title: Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni.
abstract: Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10(-8) solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.
authors.affiliation: Instituto de Astrofísica de Canarias, E-38205 La Laguna, Santa Cruz de Tenerife, Spain.|Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Santa Cruz de Tenerife, Spain.
authors.forename: T
authors.initials: T
authors.lastname: Muñoz-Darias
authors.affiliation: Instituto de Astrofísica de Canarias, E-38205 La Laguna, Santa Cruz de Tenerife, Spain.|Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Santa Cruz de Tenerife, Spain.|Department of Physics, Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
authors.forename: J
authors.initials: J
authors.lastname: Casares
authors.affiliation: Instituto de Astrofísica de Canarias, E-38205 La Laguna, Santa Cruz de Tenerife, Spain.|Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Santa Cruz de Tenerife, Spain.
authors.forename: D
authors.initials: D
authors.lastname: Mata Sánchez
authors.affiliation: Department of Physics, Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
authors.forename: R P
authors.initials: RP
authors.lastname: Fender
authors.affiliation: Instituto de Astrofísica de Canarias, E-38205 La Laguna, Santa Cruz de Tenerife, Spain.|Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Santa Cruz de Tenerife, Spain.|Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Armas Padilla
authors.affiliation: Instituto de Astrofísica de Canarias, E-38205 La Laguna, Santa Cruz de Tenerife, Spain.|Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Santa Cruz de Tenerife, Spain.|Institutt for Fysikk, Norges Teknisk-Naturvitenskapelige Universitet (NTNU), Trondheim, Norway.
authors.forename: M
authors.initials: M
authors.lastname: Linares
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching bei München, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Ponti
authors.affiliation: Department of Physics, Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.|School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK.
authors.forename: P A
authors.initials: PA
authors.lastname: Charles
authors.affiliation: Department of Physics, Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
authors.forename: K P
authors.initials: KP
authors.lastname: Mooley
authors.affiliation: Laboratoire Astrophysique Instrumentation Modélisation (AIM), UMR 7158, CEA/CNRS/Université Paris Diderot, CEA DRF/IRFU/SAp, 91191 Gif-sur-Yvette, France.
authors.forename: J
authors.initials: J
authors.lastname: Rodriguez