Search

High-energy transient events can be used to probe the extreme physics that power them, as well as the properties of matter in violent astrophysical environments such as in the vicinity of compact objects or the birthplaces of black holes. X-ray binary outbursts provide important insights into the physics of accretion and the nature of black holes and neutron stars. Meanwhile, gamma-ray bursts (GRBs) carry signatures of the powerful progenitors that produce them, yet the origins of their prompt emission and their jet structure remain unclear. Spectral and timing analyses have been effective investigative tools for these extreme settings, especially as telescopes advance and models increase in predictive power. Pairing them with linear polarization analyses of high-energy emissions can add even more information about the emission mechanisms and source geometries of accreting black holes and GRBs.The Compton Spectrometer and Imager (COSI) is a soft gamma-ray (0.2-5 MeV) telescope designed to study astrophysical sources including accreting black holes and GRBs. It has significant heritage as a balloon-borne telescope, and was selected as a NASA Small Explorer (SMEX) slated to launch on a satellite in 2027. COSI employs a compact Compton telescope designed to conduct high-resolution spectroscopy, imaging over a wide field-of-view, polarization studies, and effective suppression of background events. Compton telescopes detect multiple interactions from individual incoming photons, allowing for polarization information to be captured through measurements of the distribution of azimuthal angles. While the standard method relies on binning the photons to produce and fit an azimuthal scattering angle distribution, improved polarization sensitivity is obtained by using additional information to more accurately weigh each event’s contribution to the likelihood statistics. In this work, we report validations of COSI’s capabilities as a polarimeter. Furthermore, we develop tools that enable future spectral and polarimetric analyses of COSI GRB observations.We also present the first observed outburst from the transient X-ray binary source MAXI J0637-430, based on observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT). We study the source's transition from a soft state dominated by disk-blackbody emission to a hard state dominated by a power-law or thermal Comptonization component, with NuSTAR providing the first coverage of MAXI J0637-430 above 10 keV. These broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test alternative excess emission models such as blackbody emission from the plunging region, a reflection component with a Comptonization continuum, and a reflection component of a blackbody illuminating the disk. The study demonstrates the importance of broadband spectral analyses of accreting compact objects. We include a discussion on how joint spectral and polarization analyses could be conducted for long-duration transient sources with COSI in the future.

Gamma-ray Bursts (GRBs) are the most energetic and luminous explosions in the Universe since the Big Bang, enabling them to be observed out to extremely large redshifts (z~9). Consequently, this makes them a promising cosmological standard candle candidate. Unfortunately, however, they have proven to be quite challenging to standardize. The GRB community has worked tirelessly at this task, and to date, has put forth several luminosity-distance relations, some more propitious than others. The most prevailing problem with these relations is in their sizable amount of scatter, likely due to measurement inconsistencies and errors in the variables they employ. This arises when the results from many independent analysts are combined, which cumulates their individual choices in methods and methodologies. In this dissertation I examine the systematical uncertainties associated with the calculation of a GRB's isotropical equivalent energy, Eiso. This energetic is attributed to the total energy that is released by a GRB’s central engine in the form of X-rays and gamma-rays, before becoming reprocessed down-jet into radiation spanning the entire electromagnetic spectrum. Not only does Eiso have physical implications for GRBs, but it is employed by two of the most widely recognizable luminosity-distance relations: the Amati and the Ghirlanda relations. Our greatest concern with Eiso is that, historically, it has not been measured with a high level of consistency, which may be leading to significant scatter and bias for these two relations. Whenever a relation’s variables suffer from significant measurement errors, the relations themselves also accrue these errors, thereby increasing their scatter and decreasing their reliability. In this dissertation I present the most comprehensive exploration into the systematical uncertainties of Eiso that has ever been done. In this work I focus on a few particular analytical choices that any analyst can make differently from the next, leading to inconsistent Eiso values measured for the same GRB event. I act as two independent analysts by altering a single analytical choice between the two analyses that are performed on the same subset of GRBs. This provides us with a measure of uncertainty that is associated with each particular analytical choice. In addition to quantifying uncertainties, I examine and discuss the minutiae of the spectral modeling parameters that are used to characterize a GRB’s fluence spectrum and thus its Eiso. To achieve our goals, I employed Fermi Gamma-ray Space Telescope GRB data for a sample of events with redshifts measured from ground-based follow-up optical/NIR spectroscopy. To ensure that data preparation uncertainties remained minimal, we performed all of our own data reductions, fluence spectral modeling, and Eiso calculations for this work, rather than adopting values from the literature.

