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Correlated Optical and X-ray variability in V404 Cyg

ATel #7727; P. Gandhi, A. W. Shaw (Southampton), M. Pahari (IUCAA/Southampton), S. Littlefair, L. Hardy, V. Dhillon (Sheffield), T. R. Marsh (Warwick), P. Casella (INAF-Rome), P. A. Charles (Southampton), C. Done (Durham), D. M. Russell (NYU Abu Dhabi), T. Shahbaz, J. Casares, T. Munoz-Darias (IAC), J. Tomsick (Berkeley), K. O'Brien (Oxford), E. Kuulkers (ESAC/ESA), D. Altamirano, C. Knigge (Southampton), S. Motta (Oxford), D. Walton (Caltech)
on 29 Jun 2015; 00:04 UT
Credential Certification: Poshak Gandhi (p.gandhi@soton.ac.uk)

Subjects: Optical, Ultra-Violet, X-ray, Binary, Black Hole, Transient

Referred to by ATel #: 7734, 7736, 7740, 7959

We report first results of cross-correlating simultaneous optical (U) and X-ray (0.3-10 keV) light curves in the current outburst of V404 Cyg. The data were obtained from a Swift UVOT and X-ray event mode observation on UT 2015 Jun 21. The observing windows are UT03:56-04:04 and UT04:55-05:08. The XRT and UVOT exposures total 993s and 986s, respectively. We note that this observation was coordinated with a simultaneous timing observation with the ULTRACAM fast imager (Dhillon et al. 2007 MNRAS 378 825) on the William Herschel Telescope, described in ATel #7686 and used for comparison below.

A section of the simultaneous light curves sampled at 5 s time resolution and the cross-correlation function (CCF) are shown at the link below. The light curves appear to be broadly correlated, even by eye. Cross-correlating the light curves over strictly simultaneous sections shows a significant peak with a positive correlation between the optical and X-rays. The CCF shows a broad optical delay peaking in the range of ~0-50 s with respect to X-rays. We caution that the exact delay has some uncertainty on the absolute time correction, though we have carried out tests to account for this (see Additional Note below).

The source is a wide low mass X-ray binary with a period of 6.47 days (system parameters are critically reviewed in Casares & Jonker 2015 Space Science Review 183 223). A broad lag could be consistent with reprocessed emission over an extended region such as an accretion disc (cf. Hynes et al. 1998 MNRAS 299 L37, Munoz-Darias et al. 2007 MNRAS 379 1637), though we cannot rule out some contribution from the secondary star (the latest ephemeris of Casares & Charles 1994 MNRAS 271 L5 shows that the source was at a phase of close to 0.01 during our observations). Alternatively, this may be linked to reprocessing in optically-thick envelopes which are thought to form in Eddington outburst sources (e.g. Revnivtsev et al. 2002 A&A 391 1013). Finally, we emphasise that the observations were carried out when the source showed little rapid optical flickering and no extreme flares (Atel #7686). The strongest and fastest flares may have other contributions than the reprocessing lags we probe here.

Multi-wavelength observations are being coordinated where possible. We encourage users interested in multi-wavelength coordination to sign up to the V404 Cyg multi-wavelength campaign mailing list by sending an email to majordomo@lists.soton.ac.uk and putting "subscribe v404-mwc" in the BODY of the message (not the header).

We are grateful to the Swift team for scheduling these observations and answering our queries, specifically Neil Gehrels, Jamie Kennea, Daniele Malesani, Boris Sbarufatti and David Burrows.

Additional Note: The Swift light curves have been extracted using standard ftools and cover the full XRT band and the U filter, though we caution that these results are based upon the Swift quicklook data, processed using the standard ftools xrtpipeline and coordinator packages. We could not filter the UVOT quicklook event data and our CCF is based upon the unfiltered events, though this should not affect the CCF significantly. Additionally, while cross-checking against the WHT/ULTRACAM data, we found that we needed to apply an approximate absolute time offset of 20 s to align the UVOT barycentric light curve with the ULTRACAM barycentric light curve. However, the Swift UVOT vs. XRT CCF should be free of absolute time cross-calibration uncertainties between missions.

V404 Cyg Optical/X-ray cross-correlation