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Session 33 - Gamma-ray Burst Counterparts and Afterglows.
Topical, Oral session, Tuesday, June 09
Friars,
GRB 971214 is the third of the BeppoSAX \gamma-ray bursts with a confirmed optical counterpart. We acquired the first optical images of the field 12 hours after the event on the 2.4m Hiltner telescope of the MDM Observatory. The rapid fading of the detected optical counterpart with power-law index \delta = -1.4 dispels any lingering doubt that it and the two previous optical transients, which declined in a similar fashion, are the afterglows of their respective \gamma-ray events. The Dec. 14 optical transient was the faintest of the three, with a peak I magnitude of 21.30 \pm 0.04. It is also much redder than the others: V-I = 1.35 \pm 0.12, equivalent to \alpha = -2.5 \pm 0.2, in comparison with V-I \approx 0.7-0.9 for GRB 970228 and GRB 970508. The BeppoSAX detection of a simultaneous X-ray afterglow suggests that the steep optical slope is probably due to reddening, as opposed to synchrotron losses by the emitting electrons, because the slope between the I band and the X-ray is -0.7 \pm 0.1, a value which is consistent with the typical X-ray spectral indices of afterglows. Afterglow theory also indicates that an intrinsic spectrum as steep as \alpha = -2.5 is inconsistent with the simplest form of the external relativistic blast wave model, which predicts that the flux density at a fixed observed frequency evolves as F_\nu(t) \propto \nu^\alpha\,t^\delta where \delta = 3\alpha/2. The reddening is probably due to interstellar dust along the line of sight; we estimate E(B-V) \approx 0.25-0.4 for a reasonable range of redshifts. It is more likely that this extinction arises in a host galaxy of the GRB rather than in an intervening galaxy or in the Milky Way (\ell = 132^\circ,b = 51^\circ). Although there are two galaxies within 5^\prime\prime of the optical transient, with I = 22.0 and 22.8 respectively, it is unlikely that either contains the needed column density of material \geq 20 kpc from its center (assuming z \geq 0.2, H_0 = 50 km s^-1 Mpc^-1), which likewise disfavors either as the birthplace of the GRB progenitor. We thereby infer that a fainter host galaxy (I > 24.7) is present at the position of the transient, and that the burst's progenitor did not stray far from the dense ISM in which it was born.
