GRB Spikes Could Resolve Stars

GRBs vary more rapidly than any other known cosmological phenomena. The lower limits of this variability have not yet been explored. Improvements in detectors would reveal or limit the actual rate of short GRBs. Were microsecond "spike" GRBs to exist and be detectable, they would time-resolve stellar mass objects throughout the universe by their gravitational microlensing effect. Analyzing the time structure of sufficient numbers of GRB spikes would reveal or limit $\Omega_{star}$, $\Omega_{MACHO}$, and/or $\Omega_{baryon}$.Comment: 18 pages, 2 figures, in press: ApJ (Letters

On Suggestive Correlations Between GRBs and Clusters of Galaxies

Recent claims of angular correlations between gamma-ray bursts (GRBs) and clusters of galaxies are evaluated in light of existing but previously uncorrelated GRB positional data. Additional GRB data sets we use include sub-samples of soft BATSE 3B bursts, bursts located by the Interplanetary Network (IPN), and GRBs localized by COMPTEL. We confirm a previously reported excess by Rood and Struble (1996) of the 185 rich, nearby clusters of galaxies (Abell, Corwin, and Olowin 1989, ACO) in the 1-$\sigma$ error circles of 74 BATSE 3B positions, but find a typical correlation strength of only $~$2.5-$\sigma$ for typical sub-samples. However, none of the 185 ACO clusters occur in the 1-$\sigma$ error boxes of 40 IPN GRBs or 18 COMPTEL GRBs. When all ACO clusters are correlated with BATSE 3B GRBs however, we find an increasingly strong correlation for GRBs with decreasingly small error boxes, reaching above the 3.5-$\sigma$ level. We also find a slight excess of {\it soft} BATSE GRBs near the positions of 185 rich, nearby ACO clusters, but the significance of the correlation averages only $~$2.5-$\sigma$ for sub-samples delineated by softness. We caution that the statistical significance of all these correlations is marginal, and so conclude that the excess is at best only suggestive of a physical association. Statistical fluke is still a strong possibility. BATSE could confirm or refute such correlations in a 10-year lifetime.Comment: 17 pages in LateX including 2 postcript figures. To be published in ApJ. One affiliation has been change

Limits on the cosmological abundance of supermassive compact objects from a millilensing search in gamma-ray burst data

A new search for the gravitational lens effects of a significant cosmological density of supermassive compact objects (SCOs) on gamma-ray bursts has yielded a null result. We inspected the timing data of 774 BATSE-triggered GRBs for evidence of millilensing: repeated peaks similar in light-curve shape and spectra. Our null detection leads us to conclude that, in all candidate universes simulated, $\Omega_{SCO} < 0.1$ is favored for $10^5 < M_{SCO}/M_{\odot} < 10^9$, while in some universes and mass ranges the density limits are as much as 10 times lower. Therefore, a cosmologically significant population of SCOs near globular cluster mass neither came out of the primordial universe, nor condensed at recombination.Comment: 14 pages including 3 figures, appeared 2001 January 2

Correlations Between Lag, Luminosity, and Duration in Gamma-ray Burst Pulses

We derive a new peak lag vs. peak luminosity relation in gamma-ray burst (GRB) pulses. We demonstrate conclusively that GRB spectral lags are pulse rather than burst properties and show how the lag vs. luminosity relation determined from CCF measurements of burst properties is essentially just a rough measure of this newly derived relation for individual pulses. We further show that most GRB pulses have correlated properties: short-lag pulses have shorter durations, are more luminous, and are harder within a burst than long-lag pulses. We also uncover a new pulse duration vs. pulse peak luminosity relation, and indicate that long-lag pulses often precede short-lag pulses. Although most pulse behaviors are supportive of internal shocks (including long-lag pulses), we identify some pulse shapes that could result from external shocks.Comment: 14 pages, 4 figures, 1 table; accepted for publication in Astrophysical Journal Letter

The nature of the universe debate in 1998

A public program titled the Nature of the Universe Debate: Cosmology Solved? was held at the Smithsonian National Museum of National History on 1998 October 4. P. James E. Peebles and Michael S. Turner debated whether a fundamental set of cosmological parameters is emerging that provides a consistent geometric setting for the interpretation of cosmological observations. Introductory lectures were given by Owen Gingerich and Joseph Silk, and the program was moderated by Margaret J. Geller. This paper briefly summarizes the background, organization, and some events leading up to the program. The preceding papers are based on the actual lectures and arguments presented. The event was held in honor of David N. Schramm

Ultraviolet observations of the gamma-ray blazar 3C 279 following the gamma-ray flare of 1991 june

Ultraviolet observations of the gamma-ray blazar 3C 279 were carried out in 1991 July with the International Ultraviolet Explorer (IUE) satellite, 28 days after the outburst of intense gamma-ray emission detected from this source with the high-energy EGRET instrument aboard the Compton Gamma-Ray Observatory. IUE observations were conducted over the wavelength range 1200-3200 Å (5-10 eV) and are compared with archival UV measurements spanning the period 1988-1991. This set of observations was analyzed with a uniform, standardized data-reduction procedure. No significant variability in the UV spectrum of 3C 279 is noted over timescales of hours, though variations at the 3 σ level or higher exist on longer timescales of months to years. The UV observations which most closely bracket the γ-ray flare detected by EGRET show ∼3 σ differences in flux and spectral index, consistent with (but not definitive proof of) variations in the UV flux during the period of the γ-ray flare. Statistically marginal evidence for a monotonie correlation is found between UV spectral index and the emission observed from 3C 279, in that a harder spectrum is associated with increased UV flux

The Scale of the Universe debate in 1996

On April 21, 1996, Sidney van den Bergh and Gustav Tammann engaged in a public debate titled The Scale of the Universe. The arguments they presented focused on recently determined and still controversial values of the Hubble Constant. The program was moderated by John Bahcall, with lectures on the background and history behind humanity\u27s quest for the scale of the Universe given by Owen Gingerich and Virginia Trimble. These introductory lectures along with the arguments presented by the 1996 debaters are recreated in the following papers. This debate was of the same title and held in the same auditorium as the Great Debate between Heber Curtis and Harlow Shapley in 1920. Here we discuss some of the issues surrounding the organization and inspiration for the 1996 debate. Like the 1920 debate, the 1996 debate was not intended to resolve a disagreement instantly. Instead it is hoped that this debate and the written contributions will stand as educational tools, summarizing the arguments behind today\u27s Hubble Constant controversy, and will help provide a framework for evaluating progress in this field as the century which saw its creation draws to a close. Together with last year\u27s debate, The Distance Scale to Gamma Ray Bursts (Nemiroff, R. J. 1995, PASP. 107, 1131), these papers may also provide a clue as to how scientists think

Gamma-Ray Burst Intensity Distributions

We use the lag-luminosity relation to calculate self-consistently the redshifts, apparent peak bolometric luminosities LB, and isotropic energies Eiso for a large sample of BATSE bursts. We consider two different forms of the lag-luminosity relation; for both forms the median redshift for our burst database is 1.6. We model the resulting sample of burst energies with power law and Gaussian distributions, both of which are reasonable models. The power law model has an index of α = 1.76 ± 0.05 (95 % confidence) as opposed to the index of α = 2 predicted by the simple universal jet profile model; however, reasonable refinements to this model permit much greater flexibility in reconciling predicted and observed energy distributions. Subject headings: gamma-rays: bursts 1