[vsnet-grb-info 8517] GRB 091010: optical afterglow candidate rejection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10027
SUBJECT: GRB 091010: optical afterglow candidate rejection
DATE:    09/10/16 15:16:32 GMT
FROM:    Cristiano Guidorzi at Ferrara U,Italy  <guidorzi at fe.infn.it>

C. Guidorzi (U. Ferrara), A. Melandri, C.G. Mundell, D. Bersier,
Z. Cano, N.R. Clay, S. Kobayashi, C.J. Mottram, R.J. Smith,
I.A. Steele (Liverpool JMU), A. Gomboc (U. Ljubljana),
N.R. Tanvir (U. Leicester) on behalf of a large collaboration
report:

We re-observed the optical afterglow candidate (Rossi et al.
GCN Circ. 10008) of the SuperAGILE GRB 091010 (Donnarumma
et al. GCN Circ. 10004) with the 2-m Faulkes Telescope South
(FTS) on October 16, 09:27:33 UT (6.3 days post burst)
with the Ri' filters.

The source is still clearly detected in both filters.
Comparison with our previous values obtained at 1 day
post burst (Guidorzi et al. GCN Circ. 10017) shows that
the source did not fade appreciably.
We derived an upper limit to the possible power-law
decay index of alpha<0.5 (90% confidence) from 1 to
6 days post burst, flux~t^(-alpha).
Given that a comparably shallow and long-lasting decay
has never been observed, we can confidently rule out
this source as the optical afterglow of GRB 091010.

Telescope  Mid Time   Exposure  Filter   Mag
           (days)     (s)
-----------------------------------------------------
FTS         6.29       6x300       i'     22.2 +- 0.3
FTS         6.31       6x300       R      22.3 +- 0.3
-----------------------------------------------------

Calibration was the same as that used in our previous
report (GCN Circ. 10017).