In studying the chemical abundance evolution of star-forming galaxies across many cosmic epochs, we will be able to understand how the chemical abundance of the Milky Way came to be. Determining chemical abundances (often generalized as “the metallicity”) of galaxies is relatively straight forward to do but can become difficult at high redshift (z). Gamma-Ray Bursts (GRBs) offer a unique way to spectroscopically view high redshift galaxies. Before GRBs can be confidently used as probes of the high-redshift star-forming universe, it must be first determined if GRBs trace star formation. In the local universe, it has been shown that GRBs prefer metal-poor environments and this preference does not evolve much with redshift. On the other hand, the Mass-Metallicity (M-Z) relation of star-forming galaxies evolves toward lower metallicities with increasing redshift. It is theorized that after z ~ 3, GRBs should be unbiased tracers of star-formation and could be used as probes of otherwise unobservable aspects of the high redshift universe. Presented here is the analysis of a sample of 11 GRB host galaxies at redshift z ~ 4.7. Though the larger project consists of a sample of 23 GRB host galaxies, at this time, only 11 had sufficient data for analysis. Analysis of the remaining 12 galaxies in the sample is ongoing. Spitzer Space Telescope images exist for all 23 hosts, while Hubble Space Telescope (HST) images were taken of 19 of the hosts over the last two years. Archival HST images will be used for the remaining four galaxies. Masses and metallicities for these 11 hosts were collected from the literature. Metallicities were converted to gas-phase oxygen abundances for comparison to M-Z relations of star-forming galaxies. Our collaborators measured observed-frame Infrared (IR) magnitudes for 5 of the galaxies in our sample, though further analysis for the remaining sample is ongoing. These magnitudes were converted to masses using a mass-to-light ratio for star-forming galaxies at z ~ 4. Despite theoretical prediction that GRBs should be unbiased tracers of star-formation after z ~ 3, we find that even at z ~ 4.7, GRBs prefer metal-poor host galaxies. The offset from the closest M-Z relation at z ~ 3.5 is on the order of 1 dex. There does seem to be some agreement with our sample and the M-Z relation of star-forming galaxies at z ~ 8, though further analysis is necessary due to the large scatter of the relation and incomplete GRB host sample.

In this thesis, we present our work towards understanding the radiation mechanism of Gamma Ray Bursts (GRBs) during prompt emission phase. We study the spectra of photospheric emission from highly relativistic gamma-ray burst outflows using a Monte Carlo (MC) code. We consider the Comptonization of photons with a fast cooled synchrotron spectrum in a relativistic jet with realistic photon to electron number ratio N [subscript γ] / N [subscript e] = 10⁵ using mono-energetic protons which interact with thermalised electrons through Coulomb interaction. The photons, electrons and protons are cooled adiabatically as the jet expands outwards. ...

Gamma-ray bursts (GRBs) are relativistic explosions which originate at cosmological distances, and are among the most luminous transients in the universe. Following the prompt gamma-ray emission, a fading synchrotron "afterglow" is detectable at lower energies. While long-duration GRBs (duration > 2 sec) are linked to the deaths of massive stars, the progenitors of short-duration GRBs (duration < 2 sec) have remained elusive. Theoretical predictions formulated over the past two decades have suggested that they are the mergers of two compact objects, involving either two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH). Such merging systems are also important to understand because they are premier candidates for gravitational wave detections with upcoming facilities and are considered likely sites of heavy element nucleosynthesis. The launch of the Swift satellite in 2004, with its rapid multi-wavelength monitoring and localization capabilities, led to the first discoveries of short GRB afterglows and therefore robust associations to host galaxies. At a Swift detection rate of ~8 events per year, the growing number of well-localized short GRBs enables comprehensive population studies of their afterglows and environments for the first time. In this thesis, I undertake a multi-wavelength observational campaign to address testable predictions for the progenitors of short GRBs. From their local environments, I show that short GRBs explode in diffuse regions of their host galaxies and are weakly correlated with the distribution of stellar mass and star formation in their host galaxies. I study the host galaxy demographics for the entire population and find that ~20-40% of short GRBs originate from elliptical galaxies, implying an older stellar progenitor. From their afterglows, I present evidence that some short GRBs are collimated in narrow jets of ~5-10 degrees, directly affecting the true energy scale and event rate. Finally, taking advantage of a decade of broad-band afterglow observations at radio through X-ray wavelengths, I find that short GRBs have median isotropic-equivalent energies of ~10^51 erg and that their local environments have low densities, ~10^-3-10^-2 cm^-3. Taken together, this thesis comprises several lines of independent evidence to demonstrate that short GRBs originate from the mergers of two compact objects, and also provides the first constraints on the explosion properties for a large sample of events. With the direct detection of gravitational waves from compact object mergers on the horizon, these studies provide necessary inputs to inform the next decade of joint electromagnetic-gravitational wave search strategies.

