arXiv subjects

J. P. Norris

Publications and source records attributed to J. P. Norris.

51 records · Page 3Linked to original sources

arXiv1999

"No High Energy Emission" GRB Class Is Attributable to Brightness Bias

The inhomogeneous brightness distribution of BATSE detected gamma-ray bursts has been considered strong evidence for their cosmological origin. However, subclasses of gamma-ray bursts have been shown to have significantly more homogeneous brightness distributions. Pendleton et al. (1997) have found such a result for gamma-ray bursts with no detectable emission at energies >300 keV. Accordingly, it has been suggested that these no high energy (NHE) emission bursts represent an underluminous population of nearby sources. A distinct homogeneous NHE brightness distribution has also been considered as evidence for beaming of different spectral components of the prompt burst emission. We synthesize observed distributions of gamma-ray bursts based on a sample of typical bright BATSE bursts with intrinsic high energy emission and adopt a single cosmological distance scale for all sources. We find that the resulting synthetic NHE bursts do indeed have a more nearly homogeneous intensity distribution when an appropriate decrease in signal to noise and redshifted spectrum is incorporated. We argue that the definition of NHE bursts, and soft-spectrum bursts in general, naturally produces a steep distribution. The NHE class of gamma-ray bursts is therefore likely due to brightness bias.

astro-ph↗

arXiv1999

Beam Test of Gamma-ray Large Area Space Telescope Components

A beam test of GLAST (Gamma-ray Large Area Space Telescope) components was performed at the Stanford Linear Accelerator Center in October, 1997. These beam test components were simple versions of the planned flight hardware. Results on the performance of the tracker, calorimeter, and anti-coincidence charged particle veto are presented.

physics.ins-det↗

arXiv1998

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_{\odot} $. A stronger limit is found for a much discussed $Ω= 0.15$ universe dominated by black holes of masses $\sim 10^{6.5} M_{\odot}$, which is ruled out at the $\sim$ 90% confidence level.

astro-ph↗

arXiv1998

Constraints on Association of Single-pulse Gamma-ray Bursts and Supernovae

We explore the hypothesis, similar to one recently suggested by Bloom and colleagues, that some nearby supernovae are associated with smooth, single-pulse gamma-ray bursts, possibly having no emission above ~ 300 keV. We examine BATSE bursts with durations longer than 2 s, fitting those which can be visually characterized as single-pulse events with a lognormal pulse model. The fraction of events that can be reliably ascertained to be temporally and spectrally similar to the exemplar, GRB 980425 - possibly associated with SN 1998bw - is 4/1573 or 0.25%. This fraction could be as high as 8/1573 (0.5%) if the dimmest bursts are included. Approximately 2% of bursts are morphologically similar to GRB 980425 but have emission above ~ 300 keV. A search of supernova catalogs containing 630 detections during BATSE's lifetime reveals only one burst (GRB 980425) within a 3-month time window and within the total 3-sigma BATSE error radius that could be associated with a type Ib/c supernova. There is no tendency for any subset of single-pulse GRBs to fall near the Supergalactic Plane, whereas SNe of type Ib/c do show this tendency. Economy of hypotheses leads us to conclude that nearby supernovae generally are not related to smooth, single-pulse gamma-ray bursts.

astro-ph↗

arXiv1996

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-$σ$ error circles of 74 BATSE 3B positions, but find a typical correlation strength of only $~$2.5-$σ$ for typical sub-samples. 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 {\it soft} BATSE GRBs near the positions of 185 rich, nearby ACO clusters, but the significance of the correlation averages only $~$2.5-$σ$ 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.

astro-ph↗

arXiv1996

Time-Dilation, Log N - Log P, and Cosmology

We investigate whether a simple cosmology can fit GRB results in both time dilation and Log N - Log P simultaneously. Simplifying assumptions include: all GRBs are spectrally identical to BATSE trigger 143, $Ω=1$ universe, and no luminosity and number density evolution. Observational data used includes: the BATSE 3B peak brightness distribution (64-ms time scale), the Pioneer Venus Orbiter (PVO) brightness distribution, and the Norris et al. time dilation results for peak aligned profiles presented at this meeting. We find acceptable cosmological fits to the brightness distributions when placing BATSE trigger 143 at a redshift of 0.15 $\pm$ 0.10. This translates into a $(1 + z_{dim}) / (1 + z_{bright})$ factor of about 1.50 $\pm$ 0.50 between selected brightness extremes of the Norris et al. sample. Norris et al. estimate, however, that $(1 + z_{dim}) / (1 + z_{bright})$ $\approx$ 2.0 $\pm$ 0.5 when considering duration tests. The difference is marginal and could be accounted for by evolution. We therefore find that evolution of GRBs is preferred but not demanded.

astro-ph↗

arXiv1996

Calibration of Tests for Time Dilation in GRB Pulse Structures

Two tests for cosmological time dilation in $γ$-ray bursts -- the peak alignment and auto-correlation statistics -- involve averaging information near the times of peak intensity. Both tests require width corrections, assuming cosmological origin for bursts, since narrower temporal structure from higher energy would be redshifted into the band of observation, and since intervals between pulse structures are included in the averaging procedures. We analyze long ($>$ 2 s) BATSE bursts and estimate total width corrections for trial time-dilation factors (TDF = [1+$z_{\rm dim}$]/[1+$z_{\rm brt}$]) by time-dilating and redshifting bright bursts. Both tests reveal significant trends of increasing TDF with decreasing peak flux, but neither provides sufficient discriminatory power to distinguish between actual TDFs in the range 2--3.

