A black-hole binary in outburst: INTEGRAL/OMC optical observations of V404 Cyg
On 15 June 2015, the black-hole binary X-ray transient V404 Cyg (GS 2023+338)
went into outburst after a long period of being quiet (see
Picture of the Month of July 2015
and references therein). It showed an extreme flaring activity at all
wavelengths, from radio to gamma-rays. INTEGRAL started monitoring the
flaring black hole binary on 17 June. Observations continued until 13 July,
when the source was too faint to be detected by the INTEGRAL instruments.
In quiescence, the V404 Cyg optical V-band emission is dominated by the K star
companion, showing a brightness in V of about 17.3 mag. During the last outburst
in June it reached 11 mag in V, i.e., 250 times brighter than in quiescence. The
evolution of the optical emission as seen by INTEGRAL/OMC during the recent
outburst is shown in the image (the image at the left has a size of 3.2 x 3.2 arcmin;
the other bright source in the image at 1.38 arcmin from V404 Cyg corresponds to a
known star, e.g., in the USNO CCD Astrograph Catalog 4: UCAC 620-101865). Significant
flaring activity from V404 Cyg is evident from the OMC optical light curve, with
flares typically lasting from several minutes to few hours (the start time of the
light curve displayed on the right is on IJD 5647 = MJD 57191 = UT 2015, June 18).
When comparing the X-ray and optical emission (see
J. Rodriguez et al. 2015, A&A 581, L9)
some of the optical flares are found to show shapes similar to the high-energy ones,
but in other cases there is no clear correlation between high-energy and optical
rising/fading. In addition, some flares take place simultaneously (within the OMC
time resolution) at all energy bands (from optical V to about 100 keV), but in some
other cases the optical flare is clearly delayed with respect to the high-energy one,
by few to tens of minutes. This behaviour suggests different mechanisms being
responsible for the optical emission. For nearly simultaneous flares the optical
emission could be related to X-ray reprocessing, either by the accretion disk or
by the reheated stellar companion surface. When the optical flares present delays,
the optical emission could be related to the jet, either as their intrinsic
synchrotron emission, or from interaction with the surrounding medium.
Credit:
OMC Team at CAB - Centro de Astrobiología (INTA-CSIC)