The BAT Slew Survey (BATSS) is the first wide-field survey of the hard X-ray sky (15–150 keV) with a slewing coded aperture imaging telescope. Its fine time resolution, high sensitivity and large sky coverage make it particularly well-suited for detections of transient sources with variability timescales in the \(\sim 1 sec–1 hour\) range, such as Gamma-Ray Bursts (GRBs), flaring stars and Blazars. As implemented, BATSS observations are found to be consistently more sensitive than their BAT pointing-mode counterparts, by an average of 20% over the 10 sec–3 ksec exposure range, due to intrinsic systematic differences between them. The survey’s motivation, development and implementation are presented, including a description of the software and hardware infrastructure that made this effort possible. The analysis of BATSS science data concentrates on the results of the 4.8-year BATSS GRB survey, beginning with the discovery of GRB 070326 during its preliminary testing phase. A total of nineteen (19) GRBs were detected exclusively in BATSS slews over this period, making it the largest contribution to the Swift GRB catalog from all ground-based analysis. The timing and spectral properties of prompt emission from BATSS GRBs reveal their consistency with Swift long GRBs (L-GRBs), though with instances of GRBs with unusually soft spectra or X-Ray Flashes (XRFs), GRBs near the faint end of the fluence distribution accessible to Swift-BAT, and a probable short GRB with extended emission, all uncommon traits within the general Swift GRB population. In addition, the BATSS overall detection rate of 0.49 GRBs/day of instrument time is a significant increase (45%) above the BAT pointing detection rate. This result was confirmed by a GRB detection simulation model, which further showed the increased sky coverage of slews to be the dominant effect in enhancing GRB detection probabilities. A review of lessons learned is included, with specific proposals to broaden both the number and range of astrophysical sources found in future enhancements. The BATSS survey results provide solid empirical evidence in support of an all-slewing hard X-ray survey mission, a prospect that may be realized with the launch of the proposed MIRAX-HXI mission in 2017.

Gamma-ray bursts (GRBs), thought to be produced by the core-collapse of massive stars or merging compact objects, are the most luminous events observed since the Big Bang. GRBs are intrinsically interesting as laboratories to study physical processes at energies much higher than can be produced in the largest particle accelerators on Earth. A better understanding of GRBs may also allow for their use as cosmological tools - backlights for the study of the evolution of the Universe back to the era of the first gravitationally-bound structures. In this work, results from observations of satellite-detected GRBs with the Milagro and VERITAS very high energy (VHE, >100 GeV) gamma-ray telescopes are presented. No significant flux of VHE gamma rays associated with any of the 144 GRBs observed was detected. The limits on VHE gamma-ray emission during the GRB early afterglow phase obtained from the VERITAS observations are among the most constraining to date and the interpretation of these non-detections in the context of GRB emission models is discussed. Results from observation of the "naked-eye burst" GRB 080319B with Milagro are shown to rule out the popular synchrotron self-Compton model of emission over a broad range of energy space. Finally, the prospects for GRB observations with both current and future-generation VHE observatories are examined.

