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Discovery of a soft X-ray lag in the ultraluminous X-ray source NGC 1313 X-1

Kara, E. and Pinto, C. and Walton, D. J. and Alston, W. N. and Bachetti, M. and Barret, D. and Brightman, M. and Canizares, C. R. and Earnshaw, H. P. and Fabian, A. C. and Fürst, F. and Kosec, P. and Middleton, M. J. and Roberts, T. P. and Soria, R. and Tao, L. and Webb, N. A. (2020) Discovery of a soft X-ray lag in the ultraluminous X-ray source NGC 1313 X-1. Monthly Notices of the Royal Astronomical Society, 491 (4). pp. 5172-5178. ISSN 0035-8711. doi:10.1093/mnras/stz3318.

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Ultraluminous X-ray sources (ULXs) provide a unique opportunity to probe the geometry and energetics of super-Eddington accretion. The radiative processes involved in super-Eddington accretion are not well understood, and so studying correlated variability between different energy bands can provide insights into the causal connection between different emitting regions. We present a spectral-timing analysis of NGC 1313 X-1 from a recent XMM–Newton campaign. The spectra can be decomposed into two thermal-like components, the hotter of which may originate from the inner accretion disc, and the cooler from an optically thick outflow. We find correlated variability between hard (2–10 keV) and soft (0.3–2 keV) bands on kilosecond time-scales, and find a soft lag of ∼150 s. The covariance spectrum suggests that emission contributing to the lags is largely associated with the hotter of the two thermal-like components, likely originating from the inner accretion flow. This is only the third ULX to exhibit soft lags. The lags range over three orders of magnitude in amplitude, but all three are ∼5–20 per cent of the corresponding characteristic variability time-scales. If these soft lags can be understood in the context of a unified picture of ULXs, then lag time-scales may provide constraints on the density and extent of radiatively driven outflows.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Kara, E.0000-0003-0172-0854
Pinto, C.0000-0003-2532-7379
Walton, D. J.0000-0001-5819-3552
Alston, W. N.0000-0003-2658-6559
Bachetti, M.0000-0002-4576-9337
Barret, D.0000-0002-0393-9190
Brightman, M.0000-0002-8147-2602
Earnshaw, H. P.0000-0001-5857-5622
Fabian, A. C.0000-0002-9378-4072
Fürst, F.0000-0003-0388-0560
Middleton, M. J.0000-0002-8183-2970
Roberts, T. P.0000-0001-8252-6337
Soria, R.0000-0002-4622-796X
Additional Information:© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2019 November 24. Received 2019 November 21; in original form 2019 November 4. Published: 03 December 2019. Authors thank the referee, Thomas Dauser, for helpful comments that improved this paper. EK acknowledges support from NASA Award Number 80NSSC19K0176. CP is supported by ESA Research Fellowships. TPR thanks STFC for support as part of the consolidated grant ST/K000861/1. CRC acknowledges support by the Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 to MIT, which is in turn supported by NASA under contract NAS8-03060.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
European Space Agency (ESA)UNSPECIFIED
Science and Technology Facilities Council (STFC)ST/K000861/1
Smithsonian Astrophysical ObservatorySV3-73016
Subject Keywords:accretion, accretion discs –X-rays: binaries – X-rays: individual: NGC 1313 X-1
Issue or Number:4
Record Number:CaltechAUTHORS:20200228-123042359
Persistent URL:
Official Citation:E Kara, C Pinto, D J Walton, W N Alston, M Bachetti, D Barret, M Brightman, C R Canizares, H P Earnshaw, A C Fabian, F Fürst, P Kosec, M J Middleton, T P Roberts, R Soria, L Tao, N A Webb, Discovery of a soft X-ray lag in the ultraluminous X-ray source NGC 1313 X-1, Monthly Notices of the Royal Astronomical Society, Volume 491, Issue 4, February 2020, Pages 5172–5178,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101638
Deposited By: Tony Diaz
Deposited On:28 Feb 2020 22:36
Last Modified:16 Nov 2021 18:04

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