[vsnet-grb-info 10732] GRB 110709B: Additional Gemini observations and deep limits on an optical afterglow

[GCN Circulars]

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  <wfong at cfa.harvard.edu>

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."