Astrophysical phenomena such as exploding primordial black holes (PBHs), gamma-ray bursts (GRBs), compact object mergers, and supernovae, are expected to produce a single pulse of electromagnetic radiation detectable at the low-frequency end of the radio spectrum. Detection of any of these pulses would be significant for the study of the objects themselves, their host environments, and the interstellar/intergalactic medium. Furthermore, a detection of a radio transient from an exploding PBH could be a signature of an extra spatial dimension, which would drastically alter our perception of spacetime. However, even upper limits on the existence of PBHs, from transient searches, would be important to discussions of cosmology. We describe a method to carry out an agnostic single-dispersed-pulse search, and apply it to data collected with the Eight-meter-wavelength Transient Array (ETA). ETA is a radio telescope dedicated to searching for transient pulses. It consists of 12 crossed-dipole antenna stands with Galactic-noise-limited performance from 29-47 MHz. There is a vast amount of data collected from an ETA observation. It is therefore greatly beneficial to use a computer cluster, which works in parallel on different parts of a data set, in order to carry out a single-pulse search quickly and efficiently. Each spectrogram in a data set needs to be analyzed individually, without reference to the rest, in order to utilize a computer cluster's capabilities. The data reduction software has been developed for single-dispersed-pulse searches, and is described in this thesis. The data reduction involves sweeping through the collected data with a dedispersion routine assuming a range of dispersion measures. The resulting time series are searched with multiple matched filters for signals above a signal-to-noise threshold. Applying the single pulse search to ~ 30 hours of ETA data did not yield a compelling detection of an astrophysical signal. However, from ≈ 5 hours of interference-free data we find an observational upper limit to the rate of exploding PBHs of r ≈ 4.8 × 10⁻⁷ pc⁻³ y⁻¹ for a PBH with a fireball Lorentz-factor f = 104.3. This limit is applicable to PBHs in the halo of the Galaxy to distances ≲ 2 kpc, and dispersion measures ≲ 80 pc cm⁻³ . We also find a source-agnostic rate limit ≲ 0.25 events y⁻¹ deg⁻² for pulses of duration < 3 s, and having apparent energy densities ≳ 2.6 × 10⁻²³ J m⁻² Hz⁻¹ at 38 MHz.

Gamma-ray Bursts (GRBs) are powerful explosions that emit most of their energy, as their name suggests, in gamma-rays of typical energies of about 1 MeV. This emission lasts for about two minutes or less and it is called the prompt emission. The isotropic energy radiated in GRBs is equivalent to the energy that the Sun will radiate in its entire lifetime. After decades of studying this cosmological phenomenon, we have come to learn that it involves a collimated and relativistic jet. Also, we know that they radiate energy in the X-ray, optical and radio bands for days, weeks and years, respectively, which is called the afterglow. Recently, NASA's Fermi Satellite was launched and, in addition to MeV photons, it detected GeV photons from these astrophysical sources. We show that these GeV photons are produced when the GRB jet interacts with the medium that surrounds it: the external forward shock model. We arrive at this conclusion not only by studying the GeV emission, but also by studying the afterglow observations (Chapter 2). We corroborate this model by studying the electron acceleration in the external forward shock model and find that electrons can radiate at the maximum observed energy of ~ 10 GeV (Chapter 3). We also provide an extensive analysis of the most recent afterglow observations of GRB 090902B within the same framework of an external forward shock origin. We find that the data for this burst requires a small deviation from the traditionally used power-law electron energy distribution, however, our previous results remain unchanged (Chapter 4). To conclude, we use the end of the prompt emission phase, which exhibits a steep X-ray temporal decay, to constrain the behavior of the central engine responsible for launching the relativistic jet (Chapter 5).

