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Sub-second multi-band optical timing of V404 Cyg with ULTRACAM

ATel #7686; P. Gandhi (Southampton), S. Littlefair, L. Hardy, V. Dhillon (Sheffield), T. R. Marsh (Warwick), A. W. Shaw (Southampton)
on 22 Jun 2015; 10:49 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

Subjects: Optical, Request for Observations, Binary, Black Hole, Transient

Referred to by ATel #: 7688, 7708, 7710, 7714, 7718, 7721, 7722, 7725, 7727, 7734, 7740, 7959

The black hole X-ray binary V404 Cyg is currently showing prolific broadband activity, with many facilities reporting strong variations on timescales ranging from ~10s to days (GCN #17929, #17931, #17933, #17938, #17940, #17944, #17945, #17946, #17948, #17949; ATel #7646, #7647, #7650, #7655, #7658, #7659, #7661, #7662, #7663, #7665, #7666, #7667, #7669, #7671, #7674, #7677, #7678, #7681).

This includes reports of a bright inverted radio spectrum and detection of significant optical polarisation (#7667, #7674, #7678). These may be consistent with the presence of a compact jet which has been associated with rapid optical and infrared flickering on timescales of ~1 s or shorter in other X-ray binaries such as XTE J1118+480 and GX 339-4 (Kanbach et al. 2001 Nature 414 180, Gandhi et al. 2010 MNRAS 407 2166, Casella et al. 2010 MNRAS 404 L21, Gandhi et al. 2011 ApJ 740 L13).

In order to search for corresponding fast optical variations in V404 Cyg, we are carrying out observations of the source with the ULTRACAM fast imaging camera on the 4.2m William Herschel Telescope on La Palma (Dhillon et al. 2007 MNRAS 378 825). Two initial observations were performed on 2015 Jun 20 between UT04:10 and UT05:24, and on Jun 21 between UT03:40 and UT05:33, including nautical twilight. The u', g' and r' filters were used and the fastest cycle time was 77.8 ms and 35.8 ms on the two nights, respectively (dead time is very small). The u' band data were co-added to improve signal-to-noise and have an effective time resolution of 466.8 ms and 537 ms, respectively. A field comparison star was observed simultaneously for monitoring and correcting atmospheric transparency variations. The nights were largely clear with seeing of approximately 1 arcsec. Observations were carried out between airmasses of 1.0 and 1.1.

The lightcurves are shown here: Jun 20 and Jun 21. They appear smooth in all bands, with the variability being dominated by timescales longer than tens of seconds. The corresponding power spectra of the ULTRACAM data have a steep fall off to high Fourier frequency, with the r.m.s. at frequencies above 1 Hz being only ~1% or less of total. ATel #7677 have also reported a steep power spectrum at lower Fourier frequencies using data with 2s time resolution. We find that the variations in all bands are closely correlated, with slightly higher total r.m.s. in the bluer bands. This is evident from the stronger peak-to-peak variations of the bluer light curves. The absence of strong sub-second red flickering in these data suggests a distinct optical behaviour of the compact jet in V404 Cyg to that seen in other typical hard state sources.

It is noteworthy that the baseline optical brightness of the source appears to be increasing (Atel #7681), and it could be that the rapid flickering is associated with fainter flux levels (see also Atel #7677). We also note that extrapolation of the optically-thick radio spectrum in Atel #7667 to the optical would predict a much higher optical flux than seen in the present outburst. This indicates that the source may be ungergoing strong variations in its jet flux and/or jet spectral index, or that the jet did not extend to the optical during our observations. Longer monitoring is required to search for flickering in different apparent states which the source may be frequently transitioning through.

We have further ULTRACAM observations scheduled on Jun 25 around UT01h and UT03:30h for about an hour each, and encourage multiwavelength follow-up, especially in the X-rays, infrared and the radio.