Robert J. Nemiroff

Gamma-ray burst peak duration as a function of energy

Gamma-ray burst time histories often consist of many peaks. These peaks tend to be narrower at higher energy. If gamma-ray bursts are cosmological, the energy dependence of gamma-ray burst timescales must be understood in order to correct the timescale dependence due to the expansion of the universe. By using the average autocorrelation function and the average pulse width, we show that the narrowing with energy follows, quite well, a power law. The power-law index is ~-0.4. This is the first quantitative relationship between temporal and spectral structure in gamma-ray bursts. It is unclear what physics causes this relationship. The average autocorrelation has a universal shape such that one energy range scales linearly with time into all other energy ranges. This shape is approximately the sum of two exponentials.

Detection of signature consistent with cosmological time dilation in gamma-ray bursts

If gamma-ray bursters are at cosmological distances as suggested by their isotropic distribution on the sky and by their number-intensity relation then the burst profiles will be stretched in time, by an amount proportional to the redshift, 1 + z. We have tested data from the Compton Gamma Ray Observatory’s Burst and Transient Source Experiment (BATSE) for such time dilation. Out of 590 bursts observed by BATSE, 131 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. We used three tests to compare the timescales of bright and dim bursts, the latter, on average, being more distant than the former. Our measures of timescale are constructed to avoid selection effects arising from intensity differences by rescaling all bursts to fiducial levels of peak intensity and noise bias. (1) We found that the total rescaled count above background for the dim burst ensemble is approximately twice that for the brightest bursts translating into longer durations for the dim bursts. (2) Wavelet-transform decompositions of the burst profiles confirmed that this dilation operates over a broad range of timescales. (3) Structure on the shortest timescales was examined using a procedure which aligns the highest peaks of profiles from which the noise has been optimally removed using a wavelet thresholding technique. In all three tests, the dim bursts are stretched by a factor of ∼ 2 relative to the bright ones, over seven octaves of timescale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright BATSE bursts. Results are consistent with bursts at BATSE’s peak-flux completeness limit being at cosmological distances corresponding to z ∼ 1, and thus with independent cosmological interpretations of the BATSE number-intensity relation. Alternative explanations of our results, arising from the nature of physical processes in bursts, are still possible. Subject headings: cosmology: theory gamma rays: bursts

Duration distributions of bright and dim BATSE gamma-ray bursts

We have measured the T(sub 90) and T(sub 50) durations of bright and dim gamma-ray bursts detected by the Compton Gamma-Ray Observatorys (CGRO) Burst and Transient Source Experiment (BASTE). The T(sub 90) T(sub 50) duration is defined as the interval over which 5% (25%) to 95% (75%) of the burst counts accumulate. Out of 775 bursts observed by BATSE 159 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. A Kolmogorov-Smirnov test yields a probability of 6 x 10(exp -5) that the T(sub 50) durations of the dim and bright samples are drawn from the same parent population. We find that the centroid and extent of the duration distribution for the dim sample are scaled by approximately a factor of 2 relative to those of the bright sample. The measured time-dilation factor is not sensitive to choice of energy band. These results are quantitatively consistent with previous tests for time dilation in a smaller sample of BATSE bursts. The sources of dimmer bursts, if cosmological, would lie at redshifts of order 2.

Gamma-ray bursts are time-asymmetric

A simple test for time asymmetry is devised and carried out on the brightest gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory. We show evidence that individual bursts are time-asymmetric on all timescales tested, from a timescale shorter than that of pulses which compose GRBs to a timescale similar to a greater envelope that contains these pulses. We also find bursts which manifest significant asymmetry only on timescales comparable to the duration of burst, and bursts for which no clear asymmetry on any timescale is present. The sense of the asymmetry is that bursts and/or component structures rise in a shorter time than they decay. We also find that our whole sample of bursts taken together is time-asymmetric, in that there are sifnificantly more bursts and pulses where the rise is more rapid than the decay, on all timescales tested and for all energy bands tested. When our whole GRB sample is binned at 64 ms and integrated over all BATSE energies, the statistical significance is at the 6 sigma level. Models that predict time symmetry are therefore excluded.

