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A spectral-timing model for ULXs in the supercritical regime

Middleton, Matthew J. and Heil, Lucy and Pintore, Fabio and Walton, Dominic J. and Roberts, Timothy P. (2015) A spectral-timing model for ULXs in the supercritical regime. Monthly Notices of the Royal Astronomical Society, 447 (4). pp. 3243-3263. ISSN 0035-8711.

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Ultraluminous X-ray sources (ULXs) with luminosities lying between ∼3 × 10^(39) and 2 × 10^(40) erg s^(−1) represent a contentious sample of objects as their brightness, together with a lack of unambiguous mass estimates for the vast majority of the central objects, leads to a degenerate scenario where the accretor could be a stellar remnant (black hole or neutron star) or intermediate-mass black hole (IMBH). Recent, high-quality observations imply that the presence of IMBHs in the majority of these objects is unlikely unless the accretion flow somehow deviates strongly from expectation based on objects with known masses. On the other hand, physically motivated models for supercritical inflows can re-create the observed X-ray spectra and their evolution, although have been lacking a robust explanation for their variability properties. In this paper, we include the effect of a partially inhomogeneous wind that imprints variability on to the X-ray emission via two distinct methods. The model is heavily dependent on both inclination to the line of sight and mass accretion rate, resulting in a series of qualitative and semiquantitative predictions. We study the time-averaged spectra and variability of a sample of well-observed ULXs, finding that the source behaviours can be explained by our model in both individual cases as well as across the entire sample, specifically in the trend of hardness-variability power. We present the covariance spectra for these sources for the first time, which shed light on the correlated variability and issues associated with modelling broad ULX spectra.

Item Type:Article
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URLURL TypeDescription Paper
Middleton, Matthew J.0000-0002-8183-2970
Walton, Dominic J.0000-0001-5819-3552
Roberts, Timothy P.0000-0001-8252-6337
Additional Information:© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2014 December 12. Received 2014 December 12; in original form 2014 January 28. First published online January 16, 2015. The authors thank the anonymous referee for helpful suggestions, and Tom Maccarone, Phil Uttley, Adam Ingram and Andrew King for useful discussion. MJM acknowledges support via a Marie Curie FP7 Postdoctoral scholarship. DJW is supported by ORAU under the NASA Postdoctoral Programme. TPR was funded as part of the STFC consolidated grant ST/K000861/1. This work is based on observations obtained with XMM–Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.
Funding AgencyGrant Number
Marie Curie FellowshipUNSPECIFIED
NASA Postdoctoral ProgramUNSPECIFIED
Science and Technology Facilities Council (STFC)ST/K000861/1
Subject Keywords:accretion, accretion discs; X-rays: binaries
Issue or Number:4
Record Number:CaltechAUTHORS:20150403-092443069
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Official Citation:Matthew J. Middleton, Lucy Heil, Fabio Pintore, Dominic J. Walton, and Timothy P. Roberts A spectral-timing model for ULXs in the supercritical regime MNRAS (March 11, 2015) Vol. 447 3243-3263 doi:10.1093/mnras/stu2644 First published online January 16, 2015
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:56336
Deposited By: Tony Diaz
Deposited On:03 Apr 2015 19:25
Last Modified:09 Mar 2020 13:19

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