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Discovery of a soft X-ray lag in the Ultraluminous X-ray Source NGC 1313 X-1

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

Discovery of a soft X-ray lag in the Ultraluminous X-ray Source NGC 1313 X-1 Thumbnail


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. Furst

P. Kosec

M.J. Middleton

R. Soria

L. Tao

N.A. Webb


Ultraluminous X-ray Sources (ULXs) provide a unique opportunities 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 timescales, and find a soft lag of ∼150 seconds. 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 to ∼20 percent of the corresponding characteristic variability timescales. If these soft lags can be understood in the context of a unified picture of ULXs, then lag timescales may provide constraints on the density and extent of radiatively-driven outflows.


Kara, E., Pinto, C., Walton, D., Alston, W., Bachetti, M., Barret, D., …Webb, N. (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), 5172-5178.

Journal Article Type Article
Acceptance Date Nov 24, 2019
Online Publication Date Dec 3, 2019
Publication Date Feb 28, 2020
Deposit Date Nov 26, 2019
Publicly Available Date Jan 2, 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 491
Issue 4
Pages 5172-5178
Related Public URLs


Published Journal Article (633 Kb)

Copyright Statement
This article has been accepted for publication in the Monthly notices of the Royal Astronomical Society ©: 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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