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GCN Circular 6934

GRB 071010A: Late-time Keck/LRIS photometry - possible host galaxy and jet break
2007-10-18T21:59:38Z (17 years ago)
Daniel Perley at U.C. Berkeley <>
D. A. Perley, A. V. Filippenko, J. M. Silverman, R. J. Foley, M. Modjaz, 
D. Kocevski, and J. S. Bloom report:

We acquired an additional series of imaging observations of the field of 
GRB 071010A (Moretti et al., GCN 6859) with Keck I + LRIS starting at 
5:01 UT, 2007-10-16 (6.05 days after the trigger), in g and R filters.

The optical afterglow (Klotz et al. GCN 6860) has faded substantially. 
An object is observed at the GRB position, resolvable into two regions: 
a brighter, redder source to the east and a fainter, bluer source to the 
west.  Comparison with our previous Keck imaging shows the afterglow 
position to be consistent only with the fainter, western source.  The 
two sources may be a bright elongated host galaxy, a compact host galaxy 
with the afterglow offset from the center, or a foreground star with the 
afterglow coincidentally located very nearby.  An image of the field is 
posted at:

Aperture photometry shows the combined complex of both sources to have a 
magnitude of R=22.5, using the same calibration system in previous 
circulars.  The contribution from the afterglow is limited to R>23.3, 
depending on the uncertain contribution of a possible bright host galaxy.

Refined photometry of our imaging starting at 2007-10-11 UT 04:47 (GCN 
6885) shows the afterglow magnitude at that time to be R = 19.82+/-0.02. 
  This indicates that the afterglow decay underwent a sharp break, from 
alpha < 0.5 between the first and second night to a minimum of alpha > 
1.7 over the following five days.

Comparison with the XRT light curve at shows the 
X-ray afterglow to have undergone a break at 10^5 seconds (roughly 
coincident with our measurement on 2007-10-11) from approximately flat 
evolution before this point to a rapidly decaying power law of alpha ~ 
1.8 afterward.  This suggests that this sharp break may be achromatic, 
and possibly indicative of a jet break.

[GCN OPS NOTE(18oct07):  The "and jet break" was added back on to the 
Subject-line of this circular.  It was chopped off during processing
because the mail sending/delivery system chopps wrapped Subject lines.]
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