Central Bureau for Astronomical Telegrams
IAUC 6747: GRB 970228; 103P
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Circular No. 6747
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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GRB 970228
A. Fruchter, M. Livio, D. Macchetto, L. Petro, and K. Sahu,
Space Telescope Science Institute; E. Pian and F. Frontera, ITESRE,
CNR, Bologna; S. Thorsett, Princeton University; and M. Tavani,
Columbia University and IFCTR, Milan, report: "The optical
counterpart of GRB 970228 (cf. IAUC 6584, 6588; van Paradijs et al.
1997, Nature 386, 686) was imaged via eight exposures of 575 s with
the Hubble Space Telescope (HST) STIS CCD (+ clear filter) between
Sept. 4.65 and 4.76 UT. Both the nebula and point-source optical
transient found by Sahu et al. (loc. cit. 387, 476) in earlier HST
WFC2 observations are detected in the STIS images. The nebula is
roughly circular with an apparent diameter of about 0".8, and the
optical transient lies within the nebula but near its southern
edge. We determine V = 28.0 +/- 0.25 for the point source and V =
25.7 +/- 0.15 for the nebula. Reanalysis of the earlier HST WFPC2
F606W observations of this source taken on Mar. 26 and Apr. 7,
using an aperture determined from the STIS image, gives V = 25.6
+/- 0.25 for the nebula. The diameter of the nebula is comparable
to galaxies of similar magnitude found in the Hubble Deep Field,
especially if one takes into account a possible visual extinction
in the direction of GRB 970228 of at least one magnitude (D. Lamb,
Huntsville Symposium). The position of the point source in the
STIS images shows a proper motion with respect to earlier HST
images of 25 +/- 15 mas (see also IAUC 6629); we interpret this as
a 2-sigma upper limit to any proper motion of about 100 mas per
year. The temporal decay of the optical transient appears to be
well-fitted by a power-law with index of about -1.1, with the
exception of data points taken between Mar. 6 and 13, all of which
fall below the power-law (see Galama et al., loc. cit. 387, 479).
However, the long-term agreement between the GRB lightcurve and the
power law supports the relativistic fireball model, and thus
implies that GRB 970228 is an extragalactic phenomenon."
COMET 103P/HARTLEY 2
Total visual magnitude estimates: Aug. 30.02 UT, 13.9 (J.
Carvajal, Avila, Spain, 0.32-m reflector); Sept. 5.79, 14.9 (G.
Bakos, Raktanya, Hungary, 0.30-m reflector); 7.87, 13.2 (R. J.
Bouma, Groningen, The Netherlands, 0.25-m reflector); 8.84, 13.6
(W. Hasubick, Buchloe, Germany, 0.44-m reflector); 21.82, 12.6
(Bouma).
(C) Copyright 1997 CBAT
1997 September 22 (6747) Daniel W. E. Green
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