34956928
Spectral Hardness Decay with Respect to Fluence in BATSE Gamma‐Ray Bursts
We have analyzed the evolution of the spectral hardness parameter Epk (the maximum of the νFν spectrum) as a function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with the trend reported by Liang & Kargatis, which found that Epk decays exponentially with respect to photon fluence Φ(t). We also fit these pulses with a slight modification of this trend, where Epk decays linearly with energy fluence. In both cases, we found the set of 41 pulses to be consistent with the trend. For the latter trend, which we believe to be more physical, the distribution of the decay constant Φ0 is roughly lognormal, where the mean of log10 Φ0 is 1.75 ± 0.07 and the FWHM of log10 Φ0 is 1.0 ± 0.1. Regarding an earlier reported invariance in Φ0 among different pulses in a single burst, we found probabilities of 0.49-0.84 (depending on the test used) that such invariance would occur by coincidence, most likely because of the narrow distribution of Φ0 values among pulses.
18512052
EVOLUTION OF THE LOW-ENERGY PHOTON SPECTRA IN GAMMA-RAY BURSTS
We report evidence that the asymptotic low-energy power-law slope a(below the spectral break) of BATSE gamma-ray burst (GRB) photon spectra evolves with time rather than remaining constant. Wefind that a high degree of positive correlation exists between the time-resolved spectral break energyEpkand a. In samples of 18 “hard-to-soft” and 12 “tracking” pulses, evolution of a was found to correlate with that of the spectral break energy Epk at the 99.7% and 98% confidence levels, respectively. We also find that in the flux rise phase of hard-to-softpulses,themeanvalueof aisoftenpositive,andinsomeburststhemaximumvalueof aisconsistent withavalue .1 1.BATSEburst3B910927,forexample,hasan amaxequalto1.6H0.3.Thesefindingschallenge GRB spectral models in which amust be negative or remain constant. Subject headings: gamma rays: bursts—gamma rays: observations—methods: statistical
119334154
BATSE Observations of Gamma-Ray Burst Spectra. IV. Time-resolved High-Energy Spectroscopy
We report on the temporal behavior of the high-energy power-law continuum component of gamma-ray burst spectra with data obtained by the Burst and Transient Source Experiment. We have selected 126 high-fluence and high-flux bursts from the beginning of the mission up until the present. Much of the data were obtained with the Large Area Detectors, which have nearly all-sky coverage, excellent sensitivity over 2 decades of energy, and moderate energy resolution, ideal for continuum spectra studies of a large sample of bursts at high time resolution. At least eight spectra from each burst were fitted with a spectral form that consisted of a low-energy power law, a spectral break at middle energies, and a high-energy continuum. In most bursts (122), the high-energy continuum was consistent with a power law. The evolution of the fitted high-energy power-law index over the selected spectra for each burst is inconsistent with a constant for 34% of the total sample. The sample distribution of the average value for the index from each burst is fairly narrow, centered on -2.12. A linear trend in time is ruled out for only 20% of the bursts, with hard-to-soft evolution dominating the sample (100 events). The distribution for the total change in the power-law index over the duration of a burst peaks at the value -0.37 and is characterized by a median absolute deviation of 0.39, arguing that a single physical process is involved. We present analyses of the correlation of the power-law index with time, burst intensity, and low-energy time evolution. In general, we confirm the general hard-to-soft spectral evolution observed in the low-energy component of the continuum, while presenting evidence that this evolution is different in nature from that of the rest of the continuum.
3036852
BATSE observations of gamma-ray burst spectra. 2: Peak energy evolution in bright, long bursts
We investigate spectral evolution in 37 bright, long gamma-ray bursts observed with the BATSE Spectroscopy Detectors. High resolution spectra are characterized by the energy of the peak of \nfn~and the evolution of this quantity is examined relative to the emission intensity. In most cases it is found that this peak energy either rises with or slightly precedes major intensity increases and softens for the remainder of the pulse. Inter-pulse emission is generally harder early in the burst. For bursts with multiple intensity pulses, later spikes tend to be softer than earlier ones indicating that the energy of the peak of \nfn~is bounded by an envelope which decays with time. Evidence is found that bursts in which the bulk of the flux comes well after the event which triggers the instrument tend to show less peak energy variability and are not as hard as several bursts in which the emission occurs promptly after the trigger. Several recently proposed burst models are examined in light of these results and no qualitative conflicts with the observations presented here are found.
