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

Authors

P. Gandhi, University of Southampton
S. P. Littlefair, The University of Sheffield
L. K. Hardy, The University of Sheffield
V. S. Dhillon, The University of Sheffield
T. R. Marsh, University of Warwick
A. W. Shaw, University of Southampton
D. Altamirano, University of Southampton
M. D. Caballero-Garcia, Astronomical Institute, Academy of Sciences of the Czech Republic v.v.i.
J. Casares, Instituto Astrofisico de Canarias
P. Casella, Osservatorio Astronomico di Roma
A. J. Castro-Tirado, Instituto de Astrofísica de Andalucía - CSIC
P. A. Charles, University of Southampton
Y. Dallilar, University of Florida
S. Eikenberry, University of Florida
R. P. Fender, University of Oxford
R. I. Hynes, Louisiana State University
C. Knigge, University of Southampton
E. Kuulkers, European Space Astronomy Centre
K. Mooley, University of Oxford
T. Muñoz-Darias, Instituto Astrofisico de Canarias
M. Pahari, Inter-University Centre for Astronomy and Astrophysics India
F. Rahoui, European Southern Observatory
D. M. Russell, NYU Abu Dhabi
J. V. Hernández Santisteban, University of Southampton
T. Shahbaz, Instituto Astrofisico de Canarias
D. M. Terndrup, The Ohio State University
J. Tomsick, Space Sciences Laboratory at UC Berkeley
D. J. Walton, Jet Propulsion Laboratory

Document Type

Article

Publication Date

6-11-2016

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.

Publication Source (Journal or Book title)

Monthly Notices of the Royal Astronomical Society

First Page

554

Last Page

572

Recommended Citation

Gandhi, P., Littlefair, S., Hardy, L., Dhillon, V., Marsh, T., Shaw, A., Altamirano, D., Caballero-Garcia, M., Casares, J., Casella, P., Castro-Tirado, A., Charles, P., Dallilar, Y., Eikenberry, S., Fender, R., Hynes, R., Knigge, C., Kuulkers, E., Mooley, K., Muñoz-Darias, T., Pahari, M., Rahoui, F., Russell, D., Hernández Santisteban, J., Shahbaz, T., Terndrup, D., Tomsick, J., & Walton, D. (2016). Furiously fast and red: Sub-second optical flaring in V404 Cyg during the 2015 outburst peak. Monthly Notices of the Royal Astronomical Society, 459 (1), 554-572. https://doi.org/10.1093/mnras/stw571