Gamma-Ray Bursts (GRBs) are the most luminous objects in the universe. They herald a catastrophic energy release which manifests itself in tenths to hundreds of seconds of irregular gamma-ray emission. This initial "prompt" emission is followed by "afterglow" emission at other wavelengths that fades smoothly over hours to years. GRB prompt emission has been observed with ever-increasing sophistication for more than four decades, but many details of its origin remain unknown. While GRBs are under-stood to result from relativistic jets produced by violent reconfigurations of compact objects, the composition of the outflow, the means of energy dissipation, and the radiative processes underlying the observed emission are all uncertain. I review the present understanding of all facets of GRB science in Chapter 1. Gamma-ray spectroscopy and polarimetry provide two channels for testing models of GRB prompt emission with observed data. In Chapters 2-4, I employ the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) to conduct broad-band time-resolved spectroscopy of bright GRBs. RHESSI is a solar observatory that uses nine coaxial germanium detectors for imaging and spectroscopy of solar flares. Because the detectors are unshielded, RHESSI also records counts from off-axis sources like GRBs. In Chapter 2, I detail the methods I use for analyzing RHESSI GRB data. In Chapter 3, I conduct joint spectral analysis of bursts co-observed by RHESSI and Swift-BAT, enabling spectral modeling over a wide 15 keV-17 MeV band. These results reveal the difficulty of predicting the peak spectral energy of a burst from BAT observations alone. While GRB spectra commonly have been assumed to be non-thermal and have been fit by purely phenomenological models, some authors have proposed that thermal emission from the GRB photosphere may be the source of the GRB spectral peak. In Chapter 4, I perform time-resolved spectroscopy of bright GRBs observed by RHESSI and compare the fit results of several phenomenological and quasi-thermal models. The fit results for the quasi-thermal models are poorer than those reported for fits in more narrow energy bands, and the fit parameters show significant dependence on the fit band. More sophisticated models and additional data are needed to determine the relevance of the photospheric emission.Polarimetric studies of GRBs are in their infancy, but they have the potential to distinguish GRB emission models and probe the structure of the relativistic jet. Observations to date have exploited the polarimetric signatures produced by Compton scattering in gamma-ray observatories. Since these instruments have poor spatial resolution and are not calibrated for polarimetric observations, only a few bright GRBs have been observed, each with significant systematic effects. In Chapters 5-8, I present development work for a balloon-borne Compton telescope, the Nuclear Compton Telescope (NCT), which is capable of measuring GRB polarization. NCT uses planar cross-strip germanium detectors for Compton imaging (200 keV-10 MeV) and polarimetry (200 keV-2 MeV). Chapter 5 describes NCT and its flight history. In Chapter 6, I discuss calibrations of the effective area of NCT during the 2009 New Mexico and 2010 Australia balloon campaigns. By placing radioactive sources of known activity throughout the NCT field of view, it is possible to determine the efficiency of the telescope. I compare these values to those derived from detailed Monte Carlo simulations. The values derived from the real data show a roughly constant deficit relative to those derived from the Monte Carlo simulations which may be related to known deficiencies in the lower-level event calibrations. Chapter 7 assesses NCT's polarimetric performance. I generated partially polarized beams throughout NCT's field of view by scattering photons off of a scintillator slab triggered in coincidence with the NCT detectors. The measured polarimetric modulation showed good agreement with Monte Carlo simulations for some calibrations, although systematic geometric effects are present. NCT's polarimetric performance compares well to that of other Compton telescopes. Finally, in Chapter 8 I calculate NCT's sensitivity to GRBs and expected detection rate. Using single-site and incompletely-absorbed events, NCT has about one-fourth the sensitivity of BATSE LAD. NCT could detect about half of the bursts in the BATSE catalog if they occurred in its field of view. Bursts bright enough for polarimetric analysis are too rare to detect effectively from a balloon platform, however, so a satellite instrument with NCT-like detectors would be needed to perform a useful census of GRB polarization. Better understanding of the GRB prompt emission will clarify the place of GRBs in stellar life cycles, provide new insights into the physics of collisionless shocks and relativistic jets, and facilitate the use of GRBs as probes of the early universe and electromagnetic counterparts to gravitational wave and neutrino signals. The complex phenomenology of GRBs creates both a challenge and a rich opportunity to explain and utilize these enigmatic events.

I present results from several years of concerted observations of the afterglows and host galaxies of gamma-ray bursts (GRBs), the most energetic explosions in the Universe. Short gamma-ray bursts originate from a wide variety of environments, including disk galaxies, elliptical galaxies, galaxy haloes, and intracluster and intergalactic space. Long gamma ray bursts associate almost exclusively with star-forming hosts, but the properties of these galaxies also vary widely. Some are hosted in extremely small galaxies, difficult to identify directly in emission or infer from the absorption of afterglow light, but the host luminosity distribution extends up to very luminous (>L*) systems as well. A significant fraction of long GRBs are observed along highly dust-obscured sightlines through their host medium. Some of these events are hosted within conspicuously dusty galaxies, although the hosts of other dust-obscured events show no outward signs of significant internal dust content. By measuring the wavelength dependence of dust absorption profiles using a few well-observed GRB afterglows, I provide evidence for ordinary dust with properties similar to those of dust in the Milky Way in a system at z~3, but a very different absorption profile from the dust in a galaxy at z~5, providing tentative evidence to support a transition in dust composition early in the history of the Universe. I present an observationally-determined redshift distribution for Swift GRBs, showing few to originate from high redshifts (z>5). I also provide the first photometric and spectroscopic catalogs from one of the largest GRB host-galaxy surveys ever conducted, including observations of almost 150 distinct GRB fields.

