Robert W. Goodrich

BEAMING IN GAMMA-RAY BURSTS: EVIDENCE FOR A STANDARD ENERGY RESERVOIR

We present a comprehensive sample of all gamma-ray burst (GRB) afterglows with known distances, and we derive their conical opening angles based on observed broadband breaks in their light curves. Within the framework of this conical jet model, we correct for the geometry and we find that the gamma-ray energy release is narrowly clustered around 5 × 10^(50) ergs. We draw three conclusions. First, the central engines of GRBs release energies that are comparable to ordinary supernovae. Second, the broad distribution in fluence and luminosity for GRBs is largely the result of a wide variation of opening angles. Third, only a small fraction of GRBs are visible to a given observer, and the true GRB rate is several hundred times larger than the observed rate.

THE TEAM KECK TREASURY REDSHIFT SURVEY OF THE GOODS-NORTH FIELD

We report the results of an extensive imaging and spectroscopic survey in the Great Observatories Origins Deep Survey (GOODS)-North field completed using DEIMOS on the Keck II telescope. Observations of 2018 targets in a magnitude-limited sample of 2911 objects to RAB = 24.4 yield secure redshifts for a sample of 1440 galaxies and active galactic nuclei (AGNs) plus 96 stars. In addition to redshifts and associated quality assessments, our catalog also includes photometric and astrometric measurements for all targets detected in our R-band imaging survey of the GOODS-North region. We investigate various sources of incompleteness and find the redshift catalog to be 53% complete at its limiting magnitude. The median redshift of z = 0.65 is lower than in similar deep surveys because we did not select against low-redshift targets. Comparison with other redshift surveys in the same field, including a complementary Hawaii-led DEIMOS survey, establishes that our velocity uncertainties are as low as σ ≈ 40 km s-1 for red galaxies and that our redshift confidence assessments are accurate. The distributions of rest-frame magnitudes and colors among the sample agree well with model predictions out to and beyond z = 1. We will release all survey data, including extracted one-dimensional and sky-subtracted two-dimensional spectra, thus providing a sizable and homogeneous database for the GOODS-North field, which will enable studies of large-scale structure, spectral indices, internal galaxy kinematics, and the predictive capabilities of photometric redshifts.

Spectroscopy of the Host Galaxy of the Gamma-Ray Burst 980703

We present spectroscopic observations of the host galaxy of the -ray burst (GRB) 980703. Several emission and absorption features are detected, making the redshift, z = 0.966, completely unambiguous. This is only the third known redshift for a GRB host. The implied isotropic -ray energy release from the burst is in excess of 10 53 erg, for a reasonable choice of cosmological parameters. The spectroscopic properties of the host galaxy are typical for a star formation powered object. Using the observed value of the Balmer decrement, we derived the extinction in the galaxy’s restframe, AV � 0.3±0.3 mag. Using three different star formation rate indicators, we estimate SFR � 10 M⊙ yr −1 , or higher, depending on the extinction, with a lower limit of SFR > 7 M⊙ yr −1 . This is the highest value of the star formation rate measured for a GRB galaxy so far, and it gives some support to the idea that GRBs are closely related to massive star formation. Subject headings: cosmology: miscellaneous — cosmology: observations — gamma rays: bursts

Polarization Clues to the Structure of Broad Absorption Line Quasi-Stellar Objects

Spectropolarimetry of two broad absorption line (BAL) QSOs, PHL 5200 and H1413+117, is presented. In both cases we find that the continuum polarization rises gently to the blue, and the emission lines are polarized less than, but at the same position angle as, the continuum. This implies that scattering is the polarization mechanism. In H1413+117 we also find a dramatic rise in polarization through the absorption troughs, which suggests that the scattered light is less absorbed than the direct light. We are led to a picture in which the scatterers lie near the equatorial plane of the QSO and are physically associated with either the broad emission line region (BELR) or broad absorption line region (BALR). The direct light and the emission lines appear to be attenuated by passage through the BALR and/or scattering region. The scattered light passes through a region with less absorption, both in the continuum and in the BAL troughs.

