[vsnet-grb-info 1938] GRB 050730, optical observation

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3778
SUBJECT: GRB 050730, optical observation
DATE:    05/08/06 19:15:13 GMT
FROM:    Peter Garnavich at U of Notre Dame  <pgarnavi at nd.edu>

S. Kannappan (U. Texas), P. Garnavich (Notre Dame),
    K.Z. Stanek (Ohio State), D. Christlein (Yale), and
    D. Zaritsky (U. Arizona)

We imaged the position of the GRB 050730 afterglow (Holland et al.
GCN 3704) with the Magellan Observatory Baade Telescope and IMACS
imaging spectrograph on 2005 Aug. 3 23:40 (UT) which is 99.7 hours
after the burst. Three,  300 sec exposures were obtained in the
R band and combined into a deep image. The afterglow (+possible host
galaxy) is detected and the brightness is estimated to be
R=23.4+/-0.1 mag based on the calibration by Holman et al. (GCN 3727).

The power-law decay index between the IMACS R-band observation obtained
3 hours after the burst and 100 hours is -1.5 which confirms the light
curve break suggested by Holman et al. (GCN 3727). We note that the
strong Lyman absorption (GCN 3709,3710,3716) falls at the peak of the
standard R-band transmission making comparison between magnitude
estimates made with different detector/filter combinations difficult.