Continued photometric monitoring of the gravitational lens system 0957+561A,B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g band event in the trailing (B) image light curve at the precise time predicted in an earlier paper. The prediction was using gravitational lenses and some other possible implications and uses of the 0957+561A,B light curves.

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Knowledge of the properties of ␥-ray bursts has increased substantially following recent detections of counterparts at X-ray, optical and radio wavelengths. But the nature of the underlying physical mechanism that powers these sources remains unclear. In this context, an important question is the total energy in the burst, for which an accurate estimate of the distance is required. Possible host galaxies have been identified for the first two optical counterparts discovered, and a lower limit obtained for the redshift of one of them, indicating that the bursts lie at cosmological distances. A host galaxy of the third optically detected burst has now been identified and its redshift determined to be z ¼ 3:42. When combined with the measured flux of ␥-rays from the burst, this large redshift implies an energy of 3 ؋ 10 53 erg in the ␥-rays alone, if the emission is isotropic. This is much larger than the energies hitherto considered, and it poses a challenge for theoretical models of the bursts.Ever since their discovery nearly three decades ago 1 , it was understood that progress in solving the puzzle of ␥-ray bursts (GRBs) depends on their identification at other-preferably opticalwavelengths, so that the distances could be measured using standard spectroscopic techniques. From distances and flux measurements one can then infer luminosities and other physical parameters, which can then be used to test theoretical models of the bursts and their origins.A recent breakthrough in this field was the precise localization of bursts by the BeppoSAX satellite 2 , which has led to the first identifications of GRBs at other wavelengths: X-rays 3 , optical 4 and radio 5 . This has further led to the determination of the distance scale of GRBs, with the detection of intergalactic absorption lines 6 in the optical transient 7,8 (OT) of GRB970508 (refs 9, 10). Apparent host galaxies have been detected for the first two optical afterglows found 11-14 .Here we report follow-up studies of the OT 15 of a relatively bright burst, GRB971214 (refs 16-18). As the OT faded away, we found an extended object with a red-band magnitude R ¼ 25:6 Ϯ 0:15 at the position of the OT. Based on the excellent positional coincidence, 0:06 Ϯ 0:06 arcsec, we argue here that this is the host galaxy of GRB971214. Spectroscopic observations show that the host is a typical star-forming galaxy 19-21 at a redshift z ¼ 3:418.Given this high redshift, the ␥-ray energy release of this burst is unexpectedly large, about 3 ϫ 10 53 erg, assuming isotropic emission, corresponding to about 16% of the rest-mass energy of our Sun. Energy released in other forms of radiation, for example, neutrinos or gravity waves, is not included in this energy budget. Nonetheless, the inferred energy release in ␥-rays alone is so substantial that it may present difficulties for some of the currently popular theoretical models for the origin of the bursts (coalescence of neutron stars). We may be forced to consider even more energetic possibilities 22,23 or to find ways of extracting...

In a number of multiply imaged quasar systems, a significant contribution to the lensing potential is provided by groups and clusters of galaxies associated with the primary lens. As part of an ongoing effort to gather observational data on these systems, we present spectroscopy and near-infrared and optical photometry of galaxies in the field of the quadruple lens system B 1422+231. The spectra show that the primary lens and five nearby galaxies belong to a compact group at z = 0.338. The median projected radius of this group is 35 h −1 kpc and its velocity dispersion is ∼ 550 km s −1 . A straightforward application of the virial theorem yields a group mass of 1.4 × 10 13 h −1 M ⊙ , which provides sufficient external shear to produce the observed image configuration. This data rules out a class of models and improves the system's prospects for a measurement of the Hubble constant.

The phenomenon of \tidal{shock relaxation" is de ned and quantitatively estimated. We show that the second order term h(E) 2 i ts , which has usually been neglected in the treatment of tidal shocks is typically far more important than the rst order term h(E)i ts , which has been found by Aguilar, Ostriker & Hut (1988) to be the dominant physical process driving the evolution of the Galactic system of globular clusters. The reason is simply that jv vj, which contributes to the second order term, is usually much larger than j vj 2 , the basis of the rst order term. Near the tidal radius the tidal{shock relaxation term h(E) 2 i ts will accelerate mass loss, and near the half mass radius it competes with the two-body relaxation h(E) 2 i rel in driving the evolution of the internal structure in the cluster. Formulae for the evaluation of the second order term are computed for the idealized case treated by Spitzer (1987) of stars in an harmonic potential. For typical parameters of globular clusters we nd that even at the half mass point, tidal{shock relaxation may be competitive with two-body relaxation.

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The quadruple system PG 1115+080 is the second gravitational lens with a reported measurement of the Hubble constant. In addition to the primary lens, three nearby galaxies are believed to contribute significantly to the lensing potential. In this paper we report accurate redshifts for all four galaxies and show that they belong to a single group at z d = 0.311. This group has very similar properties to Hickson's compact groups of galaxies found at lower redshifts. We briefly discuss implications for the existing lens models and derive H 0 = 52 ± 14 km s −1 Mpc −1 .

We present new results from a continuing Keck program to study gravitational lens systems. We have obtained redshifts for three lens systems, SBS 0909+532, HST 1411+5211, and CLASS B2319+051. For all of these systems, either the source or lens redshift (or both) has been previously unidentified. Our observations provide some of these missing redshifts. We find (z ℓ , z s ) = (0.830, 1.377) for SBS 0909+532; (z ℓ , z s ) = (0.465, 2.811) for HST 1411+5211, although the source redshift is still tentative; and (z ℓ 1 , z ℓ 2 ) = (0.624, 0.588) for the two lensing galaxies in CLASS B2319+051. The background radio source in B2319+051 has not been detected optically; its redshift is, therefore, still unknown. We find that the spectral features of the central lensing galaxy in all three systems are typical of an early-type galaxy. The observed image splittings in SBS 0909+532 and HST 1411+5211 imply that the masses within the Einstein ring radii of the lensing galaxies are 1.4 × 10 11 and 2.0 × 10 11 h −1 M ⊙ , respectively. The resulting B band mass-to-light ratio for HST 1411+5211 is 41.3 ± 1.2 h (M/L) ⊙ , a factor of ∼ 5 times higher than the average early-type lensing galaxy. This large mass-to-light is almost certainly the result of the additional mass contribution from the cluster CL 3C295 at z = 0.46. For the lensing galaxy in SBS 0909+532, we measure (M/L) B = 4 +11 −3 h (M/L) ⊙ where the large errors are the result of significant uncertainty in the galaxy luminosity. While we cannot measure directly the mass-to-light ratio of the lensing galaxy in B2319+051, we estimate that (M/L) B is between 3 − 7 h (M/L) ⊙ .