astro-ph↗

arXiv1996

Test for Time Dilation of Intervals Between Pulse Structures in GRBs

If $γ$-ray bursts are at cosmological distances, then not only their constituent pulses but also the intervals between pulses should be time-dilated. Unlike time-dilation measures of pulse emission, intervals would appear to require negligible correction for redshift of narrower temporal structure from higher energy into the band of observation. However, stretching of pulse intervals is inherently difficult to measure without incurring a timescale-dependent bias since, as time profiles are stretched, more structure can appear near the limit of resolution. This problem is compounded in dimmer bursts because identification of significant structures becomes more problematic. We attempt to minimize brightness bias by equalizing signal-to-noise (s/n) level of all bursts. We analyze wavelet-denoised burst profiles binned to several resolutions, identifying significant fluctuations between pulse structures and interjacent valleys. When bursts are ranked by peak flux, an interval time-dilation signature is evident, but its magnitude and significance are dependent upon temporal resolution and s/n level.

astro-ph↗

arXiv1996

Canonical Timescales In GRBs--199

Understanding the bimodal duration distribution (dynamic range $>$ 10$^{4}$) of $γ$-ray bursts is central to determining if the phenomenon is in fact a singular one. A unifying concept, beyond isotropy and inhomogeneity of the two groups separately, is that bursts consist of pulses, organized in time and energy: wider pulses are more asymmetric, their centroids are shifted to later times at lower energies, and shorter, more symmetric pulses tend to be spectrally harder. Long bursts tend to have many pulses while short bursts usually have few, relatively narrow pulses. Two factors, viewing angle and beaming, may account for pulse asymmetry and the large dynamic range ($\sim$ 200) in pulse widths. A cosmological time-dilation signature, with an expected dynamic range of order two, would be difficult to measure against these large intrinsic variations and low signal-to-noise levels of dimmer bursts. Some statistics ($T_{\rm 90}$, pulse intervals) are particularly sensitive to brightness bias, noise, and apparently minor variations in definition. Also, spectral redshift would move narrower, high-energy emission from dim bursts into the band of observation, constituting a countering effect to time dilation. With analyses restricted to bursts longer than $\sim$ 2 s, tests for time dilation that are constructed to be free of brightness bias have yielded time-dilation factors $\sim$ 2--3, for pulse structures, intervals between structures, and durations.

astro-ph↗

arXiv1995

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 time scales must be understood in order to correct the time scale 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 about -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 exponential.

astro-ph↗

arXiv1994

Duration Distributions of Bright and Dim BATSE Gamma-Ray Bursts

We have measured the T90 and T50 durations of bright and dim gamma-ray bursts (GRBs) detected by the Compton Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE). The T90 (T50) 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 $10^{-5}$ that the T50 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 two 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 dimmer bursts, if cosmological, would lie at redshifts of order two.

astro-ph↗

arXiv1994

Gross Spectral Differences between Bright and Dim Gamma-Ray Bursts

We find that dim gamma-ray bursts (GRBs) are softer than bright GRBs, as indicated on average by data from the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory. We show that this correlation is statistically significant with respect to variations due to random differences between GRBs. This effect is discernable using a variety of methods and data sets, including public domain data. We analyze several types of systematic errors and selection effects in the BATSE data and conclude that the observed effect is not dominated by any of them. We therefore assert that this dim/soft effect is a real property of GRBs. It is possible that this correlation is a consequence of the time dilation detected by Norris et al. (1994) and that this is additional evidence that burst sources are located at cosmological distances.

astro-ph↗

arXiv1994

Null Result in Gamma-Ray Burst Lensed Echo Search

We have searched for gravitational-lens induced echoes between gamma-ray bursts (GRBs) in NASA's orbiting {\it Compton} Gamma Ray Observatory's 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 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 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.

astro-ph↗

arXiv1993

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 {\it Compton} Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE) for such time dilation. Our measures of time scale are constructed to avoid selection effects arising from intensity differences by rescale all bursts to fiducial levels of peak intensity and noise bias. The three tests involved total count rate above background, wavelet decomposition, and alignment of the highest peaks. In all three tests, the dim bursts are stretched by a factor of about two relative to the bright ones, over seven octaves of time scale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright BATSE bursts. Results are consistent with bursts of BATSE's peak-flux completeness limit being at cosmological distances corresponding to $z \sim 1$, and thus with independent cosmological interpretations of the BATSE number-intensity relation.

astro-ph↗

arXiv1993

Gamma-Ray Bursts are Time-Asymmetric

A simple test for time-asymmetry is devised and carried out on the brightest gamma-ray bursts detected by the Burst and Transient Source Experiment (BATSE) on board the {\it Compton} Gamma Ray Observatory. We show evidence that individual bursts are time-asymmetric on all time scales tested, from a time scale shorter than that of pulses which compose GRBs to a time scale similar to a greater envelope that contains these pulses. We also find bursts which manifest significant asymmetry only on time scales comparable to the duration of burst, and bursts for which no clear asymmetry on any time scale 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 significantly more bursts and pulses where the rise is more rapid than the decay, on all time scales 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 $σ$ level. Models that predict time-symmetry are therefore excluded.

astro-ph↗