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THE ASTRONOMER Electronic Circular No 3084      2015 Jun 18 19.55UT
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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V404 CYGNI (=GS 2023+338)
S. D. Barthelmy (GSFC) etl al., report on GCN 17929 that at  18:31:38 UT,
the Swift Burst Alert Telescope (BAT) triggered and located V404 Cyg.  Swift
slewed immediately to the source. 
The BAT on-board calculated location as:
RA 20h 24m 05s Dec +33d 50' 59" (2000) with an uncertainty of 3 arcmin
(radius, 90% containment, including systematic uncertainty). 
The XRT began observing the field at 18:34:37.3 UT, 179.1 seconds after the
BAT trigger. Using promptly downlinked data they find an X-ray source with
an enhanced position: 
RA 20h 24m 03.89s Dec +33d 52' 02.4" (2000).

Kosmas Gazeas relays via GCN 17931 Swift trigger 643949 occurred June 15,
2015 and followed a few minutes afterwards with the 0.40 m f/8 robotic
telescope at the University of Athens in R-band. Data collection has started
on June 15, 18:58:37 UT and finishing on June 15 (UT) 20:28:09 (mid-exposure
time). The bright GRB seemed to be the X-ray binary V404 Cyg, consisting of
a black hole and a late G-type companion. A sum of 30 exposures of 180 sec
each was collected. Photometry with a 3 pixel (approximately 4 arcsec)
radius aperture yields an R magnitude estimation of R = 12.65 +/- 0.01 mag
in the beginning of data acquisition and R = 15.43 +/- 0.03 mag at the end.
There is a luminosity fading trend of ~2.7 mag/hour as observed during the
first hour of acquisition. Fading rate dramatically decreased afterwards,
reaching the value of ~0.2 mag/hour. Differential photometry was performed
utilizing the nearby stars, namely USNO 1200-15039207 (Rmag=12.9) and USNO
1200-15046396 (Rmag=12.7), located 140 arcsec west and 96 arcsec southeast
of the source, respectively. The extracted light curve can be found under:
https://sites.google.com/site/astrofridaysmeetings/paratereseis/grb

Eddy Muyllaert, Belgium confirmed the optical outburst on 2015 June
16.1588UT at magnitude 16.18C using the Bradford Robotic Telescope.

Martin Mobberley also confirmed the outburst using an unfiltered ITEL
California. An image was measured with AIP$WIN using the AAVSO VSP chart
giving a magnitude of 15.7 on June 17 at 08h59mUT. 

AAVSO Alert Notice 520 carries news that V404 Cyg, an X-ray nova and a low
mass X-ray binary (LMXB) with black hole component, is undergoing its first
reported X-ray and optical outburst since 1989. Large scale, rapid
variations are being reported in wavelengths from X-ray to radio by
professional and amateur astronomers worldwide.

In the 1989 outburst, which was detected by Ginga on May 22
(IAUC 4782), V404 Cyg remained brighter than visual magnitude 15.3 for at
least 37 days. The AID shows a maximum visual magnitude of 14.0 on 1989 June
3.021 UT (JD 2447680.521; S. Korth, Germany); the last positive observation
was 15.3 on 1989 July 10.2 (2447717.7; C. Scovil, Stamford, CT).

Coordinates: 20 24 03.83 +33 52 02.2 (J2000.0)
Charts with comparison star sequence for V404 Cyg may be created using the
AAVSO Variable Star Plotter (VSP, http::www.aavso.org/vsp).

Guy M Hurst