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Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion

Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion
Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion
The black-hole binary V404 Cyg entered the outburst phase in June 2015 after 26 years of X-ray quiescence, and with its behaviour broke the outburst evolution pattern typical of most black-hole binaries. We observed the entire outburst with the Swift satellite and performed time-resolved spectroscopy of its most active phase, obtaining over a thousand spectra with exposures from tens to hundreds of seconds. All the spectra can be fitted with an absorbed power law model, which most of the time required the presence of a partial covering. A blue-shifted iron-Kalpha line appears in 10% of the spectra together with the signature of high column densities, and about 20% of the spectra seem to show signatures of reflection. None of the spectra showed the unambiguous presence of soft disk-blackbody emission, while the observed bolometric flux exceeded the Eddington value in 3% of the spectra. Our results can be explained assuming that the inner part of the accretion flow is inflated into a slim disk that both hides the innermost (and brightest) regions of the flow, and produces a cold, clumpy, high-density outflow that introduces the high-absorption and fast spectral variability observed. We argue that the black hole in V404 Cyg might have been accreting erratically or even continuously at Eddington/Super-Eddington rates - thus sustaining a surrounding slim disk - while being partly or completely obscured by the inflated disk and its outflow. Hence, the largest flares produced by the source might not be accretion-driven events, but instead the effects of the unveiling of the extremely bright source hidden within the system.
astro-ph.HE
0035-8711
1797-1818
Motta, S.E.
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Kajava, J.J.E.
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Sánchez-Fernández, C.
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Beardmore, A.P.
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Sanna, A.
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Page, K.L.
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Fender, R.
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Altamirano, D.
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Charles, P.
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Giustini, M.
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Knigge, C.
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Kuulkers, E.
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Oates, S.
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Osborne, J.P.
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Motta, S.E.
b6813310-4162-4bb5-985e-c6daaa33f2e1
Kajava, J.J.E.
9369f181-1108-4d22-aa1a-9645dbc9e74a
Sánchez-Fernández, C.
8671413a-270a-4942-aacf-c819129cb01d
Beardmore, A.P.
d72591c2-c08f-42b6-9f46-645aacb4e3e0
Sanna, A.
b925b305-171b-461f-bbf7-1c8fe246293c
Page, K.L.
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Fender, R.
c802ddfc-25a3-4c0e-899d-11c405c705d1
Altamirano, D.
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Charles, P.
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Giustini, M.
95252c16-b214-4a63-bc56-1614149c2e38
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Kuulkers, E.
bbe4b980-0d14-46c7-92a4-7bc8149a1947
Oates, S.
ad6dac21-5a31-4119-a643-849046998b86
Osborne, J.P.
8fbc82c5-b7d3-46f7-be06-e1b8128d512f

Motta, S.E., Kajava, J.J.E., Sánchez-Fernández, C., Beardmore, A.P., Sanna, A., Page, K.L., Fender, R., Altamirano, D., Charles, P., Giustini, M., Knigge, C., Kuulkers, E., Oates, S. and Osborne, J.P. (2017) Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion. Monthly Notices of the Royal Astronomical Society, 471 (2), 1797-1818. (doi:10.1093/mnras/stx1699).

Record type: Article

Abstract

The black-hole binary V404 Cyg entered the outburst phase in June 2015 after 26 years of X-ray quiescence, and with its behaviour broke the outburst evolution pattern typical of most black-hole binaries. We observed the entire outburst with the Swift satellite and performed time-resolved spectroscopy of its most active phase, obtaining over a thousand spectra with exposures from tens to hundreds of seconds. All the spectra can be fitted with an absorbed power law model, which most of the time required the presence of a partial covering. A blue-shifted iron-Kalpha line appears in 10% of the spectra together with the signature of high column densities, and about 20% of the spectra seem to show signatures of reflection. None of the spectra showed the unambiguous presence of soft disk-blackbody emission, while the observed bolometric flux exceeded the Eddington value in 3% of the spectra. Our results can be explained assuming that the inner part of the accretion flow is inflated into a slim disk that both hides the innermost (and brightest) regions of the flow, and produces a cold, clumpy, high-density outflow that introduces the high-absorption and fast spectral variability observed. We argue that the black hole in V404 Cyg might have been accreting erratically or even continuously at Eddington/Super-Eddington rates - thus sustaining a surrounding slim disk - while being partly or completely obscured by the inflated disk and its outflow. Hence, the largest flares produced by the source might not be accretion-driven events, but instead the effects of the unveiling of the extremely bright source hidden within the system.

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Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion - Accepted Manuscript
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Accepted/In Press date: 4 July 2017
e-pub ahead of print date: 7 July 2017
Published date: October 2017
Keywords: astro-ph.HE

Identifiers

Local EPrints ID: 413123
URI: http://eprints.soton.ac.uk/id/eprint/413123
ISSN: 0035-8711
PURE UUID: 01e0947f-7ab2-41b3-8efd-9e1c18513a97
ORCID for D. Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

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Date deposited: 15 Aug 2017 16:30
Last modified: 07 Oct 2020 02:05

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Contributors

Author: S.E. Motta
Author: J.J.E. Kajava
Author: C. Sánchez-Fernández
Author: A.P. Beardmore
Author: A. Sanna
Author: K.L. Page
Author: R. Fender
Author: D. Altamirano ORCID iD
Author: P. Charles
Author: M. Giustini
Author: C. Knigge
Author: E. Kuulkers
Author: S. Oates
Author: J.P. Osborne

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