TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3531 SUBJECT: GRB 050607: Optical decay and spectral slope DATE: 05/06/07 18:26:06 GMT FROM: James Rhoads at STScI James Rhoads reports on behalf of a larger collaboration: We have measured the flux of the GRB 050607 optical afterglow (Rhoads 2005, GCNC 3527) in our five I band images from t=640 to t=6330 seconds post-burst. A power law decay with slope -0.5 gives a marginally acceptable fit to the I band light curve. A better fit can be obtained by a two-part power law, with an initial steep decay (exponent near -1) up to t ~ 1700 seconds, followed by a flatter decay (exponent near -0.3) from 1700 to 6300 seconds. The overall optical decay slope is in good agreement with the X-ray slope reported by Pagani et al (GCNC 3528). We also obtained images of Landolt standard field SA 113. We found a zero point error in our earlier photometry (GCNC 3527). Our new zero point based on our standard star observations gives a brighter afterglow magnitude at t=640 seconds: I = 21.46 +- 0.1 (statistical) +- 0.3 (systematic) The remaining systematic error will go away with a more careful reduction. As a consistency check we find magnitude I=16.2 for the star USNO 0975-17511046 (our astrometric reference at 20:00:42.703 +9:08:34.98), which has USNO tabulated magnitude Rmag=16.6 and B-R=0.7. In addition to our I band data, we also monitored the burst in B band and took smaller amounts of data in R, z, and g. All data discussed here were obtained with the Mosaic 1 camera on the 4m Mayall Telescope of the Kitt Peak National Observatory. Observations started at 2005 June 7 UT 09:20:46, and continued until 11:18 UT. The transient is faint in the B filter. Our first B band epoch, of 150 seconds centered at t=1384 seconds, does not show the afterglow clearly. A formal aperture flux measurement at the transient location yields flux 0.7 +- 0.25 microJansky, or B ~ 24.4 +- 0.4. This gives an estimated spectral power law slope of f_nu ~ nu^-1.5 or steeper, where we have accounted for the Galactic reddening based on the Schlegel et al prediction (A_V = 0.6 mag, E(B-I) = 0.44 mag). The faint B image is thus suggestive of a Lyman break in the spectrum, though we cannot rule out a red power law with our present analysis. We detect the transient at R band, placing a rough redshift bound z<5, We do not detect it in our less sensitive z band images (suggesting that the power law slope is not extremely red). An I band finding chart and light curve are now available at http://www.stsci.edu/~rhoads/GRB050607/ .