[vsnet-grb-info 4134] GRB 061218: TORTOREM optical upper limits

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  5941
SUBJECT: GRB 061218: TORTOREM optical upper limits
DATE:    06/12/19 16:05:22 GMT
FROM:    Giuseppe Greco at U Bologna  <giuseppe.greco2 at studio.unibo.it>

S. Karpov, G. Beskin, S. Bondar, C. Bartolini, G. Greco, A. Guarnieri,
D. Nanni, A. Piccioni, F. Terra, E. Molinari, G. Chincarini, F.M. Zerbi,
S. Covino, V. Testa, G. Tosti, F. Vitali, L.A. Antonelli, P. Conconi, G.
Cutispoto, G. Malaspina, L. Nicastro, E. Palazzi, E. Meurs, P.  Goldoni

report on behalf of the TORTOREM team:

The field of GRB 061218 (Zane et al. GCN 5936) has been observed by the
TORTORA (Telescopio Ottimizzato per la Ricerca di Transienti Ottici
RApidi) wide-field fast camera (12 cm diameter, 20x25 deg FOV) mounted
on REM robotic 60-cm telescope located at La Silla (Chile). The burst
was initially outside the camera field of view. The system was repointed
and the TORTORA began to acquire frames at 04:07:03 UT (118 sec after
trigger) with 7.5 Hz frame frequency (0.128 s exposure).

The summation of 100 frames with 12.8 s. effective exposure (middle time
04:07:16 UT) did not reveal any source down to the B = 11.3 mag (3-sigma).

We performed the Fourier analysis of 23 min data set (10000 successive
frames) to search for the periodic signal at the GRB position. The upper
limit for the amplitude of sinusoidal variability is B=16.4 (3-sigma)
over the 0.01 - 3.5 Hz range.

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