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Title: EVENTS LEADING UP TO THE 2015 JUNE OUTBURST OF V404 CYG

On 2015 June 15 the burst alert telescope (BAT) on board Swift detected an X-ray outburst from the black hole (BH) transient V404 Cyg. We monitored V404 Cyg for the last 10 years with the 2-m Faulkes Telescope North in three optical bands (V, R, and i′). We found that, one week prior to this outburst, the optical flux was 0.1–0.3 mag brighter than the quiescent orbital modulation, implying an optical precursor to the X-ray outburst. There is also a hint of a gradual optical decay (years) followed by a rise lasting two months prior to the outburst. We fortuitously obtained an optical spectrum of V404 Cyg 13 hr before the BAT trigger. This too was brighter than quiescence, and showed spectral lines typical of an accretion disk, with characteristic absorption features of the donor being much weaker. No He ii emission was detected, which would have been expected had the X-ray flux been substantially brightening. This, combined with the presence of intense Hα emission, about seven times the quiescent level, suggests that the disk entered the hot, outburst state before the X-ray outburst began. We propose that the outburst is produced by a viscous–thermal instability triggered close to themore » inner edge of a truncated disk. An X-ray delay of a week is consistent with the time needed to refill the inner region and hence move the inner edge of the disk inwards, allowing matter to reach the central BH, finally turning on the X-ray emission.« less
Authors:
; ;  [1] ; ;  [2] ;  [3] ;  [4] ;  [5]
  1. New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi (United Arab Emirates)
  2. Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ (United Kingdom)
  3. Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, The Parade, CF24 3AA, Cardiff, Wales (United Kingdom)
  4. Institut d’Astrophysique de Paris, CNRS et Sorbonne Universités, UPMC Paris 06, UMR 7095, 98bis Bd Arago, F-75014 Paris (France)
  5. Instituto de Astrofísica de Canarias, E-38205 La Laguna, Santa Cruz de Tenerife (Spain)
Publication Date:
OSTI Identifier:
22518618
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 818; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; ACCRETION DISKS; BINARY STARS; BLACK HOLES; EMISSION; MODULATION; PRECURSOR; QUASARS; TELESCOPES; TRANSIENTS; X RADIATION