Mathematics – Logic
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........23w&link_type=abstract
PhD Dissertation, Pennsylvania Univ. Philadelphia, PA United States
Mathematics
Logic
Gamma Rays, Gravitational Lenses, Bursts, Gamma Ray Observatory, Cosmology, Luminosity, Color-Color Diagram, Background Radiation, Astronomical Models
Scientific paper
It is argued that the most natural explanation of recent observations of gamma-ray bursts (GRBs) is that they are cosmological. Assuming that they are standard candles with no number or luminosity evolution in co-moving coordinates, the deficiency of faint GRBs observed by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory can be successfully explained in the context of standard Friedmann-Lemaitre cosmological models. However, GRBs cannot distinguish among different cosmologies; the parameter space within which a successful theoretical model of the population of GRBs could be established is quite large. It is also not possible to say whether GRBs evolve or not. At any rate, the most simple, flat cosmological model fits the data very well The luminosity of a standard candle burst with no beaming is estimated to be approximately 1050erg/s. The average redshift of BATSE GRBs is estimated to be of the order of unity. An estimate of the contribution of GRBs to the diffuse gamma ray background yields an upper limit to the luminosity of a standard candle burst to be is less than 1054 erg/s. It is also concluded that the rate of GRBs in the entire observable universe should be approximately 3100 yr-1 if the source beaming is neglected BATSE GRBs have been searched for possible gravitational lensing. An analysis of 44 GRBs showed that none was lensed. This null result implies an exclusion of compact masses in the universe over the range 106.5 - 108.1 solar mass. The possibility of the determination of lens parameters from lensing of GRBs is discussed. A lensed GRB uniquely determines (1 + zd)M, where zd is the redshift of a compact mass lens of mass M. The mass determination via this method b accurate only up to a factor of 2 because usually the redshift of the lens is not observed. Theoretically, it is also possible to estimate the Hubble constant from lensing of GRBs. However, it is unlikely that present BATSE data will yield an accurate value because of the poor angular resolution of BATSE detectors; the determination of the Hubble constant is involved with the angular positions of images of a lensed GRB. As the second phase of the lens search, more than 600 GRBs were tested to see whether they had undergone gravitational lensing. This result was also null From this null result, the maximum redshift limit of BATSE GRBs was estimated to be approximately less than 2.4 - 6.2. The rate of discovering gravitationally lensed GRBs by BATSE was estimated to be one every S-24 years. The probability of lensing of GRBs as calculated from the null result of the lensing search is approximately 1%. Color-color diagrams (CCDs) have been introduced as a possible classification scheme for GRB spectra. The CCDs show that GRBs are consistent with a cosmological interpretation of GRBs and there is a high degree of diversity among their spectra. The construction of a theoretical grid on CCDs is found to be an efficient way to identify spectra with different shapes.
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