[vsnet-grb-info 4797] GRB 070612A: Rapid Decay

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6534
SUBJECT: GRB 070612A: Rapid Decay
DATE:    07/06/14 04:45:10 GMT
FROM:    Nestor Mirabal at Columbia U  <mirabal at astro.columbia.edu>

N. Mirabal, I. D. McGreer and J. P. Halpern (Columbia U.)
report on behalf of the MDM GRB follow-up team:

"We have re-observed the optical afterglow of Swift
GRB 070612A (Updike et al. GCN 6515, Cenko et al.
GCN 6525) in the R and i' filters using the same
instrumental setup as in Mirabal et al. (GCN  6526).
Preliminary calibration referenced to the SDSS photometry
of field stars (Cool et al. GCN 6510) shows that the
afterglow has faded by 1.88 mag since the previous night.
This corresponds to a decay rate steeper than ~ -2.5, which
might represent the end of an optical flash from reverse shock emission
(Meszaros & Rees 1997, ApJ, 306, 301; Sari & Piran 1999,
ApJ, 496, L1). Alternatively, the steepening could be the
signature of a "jet break". Further observations are encouraged
to distinguish between various possibilities."