Gamma-ray bursts (GRBs) are the mysterious, short and intense flashes of gamma-rays in the space, and are believed to originate from the rare, explosively devastating, stellar events that happens at cosmological distances. Enormous progress has been made from four decades of GRB research endeavor but the ultimate understanding of their origins has yet to arrive. Recently revealed features in their early afterglows broadened the opportunity space for exploration. We have carried out extensive studies on various physical processes in GRBs. We showed that the distribution of electrons' energy spectral index in GRBs and other relativistic sources is inconsistent with the prediction from the first-order Fermi theory of the shock particle acceleration. We investigated the photon scattering processes within the relativistic outflow that produces the GRB and calculated the resultant emission flux from it. We showed the scattering of the GRB prompt photons by the circum-burst dust, although an attractive possibility, can not explain the puzzling plateau component in the GRB afterglow light curve. We made meaningful constraint on the GRB prompt emission radius, R [greater-than or equal to] 10¹⁴, by studying the synchrotron self absorption for a small sample of bursts with good data. We showed that a late jet, which is thought to be producing the late X-ray flares in GRB afterglows, will produce detectable emissions from its interactions with other components in the explosive event of GRB, and identification of these emissions could verify the existence of the late jet and further prove the massive star origin of long-duration GRBs.

Astrophysical flows have an enormous dynamic range of relevant length scales. The physics occurring on the smallest scales often influences the physics of the largest scales, and vice versa. I present a detailed study of the multiscale and multidimensional behavior of three high-energy astrophysical flows: jet-driven supernovae, massive black hole accretion, and current-driven instabilities in gamma-ray burst external shocks. Both theory and observations of core-collapse supernovae indicate these events are not spherically-symmetric; however, the observations are often modeled assuming a spherically-symmetric explosion. I present an in-depth exploration of the effects of aspherical explosions on the observational characteristics of supernovae. This is accomplished in large part by high-resolution, multidimensional numerical simulations of jet-driven supernovae. The existence of supermassive black holes in the centers of most large galaxies is a well-established fact in observational astronomy. How such black holes came to be so massive, however, is not well established. In this work, I discuss the implications of radiative feedback and multidimensional behavior on black hole accretion. I show that the accretion rate is drastically reduced relative to the Eddington rate, making it unlikely that stellar mass black holes could grow to supermassive black holes in less than a Hubble time. Finally, I discuss a mechanism by which magnetic field strength could be enhanced behind a gamma-ray burst external shock. This mechanism relies on a current-driven instability that would cause reorganization of the pre-shock plasma into clumps. Once shocked, these clumps generate vorticity in the post-shock plasma and ultimately enhance the magnetic energy via a relativistic dynamo process.

We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB [gamma]-ray and X-ray light curve, which lends support to the proposal by Kumar et al. (2008a,b) that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of [scientific symbols]. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rate Ṁ [proportional to] t⁻²̇⁸, which resembles the L[subscript x] [proportional to] t⁻³ decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low-angular-momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows down, which is potentially suggestive of the "plateau" phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase can be ~ 20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 10¹⁰ K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent; GRBs with longer prompt [gamma]-ray emission should have more slowly rotating envelopes.

This work describes two experiments which searched for contemporaneous optical counterparts to gamma ray bursts (GRBs), GROCSE and ROTSE. The Gamma-Ray Optical Counterpart Search Experiment (GROCSE) used an automated rapidly slewing wide field of view optical telescope at Lawrence Livermore National Laboratory. The telescope was triggered in real time by the Burst And Transient Source Experiment (BATSE) data telemetry stream as processed and distributed by the BATSE COordinates DIstribution NEtwork (BACODINE), recently renamed the GRB Coordinates Network (GCN). GROCSE recorded sky images for 28 GRB triggers between January 1994 and June 1996. The analysis of the 12 best events is presented here, half of which were recorded during detectable gamma ray emission. No optical counterparts were detected to limiting magnitudes m V ≤ 8.5 despite near complete coverage of burst error boxes. The Robotic Optical Transient Search Experiment (ROTSE) continued this work with a faster, more sensitive, and more robust instrument, ROTSE-I. Prom the beginning of its run in March of 1998 through April 1999, ROTSE-I responded to 31 GRB triggers. On January 23, 1999 UTC, ROTSE-I detected the first-ever contemporaneous optical counterpart to a GRB, a bright transient with peak magnitude 8.86. The observations of this event and limits from four additional events are presented here.