Skip to main content

Research Repository

Advanced Search

X-ray time lags and non-linear variability in the ultraluminous X-ray sources NGC 5408 X-1 and NGC 6946 X-1

Hernández-García, L.; Vaughan, S.; Roberts, T.P.; Middleton, M.

X-ray time lags and non-linear variability in the ultraluminous X-ray sources NGC 5408 X-1 and NGC 6946 X-1 Thumbnail


Authors

L. Hernández-García

S. Vaughan

M. Middleton



Abstract

We present our analysis of the X-ray variability of two ultraluminous X-ray sources (ULXs) based on multiple XMM–Newton observations. We show that the linear rms–flux relation is present in eight observations of NGC 5408 X-1 and also in three observations of NGC 6946 X-1, but data from other ULXs are generally not sufficient to constrain any rms–flux relation. The presence of this relation was previously reported in only two observations of NGC 5408 X-1; our results show that this is a persistent property of the variability of NGC 5408 X-1 and extends to at least one other variable ULX. We speculate this is a ubiquitous property of ULX variability, as it is for X-ray variability in other luminous accreting sources. We also recover the time delay between hard and soft bands in NGC 5408 X-1, with the soft band (<1 keV) delayed with respect to the hard band (>1 keV) by up to ∼10 s (∼0.2 rad) at frequencies above ∼few mHz. For the first time, we extend the lag analysis to lower frequencies and find some evidence for a reversal of the lag, a hard lag of ∼1 ks at frequencies of ∼0.1 mHz. Our energy-resolved analysis shows that the time delays are energy dependent. We argue that the lag is unlikely to be a result of reflection from an accretion disc (‘reverberation’) based on the lack of reflection features in the spectra, and the large size of the reflector inferred from the magnitude of the lag. We also argue that associating the soft lag with a quasi-periodic oscillation (QPO) in these ULXs – and drawing an analogy between soft lags in ULXs and soft lags seen in some low-frequency QPOs of Galactic X-ray binaries – is premature.

Citation

Hernández-García, L., Vaughan, S., Roberts, T., & Middleton, M. (2015). X-ray time lags and non-linear variability in the ultraluminous X-ray sources NGC 5408 X-1 and NGC 6946 X-1. Monthly Notices of the Royal Astronomical Society, 453(3), 2877-2884. https://doi.org/10.1093/mnras/stv1830

Journal Article Type Article
Acceptance Date Aug 5, 2015
Publication Date Nov 1, 2015
Deposit Date Sep 11, 2015
Publicly Available Date Mar 28, 2024
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 453
Issue 3
Pages 2877-2884
DOI https://doi.org/10.1093/mnras/stv1830
Keywords X-rays: general, X-rays: individual: NGC 5408 X-1, X-rays: individual: NGC 6946 X-1.
Related Public URLs http://arxiv.org/abs/1508.01328

Files

Published Journal Article (328 Kb)
PDF

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society ©: 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.





You might also like



Downloadable Citations