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Recent Optical Observations of V404 Cyg

ATel #8501; Liam Hardy (Sheffield), Poshak Gandhi (Southampton), Vik Dhillon (Sheffield, IAC), Stuart Littlefair (Sheffield), Tim Butterley (Durham), Richard Wilson (Durham)
on 3 Jan 2016; 17:50 UT
Credential Certification: Liam Hardy (liam.hardy@sheffield.ac.uk)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 8515, 8531

On 21st Dec 2015, V404 Cyg began a new phase of activity following on from the bright June 2015 outburst (ATels #8453, #8454, #8455, #8457, #8458, #8462, #8466, #8475, #8482, #8489, #8494, #8499, #8500; GCN #18716, #18717, #18718, #18721, #18767, #18769, #18777, #18778, #18783, #18785). Here we report nightly optical observations of V404 Cyg with pt5m, a 0.5m robotic telescope on La Palma (Hardy et al 2015. MNRAS, 454, 4316).

Observations were conducted at airmass 2-3 for approximately one hour from ~19:30UT on the nights of 23rd, 24th, 25th, 27th, 29th, 30th, 31st Dec 2015 and 1st, 2nd Jan 2016. Exposures of 60s were obtained in the V-band with 6s deadtime between frames. A light curve spanning 11 nights is available at http://www.southampton.ac.uk/~pg3e14/v404/v404_pt5m_multinight.png .

Other optical observers have reported bright states and flares during the gaps in our light curve (GCN #18721, #18767, #18783). We did not observe any major variability or brightening until the night of 31st Dec when the source reached a peak magnitude of V=15.6. V404 Cyg was brighter again the following night (1st Jan), with a peak magnitude of V=13.2. Our observations of this flare are temporally coincident with the sub-millimetre observations reported in ATel #8499, and also simultaneous with the bright X-ray state in INTEGRAL revolution 1628 reported in ATel #8500. Our light curve from 1st Jan (brightest state observed by pt5m) is available at http://www.southampton.ac.uk/~pg3e14/v404/v404_pt5m_2016jan1.png . The source appeared relatively quiescent again on the night of 2nd Jan.

Fluxes were calibrated using the nearby comparison star USNO-B1.0 1238-0435227. This star has a V-band magnitude of 12.817 reported in Udalski & Kaluzny (1991, PASP, 103, 198), with a quoted uncertainty of 0.05 mags. Other V-band entries for this star in Vizier catalogues show a spread of 0.4 mags. We therefore assign a very conservative systematic error of 0.4 mags, but expect the magnitudes for these observations to be more precise than this.

We intend to continue monitoring V404 Cyg with pt5m for ~5 more days, after which it will be too close to the Sun.