TITLE: GCN CIRCULAR NUMBER: 7678 SUBJECT: GRB 080503: Late-time Optical rebrightening (possible mini-SN) DATE: 08/05/05 04:49:49 GMT FROM: Daniel Perley at U.C. Berkeley D. A. Perley, J. S. Bloom (UC Berkeley), and J. X. Prochaska (UCO/Lick) report: Further to Bloom et al. (GCN 7667), we acquired additional imaging of the probable short-hard (Ukwatta et al., GCN 7677) GRB 080503 (Mao et al., GCN 7665) 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) 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.