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The unusual broad-band X-ray spectral variability of NGC 1313 X-1 seen with XMM–Newton, Chandra, and NuSTAR

Walton, D.J.; Pinto, C.; Nowak, M.; Bachetti, M.; Sathyaprakash, R.; Kara, E.; Roberts, T.P.; Soria, R.; Brightman, M.; Canizares, C.R.; Earnshaw, H.P.; Fürst, F.; Heida, M.; Middleton, M.J.; Stern, D.; Tao, L.; Webb, N.; Alston, W.N.; Barret, D.; Fabian, A.C.; Harrison, F.A.; Kosec, P.

The unusual broad-band X-ray spectral variability of NGC 1313 X-1 seen with XMM–Newton, Chandra, and NuSTAR Thumbnail


D.J. Walton

C. Pinto

M. Nowak

M. Bachetti

R. Sathyaprakash

E. Kara

R. Soria

M. Brightman

C.R. Canizares

H.P. Earnshaw

F. Fürst

M. Heida

M.J. Middleton

D. Stern

L. Tao

N. Webb

W.N. Alston

D. Barret

A.C. Fabian

F.A. Harrison

P. Kosec


We present results from the major coordinated X-ray observing programme on the ULX NGC 1313 X-1 performed in 2017, combining XMM–Newton, Chandra, and NuSTAR, focusing on the evolution of the broad-band (∼0.3–30.0 keV) continuum emission. Clear and unusual spectral variability is observed, but this is markedly suppressed above ∼10–15 keV, qualitatively similar to the ULX Holmberg IX X-1. We model the multi-epoch data with two-component accretion disc models designed to approximate super-Eddington accretion, allowing for both a black hole and a neutron star accretor. With regards to the hotter disc component, the data trace out two distinct tracks in the luminosity–temperature plane, with larger emitting radii and lower temperatures seen at higher observed fluxes. Despite this apparent anticorrelation, each of these tracks individually shows a positive luminosity–temperature relation. Both are broadly consistent with L ∝ T4, as expected for blackbody emission with a constant area, and also with L ∝ T2, as may be expected for an advection-dominated disc around a black hole. We consider a variety of possibilities for this unusual behaviour. Scenarios in which the innermost flow is suddenly blocked from view by outer regions of the super-Eddington disc/wind can explain the luminosity–temperature behaviour, but are difficult to reconcile with the lack of strong variability at higher energies, assuming this emission arises from the most compact regions. Instead, we may be seeing evidence for further radial stratification of the accretion flow than is included in the simple models considered, with a combination of winds and advection resulting in the suppressed high-energy variability.


Walton, D., Pinto, C., Nowak, M., Bachetti, M., Sathyaprakash, R., Kara, E., …Kosec, P. (2020). The unusual broad-band X-ray spectral variability of NGC 1313 X-1 seen with XMM–Newton, Chandra, and NuSTAR. Monthly Notices of the Royal Astronomical Society, 494(4), 6012-6029.

Journal Article Type Article
Acceptance Date Apr 20, 2020
Online Publication Date Apr 26, 2020
Publication Date 2020-06
Deposit Date Jun 24, 2020
Publicly Available Date Jun 24, 2020
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 494
Issue 4
Pages 6012-6029


Published Journal Article (1.1 Mb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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