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

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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. R. 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). pp. 1797-1818. doi:10.1093/mnras/stx1699

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Official URL: http://dx.doi.org/10.1093/mnras/stx1699

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Abstract

The black hole (BH) binary V404 Cyg entered the outburst phase in 2015 June after 26 yr of X-ray quiescence, and with its behaviour broke the outburst evolution pattern typical of most BH 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 blueshifted iron-Kα line appears in 10 per cent of the spectra together with the signature of high column densities, and about 20 per cent of the spectra seem to show signatures of reflection. None of the spectra showed the unambiguous presence of soft disc–blackbody emission, while the observed bolometric flux exceeded the Eddington value in 3 per cent of the spectra. Our results can be explained assuming that the inner part of the accretion flow is inflated into a slim disc 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 BH in V404 Cyg might have been accreting erratically or even continuously at Eddington/super-Eddington rates – thus sustaining a surrounding slim disc – while being partly or completely obscured by the inflated disc 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.

Item Type: Journal Article
Subjects: Q Science > QB Astronomy
Divisions: Faculty of Science, Engineering and Medicine > Science > Physics
Library of Congress Subject Headings (LCSH): Black holes (Astronomy), X-ray astronomy
Journal or Publication Title: Monthly Notices of the Royal Astronomical Society
Publisher: Oxford University Press
ISSN: 0035-8711
Official Date: 21 October 2017
Dates:
DateEvent
21 October 2017Published
7 July 2017Available
4 July 2017Accepted
Volume: 471
Number: 2
Page Range: pp. 1797-1818
DOI: 10.1093/mnras/stx1699
Status: Peer Reviewed
Publication Status: Published
Access rights to Published version: Restricted or Subscription Access
RIOXX Funder/Project Grant:
Project/Grant IDRIOXX Funder NameFunder ID
UNSPECIFIEDScience and Technology Facilities Councilhttp://dx.doi.org/10.13039/501100000271
Grants 268740 Academy of Finlandhttp://dx.doi.org/10.13039/501100002341
Grants 295114 Academy of Finlandhttp://dx.doi.org/10.13039/501100002341
Research fellowship programmeEuropean Space Agencyhttp://dx.doi.org/10.13039/501100000844
UNSPECIFIEDRoyal Societyhttp://dx.doi.org/10.13039/501100000288
Emeritus Fellowship Leverhulme Trusthttp://dx.doi.org/10.13039/501100000275
Early Career FellowshipLeverhulme Trusthttp://dx.doi.org/10.13039/501100000275
UNSPECIFIEDNederlandse Organisatie voor Wetenschappelijk Onderzoekhttp://dx.doi.org/10.13039/501100003246
UNSPECIFIEDViolette and Samuel Glasstone Research Fellowship programmeUNSPECIFIED

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