Gabriela Fabiana Marani

Gravitationally Lensed Gamma-Ray Bursts as Probes of Dark Compact Objects

If dark matter in the form of compact objects comprises a large fraction of the mass of the universe, then gravitational lensing effects on gamma-ray bursts are expected. We utilize BATSE and Ulysses data to search for lenses of different mass ranges, which cause lensing in the milli, pico, and femto regimes. Null results are used to set weak limits on the cosmological abundance of compact objects in mass ranges from 10-16 to 10-9 M☉. A stronger limit is found for a much-discussed Ω=0.15 universe dominated by black holes of mass ~106.5 M☉, which is ruled out at the ~90% confidence level.

Gamma-Ray Burst Spikes Could Resolve Stars

Gamma-ray bursts (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 ?star, ?MACHO, and ?baryon.

On Suggestive Correlations between Gamma-Ray Bursts 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 subsamples 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 & Struble (1996) of the 185 rich, nearby clusters of galaxies (Abell, Corwin, & Olowin 1989; ACO clusters) in the 1 σ error circles of 74 BATSE 3B positions but find a typical correlation strength of only 2.5 σ for typical subsamples. However, none of the 185 ACO clusters occur in the 1 σ 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 σ level. We also find a slight excess of soft BATSE GRBs near the positions of 185 rich, nearby ACO clusters, but the significance of the correlation averages only 2.5 σ for subsamples 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.

Correlations with gamma‐ray bursts

We performed correlation function analyses involving the first 260 BATSE detected gamma‐ray bursts (GRBs) searching for evidence of bunching or repetition. The BATSE GRB two‐point angular auto‐correlation function shows excesses at small and high angles as noted previously by Quashnock and Lamb (1993), and Narayan and Piran (1993). The BATSE GRB two‐point temporal correlation function shows no significant excesses at any times but does show a significant dip on the time scale of hours. This dip is real and corresponds to BATSE ignoring dim bursts while processing bright bursts. Even when constrained to bursts with recorded angular separations less than 20°, no temporal excess is found. Therefore if GRBs repeat, this analysis was unable to locate any obvious repetition timescale. We have computed the general GRB‐Abell cluster angular cross‐correlation function and again find no significant peaks. In sum, the times and positions of the first 260 GRBs released into the public domain show no significant evide...

A comprehensive search for gravitational lensing of GRBs

Different types of gravitational lensing may affect GRBs if they lie at cosmological distances. Given the non-detection of lensing events due to the known galaxy field and assuming that QSO images are distorted by the same lens field, we have derived a very conservative upper limit on the redshift of zgrbmax∼4.06. If dark matter comprises a certain fraction of the mass of the universe and is composed by individual objects, the lack of mega-, micro-, pico-, and femto-lensing events is also used to set limits on the cosmological abundance of these compact objects.

What is 2CG 010‐31?

As recorded by COS‐B, 2CG 010‐31 was the brightest gamma‐ray source above 100 MeV out of the galactic plane, brighter than either 3C 273 or ρ Oph. 2CG 010‐31 has never been identified ‐ the only catalogued objects within 20 arcminutes are a 7th magnitude F star and a 15th magnitude galaxy (as determined by searches in SIMBAD and NED). Within the 1.5 degree COS‐B error circle lie 76 stars, 14 IRAS sources, 11 individual galaxies, 2 QSOs, 2 radio sources, 1 galaxy pair and 1 cluster of galaxies. Therefore this previously unidentified source was observed with OSSE, EGRET and COMPTEL during Compton observation period 209 in an effort to classify and understand its nature. Data reduction is continuing although preliminary results are now available. Two weeks of OSSE exposure failed to detect the object even though a significant detection was expected. Two weeks of EGRET exposure also failed to detect this object where a detection was again expected. COMPTEL results are pending. Inspection of the COS‐B detectio...

A new gravitational lens search for gamma ray bursts

In the new 3rd BATSE catalog, many new gamma ray bursts have been added, and all GRB positions and error boxes have been recomputed, yielding many significantly different GRB locations and smaller error boxes. These improvements mandate a new search for GRB gravitational lensing in the new catalog. We are conducting an automated search for echo signals between the 834 triggered bursts with recorded T90s and angular positions within 3σ of their combined positional error. The GRB light curves are compared using different statistical techniques. So far, no convincing gravitational lens pairs have been found.

Cosmology with GRBs

Given the duration and peak flux distribution of GRBs in the BATSE 3B catalog, inherent detection thresholds of BATSE, and the assumption that peak flux is a standard candle irrespective of duration, rates are estimated for GRBs of cosmologically interesting durations and peak fluxes.