Georgia Ann Richardson

The Fourth BATSE Gamma-Ray Burst Catalog (Revised)

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) has triggered on 1637 cosmic gamma-ray bursts between 1991 April 19 and 1996 August 29. These events constitute the Fourth BATSE burst catalog. The current version (4Br) has been revised from the version first circulated on CD-ROM in 1997 September (4B) to include improved locations for a subset of bursts that have been reprocessed using additional data. A significant difference from previous BATSE catalogs is the inclusion of bursts from periods when the trigger energy range differed from the nominal 50-300 keV. We present tables of the burst occurrence times, locations, peak fluxes, fluences, and durations. In general, results from previous BATSE catalogs are confirmed here with greater statistical significance.

Intrinsic dependence of gamma-ray burst durations on energy

We have measured T90 and T50 as a function of energy for a set of bright BATSE gamma-ray bursts, selected on the basis of their peak photon flux on the 64 ms time scale. These events lie mainly on the −3/2 portion of the log N(>P)–log P curve; thus, in the cosmological scenario their measured characteristics may be relatively free from redshift and time dilation effects, so that the measured dependence of duration on energy may reflect an intrinsic dependence. We examine the compatibility of our results with recent work (1), which showed that the functional dependence of duration on energy is well-represented by a power law.

Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Shocks

Abstract Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3D relativistic electromagnetic particle code, we have investigated particle acceleration associated with a relativistic jet front propagating into an ambient plasma. We find small differences in the results for no ambient and modest ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The small scale magnetic field structure generated by the Weibel instability is appropriate to the generation of “jitter” radiation from defected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in γ-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Relativistic Shocks: Particle Acceleration, Magnetic Field Generation, and Emission

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two‐stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3‐D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating into an ambient plasma with and without initial magnetic fields. We find small differences in the results for no ambient and modest ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The non‐linear fluctuation amplitudes of densities, currents, electric, and magnetic fields in the electron‐positron shock are larger than those found in the electron‐ion shock at the same simulation time. This comes from the fac...

Scientific capabilities of SIFTER for discovering and monitoring gamma-ray bursts and active galactic nuclei

An exciting possibility for the GLAST main instrument is a scintillating fiber system where the properties of both a tracker and a calorimeter are combined in one type of detector module. This instrument provides all the detector capabilities required to achieve the science goals of the GLAST mission, at a substantially reduced cost compared to the baseline technology, and with the benefit of increased effective area and superior low energy angular resolution.

FiberGLAST: a scintillating fiber approach to the GLAST mission

FiberGLAST is a scintillating fiber gamma-ray detector designed for the GLAST mission. The system described below provides superior effective area and field of view for modest cost and risk. An overview of the FiberGLAST instrument is presented, as well as a more detailed description of the principle elements of the primary detector volume. The triggering and readout electronics are described, and Monte Carlo Simulations of the instrument performance are presented.

Particle acceleration in electron-ion jets

Weibel instability created in collisionless shocks is responsible for particle (electron, positron, and ion) acceleration. Using a 3‐D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic electron‐ion jet fronts propagating into an ambient plasma without initial magnetic fields with a longer simulation system in order to investigate nonlinear stage of the Weibel instability and its acceleration mechanism. The current channels generated by the Weibel instability induce the radial electric fields. The z component of the Poynting vector (E × B) become positive in the large region along the jet propagation direction. This leads to the acceleration of jet electrons along the jet. In particular the E × B drift with the large scale current channel generated by the ion Weibel instability accelerate electrons effectively in both parallel and perpendicular directions.

3-D GRMHD Simulations of Disk-Jet Coupling and Emission

We have performed a fully three‐dimensional general relativistic magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion disk around a Schwarzschild black hole with a free‐falling corona. The initial simulation results show that a bipolar jet (velocity ≈ 0.3c) is created as shown by previous two‐dimensional axisymmetric simulations with mirror symmetry at the equator. The 3‐D simulation ran over one hundred light‐crossing time units (τS = rS/c where rS ≡ 2GM/c2) which is considerably longer than the previous simulations. We show that the jet is initially formed as predicted due in part to magnetic pressure from the twisting the initially uniform magnetic field and from gas pressure associated with shock formation in the region around r = 3rS. At later times, the accretion disk becomes thick and the jet fades resulting in a wind that is ejected from the surface of the thickened (torus‐like) disk. It should be noted that no streaming matter from a donor is included at the outer boundar...

Particle Acceleration and Radiation Associated with Magnetic Field Generation from Relativistic Collisionless Shocks

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, two‐streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3‐D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating through an ambient plasma with and without initial magnetic fields. We find only small differences in the results between no ambient and weak ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates particles perpendicular and parallel to the jet propagation direction. The simulation results show that this instability is responsible for generating and amplifying highly nonuniform, small‐scale magnetic fields, which contribute to the electron’s transverse deflection behind the jet head. T...

Flow-field dependent variation method for complex relativistic fluids

Many current high-energy astrophysics problems, particularly those containing shock waves and high-speed flow, do not take advantage of new computational fluid dynamics (CFD) techniques available in such fields as aerospace engineering. We will present the flow-field dependent variation (FDV) method to accurately solve very high-speed flow problems, as well as capture relativistic shocks, all while allowing the user to apply their familiar finite difference method (FDM) or finite element method (FEM). This method is also versatile enough to apply the non-relativistic Naiver-Stokes equations to solve low speed flows. In the FDV method, numerical schemes are automatically adjusted from the current flow field information reflecting shock discontinuities and/or effects of viscosity in boundary layers. To demonstrate the validity of this theory, the shock tube using the relativistic hydrodynamic equations has been applied.