On the Afterglow of the X-ray Flash of 2003 July 23: Photometric evdence for an Off-Axis Gamma-Ray burst with an Associated Supernova

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HST/STIS Observations of the Optical Counterpart to GRB 970228

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made on 4~September~1997 using the STIS CCD on the Hubble Space Telescope. The unresolved counterpart is detected at V=28 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. No proper motion is detected, in contradiction of some earlier claims. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude 25.7 +/- 0.25. Comparison with WFPC2 data taken one month after the initial burst and NTT data taken on March 13 shows no evidence for fading of the extended emission. After adjusting for the probable Galactic extinction in the direction of GRB 970228 of A_v=0.7, we find that the observed nebula is consistent with the sizes of galaxies of comparable magnitude found in the Hubble Deep Field and other deep HST images, and that only 2% of the sky is covered by galaxies of similar or greater surface brightness. Therefore, the extended source observed about GRB 970228 is most likely a galaxy at moderate redshift, and is almost certainly the host of the gamma-ray burst

Optical and near-infrared observations of the afterglow of GRB 980329 from 15 hours to 10 days

We report I-band observations of the GRB 980329 field made on 1998 March 29 with the 1.34 m Tautenberg Schmidt telescope, R-, J- and K-band observations made on 1998 April 1 with the APO 3.5 m telescope, R- and I-band observations made on 1998 April 3 with the Mayall 4 m telescope at KPNO, and J- and K-band observations made 1998 April 6-8 with the Keck-I 10 m telescope. We show that these and other reported measurements are consistent with a power-law fading of the optical/near-infrared source that is coincident with the variable radio source VLA J0702+3850. This firmly establishes that this source is the afterglow of GRB 980329

Gamma-Ray Bursts: The Underlying Model

A pedagogical derivation is presented of the ``fireball'' model of gamma-ray bursts, according to which the observable effects are due to the dissipation of the kinetic energy of a relativistically expanding wind, a ``fireball.'' The main open questions are emphasized, and key afterglow observations, that provide support for this model, are briefly discussed. The relativistic outflow is, most likely, driven by the accretion of a fraction of a solar mass onto a newly born (few) solar mass black hole. The observed radiation is produced once the plasma has expanded to a scale much larger than that of the underlying ``engine,'' and is therefore largely independent of the details of the progenitor, whose gravitational collapse leads to fireball formation. Several progenitor scenarios, and the prospects for discrimination among them using future observations, are discussed. The production in gamma- ray burst fireballs of high energy protons and neutrinos, and the implications of burst neutrino detection by kilometer-scale telescopes under construction, are briefly discussed.Comment: In "Supernovae and Gamma Ray Bursters", ed. K. W. Weiler, Lecture Notes in Physics, Springer-Verlag (in press); 26 pages, 2 figure