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A spectral-timing model for ULXs in the supercritical regime

A spectral-timing model for ULXs in the supercritical regime
A spectral-timing model for ULXs in the supercritical regime
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.
0035-8711
3243-3263
Middleton, Matthew
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Heil, Lucy
db5c0f2e-dc1e-41a8-8725-20c308f6af2d
Pintore, Fabio
72e13609-e283-4888-9d6d-a07279db9cd3
Walton, Dominic J.
3cf6ea78-7dd5-4765-8c09-ba2af76f11e7
Roberts, Timothy P.
37fd30f2-b7da-4268-80ab-919c503925c3
Middleton, Matthew
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Heil, Lucy
db5c0f2e-dc1e-41a8-8725-20c308f6af2d
Pintore, Fabio
72e13609-e283-4888-9d6d-a07279db9cd3
Walton, Dominic J.
3cf6ea78-7dd5-4765-8c09-ba2af76f11e7
Roberts, Timothy P.
37fd30f2-b7da-4268-80ab-919c503925c3

Middleton, Matthew, Heil, Lucy, Pintore, Fabio, 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), 3243-3263. (doi:10.1093/mnras/stu2644).

Record type: Article

Abstract

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.

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More information

Accepted/In Press date: 12 December 2014
e-pub ahead of print date: 16 January 2015
Published date: 11 March 2015

Identifiers

Local EPrints ID: 412259
URI: http://eprints.soton.ac.uk/id/eprint/412259
ISSN: 0035-8711
PURE UUID: c3b15b96-c9d9-4669-bd27-8f70a81e2dd3

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Date deposited: 14 Jul 2017 16:30
Last modified: 29 Jan 2021 17:37

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Contributors

Author: Lucy Heil
Author: Fabio Pintore
Author: Dominic J. Walton
Author: Timothy P. Roberts

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