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The nova-like nebular optical spectrum of V404 Cygni at the beginning of the 2015 outburst decay

Rahoui, Farid and Tomsick, J. A. and Gandhi, P. and Casella, P. and Fürst, F. and Natalucci, L. and Rossi, A. and Shaw, A. W. and Testa, V. and Walton, D. J. (2017) The nova-like nebular optical spectrum of V404 Cygni at the beginning of the 2015 outburst decay. Monthly Notices of the Royal Astronomical Society, 465 (4). pp. 4468-4481. ISSN 0035-8711.

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We report on FORS2 optical spectroscopy of the black hole X-ray binary V404 Cygni, performed at the very beginning of its 2015 outburst decay, complemented by quasi-simultaneous Swift X-ray and ultraviolet as well as Rapid Eye Mountain near-infrared observations. Its peculiar spectrum is dominated by a wealth of emission signatures of H i, He i, and higher ionization species, in particular Fe ii. The spectral features are divided between broad redshifted and narrow stationary varieties, the latter being emitted in the outer regions. Continuum and line variability at short time-scale is high, and we find Baldwin effect-like anticorrelations between the full widths at half-maximum and equivalent widths of the broad lines with their local continua. The Balmer decrement H α/H β is also abnormally large at 4.61 ± 0.62. We argue that these properties hint at the broad lines being optically thick and arising within a circumbinary component in which shocks between faster optically thick and slower optically thin regions may occur. We associate it to a nova-like nebula formed by the cooling remnant of strong accretion disc winds that turned off when the mass-accretion rate dropped following the last major flare. The Fe ii lines likely arise from the overlap region between this nebula and the companion star winds, whereas we favour the shocks within the nebula as responsible for the optical continuum via self-absorbed optically thin bremsstrahlung. The presence of a near-infrared excess also points towards the contribution of a strongly variable compact jet or a dusty component.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Rahoui, Farid0000-0001-7655-4120
Tomsick, J. A.0000-0001-5506-9855
Gandhi, P.0000-0003-3105-2615
Fürst, F.0000-0003-0388-0560
Natalucci, L.0000-0002-6601-9543
Walton, D. J.0000-0001-5819-3552
Additional Information:© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 November 7. Received 2016 November 4; in original form 2016 June 17. We are very thankful to the referee for his/her very insightful comments and suggestions that helped improve this paper a lot. We also thank Mariko Kimura for kindly providing her I-band photometry. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. PC acknowledges support by a Marie Curie FP7-Reintegration-Grants under contract no. 2012-322259. This research has made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center. This research has made use of NASA's Astrophysics Data System, of the SIMBAD, and VizieR data bases operated at CDS, Strasbourg, France.
Funding AgencyGrant Number
Marie Curie Fellowship2012-322259
Subject Keywords:accretion, accretion discs, binaries: close, stars: individual: V404 Cygni, ISM: jets and outflows, X-rays: binaries
Issue or Number:4
Record Number:CaltechAUTHORS:20170331-120251653
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Official Citation:Farid Rahoui, J. A. Tomsick, P. Gandhi, P. Casella, F. Fürst, L. Natalucci, A. Rossi, A. W. Shaw, V. Testa, D. J. Walton; The nova-like nebular optical spectrum of V404 Cygni at the beginning of the 2015 outburst decay. Mon Not R Astron Soc 2017; 465 (4): 4468-4481. doi: 10.1093/mnras/stw2890
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:75590
Deposited By: George Porter
Deposited On:31 Mar 2017 19:38
Last Modified:09 Mar 2020 13:19

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