GCN Circular 136

GRB 980703 Transient Optical Follow-Up

J. S. Bloom, S. J. Djorgovski, S. R. Kulkarni (CIT) and D. A. Frail (NRAO)
report on behalf of the Caltech-NRAO GRB collaboration: "In the GRB 980703
field (IAUC #6966; GCN #126
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; GCN #127
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) of the radio/optical transient
discovered by Frail et al. (GCN #128
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), R-band images (10 min) were
obtained at the Palomar 60 inch on July 4,5 by R. O. Marzke (Carnegie) and
D. R. Patton (U. Victoria) and at the Keck-II 10-m on LRIS by J. B. Oke
(DAO), K. D.  Horne (U. St Andrews), and R. Gomer.  Photometry, based on
an approximate LRIS zero-point (which is uncertain to ~0.2 mag), results
in the following derived magnitudes for the transient:
 
Date (UT)    Intr     Mag OT Rc
-----------------------------------
Jul 4.477    P60    21.3 +/- 0.2
Jul 5.482    P60    21.8 +/- 0.3   
Jul 6.607    LRIS   22.03 +/- 0.02  (+ zero point uncertainty 0.2mag
                                       for all three points)  

The zero-point may change with expected calibrations but the slope will
not. The fading a power-law decline of -0.74, consistent with the I-band
fading (-0.84) found from data in Vreeswijk et al. (GCN 132
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), would make
this OT the slowest fading counterpart to a GRB yet.  However, since the
decay appears somewhat stronger at earlier epochs, we believe that the
light may be contaminated by an underlying host.  If true, the light curve
should show even more flattening.  This hypothesis appears corroborated by
the fact that the spectral indices derived from the I/R/H-band data are
not consistent with a simple fireball model prediction from the decay
constant. A summary light curve with all reported I/R-band data may be
found at http://astro.caltech.edu/~jsb/grb980703_ltcurve.ps"

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