A. Crider

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

PHYSICAL MODEL OF GAMMA-RAY BURST SPECTRAL EVOLUTION

We propose a physical model of gamma-ray burst spectral evolution in which impulsively accelerated nonthermal leptons cool by saturated Compton upscattering of soft photons. This model naturally explains the recently discovered exponential decay of the spectral break energy with photon fluence, the hard-to-soft spectral evolution patterns, and other spectral and temporal properties of gamma-ray bursts.

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.

GRB 990123: The Case for Saturated Comptonization

We find that the optical magnitudes of GRB 990123 observed by the Robotic Optical Transient Search Experiment move in tandem with the magnitudes extrapolated from the simultaneous BATSE spectra, strongly suggesting that the optical, X-ray, and gamma-ray photons originate from a single source. We then show that the broadband optical-gamma-ray spectra can be naturally fit by the saturated Compton model. We also derive the parameters of the Compton emitting shell from first principles.

Multiwavelength observations of gx 339-4 in 1996. I. daily light curves and x-ray and gamma-ray spectroscopy

As part of our multiwavelength campaign of GX 339-4 observations in 1996, we present our radio, X-ray, and gamma-ray observations made in July, when the source was in a hard state (=soft X-ray low state). The radio observations were made at the time when there was a possible radio jet. We show that the radio spectrum was flat and significantly variable and that the radio spectral shape and amplitude at this time were not anomalous for this source. Daily light curves from our pointed observation on July 9-23 using the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory (CGRO), from the Burst and Transient Source Experiment (BATSE) on CGRO, and from the All-Sky Monitor on the Rossi X-Ray Timing Explorer (RXTE) also show that there was no significant change in the X-ray and gamma-ray flux or hardness during the time when the possible radio jet-like feature was seen. The higher energy portion of our pointed RXTE observation made on July 26 can be equally well fitted using simple power law times exponential (PLE) and Sunyaev-Titarchuk (ST) functions. An additional soft component is required, as well as a broad emission feature centered on ~6.4 keV. This may be an iron line that is broadened by orbital Doppler motions and/or scattering off a hot medium. Its equivalent width is ~600 eV. Our simplistic continuum fitting does not require an extra reflection component. Both a PLE and an ST model also fit our OSSE spectrum on its own. Although the observations are not quite simultaneous, combining the RXTE and CGRO spectra we find that the PLE model easily fits the joint spectrum. However, the ST model drops off too rapidly with increasing energies to give an acceptable joint fit.

Confronting synchrotron shock and inverse Comptonization models with GRB spectral evolution

The time-resolved spectra of gamma-ray bursts (GRBs) remain in conflict with many proposed models for these events. After proving that most of the bursts in our sample show evidence for spectral “shape-shifting”, we discuss what restrictions that BATSE time-resolved burst spectra place on current models. We find that the synchrotron shock model does not allow for the steep low-energy spectral slope observed in many bursts, including GRB 970111. We also determine that saturated Comptonization with only Thomson thinning fails to explain the observed rise and fall of the low-energy spectral slope seen in GRB 970111 and other bursts. This implies that saturated Comptonization models must include some mechanism which can cause the Thomson depth to increase initially in pulses.

Testing the Gamma-Ray Burst Blast-Wave Model: A Primer

The discovery of optical gamma-ray burst (GRB) counterparts with measurable redshifts has shown that the sources are indeed cosmological in origin. The energy source and emission mechanism remain a mystery. A common aspect of many GRB models is the collision of two compact objects, typically neutron stars. The energy released forms a relativistic blast wave that eventually converts its kinetic energy into radiation via synchrotron shocks. The Lorentz boosting of the radiation makes it appear in the gamma-ray regime. The observed X-ray and optical counterparts match some predictions of an external shock model, in which the blast wave collides with the surrounding ambient medium. However, recent studies of burst time histories suggest that the gamma-ray active phase of bursts cannot result from external shocks. Features found in the time-resolved spectra of gamma-ray bursts also suggest that their emission mechanism is not optically thin synchrotron. In this paper, we will review the more important gamma-ray burst observations, discuss briefly the development of the synchrotron shock model for GRBs, and propose that this model cannot currently explain the prompt emission of gamma-ray bursts.

A thermal-nonthermal inverse Compton model for Cyg X-1

Using Monte Carlo methods to simulate the inverse Compton scattering of soft photons, we model the spectrum of the Galactic black hole candidate Cyg X-1, which shows evidence of a nonthermal tail extending beyond a few hundred keV. We assume an ad hoc sphere of leptons, whose energy distribution consists of a Maxwellian plus a high energy power-law tail, and inject 0.5 keV blackbody photons. The spectral data is used to constrain the nonthermal plasma fraction and the power-law index assuming a reasonable Maxwellian temperature and Thomson depth. A small but non-negligible fraction of nonthermal leptons is needed to explain the power-law tail.

Testing the invariance of cooling rate in gamma-ray burst pulses

Recent studies have found that the spectral evolution of pulses within gamma-ray bursts (GRBs) is consistent with simple radiative cooling. Perhaps more interesting was a report that some bursts may have a single cooling rate for the multiple pulses that occur within it. We determine the probability that the observed “cooling rate invariance” is purely coincidental by sampling values from the observed distribution of cooling rates. We find a 0.1–26% probability that we would randomly observe a similar degree of invariance based on a variety of pulse selection methods and pulse comparison statistics. This probability is sufficiently high to warrant skepticism of any intrinsic invariance in the cooling rate.

Experiential Education on the Edge: SETI Activities for the College Classroom

In a sophomore-level, interdisciplinary honors class, we introduced students to the Search for Extraterrestrial Intelligence through assigned readings, student presentations, classroom discussions, and multiple experiential activities. In this paper, we present four of these novel experiential activities. In the first, students suddenly find themselves trying to make contact with an unknown person who is simultaneously trying to contact them. The second is a course-long role-playing exercise patterned after a “first contact” simulation held annually at the CONTACT: Culture of the Imagination conferences. The third and fourth are parts of a unique final exam where students must respond as a group to two surreal encounters, one being a “2001”-style monolith that shows up, as in the film, entirely without warning or instructions. For the final, we also developed an assessment rubric appropriate for this kind of open-ended test. We conclude by discussing recommendations for implementing similar experiential education activities, both specifically and in spirit, in other classes.