15817563
BATSE Gamma-Ray Burst Line Search. V. Probability of Detecting a Line in a Burst
The physical importance of the apparent discrepancy between the detections by pre-BATSE missions of absorption lines in gamma-ray burst spectra and the absence of a BATSE line detection necessitates a statistical analysis of this discrepancy. This analysis requires a calculation of the probability that a line, if present, will be detected in a given burst. However, the connection between the detectability of a line in a spectrum and in a burst requires a model for the occurrence of a line within a burst. We have developed the necessary weighting for the line detection probability for each spectrum spanning the burst. The resulting calculations require a description of each spectrum in the BATSE database. With these tools, we identify the bursts in which lines are most likely to be detected. Also, by assuming a small frequency with which lines occur, we calculate the approximate number of BATSE bursts in which lines of various types could be detected. Lines similar to the Ginga detections can be detected in relatively few BATSE bursts; for example, in only ~20 bursts are lines similar to the GB 880205 pair of lines detectable. Ginga reported lines at ~20 and ~40 keV, whereas the low-energy cutoff of the BATSE spectra is typically above 20 keV; hence BATSE's sensitivity to lines is less than that of Ginga below 40 keV, and greater above. Therefore, the probability that the GB 880205 lines would be detected in a Ginga burst rather than a BATSE burst is ~0.2. Finally, we adopt a more appropriate test of the significance of a line feature.
15241714
BATSE evidence for GRB spectral features
We have developed an automatic search procedure to identify low-energy spectral features in GRBs. We have searched 133,000 spectra from 117 bright bursts and have identified 12 candidate features with significances ranging from our threshold of P=5x10{sup -5} to P=1x10{sup -7}. Several of the candidates have been examined in detail, including some with data from more than one BATSE spectroscopy detector. The evidence for spectral features appears good; however, the features have not conclusively been shown to be narrow lines.
14304615
Baryon, charged hadron, Drell-Yan and J/{psi} production in high energy proton-nucleus collisions
We show that the distributions of outgoing protons and charged hadrons in high energy proton-nucleus collisions are described rather well by a linear extrapolation from proton-proton collisions. The only adjustable parameter required is the shift in rapidity of a produced charged meson when it encounters a target nucleon. Its fitted value is 0.16. Next, we apply this linear extrapolation to precisely measured Drell-Yan cross sections for 800 GeV protons incident on a variety of nuclear targets which exhibit a deviation from linear scaling in the atomic number {ital A}. We show that this deviation can be accounted for by energy degradation of the proton as it passes through the nucleus if account is taken of the time delay of particle production due to quantum coherence. We infer an average proper coherence time of 0.4{plus_minus}0.1hfm/c, corresponding to a coherence path length of 8{plus_minus}2hfm in the rest frame of the nucleus. Finally, we apply the linear extrapolation to measured J/{psi} production cross sections for 200 and 450 GeV/c protons incident on a variety of nuclear targets. Our analysis takes into account energy loss of the beam proton, the time delay of particle production due to quantum coherence, and absorption of the J/{psi}more » on nucleons. The best representation is obtained for a coherence time of 0.5 fm/c, which is consistent with Drell-Yan production, and an absorption cross section of 3.6 mb, which is consistent with the value deduced from photoproduction of the J/{psi} on nuclear targets. {copyright} {ital 1999 American Institute of Physics.}« less
15653990
Missing Molecular Hydrogen and the Physical Conditions of GRB Host Galaxies
We examine the abundance of molecular hydrogen (H2) in the spectra of gamma ray burst afterglows (GRBs). In nearby galaxies, H2 traces the cold neutral medium (CNM) and dense molecular star-forming interstellar gas. Although H2 is detected in at least half of all sight lines toward hot stars in the Magellanic Clouds and in ≈25% of damped Lyα systems toward quasars, it is not detected in any of the five GRB environments with a similar range of neutral hydrogen column density and metallicity. We detect no vibrationally excited H2 that would imply that the GRB itself has photodissociated its parent molecular cloud, so such models are ruled out unless the parent cloud was 4 pc in radius and was fully dissociated prior to the spectroscopic observations, or the star escaped its parent cloud during its main-sequence lifetime. The low molecular fractions for the GRBs are mysterious in light of their large column densities of neutral H and expectations based on local analogs, i.e., 30 Doradus in the LMC. This surprising lack of H2 in GRB damped Lyα absorbers indicates that the destruction processes that suppress molecule formation in the LMC and SMC are more effective in the GRB hosts, most probably due to a combination of low metallicity and an FUV radiation field 10-100 times the Galactic mean field. These inferred conditions place strong constraints on the star-forming regions in these early galaxies.
119115901
Pseudo-Dirac Neutrinos in the New Standard Model
The addition of gauge-singlet fermions to the standard model Lagrangian renders the neutrinos massive and allows one to explain all that is experimentally known about neutrino masses and lepton mixing by varying the values of the Majorana mass parameters M for the gauge singlets and the neutrino Yukawa couplings {lambda}. Here we explore the region of parameter space where M values are much smaller than the neutrino Dirac masses {lambda}v. In this region, neutrinos are pseudo-Dirac fermions. We find that current solar data constrain M values to be less than at least 10{sup -9} eV, and discuss the sensitivity of future experiments to tiny gauge-singlet fermion masses. We also discuss a useful basis for analyzing pseudo-Dirac neutrino mixing effects. In particular, we identify a simple relationship between elements of M and the induced enlarged mixing matrix and new mass-squared differences. These allow one to directly relate bounds on the new mass-squared differences to bounds on the singlet fermion Majorana masses.