[vsnet-grb-info 4305] GRB 070125: Break in Optical Decay

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6096
SUBJECT: GRB 070125: Break in Optical Decay
DATE:    07/02/11 00:07:21 GMT
FROM:    Jules Halpern at Columbia U.  <jules at astro.columbia.edu>

N. Mirabal, J. Halpern (Columbia U.), & J. R. Thorstensen (Dartmouth)
report on behalf of the MDM Observatory GRB follow-up team:

"We obtained several observations of the afterglow of GRB 070125
(Cenko & Fox, GCN 6028) between 2 and 12 days after the burst using the
MDM 2.4m and 1.3m telescopes.  Results in the R-band are summarized as
follows:

             ---------------------------------------
               Date(UT)   t-t0(days)  R(mag)   +/-
             ---------------------------------------
              Jan 27.270     1.964     19.71   0.02
              Jan 28.299     2.993     20.44   0.03
              Jan 29.346     4.040     21.07   0.07
              Feb  6.307    12.001    >23.8
             ---------------------------------------

Calibration was performed with Landolt standard stars.  In addition, we
compiled
data from the GCN circulars, and placed them on a common scale using their
stated
calibrators.  MDM images and the compiled R-band light curve are shown at:

http://www.astro.columbia.edu/~jules/grb/070125/

The light curve shows an apparent plateau or rise at about 1 day.  Between 1
and 4 days, it is well described by a power law of slope -1.6.  Our upper
limit
at 12 days implies that a break occurred after 4 days, and that the slope
became steeper than -2.2.   At redshift z=1.55 (Fox et al., GCN 6071) and
fluence 1.5x10^-4 erg cm^-2 (Bellm et al., GCN 6025), the assumed isotropic
energy is 9x10^53 erg.  If we treat 4 days as a lower limit on the time
of any "jet break", then the jet opening angle is of order 6 degrees or
greater, and the beamed energy is 6x10^51 erg or greater.

This message may be cited."