GCN Circular 22542

Klotz A., Noysena K., Atteia J.L. (CNRS-OMP-IRAP),
Boer M., Eymar L. (CNRS-ARTEMIS),
Gendre B. (UVI - Etelman Obs.) report:

We imaged the field of GRB 180325A detected by SWIFT
(trigger 817564) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 26s after the GRB trigger
(14s after the notice). The elevation of the field decreased
from 37 degrees above horizon and weather conditions
were excellents.

We detect clearly the optical transcient (OT) reported by
Troja et al. (GCN 22532
Not found
 
 
), Guidorzi et al. (GCN 22534
Not found
 
 
),
Heintz et al. GCN 22535
Not found
 
 
), D'Avanzo et al. (GCNC 22536),
Watson  et al. (GCNC 22537), Garnavich  et al. (GCNC 22538),
Strobl et al. (GCNC 22541).

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
We detect the OT at a magnitude of:
t0+26s to t0+86s : r(AB) = 17.2 +/- 0.3

It is important to notice that a bright gamma flash in
Swift-BAT data occured between 70 and 92 seconds. We
do not detect any significant optical flux increase
during the range 70 to 86 seconds of the first TAROT
exposure.

After an overhead time of 13 seconds the second image
of 30s exposure in tracking mode started 99 seconds
after the trigger. In this image the OT has brightened
by about 1 magnitude. The third image shows a rapid
decrease of 1 magnitude compatible with a decay index
of about 3 followed by a mean decay of 0.45 until
1.7 hour after the trigger confirming the small decay
value published by Littlefield and Garnavich (GCNC 22538).

The photometric analysis of the TAROT images are given:

t1(h)  t2(h)  r(AB) 1sig
0.0277 0.0361 16.24 0.09
0.0391 0.0474 17.22 0.20
0.0504 0.0815 17.68 0.19
0.0843 0.1093 18.02 0.30
0.2062 0.2872 18.56 0.30
0.2951 0.4364 18.78 0.39
0.4392 1.1843 18.94 0.22
1.2400 1.6746 19.26 0.36

Magnitudes were estimated with the nearby star
NOMAD-1 1144-0181799 R=14.440 (V-R)=0.65, converted into
r(AB) system and are not corrected for galactic dust extinction.

Note that the TAROT light curve is compatible with an
optical flash occuring with a lag of 20 +/- 10 seconds
after the bright gamma emission peak as mentioned
by Strobl et al. (GCNC 22541).