Bradley E. Schaefer

BATSE observations of gamma-ray burst spectra. I: Spectral diversity

We studied the time-averaged gamma-ray burst spectra accumulated by the spectroscopy detectors of the Burst and Transient Source Experiment. The spectra are described well at low energy by a power-law continuum with an exponential cutoff and by a steeper power law at high energy. However, the spectral parameters vary from burst to burst with no universal values. The break in the spectrum ranges from below 100 keV to more than 1 MeV, but peaks below 200 keV with only a small fraction of the spectra breaking above 400 keV; it is therefore unlikely that a majority of the burst spectra are shaped directly by pair processes, unless bursts originate from a broad redshift range. The correlations among burst parameters do not fulfill the predictions of the cosmological models of burst origin. No correlations with burst morphology or the spatial distribution were found. We demonstrate the importance of using a complete spectral description even if a partial description (e.g., a model without a high-energy tail) is statistically satisfactory.

Discovery of a supernova explosion at half the age of the universe

The ultimate fate of the Universe, infinite expansion or a big crunch, can be determined by using the redshifts and distances of very distant supernovae to monitor changes in the expansion rate. We can now find large numbers of these distant supernovae, and measure their redshifts and apparent brightnesses; moreover, recent studies of nearby type Ia supernovae have shown how to determine their intrinsic luminosities—and therefore with their apparent brightnesses obtain their distances. The >50 distant supernovae discovered so far provide a record of changes in the expansion rate over the past several billion years. However, it is necessary to extend this expansion history still farther away (hence further back in time) in order to begin to distinguish the causes of the expansion-rate changes—such as the slowing caused by the gravitational attraction of the Universes mass density, and the possibly counteracting effect of the cosmological constant. Here we report the most distant spectroscopically confirmed supernova. Spectra and photometry from the largest telescopes on the ground and in space show that this ancient supernova is strikingly similar to nearby, recent type Ia supernovae. When combined with previous measurements of nearer supernovae,, these new measurements suggest that we may live in a low-mass-density universe.

New constraints on Omega(M), Omega(lambda), and w from an independent set of eleven high-redshift supernovae observed with HST

We report measurements of

Timescale stretch parameterization of Type Ia supernova B-band light curves

\Omega_M

New Constraints on ΩM, ΩΛ, and w from an Independent Set of 11 High-Redshift Supernovae Observed with the Hubble Space Telescope*

,

BATSE observations of gamma-ray burst spectra. 2: Peak energy evolution in bright, long bursts

\Omega_\Lambda

The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 Kiloparsecs and Other Overdensities in the Outer Halo

, and w from eleven supernovae at z=0.36-0.86 with high-quality lightcurves measured using WFPC-2 on the HST. This is an independent set of high-redshift supernovae that confirms previous supernova evidence for an accelerating Universe. Combined with earlier Supernova Cosmology Project data, the new supernovae yield a flat-universe measurement of the mass density

Gamma-Ray Burst Hubble Diagram to z = 4.5

\Omega_M=0.25^{+0.07}_{-0.06}

GRB 021004: A Massive Progenitor Star Surrounded by Shells

(statistical)

The distant type Ia supernova rate

\pm0.04