[vsnet-alert 18738] Re: V404 Cyg outburst

[Taichi Kato]

Re: V404 Cyg outburst

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===

TITLE:   GCN CIRCULAR
NUMBER:  17929
SUBJECT: Swift trigger 643949 is V404 Cyg
DATE:    15/06/15 18:55:32 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB),
H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

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 is 
RA, Dec 306.020, +33.850 which is 
   RA(J2000) = 20h 24m 05s
   Dec(J2000) = +33d 50' 59"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). As is usual with an image trigger,
the available BAT light curve shows no significant structure. 

The XRT began observing the field at 18:34:37.3 UT, 179.1 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 306.0162, 33.8673 which is
equivalent to:
   RA(J2000)  = 20h 24m 03.89s
   Dec(J2000) = +33d 52' 02.4"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 63 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. This
position is 0.8 arcseconds from that of a known transient low-mass X-ray 
binary V* V404 Cyg. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 8.10
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 183 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain extinction expected. 

===

TITLE:   GCN CIRCULAR
NUMBER:  17931
SUBJECT: Optical follow-up of Swift trigger on V404 Cyg
DATE:    15/06/15 22:05:04 GMT
FROM:    Kosmas Gazeas at U of Athens  <kgaze at physics.auth.gr>

Kosmas Gazeas and Konstantinos Sapountzis (National Univ. of Athens) report:

Swift trigger 643949 occurred in 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  
(UT) 18:58:37 and finishing on June 15 (UT) 20:28:09 (mid-exposure  
time). The bright GRB seems 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. No further  
absolute photometric calibration has been applied on these data. The  
extracted light curve can be found under:
https://sites.google.com/site/astrofridaysmeetings/paratereseis/grb

This message is quotable in publications.

===

TITLE:   GCN CIRCULAR
NUMBER:  17933
SUBJECT: V404 Cyg: Swift/UVOT Detection
DATE:    15/06/16 15:01:12 GMT
FROM:    Samantha Oates at MSSL  <sro at mssl.ucl.ac.uk>

S. R. Oates (IAA-CSIC/UCL-MSSL) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of V404 Cyg
184 s after the BAT trigger (Barthelmy et al., GCN Circ. 17929).
A source consistent with the XRT position is detected in the initial
UVOT exposures. The white filter is approximately constant for
the first 600s after the trigger, but a slight increase in flux is observed
in white after this time until observations cease at ~900s. The increase
in flux is also observed in the v and b filters.

Preliminary detections using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag
white (FC)         184          334          147         15.72 +/- 0.02
white              875          919           44         15.34 +/- 0.03
v                  673          693           19         14.58 +/- 0.07
v                  848          867           19         14.22 +/- 0.06
b                  599          619           19         16.17 +/- 0.08
b                  773          793           19         15.78 +/- 0.07
u (FC)             342          591         246         16.13 +/- 0.04
u                  748          768           20         16.06 +/- 0.11
w1                 722          742           19         > 17.66
m2                 698          717           19         > 17.45
w2                 649          843           39         > 18.37

The magnitudes in the table are not corrected for the large Galactic
extinction due to the reddening of E(B-V) = 2.02 in the direction of the burst
(Schlegel et al. 1998).