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GCN Circular 6829

Subject
GRB 070911, SMARTS optical/IR afterglow observations
Date
2007-09-27T20:39:59Z (17 years ago)
From
Bethany Cobb at Yale U <cobb@astro.yale.edu>
B. E. Cobb, part of the larger SMARTS consortium, reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 070911
(Krimm et al. GCN 6777Not found
 
 
) with the following mid-exposure times:
         2007-09-13 07:10:30 UT -  2.05 days post-burst
         2007-09-15 06:42:00 UT -  4.03 days post-burst
         2007-09-22 05:22:21 UT - 10.98 days post-burst
For each observation, total summed exposure times amounted to
36 minutes in I and 30 minutes in J.

A fading afterglow is detected in both I and J within the X-ray afterglow
error region (Stratta et al. 6779). This afterglow has the following 
coordinates and magnitudes:

RA/DEC: 01:43:14.4 -33:29:02.5

days post
burst           I-magnitude (or limit)  J-magnitude (or limit)
----------------------------------------------------------------
  2.05           20.9 +/- 0.1            19.8 +/- 0.2
  4.03           21.9 +/- 0.2           >19.5 +/- 0.1
10.98          >22.3 +/- 0.1           >19.7 +/- 0.1

The I-band photometry is calibrated to a number of secondary stars in the
field of GRB 070911.  The magnitude of these secondary stars was
derived using Landolt standard star observations obtained
during the first two epochs, which were photometric night at CTIO.
The J-band images are calibrated based on several 2MASS stars in the 
field.

The afterglow's behavior between the first two epochs is consistent with a
decay rate of alpha = -1.4+/-0.3.  Interestingly, no J-band afterglow was
detected by Butler et al. (GCN 6780Not found
 
 
) to J > 16.5 only 3 hours after
the burst. This indicates that a period of rebrightening might have 
occurred between 3 hours and 2 days post-burst, though steady decay
is only ruled out at the ~1 sigma level.