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astro­ph/9710346
v2
27
Apr
1998
Power Law Decays in the Optical Counterparts of
GRB 970228 and GRB 970508
M. R. Garcia 1 , P. J. Callanan 2 D. Moraru 1 , J. E. McClintock 1 , E. Tollestrup 1 , S. P. Willner 1 , C.
Hergenrother 3 , C. R. Robinson 4 , C. Kouveliotou 4 , and J. van Paradijs 5;6
ABSTRACT
We report on R c and K band observations of the optical counterpart to GRB 970508
with the Fred Lawrence Whipple Observatory (FLWO) 1.2m telescope. Eleven R c ­band
observations were obtained on 1997 May 12, and three on May 14. The counterpart
clearly faded between the two nights. On May 12 there was no evidence for variability
(! 9%) on 10­70 minute time scales based on 11 R c ­band observations. On May 19
a one hour observation set a limit on the K magnitude of K ?18.2. Comparison of
these data points with those obtained by other authors shows that the decay of the
optical counterpart can be well fitted by a power law of the form f ё t \Gammaff where ff =
1:22 \Sigma 0:03 with occasional fluctuations superposed. We note that the decay of the
optical counterpart to another burst, GRB 970228, can also be well fitted with a power
law with exponent ff = 1:0 +0:2
\Gamma0:5 with occasional fluctuations superposed. These two decay
light curves are remarkably similar in form to that predicted by cosmic­fireball models.
Subject headings: gamma rays: bursts
1 Center for Astrophysics, 60 Garden St., Cambridge, MA 02138; mgarcia, dmoraru, jmcclintock,etollestrup,swillner, @cfa.harvard.edu
2 Physics Department, University College, Cork, Ireland; paulc@ucc.ie
3 Lunar and Planetary Lab, University of Arizona, Tucson, AZ; chergen@pirl.lpl.arizona.edu
4 Universities Space Research Association and Marshall Space Flight Center, ES­62, Huntsville, AL 35812; craig.robinson@msfc.nasa.gov,
kouveliotou@eagles.msfc.nasa.gov
5 University of Amsterdam, Physics Department, 403 Kruislaan, 1098SJ Amsterdam, Netherlands; jvp@astro.uva.nl
6 Department of Physics, University of Alabama in Huntsville, AL 35899
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