Furiously fast and red: Sub-second optical flaring in V404 Cyg during the 2015 outburst peak

P. Gandhi, S. P. Littlefair, L. K. Hardy, V. S. Dhillon, T. R. Marsh, A. W. Shaw, D. Altamirano, M. D. Caballero-Garcia, J. Casares, P. Casella, A. J. Castro-Tirado, P. A. Charles, Y. Dallilar, S. Eikenberry, R. P. Fender, R. I. Hynes, C. Knigge, E. Kuulkers, K. Mooley, T. Muñoz-DariasM. Pahari, F. Rahoui, D. M. Russell, J. V. Hernández Santisteban, T. Shahbaz, D. M. Terndrup, J. Tomsick, D. J. Walton

Research output: Contribution to journalArticlepeer-review

Abstract

We present observations of rapid (sub-second) optical flux variability in V404 Cyg during its 2015 June outburst. Simultaneous three-band observations with the ULTRACAM fast imager on four nights show steep power spectra dominated by slow variations on ~100-1000 s time-scales. Near the peak of the outburst on June 26, a dramatic change occurs and additional, persistent sub-second optical flaring appears close in time to giant radio and X-ray flaring. The flares reach peak optical luminosities of ~ few × 1036 erg s-1. Some are unresolved down to a time resolution of 24 ms. Whereas the fast flares are stronger in the red, the slow variations are bluer when brighter. The redder slopes, emitted power and characteristic time-scales of the fast flares can be explained as optically thin synchrotron emission from a compact jet arising on size scales ~140-500 Gravitational radii (with a possible additional contribution by a thermal particle distribution). The origin of the slower variations is unclear. The optical continuum spectral slopes are strongly affected by dereddening uncertainties and contamination by strong Hα emission, but the variations of these slopes follow relatively stable loci as a function of flux. Cross-correlating the slow variations between the different bands shows asymmetries on all nights consistent with a small red skew (i.e. red lag). X-ray reprocessing and non-thermal emission could both contribute to these. These data reveal a complex mix of components over five decades in time-scale during the outburst.

Original languageEnglish (US)
Pages (from-to)554-572
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Volume459
Issue number1
DOIs
StatePublished - Jun 11 2016

Keywords

  • Accretion, accretion discs
  • Stars: black holes
  • Stars: individual: V404 Cyg
  • Stars: jets
  • X-rays: binaries

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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