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THE ASTRONOMER Electronic Circular No 3088      2015 Jly 05 15.27UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
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OPTICAL PRECURSOR TO V404 CYGNI OUTBURST
F. Bernardini (NYU Abu Dhabi) et. al. relay on The Astronomer Telegram 7761
that they have been monitoring V404 Cyg with the Faulkes Telescope North
(2-m, at Haleakala, Hawaii) since 2006, collecting data in the i', V, and R
optical bands (200-sec exposures). For the photometry they adopt a circular
aperture size of 3.6 arcsec to encompass V404 Cyg and an unrelated field
star located 1.4 arcsec to the north of the X-ray binary which is unresolved
from V404 Cyg in some of their images. The nearby star has mag V = 18.90 +-
0.05 (Udalski & Kaluzny 1991), and an i'-band magnitude i' = 16.92 +- 0.01
(IPHAS-DR2, http://www.iphas.org/dr2/). The reported magnitudes below are
therefore the combined measurements from V404 Cyg and this nearby star. 

Their first clear detection of the latest outburst was on 2015 June 19, four
days after the initial detection of the new outburst (GCN #17929) by
Swift/BAT in the hard X-rays. However, a faint but significant optical
precursor is visible on 2015 June 8 and June 9. They folded the data on the
orbital period (6.4714 days) using the ephemeris of Casares & Charles
(1994). The quiescent optical flux varies with the orbital period, with some
low amplitude flaring activity. Figures showing the phase-resolved i', V,
and R-band magnitudes of V404 Cyg are available at the link at the bottom of
this ATel. 

The phase-resolved V-band data up until 2015 June 2 are consistent with the
long-term orbital modulation measured in 2014. On 2015-June 8, seven days
before the outburst reported in GCN #17929, the magnitude was V = 17.74 +-
0.04, which is about 0.3 mag above the orbital modulation. On 2015-June-9,
the magnitude is V = 17.69 +- 0.04, about 0.4 mag above the modulation,
suggesting a faint precursor to the outburst. The phase-resolved R-band data
up until 2015 April 8 are consistent with the long-term orbital modulation
measured in 2014. The data between 2015 May 13 and 2015 June 2 are
marginally above the modulation, but the exact modulation curve is uncertain
and there is variability in the shape of the modulation from year to year.
On 2015 June 8 the magnitude is R = 16.78 +- 0.01, about 0.3 mag above the
long-term modulation, and on 2015 June 9 it is R = 16.89 +- 0.01, still
about 0.2 mag above, supporting the scenario of a faint precursor to the
outburst. The phase-resolved i'-band data up until 2015 June 5 are
consistent with the long-term orbital modulation measured in 2012 and 2014.
While the source is showing some variability in the shape of the modulation
from year to year, on 2015 June 9 the magnitude is i' = 15.51 +- 0.01, which
is 0.1 mag above the modulation, confirming the presence of an optical
precursor to the outburst. 

They conclude that the outburst of V404 Cyg started on or before 2015 June
8, likely between 2015 June 2 and 2015 June 8, with a slow faint optical
rise before the bright X-ray, optical, millimeter and radio flares. They
note that the X-ray flux may have also been rising at this time if the
source remained below the detection sensitivity limits of X-ray all-sky
monitors.

Optical light curves of V404 Cyg: 
http://www.faulkes-telescope.com/v404

Guy M Hurst