The results obtained with the Burst and Transient Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) show that the durations of gamma-ray bursts (GRBs) are consistent with two populations. We report here our attempts to fit lognormal distributions to various properties of GRBs. In particular it is shown that the two populations are well fit by lognormal distributions. Furthermore, the separations in time between the pulses of emission in long duration, intense GRBs are also consistent with a lognormal distribution. This fingerprint of the emission process can be used to test the hypothesis of a cosmological origin for faint GRBs. The integral number versus peak flux distribution can be fit by a truncated lognormal distribution or by functions consisting of a single lognormal distribution with a power law tail of slope -1.5. The lognormal properties of GRBs are similar to terrestial lightning and suggest that relativistic discharges between regions of charge separation may be the emission mechanism responsible for GRBs originating in the Solar System, the galaxy or at cosmological distances.
Key words: gamma rays:bursts -- lognormal -- lightning
No postscript available.
The time intervals between the bursts from the soft gamma ray repeating source, SGR 1806--20, are consistent with a lognormal distribution. The intensity of the bursts is compatible with a truncated lognormal luminosity distribution but there is no correlation between the intensity and the time interval between bursts. The large range in the time intervals between the bursts from the other two known sources, SGR 1900+14 and SGR 0526--66, may be compatible with a lognormal distribution. The time intervals between the microjumps in the Vela pulsar and the size of the microjumps are compatible with lognormal distributions and there is no correlation between the time interval and the size of the microjump. The statistical properties of the bursts from SGR 1806--20 are very similar to the microjumps observed from the Vela pulsar and this association favours models of adjustments within a neutron star for the soft repeaters. There may be a large number of soft gamma ray repeating sources to be discovered with mean time intervals between bursts much longer than SGR 1806--20. New experiments are required to search for this population.
Key words:gamma rays: bursts -- pulsars: individual: Vela -- methods: statistical
For a postscript version click here (123kb)
Broadband photometry and optical spectroscopy of the dwarf starburst galaxy Haro 3 (NGC 3353, Mkn 35) is presented. Despite its isolated location and relaxed outer morphology, Haro 3 is a dwarf galaxy dominated by multiple violent star forming regions. Emission line fluxes from the ionised gas of these starburst events show that the galaxy is underabundant in heavy elements by a factor of three compared with the Solar value. Age estimates for the starbursts calculated from emission line strengths suggest that the current episode of major star formation is under 5 million years old. Evidence for an underlying older stellar substrate is obtained from the detection of magnesium and iron absorption lines. The detection of emission features attributable to Wolf-Rayet stars in one of the HII regions also allows Haro 3 to be classified as a Wolf-Rayet galaxy. We discuss the possibilities that the current high rate of star formation was initiated by an interaction, gas cloud infall from the halo, or by stochastic self-propagating star formation.
Key words:galaxies: individual: Haro 3 -- galaxies: starburst -- galaxies: abundances -- stars: Wolf-Rayet -- ISM: HII Regions
For a postscript version click here (620kb)
The time interval between successive bursts from SGR 1806-20 and the intensity of these bursts are both consistent with lognormal distributions. Monte Carlo simulations of lognormal burst models with a range of distribution parameters have been investigated and the results are presented here. Some conclusions are drawn about a possible population of SGRs with mean recurrence times much longer than SGR 1806-20 and the results expected of a search for such SGRs are discussed.
Key words: Soft-Gamma Repeaters --- Lognormal
For a postscript version click here (108kb)
A small air shower array operating over many years has been used to search for ultra-high energy (UHE) gamma radiation ($\geq 50$ TeV) associated with gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each burst are presented for seven GRBs located with zenith angles $\theta < 20^{\circ}$. A $4.3\sigma$ excess over background was observed between 10 and 20 minutes following the onset of a GRB on 11 May 1991. The confidence level that this is due to a real effect and not a background fluctuation is 99.8\%. If this effect is real then cosmological models are excluded for this burst because of absorption of UHE gamma rays by the intergalactic radiation fields.
Key words: gamma rays:bursts -- gamma rays:high energy -- air showers
For a postscript version click here (93kb)