly powered by super-Eddington accreting neutron stars or black holes as shown by the recent discovery of X-ray pulsations and relativistic winds.

Aims: this work presents a follow-up study of the spectral evolution over two decades of the pulsing ULX NGC 1313 X-2 in order to understand the structure of the accretion disc. The primary objective is to determine the shape and nature of the dominant spectral components by investigating their variability with the changes in the source luminosity.

Methods: we performed a spectral analysis over the canonical 0.3–10.0 keV energy band of all the high signal-to-noise XMM-Newton observations (96% of the available data), and we tested a number of different spectral models, which should approximate super-Eddington accretion discs. The baseline model consists of two thermal blackbody components with different temperatures plus an exponential cutoff powerlaw.

Results: the baseline model provides a good description of the X-ray spectra. In particular, the hotter and brighter (LX ∼ 6–9 × 1039 erg s−1) thermal component describes the emission from the super-Eddington inner disc and the cutoff powerlaw describes the contribution from the accretion column of the neutron star. Instead, the cooler component describes the emission from the outer region of the disc close to the spherisation radius and the wind. The luminosity-temperature relation for the cool component follows a negative trend, which is not consistent with L ∝ T4, as is expected from a sub-Eddington thin disc of Shakura-Sunayev. This is not consistent with L ∝ T2 either, as is expected for an advection-dominated disc. However, this would rather agree with a wind-dominated X-ray emitting region. Instead, the (Lx, Tdisk) relation for the hotter component is somewhere in between the first two theoretical scenarios.

Conclusions: our findings agree with the super-Eddington scenario and provide further detail on the disc structure. The source spectral evolution is qualitatively similar to that seen in NGC 1313 X-1 and Holmberg IX X-1, indicating a common structure and evolution among archetypal ULXs.
Accretion, accretion disks, X-rays: Binaries, X-rays: Individuals: NGC 1313 X-2
0004-6361
Robba, A.
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Pinto, C.
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Walton, D.J.
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Soria, R.
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Kosec, P.
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Pintore, F.
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Roberts, T.P.
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Alston, W.N.
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Middleton, Matthew
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Cusumano, G.
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Earnshaw, H.P.
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Fürst, F.
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Sathyaprakash, R.
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Kyritsis, E.
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Fabian, A.C.
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Robba, A.
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Pinto, C.
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Walton, D.J.
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Soria, R.
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Kosec, P.
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Pintore, F.
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Roberts, T.P.
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Alston, W.N.
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Middleton, Matthew
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Cusumano, G.
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Earnshaw, H.P.
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Fürst, F.
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Sathyaprakash, R.
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Kyritsis, E.
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Fabian, A.C.
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Robba, A., Pinto, C., Walton, D.J., Soria, R., Kosec, P., Pintore, F., Roberts, T.P., Alston, W.N., Middleton, Matthew, Cusumano, G., Earnshaw, H.P., Fürst, F., Sathyaprakash, R., Kyritsis, E. and Fabian, A.C. (2021) Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2. Astronomy & Astrophysics, 652, [A118]. (doi:10.1051/0004-6361/202140884).

Record type: Article

Abstract

Context: it is thought that ultraluminous X-ray sources (ULXs) are mainly powered by super-Eddington accreting neutron stars or black holes as shown by the recent discovery of X-ray pulsations and relativistic winds.

Aims: this work presents a follow-up study of the spectral evolution over two decades of the pulsing ULX NGC 1313 X-2 in order to understand the structure of the accretion disc. The primary objective is to determine the shape and nature of the dominant spectral components by investigating their variability with the changes in the source luminosity.

Methods: we performed a spectral analysis over the canonical 0.3–10.0 keV energy band of all the high signal-to-noise XMM-Newton observations (96% of the available data), and we tested a number of different spectral models, which should approximate super-Eddington accretion discs. The baseline model consists of two thermal blackbody components with different temperatures plus an exponential cutoff powerlaw.

Results: the baseline model provides a good description of the X-ray spectra. In particular, the hotter and brighter (LX ∼ 6–9 × 1039 erg s−1) thermal component describes the emission from the super-Eddington inner disc and the cutoff powerlaw describes the contribution from the accretion column of the neutron star. Instead, the cooler component describes the emission from the outer region of the disc close to the spherisation radius and the wind. The luminosity-temperature relation for the cool component follows a negative trend, which is not consistent with L ∝ T4, as is expected from a sub-Eddington thin disc of Shakura-Sunayev. This is not consistent with L ∝ T2 either, as is expected for an advection-dominated disc. However, this would rather agree with a wind-dominated X-ray emitting region. Instead, the (Lx, Tdisk) relation for the hotter component is somewhere in between the first two theoretical scenarios.

Conclusions: our findings agree with the super-Eddington scenario and provide further detail on the disc structure. The source spectral evolution is qualitatively similar to that seen in NGC 1313 X-1 and Holmberg IX X-1, indicating a common structure and evolution among archetypal ULXs.

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Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2 - Accepted Manuscript
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More information

Accepted/In Press date: 7 June 2021
Published date: August 2021
Additional Information: arxiv is am.
Keywords: Accretion, accretion disks, X-rays: Binaries, X-rays: Individuals: NGC 1313 X-2

Identifiers

Local EPrints ID: 451714
URI: http://eprints.soton.ac.uk/id/eprint/451714
ISSN: 0004-6361
PURE UUID: 16b6f0da-0c06-49b3-aab0-f0b00f48521a

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Date deposited: 21 Oct 2021 16:31
Last modified: 25 Oct 2021 16:30

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Contributors

Author: A. Robba
Author: C. Pinto
Author: D.J. Walton
Author: R. Soria
Author: P. Kosec
Author: F. Pintore
Author: T.P. Roberts
Author: W.N. Alston
Author: G. Cusumano
Author: H.P. Earnshaw
Author: F. Fürst
Author: R. Sathyaprakash
Author: E. Kyritsis
Author: A.C. Fabian

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