GCN Circular 7678

D. A. Perley, J. S. Bloom (UC Berkeley), and J. X. Prochaska (UCO/Lick) 
report:

Further to Bloom et al. (GCN 7667
Not found
 
 
), we acquired additional imaging of 
the probable short-hard (Ukwatta et al., GCN 7677
Not found
 
 
) GRB 080503 (Mao et 
al., GCN 7665
Not found
 
 
) with extended emission the night after the burst.   A 
total of 10 exposures of 180 seconds each were taken in r-band between 
UT 14:08 and 14:46 (2008-05-04), 26 hours after the BAT trigger, using 
GMOS on Gemini-North.  In addition, we report analysis of additional 
imaging taken between 1.0-2.5 hours after the burst in g, r, i, and z 
filters.

The source noted in our previous circular, s1, does not appear to be 
variable between images, and no other sources are detected in the XRT 
error circle to a very deep limit of r >~ 26.5 during the first night. 
However, a new source ("s2") appears in the second night's observation 
that was not detected in our r-band imaging during the first night.  If 
this source is associated with GRB 080319B this would be extremely 
unusual behavior for a short gamma-ray burst.  It is possible that the 
rebrightening is the first example of the detection of a Li-Paczynski 
"mini"-supernova (1998ApJ...507L..59L; see also Kulkarni 
astro-ph/0510256). More tentatively, we note that s1 may the host galaxy 
(we note that all previous SHB's with extended emission have been seen 
at small offset from their host galaxies: Troja et al., astro-ph/0711.3034).

The source is located near the center of the enhanced XRT error circle 
(Goad et al, GCN 7669
Not found
 
 
) at:

RA=19:06:28.766  Dec=+68:47:35.53 (J2000)

(uncertainty is 250 mas in both coordinates relative to the ICRS)

And can be seen at the following links:
http://lyra.berkeley.edu/~jbloom/grb080503-comp.png
http://lyra.berkeley.edu/~jbloom/grb080503-rebrightening.mpeg
http://lyra.berkeley.edu/~dperley/080503/080503_s2b.png

We report the following photometry, calibrated relative to USNO-B1.0:

Acquisition r-band image (t=0.98 hr)
s1: R = 25.56 +/- 0.33
s2: R > 25.6

First night r-band stack (t=1.5 hr)
s1: R = 26.00 +/- 0.18
s2: R >~ 26.5

Second night r-band stack (t=26.0 hr)
s1: R = 25.70 +/- 0.12
s2: R = 25.28 +/- 0.09

UT_start exposure   filter magnitude
13:23    1x180      r      R = 24.8 +/- 0.3
13:38    1x30+2x300 R      R > 25.5 (2-sigma)
13:47    5x180      r      R = 25.4 +/- 0.2

Photometric uncertainties do not include calibration uncertainties 
relative to USNO-B1.0, which are at the level of 0.1 magnitude.  (This 
does not affect the relative magnitudes between the images).

In our images in other filters, s2 is not detected in i-band or z-band 
during the first night.  It may be marginally detected in g-band imaging 
near the limiting magnitude of that observation.

Further observations of this source are planned and strongly encouraged.