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GCN Circular 3531

GRB 050607: Optical decay and spectral slope
2005-06-07T18:26:06Z (18 years ago)
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 .
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