GCN Circular 6992

E.S. Rykoff (UCSB), F. Yuan (U Mich), B. E. Schaefer (Louisiana
State), H. Swan (U Mich), R. Quimby (Caltech), report on behalf of the
ROTSE collaboration:

ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB
071025 (Swift trigger 295301, GCN 6986
Not found
 
 
, Pagani, et al.) and detected
an optical counterpart as reported in GCN 6987
Not found
 
 
 (Rykoff, et al.).  We
have improved the photometry and recalibrated the images relative to
USNO-B1.0 R-band, which is significantly offset from the magnitudes in
USNO-A2.0 R-band.

 From our initial observations at T+80.3s we see the counterpart rise
to a peak of 17.3+/-0.2 at T+332s (midtime for a 40s coadd), and then
fade slowly over the next 1500 s.  At T+595s (midtime for a 280s
coadd) the magnitude is 17.6+/-0.1.  This integration is nearly
simultaneous with the NIR detections of H~13.8 (Covino, et al., GCN
6988
Not found
 
 
) and J=14.95 (Bloom, et al., GCN 6989
Not found
 
 
).

Although the ROTSE-III images are calibrated relative to R-band, the
unfiltered CCD has significant sensitivity redward of R.  Our
observations are thus consistent with a possible R-band dropout (Im,
et al., GCN 6991
Not found
 
 
) and UVOT non-detection (Pangani, et al., GCN 6986
Not found
 
 
).

We can assume that the intrinsic GRB spectrum as an R-J color of ~1,
and the reddening is entirely caused by the intergalactic medium
(IGM).  We then use the ROTSE-III magnitude and the integrated
ROTSE-III response curve convolved with the IGM absorption model of
Meiksin (2005) to estimate an upper limit on the burst redshift of
z~5.3 (for more details see Ruiz-Velasco, et al, 2007).  We note that
at this redshift the R-band offset would be ~1.2 mag, which may be
consistent with the limits of Im, et al.  However, if the reddening is
not caused by absorption in the IGM then the burst redshift may be
lower.