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

Robba, A. and Pinto, C. and Walton, D. J. and Soria, R. and Kosec, P. and Pintore, F. and Roberts, T. P. and Alston, W. N. and Middleton, M. and Cusumano, G. and Earnshaw, H. P. and Fürst, F. and Sathyaprakash, R. and Kyritsis, E. and Fabian, A. C. (2021) Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2. Astronomy and Astrophysics, 652 . Art. No. A118. ISSN 0004-6361. doi:10.1051/0004-6361/202140884.

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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 (L_X ∼ 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 (L_x, T_(disk)) 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Robba, A.0000-0002-4107-8475
Pinto, C.0000-0003-2532-7379
Walton, D. J.0000-0001-5819-3552
Soria, R.0000-0002-4622-796X
Pintore, F.0000-0002-3869-2925
Roberts, T. P.0000-0001-8252-6337
Alston, W. N.0000-0003-2658-6559
Middleton, M.0000-0002-8183-2970
Earnshaw, H. P.0000-0001-5857-5622
Fürst, F.0000-0003-0388-0560
Sathyaprakash, R.0000-0002-5254-3969
Fabian, A. C.0000-0002-9378-4072
Additional Information:© ESO 2021. Article published by EDP Sciences. Received 25 March 2021; Accepted 7 June 2021; Published online 20 August 2021. The authors would like to thank the anonymous referee, who provided useful suggestions for improving the final manuscript. This work is based on observations obtained with XMM-Newton, an ESA science mission funded by ESA Member States and USA (NASA). D. J. W. acknowledges support from STFC in the form of an Ernest Rutherford fellowship (ST/N004027/1). T. P. R. gratefully acknowledges support from the Science and Technology Facilities Council (STFC) as part of the consolidated grant award ST/000244/1.
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)ST/N004027/1
Science and Technology Facilities Council (STFC)ST/000244/1
Subject Keywords:accretion, accretion disks – X-rays: binaries – X-rays: individuals: NGC1313 X-2
Record Number:CaltechAUTHORS:20210930-152957940
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Official Citation:Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2. A. Robba, C. Pinto, D. J. Walton, R. Soria, P. Kosec, F. Pintore, T. P. Roberts, W. N. Alston, M. Middleton, G. Cusumano, H. P. Earnshaw, F. Fürst, R. Sathyaprakash, E. Kyritsis and A. C. Fabian. A&A, 652 (2021) A118; DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:111111
Deposited By: Tony Diaz
Deposited On:04 Oct 2021 19:15
Last Modified:04 Oct 2021 20:32

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