Columbia University Astronomy and 
Dartmouth College
Astronomy and Physics

MDM Observatory GRB Team

GRB 070125: Optical Monitoring

(J2000) 07h 51m 17.77s, +31° 09′ 04.1″

MDM 2.4m MDM 2.4m
MDM 2.4m - I - Jan. 27.30 UT MDM 2.4m - I - Jan. 28.28 UT

MDM 2.4m MDM 2.4m
MDM 2.4m - R - Jan. 27.27 UT MDM 2.4m - R - Jan. 28.30 UT

MDM 2.4m MDM 2.4m
MDM 2.4m - R - Jan. 29.35 UT MDM 2.4m - V - Jan. 28.31 UT

MDM 1.3m
MDM 1.3m - R - Feb 06.31 UT

Decay Curve

GCN Circular #6096
GRB 070125: Break in Optical Decay
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:

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."

John Thorstensen
Jules Halpern
Nestor Mirabal
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Last Modified: February 10, 2007