TITLE: GCN CIRCULAR NUMBER: 12155 SUBJECT: GRB 110709B: Additional Gemini observations and deep limits on an optical afterglow DATE: 11/07/14 01:20:27 GMT FROM: Wen-fai Fong at CFA W. Fong and E. Berger (Harvard) report: "We re-observed the location of GRB 110709B (GCNs 12122, 12124) with GMOS on the Gemini-South 8-m telescope starting on 2011 July 13.958 UT (4.1 days after the initial trigger). Images were obtained in mediocre seeing (1.35"). In a stack of 5x180 sec r-band exposures we still detect sources S1 and S2 (GCNs 12126, 12128, 12129, 12132). Neither source exhibits evidence for fading in photometry relative to several nearby stars, and in digital image subtraction using the ISIS package. We further note that both S1 and S2 are located outside of the refined XRT error circle (GCN 12136). We do not detect any other sources within the refined XRT circle in either of our Gemini epochs to a limit of >23.8 mag at delta-t=3.2 hours and >25.1 mag at 4.1 days. A comparison to the X-ray flux density at 3.2 hours (F_nu,X ~ 12 microJy**) requires a spectral index of beta_OX > 0.4 for F_nu ~ nu^beta. This is a highly unusual spectral index since typical values are beta_OX ~ -0.6 to -1, and events with beta_OX > -0.5 are considered "dark bursts". Such an unusual spectral index may be indicative of a high redshift origin (z>4.5 based on r-band non-detection) or a non-GRB origin similar to the recent GRB 110328A." ** http://www.swift.ac.uk/xrt_curves/00456967/ [GCN OPS NOTE(14jul11): Per author's request, the missing footnote was added.]