GCN Circular 207

GRB 990123 r-band Photometry

R. R. Gal, S. C. Odewahn, J. S. Bloom, S. R. Kulkarni (CIT), D. A. Frail
(NRAO) report on behalf of the Caltech-NRAO-CARA GRB Collaboration:

"We report on a detailed analysis of images obtained at the
Palomar 60-inch telescope, the initial results of which were reported
in GCN 201
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. Three images of 400-s duration each were obtained of the
field of GRB 990123 localized by BeppoSAX (GCN #200
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). These images were
obtained  in the Gunn r-band and not R band as reported in GCN #201
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.
The seeing was 1.7 arcsec (FWHM) in each image.  Using images obtained
of two Gunn standards (Feige 34 and Feige 67), we obtained a
photometric zero point for the data.  The observation times and
magnitudes for the optical transient reported in Odewahn, Bloom,
Kulkarni (GCN #202
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) and the reference star from Bloom et al. (GCN #206
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)
are:

				 Gunn-r Mag
UT (23 Jan 1999)            OT             Ref. Star
---------------------------------------------------------
13:37:20.3           18.70 +/- 0.04      19.10 +/- 0.04
13:51:03.6           18.78 +/- 0.04      19.11 +/- 0.04
14:02:56.5           18.75 +/- 0.06      19.07 +/- 0.06
--------------------------------------------------------

The uncertainty in the magnitudes include the systematic and
statistical uncertainties, although no color term was applied.  We note
that the second r-band image was taken nearly simultaneous with the
B-band image discussed in Bloom et al. (GCN #206
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).

The Galactic extinction in the direction of the optical transient (l,b
= 73.12, 54.64) is E(B-V) = 0.01597 (Schlegel, Finkbeiner, and Davis;
1998 ApJ, 500, 525).  Thus (assuming Rv = 3.1), A_B = 0.069 and A_gunnr
= 0.041. The extinction corrected magnitudes of the transient at epoch
Jan 23.578 1999 UT are thus B = 18.86 and Gunn-r = 18.74.  Assuming the
bandpass zeropoints and effective central wavelength of the Johnson B
and Gunn-r bandpasses from Fukugita, Shimasaku, and Ichikawa (1995
PASP, 107, 945) we get F(B)= 115 microJy F(r) =  94 microJy. Thus the
power law slope beta = 0.5 where beta = log F(nu/log(nu).  the
transient is blue with a spectral index of beta = 0.5 (with beta =dlog
F(nu)/dlog nu).  This spectral behavior has not been seen in previous
GRB afterglows.  In the framework of the afterglow models, this result
can be interpreted to mean that the flux from the afterglow did not
peak in the optical bands by the epoch of our second r-band image.


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