Polarization of Broad Absorption Line QSOs. I. A Spectropolarimetric Atlas

We present a spectropolarimetric survey of 36 broad absorption line quasi-stellar objects (BAL QSOs). The continuum, absorption trough, and emission-line polarization of BAL QSOs yield clues about their structure. We confirm that BAL QSOs are in general more highly polarized than non-BAL QSOs, consistent with a more equatorial viewing direction for the former than the latter. We have identified two new highly polarized QSOs in our sample (1232+1325 and 1333+2840). The polarization rises weakly to the blue in most objects, perhaps owing to scattering and absorption by dust particles. We find that a polarization increase in the BAL troughs is a general property of polarized BAL QSOs, indicating an excess of scattered light relative to direct light, and consistent with the unification of BAL QSOs and non-BAL QSOs. We have also discovered evidence of resonantly scattered photons in the red wing of the C IV broad emission lines of a few objects. In most cases, the broad emission lines have lower polarization and a different position angle than the continuum. The polarization characteristics of low-ionization BAL QSOs are similar to those of high-ionization BAL QSOs, suggesting a similar BAL wind geometry.

The Type IC supernova 1994I in M51: detection of helium and spectral evolution

We present a series of spectra of SN 1994I in M51, starting 1 week prior to maximum brightness. The nebular phase began about 2 months after the explosion; together with the rapid decline of the optical light, this suggests that the ejected mass was small. Although lines of He I in the optical region are weak or absent, consistent with the Type Ic classification, we detect strong He I λ10830 absorption during the first month past maximum. Thus, if SN 1994I is a typical Type Ic supernova, the atmospheres of these objects cannot be completely devoid of helium. The emission-line widths are smaller than predicted by the model of Nomoto and coworkers, in which the iron core of a low-mass carbon-oxygen star collapses. They are, however, larger than in Type Ib supernovae.

Spectropolarimetry of Two Broad Absorption Line Quasars with the W. M. Keck Telescope

We have observed the broad absorption line quasars PHL 5200 and 01052265 with spectropolarimetry at the Keck telescope. In PHL 5200 the continuum is consistent with a power law with constant polarization,p 55.1%. A drop in polarization in the red is explained with dilution by unpolarized Fe II emission. In both objects the permitted emission lines are unpolarized. In PHL 5200 the semiforbidden line C III] l1909 is weakly polarized; weattributethistoresonancescattering.Thepolarizationrisesinthebroadabsorptiontroughs,toapeakof12% in both objects. The high values ofpare restricted to a narrow velocity range that is well inside the absorption troughs. In each object, the polarization position angle is constant, except that in PHL 5200 there are marginally significant rotations in the C IVtrough and in the C III] emission line. We describe two simple geometries that can explain some of these features. In thefirst, there are scattered and direct rays; in the troughs the direct ray (unpolarized) is largely absorbed, and we mainly see the highly polarized scattered ray. In the other, the high trough polarization is caused by resonance scattering. Subject headings: polarization—quasars: absorption lines—quasars: emission lines— quasars: individual (PHL 5200, 01052265)

The Discovery and Broadband Follow-Up of the Transient Afterglow of GRB 980703

We report on the discovery of the radio, infrared, and optical transient coincident with an X-ray transient proposed to be the afterglow of GRB 980703. At later times when the transient has faded below detection, we see an underlying galaxy with R=22.6; this galaxy is the brightest host galaxy (by nearly 2 mag) of any cosmological gamma-ray burst (GRB) thus far. In keeping with an established trend, the GRB is not significantly offset from the host galaxy. Interpreting the multiwavelength data in the framework of the popular fireball model requires that the synchrotron cooling break was between the optical and X-ray bands on 1998 July 8.5 UT and that the intrinsic extinction of the transient is A -->V=0.9. This is somewhat higher than the extinction for the galaxy as a whole, as estimated from spectroscopy.

The Host Galaxy of GRB 990123

We present deep images of the field of GRB 990123 obtained in a broadband UV/visible bandpass with the Hubble Space Telescope (HST) and deep near-infrared images obtained with the Keck I 10 m telescope. The HST image reveals that the optical transient (OT) is offset by 0farcs67 (5.8 kpc in projection) from an extended, apparently interacting galaxy. This galaxy, which we conclude is the host galaxy of GRB 990123, is the most likely source of the absorption lines of metals at a redshift of z=1.6 seen in the spectrum of the OT. With magnitudes of Gunn-r = 24.5 ± 0.2 and K = 22.1±0.3 mag, this corresponds to an L ~ 0.5L

Scattered Nuclear Continuum and Broad Hα in Cygnus A

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