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IAUC 6658: GRB 970508

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                                                 Circular No. 6658
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
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GRB 970508
     B. Schaefer and M. Schaefer, Yale University; P. Smith and C.
Mackey, Kitt Peak National Observatory; E. Wilcots, D. J. Pisano,
and C. Howk, University of Wisconsin; E. Costa, M. Feroci, and L.
Piro, Istituto di Astrofisica Spaziale, Frascati; M. Tavani,
Columbia University and Istituto di Fisica Cosmica e Tecnologie
Relative, Milan; F. Frontera, Universita di Ferrara; and E. Palazzi
and L. Nicastro, Istituto Tecnologie e Studio Radiazioni
Extraterrestri, Bologna, report on observations with the 3.5-m WIYN
telescope on May 9.203, 10.142, and 12.135 UT:  "There are seven
sources inside the 50" BeppoSAX/NFI error box (IAUC 6656).  The R
magnitudes of these sources were obtained with respect to a star at
R.A. = 6h53m48s.5, Decl. = +79o16'32".7 (equinox 2000.0), which has
R = 19.70 (this magnitude is based on calibration of the filter and
detector on previous nights).  Following are given the position end
figures (accuracy roughly 0".5) for the other six sources, the
respective R magnitudes (accurate to typically 0.02 mag for the
brighter sources and 0.15 mag for the galaxy), and apparent object
types:  38s.9, 15'22".0, 17.25, stellar; 36s.1, 15'31".1, 18.91,
stellar; 49s.4, 16'20".0, 21.40-20.30, stellar variable; 54s.8,
16'03".7, 21.34, stellar; 41s.3, 15'43".5, 20.55, unusual; 45s.2,
15'57".0, about 22.7, galaxy.  Bond's variable (see IAUC 6654) is a
point source that has brightened from R = 21.40 +/- 0.05 on May 9
to 20.30 +/- 0.02 on May 10, fading to 20.47 +/- 0.03 on May 11.
The existence of brightening and fading creates a coincidence in
time that can causally connect the variable with the burst, and
matches various model predictions.  The 'unusual' object is an
extended source shaped like a 'Z', extending 7" from tip-to-tip;
the morphology of this source is highly unusual, although it could
be a superposition of three or more sources."
     S. G. Djorgovski, S. C. Odewahn, R. R. Gal, M. R. Metzger, S.
R. Kulkarni, and M. A. Pahre, California Institute of Technology; D.
A. Frail, National Radio Astronomy Observatory; and E. Costa, M.
Feroci, and the rest of the BeppoSAX team write:  "Observations at
Palomar indicate that the optical counterpart of GRB 970508 (IAUC
6654, 6655) has began to decline in brightness, as shown by the
following Gunn r-band magnitudes:  May 9.195 UT, 21.2 +/- 0.15;
10.178, 20.18 +/- 0.07; 11.198, 20.16 +/- 0.06; 12.195, 20.53 +/-
0.06.  It had apparently gone through a peak brightness some time
on May 10, and is now fading.  Such lightcurve behavior is expected
in some classes of theoretical models."

                      (C) Copyright 1997 CBAT
1997 May 12                    (6658)            Daniel W. E. Green

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