[vsnet-grb-info 4849] OT20050728 was not an optical GRB afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6581
SUBJECT: OT20050728 was not an optical GRB afterglow
DATE:    07/06/27 13:23:05 GMT
FROM:    Jean-Luc Atteia at Lab d Astrophys.,OMP,Toulouse  <atteia at ast.obs-mip.fr>

F. Malacrino, C. Veillet, J-L. Atteia, M. Boer, J-C. Cuillandre,
A. Klotz and K. Withington communicate

OT20050728 was an optical transient, showing many similarities
with GRB optical afterglows, which was discovered by Malacrino et al. (2007)
in the course of the CFHTLS Very Wide Survey.
The magnitudes of the transient, measured on July 28, 2005 were :
(date, SOD, magnitude)
2005-07-28, 21849, i'= 20.09
2005-07-28, 26054, i'= 18.69
2005-07-28, 30330, i'= 20.94
2005-07-28, 30631, i'= 20.66
2005-07-29, 22204, i'= 21.59

Two exposures taken in june 2005, with a g’ filter
show no object at the position of the transient down to the
limiting magnitude of the observation g’ = 24.0


On june 8th, 2007 we re-observed the position
of OT20050728. These new observations clearly show
a star at the position of the transient, with coordinates (J2000) :

RA = 15 57 16.82 (RA=239.32008)
DEC = -18 50 59.1 (DEC=-18.84975)

The magnitude of the star is i’= 21.72

We conclude that the star brightened by 3 magnitudes
on 2005 july 28th, and that the 'flare' lasted about 1 day
(the magnitude on july 29th was almost back to normal).

The galactic coordinates of the star are
L = 352.48572023
B = +25.65001461
The galactic extinction in this direction, given by
The NASA Extragalactic Database, is A_I=0.58 and A_V=1.00
The magnitude of the star in quiescence, corrected for
Galactic extinction are thus : i’ = 21.14 and g’ > 23.0
The color index g-i > 1.86

Assuming that the star is an M dwarf, as suggested by its color,
with an absolute magnitude I=9.5 (Zheng et al. 2004), we can
estimate the distance to the star: D ~ 2.1 kpc, and the height
above the disk: z ~ 900 pc. These values agree with those
found by Zheng et al. (2001) for the population of M dwarfs
detected with the HST WFC2.


What was OT20050728 ?

The majority of optical transients detected to date have been
attributed to late type flare stars (e.g. Becker et al. 2004,
Christensen et al. 2004, Kulkarni & Rau 2006).
Given the red color Of OT20050728 in quiescence this may be another
example of such flaring activity.
We nevertheless consider this possibility as unlikely for two reasons:
the rise time of OT20050728 exceeds 1 hour, and the magnitude
of the flare reaches 3 magnitudes in the i' band.
We note that flares reaching 6 magnitudes in the B band
have an amplitude in the I band which is only 1 magnitude
(Rockenfeller et al. 2006).

A second possibility is a microlensing event, but the short duration
of the transient would imply a fast moving lens.

Given the uncertainty on the origin of OT20050728, we strongly
encourage spectroscopic observations of this object which is
observable now (until august).


Consequences for Gamma-Ray Burts:

The main consequence of the stellar nature of OT20050728
is that no GRB afterglow has been found in the Very Wide Survey
of the CFHTLS. This search is one of the most extensive performed
to date (Malacrino et al. 2007), with an area of ~1000 sq. deg
down to r’ = 22.5.
Consequently, we can constrain the number of afterglows brighter than 
r’=22.5,
to be smaller than 220 in the whole sky at any time.


Acknowledgements:
We thank the TERAPIX data center located at the Institut d'Astrophysique
de Paris (http://terapix.iap.fr/) for the efficient processing of
Megacam images


References :
Becker et al. 2004, ApJ,
Christensen et al. 2004, GCNC 2849
Malacrino et al. 2007, A&A, 464, L29
Kulkarni & Rau 2006, ApJ, 644, L63
Rockenfeller et al. 2006, MNRAS, 367, 407
Zheng et al. 2001, ApJ, 555, 393
Zheng et al. 2004 ApJ, 601, 500