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Crossing the Eddington Limit: examining disk spectra at high accretion rates

Crossing the Eddington Limit: examining disk spectra at high accretion rates
Crossing the Eddington Limit: examining disk spectra at high accretion rates
The faintest ultraluminous X-ray sources (ULXs), those with 0.3–10 keV luminosities $1\lt {L}_{{\rm{X}}}/{10}^{39}\lt 3\,\,\mathrm{erg}\,{{\rm{s}}}^{-1}$, tend to have X-ray spectra that are disk-like but broader than expected for thin accretion disks. These "broadened disk (BD)" spectra are thought to indicate near- or mildly super-Eddington accretion onto stellar remnant black holes. Here we report that a sample of bright thermal-dominant black hole binaries, which have Eddington ratios constrained to moderate values, also show BD spectra in the 0.3–10 keV band at an order of magnitude lower luminosities. This broadening would be missed in studies that only look above $\sim 2\,\mathrm{keV}$. While this may suggest that BD ULXs could be powered by accretion onto massive stellar remnant black holes with close to maximal spin, we argue in favor of a scenario where they are at close to the Eddington luminosity, such that radiation pressure would be expected to result in geometrically slim, advective accretion disks. However, this implies that an additional physical mechanism is required to produce the observed broad spectra at low Eddington ratios.
0004-637X
Sutton, Andrew D.
9fed38c0-4b4b-4f5a-ac2b-cb1c287dac21
Swartz, Douglas A.
2150c09a-3f35-401f-af6a-bdaae28faabf
Roberts, Timothy P.
37fd30f2-b7da-4268-80ab-919c503925c3
Middleton, Matthew J.
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Soria, Roberto
5c123193-2dcd-49b7-a20d-4ddbf878a886
Done, Chris
f21f3c76-9341-4bc2-bbeb-d9a91c1ccee4
Sutton, Andrew D.
9fed38c0-4b4b-4f5a-ac2b-cb1c287dac21
Swartz, Douglas A.
2150c09a-3f35-401f-af6a-bdaae28faabf
Roberts, Timothy P.
37fd30f2-b7da-4268-80ab-919c503925c3
Middleton, Matthew J.
f91b89d9-fd2e-42ec-aa99-1249f08a52ad
Soria, Roberto
5c123193-2dcd-49b7-a20d-4ddbf878a886
Done, Chris
f21f3c76-9341-4bc2-bbeb-d9a91c1ccee4

Sutton, Andrew D., Swartz, Douglas A., Roberts, Timothy P., Middleton, Matthew J., Soria, Roberto and Done, Chris (2017) Crossing the Eddington Limit: examining disk spectra at high accretion rates. The Astrophysical Journal, 836 (1), [48]. (doi:10.3847/1538-4357/836/1/48).

Record type: Article

Abstract

The faintest ultraluminous X-ray sources (ULXs), those with 0.3–10 keV luminosities $1\lt {L}_{{\rm{X}}}/{10}^{39}\lt 3\,\,\mathrm{erg}\,{{\rm{s}}}^{-1}$, tend to have X-ray spectra that are disk-like but broader than expected for thin accretion disks. These "broadened disk (BD)" spectra are thought to indicate near- or mildly super-Eddington accretion onto stellar remnant black holes. Here we report that a sample of bright thermal-dominant black hole binaries, which have Eddington ratios constrained to moderate values, also show BD spectra in the 0.3–10 keV band at an order of magnitude lower luminosities. This broadening would be missed in studies that only look above $\sim 2\,\mathrm{keV}$. While this may suggest that BD ULXs could be powered by accretion onto massive stellar remnant black holes with close to maximal spin, we argue in favor of a scenario where they are at close to the Eddington luminosity, such that radiation pressure would be expected to result in geometrically slim, advective accretion disks. However, this implies that an additional physical mechanism is required to produce the observed broad spectra at low Eddington ratios.

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Crossing the Eddington Limit: Examining Disk Spectra at High Accretion Rates - Accepted Manuscript
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Accepted/In Press date: 14 November 2016
e-pub ahead of print date: 7 February 2017
Published date: 7 February 2017
Additional Information: arxiv record 1611.09416 author Middleton confirmed AM record.
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 412028
URI: http://eprints.soton.ac.uk/id/eprint/412028
ISSN: 0004-637X
PURE UUID: 864aeb41-2aca-4413-ab63-c2eaca36267e

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Date deposited: 05 Jul 2017 16:31
Last modified: 06 Oct 2020 23:50

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Contributors

Author: Andrew D. Sutton
Author: Douglas A. Swartz
Author: Timothy P. Roberts
Author: Roberto Soria
Author: Chris Done

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