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IAUC 6581: GRB 970228

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                                                 Circular No. 6581
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 970228
     E. P. Liang, A. Crider, D. Lin, and I. A. Smith, Rice
University; and M. Kusunose, Kwansei-Gakuin University, report:
"The x-ray photon fluxes of GRB 970228 and the fading counterpart
SAX J0501.7+1146 together follow a tE-1 decay law (where t
represents time) that is a robust prediction of the saturated
Comptonization model of gamma-ray bursters (Liang et al. 1997,
Ap.J. Lett., in press).  This model also predicts that the fluxes
in other bands from optical to radio, if not absorbed or
reprocessed by intervening matter, should follow a similar decay
profile.  For example, scaling from the x-ray flux, V-band searches
of the revised error box (IAUC 6578) deeper than mag 24 + 2.5 log
(t/8 hr) may still catch the afterglow."
     R. M. Wagner, Ohio State University, writes:  "Examination of
R-band CCD images obtained by M. W. Buie with the Lowell
Observatory's 1.1-m and 1.8-m telescopes on Mar. 7.23 ad 8.13 UT
confirms the presence of a faint object at R about 21 (Margon et
al., IAUC 6577), consistent with the VLA position of a radio source
(Frail et al., IAUC 6576) that lies in the BeppoSax Narrow Field x-
ray error box (Costa et al., IAUC 6576), and possibly associated
with GRB 970228 (IAUC 6572; Palmer et al., IAUC 6577).  A PPM
solution for the mean position of the optical object yields R.A. =
5h01m41s.70, Decl. = +11o47'04".1 (equinox J2000.0, +/- 0".2 in
both coordinates).  We note that the object appears nonstellar on
these images and either consists of a condensation with structure
encompassing the radio position or it is the close superposition of
two distinct sources, one of which is optically fainter and
coincident with the radio source.  Deep CCD imaging in excellent
seeing is required to discriminate between the two cases."
     Wagner and C. B. Foltz, Multiple Mirror Telescope (MMT)
Observatory; and P. Hewett, Institute of Astronomy, University of
Cambridge, report:  "We obtained a noisy spectrum (range 400--970
nm, resolution 2.2 nm) of the object at the position described
above with the MMT on Mar. 10.11 UT.  We tentatively identify the
Ca II H and K break at 592.5 nm, the G-band at 644.7 nm, and the
MgH band at 779.8 nm in the reduced spectrum as being consistent
with a galaxy of redshift 0.50 +/- 0.01.  No strong emission lines
characteristic of an AGN or QSO were seen.  We estimate that V-R =
+1.7 from the spectrum.  If the rapidly decaying x-ray source
observed by the BeppoSax LECS with the flux given on IAUC 6576 is
associated with this galaxy, then its luminosity was about 4 x
10E45 erg/s (assuming H0 = 50 km sE-1 MpcE-1), comparable to the
persistent x-ray luminosities of some AGNs and QSOs.  Additional
spectroscopic observations are required to confirm the redshift of
the galaxy."

                      (C) Copyright 1997 CBAT
1997 March 10                  (6581)            Daniel W. E. Green

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