TITLE:   GCN CIRCULAR
NUMBER:  6992
SUBJECT: GRB 071025: ROTSE-III Refined Analysis
DATE:    07/10/25 06:54:28 GMT
FROM:    Eli Rykoff at U of Michigan/ROTSE  <erykoff@umich.edu>

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, Pagani, et al.) and detected
an optical counterpart as reported in GCN 6987 (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) and J=14.95 (Bloom, et al., GCN 6989).

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) and UVOT non-detection (Pangani, et al., GCN 6986).

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.