Searching gamma-ray bursts for gravitational lensing echoes - Implications for compact dark matter

The first available 44 gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment on board the Compton Gamma-Ray Observatory have been inspected for echo signals following shortly after the main signal. No significant echoes have been found. Echoes would have been expected were the GRBs distant enough and the universe populated with a sufficient density of compact objects composing the dark matter. Constraints on dark matter abundance and GRB redshifts from the present data are presented and discussed. Based on these preliminary results, a universe filled to critical density of compact objects between 10 exp 6.5 and 10 exp 8.1 solar masses are now marginally excluded, or the most likely cosmological distance paradigm for GRBs is not correct. We expect future constraints to be able either to test currently popular cosmological dark matter paradigms or to indicate that GRBs do not lie at cosmological distances.

Magnification bias in galactic microlensing searches

It is shown that a significant amount of detectable gravitational microlensing events that could potentially be found by Massively Parallel Photometry (MAPP) project (such as the MACHO, EROS, and OGLE collaborations) will occur for stars too dim to be easily noticed individually by these projects. This is the result of a large magnification bias effect, a bias of including high-magnification events in any flux-limited sample. The probablility of detecting these events may be as high as 2.3 times the lensing probability of stars currently being monitored by MAPP collaborations.

Null Result in gamma-ray burst lensed echo search

We have searched for gravitational-lens-induced echoes between gamma-ray bursts (GRBs) in Burst and Transient Source Experiment (BATSE) data. The search was conducted in two phases. In the first phase we compared all GRBs in a brightness-complete sample of the first 260 GRBs with recorded angular positions having at least a 5% chance of being coincident from their combined positional error. In the second phase, we compared all GRB light curves of the first 611 GRBs with recorded angular positions having at least a 55% chance of being coincident from their combined positional error. No unambiguous gravitational lens candidate pairs were found in either phase, although a library of close calls was accumulated for future reference. This result neither excludes nor significantly constrains a cosmological origin for GRBs.

Soft gamma-ray telescope for space flight use

Soft (gamma) -ray imaging instruments onboard currently operating astrophysical observatories attain angular resolutions on the order of 10 arc minutes using coded apertures with uniformly redundant arrays. In crowded regions, e.g. the galactic plane and center regions, unambiguous association with candidates at other wavelengths will often require improved localizations. We discuss a (gamma) -ray telescope design using chirp-Z function codes and coarse position-sensitive image plane detectors. Image plane detector systems consisting of CsI scintillators and avalanche photodiodes are evaluated. Position sensitivity is achieved by either using a matrix of individual CsI/APD detectors or CsI scintillator rods. The aim is to design a narrow field-of-view telescope with sub-arcminute resolution to be flown on a small spacecraft.

Exploration of bi‐modality in gamma‐ray burst duration and hardness distributions

A bimodal burst duration distribution with the minimum near 1–2 s, previously reported by several investigators, is now confirmed by Compton’s Burst and Transient Source Experiment (BATSE). The burst subgroups are also distinguished by their integral spectral hardness ratios—shorter events tend to be harder. After fitting 280 pulses in 35 long, bright bursts, we find that the interval distribution exhibits a single wide mode centered near 1 s. Analysis of a sample of 12 short, bright bursts indicates that they tend to have considerably fewer pulses, while their pulse width and interval distributions are peaked on the shortward end of those distributions for long bursts. Simulations patterned after these measured attributes of pulse distributions reproduce fairly well the shortward mode and minimum in the duration distribution. The average trend of spectral evolution for long, bright bursts suggests that the difference in hardness ratios might be connected with the tendency of these bursts to soften. No ev...

Color diagrams of gamma‐ray bursts

We compute color‐color and color‐brightness diagrams for BATSE gamma‐ray bursts using peak fluxes and fluences. We describe some of the difficulties associated with color‐color diagrams due to systematic effects of data acquisition and interpretation. We find that faint bursts generally have a softer hardness ratio in the BATSE LAD middle energy channels, which cover the range from 50–300 keV. Although this may be consistent with a cosmological population of bursts, potential systematic effects prevent us from being definitive at this point. Our diagrams also indicate that there is a great deal of diversity of observed spectra of bursts and confirm that bursts cannot be explained by a universal power law.