TITLE: GCN CIRCULAR NUMBER: 10049 SUBJECT: GRB 090926A: Late-time Gemini South Observations and Possible Jet Break DATE: 09/10/20 22:10:04 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko, D. A. Perley, B. E. Cobb, J. S. Bloom, and N. R. Butler (UC Berkeley) report on behalf of a larger collaboration: We have imaged the field of the Fermi GRB090926A (Bissaldi et al., GCN 9933; Uehara et al., GCN 9934) with the Gemini Multi-Object Spectrograph mounted on the 8-m Gemini South telescope. Observations were taken in the Sloan g', r', and i' filters beginning at 2:28 UT on 19 October 2009 (~ 22.9 d after the GBM trigger). We detect a faint source in all filters at the location of the optical afterglow (Haislip et al., GCN 9937, Gronwall et al., GCN 9938). Using several unsaturated USNO-B objects in the field of view, along with the filter transformations of Jordi, Grebel, and Ammon (2006 A&A 460, 339), we measure a magnitude of r' ~ 23.7 for this source. Along with its relatively blue color (g' - i' ~ 0), the object appears marginally extended, suggesting it is likely dominated by emission from the host galaxy of GRB090926A. We caution, however, that the host candidate is partially blended with a nearby object (~ 1.5" in the SW direction, just outside the optical and X-ray afterglow error circle) of comparable magnitude, which likely affects the photometry. Comparing with the last reported R-band detection (R ~ 21.5 at t ~ 11.1 d; Haislip et al., GCN 10003), the optical decay must have significantly steepened from previous measurements (power-law index alpha >~ 2.5). A similar steepening is hinted at in the latest XRT observations of this source (see http://astro.berkeley.edu/~nat/swift/00020113/bat_xrt.jpg), suggesting a possible jet break. Using the measured redshift of z = 2.1 (Malesani et al., GCN 9942), the isotropic gamma-ray energy derived from the Konus-Wind instrument (E_iso ~ 2e54 erg; Golenetskii et al., GCN 9959), and assuming expansion into a constant density medium (n ~ 1 cm^-3) with a gamma-ray efficiency ~ 20%, we infer an opening angle ~ 0.1 rad. The collimation-corrected prompt energy release would therefore be large, E ~ 1e52 erg.