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IAUC 6577: GRB 970228

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                                                 Circular No. 6577
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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GRB 970228
     D. Palmer, T. Cline, N. Gehrels, P. Kurczynski, R. Ramaty, H.
Seifert, and B. Teegarden, Goddard Space Flight Center, NASA; and
K. Hurley, University of California at Berkeley, report: "TGRS data
have been analyzed for the most intense 3.6 s of this gamma-ray
burst, corresponding to the initial peak mentioned on IAUC 6572.
The spectrum is consistent with that of a classical burst.  The
peak countrate in the band 40-200 keV is 3.3 counts sE-1 cmE-2
(1.2-s average).  This low countrate precludes tight constraints on
spectral form and parameters, although a lack of detectable flux
above 300 keV indicates a high-energy cutoff steeper than 10E-3.
An optically-thin thermal bremsstrahlung spectral form with
temperature 100 keV < T < 150 keV fits the data adequately over the
interval 35-1000 keV.  This spectral fit gives a fluence of 2 x
10E-6 erg/cm**2 over this time interval (although a comparable
fluence may exist in the remainder of the burst).  Assuming that
the x-ray emission seen by BeppoSAX (IAUC 6576) has a 1-day
exponential decay time, the persistent 0.5-10-keV emission has a
total energy fluence that is about 10-20 percent of the prompt
gamma-ray fluence."
     I. A. Smith and E. P. Liang, Rice University; and R. A.
Gruendl and K. Y. Lo, University of Illinois at Urbana/Champaign,
report:  "Observations at 86.4 GHz (3.5 mm) using the Berkeley-
Illinois-Maryland Association array were made on Mar. 7 UT,
centered on the VLA source reported by Frail et al. (IAUC 6576).
No significant source was detected there or anywhere else inside
the x-ray error box of SAX J0501.7+1146 (Costa et al., IAUC 6576)
with a 1-sigma rms of 1.2 mJy."
     B. Margon and E. W. Deutsch, University of Washington; and J.
Secker, Washington State University, write:  "Observations with the
ARC 3.5-m telescope on Mar. 7 and 8 UT reveal a faint (R = 21)
image coincident (+/- 1") with the VLA position of the radio source
(Frail et al., IAUC 6576) that lies in the BeppoSAX Narrow Field
position (Costa et al., IAUC 6576) for the x-ray source possibly
associated with GRB 970228 (IAUC 6572).  If the x-ray and optical
sources are identical, which of course remains to be proven, then
the x-ray flux observed on Mar. 4 (IAUC 6576) implies an x-ray-to-
optical flux ratio consistent with that of an active galactic
nucleus; however, that ratio in the brighter x-ray state of Feb.
28, closer in time to the gamma-ray burst, would be extraordinary."

                      (C) Copyright 1997 CBAT
1997 March 8                   (6577)            Daniel W. E. Green

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