EVENTS LEADING UP TO THE 2015 JUNE OUTBURST OF V404 CYG

Journal Article · Wed Feb 10 04:00:00 EST 2016 · Astrophysical Journal Letters

DOI:https://doi.org/10.3847/2041-8205/818/1/L5· OSTI ID:22518618

Bernardini, F.; Russell, D. M.; Koljonen, K. I. I. ^[1]; Shaw, A. W.; Charles, P. A. ^[2]; Lewis, F. ^[3]; Lasota, J. P. ^[4]

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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)

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 the 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.

• OSTI ID: 22518618
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 818; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
ACCRETION DISKS
BINARY STARS
BLACK HOLES
EMISSION
MODULATION
PRECURSOR
QUASARS
TELESCOPES
TRANSIENTS
X RADIATION