Showing papers by "E. E. Fenimore published in 2001"

The Unique Signature of Shell Curvature in Gamma-Ray Bursts

Abstract: As a result of spherical kinematics, temporal evolution of received gammaray emission should demonstrate signatures of curvature from the emitting shell. Specifically, the shape of the pulse decay must bear a strict dependence on the degree of curvature of the gamma-ray emitting surface. We compare the spectral evolution of the decay of individual GRB pulses to the evolution as expected from curvature. In particular, we examine the relationship between photon flux intensity (I) and the peak of the νFν distribution (Epeak) as predicted by colliding shells. Kinematics necessitate that Epeak demonstrate a power-law relationship with I described roughly as: I = E(1−ζ) peak where ζ represents a weighted average of the low and high energy spectral indices. Data analyses of 24 observed gamma-ray burst pulses provide evidence that there exists a robust relationship between Epeak and I in the decay phase. Simulation results, however, show that a sizable fraction of observed pulses evolve faster than kinematics allow. Regardless of kinematic parameters, we found that the existence of curvature demands that the I − Epeak function decay be defined by ∼ (1 − ζ). Efforts were employed to break this curvature dependency within simulations through a number of scenarios such as anisotropic emission (jets) with angular dependencies, thickness values for the colliding shells, and various cooling mechanisms. Of these, the only method successful in dominating curvature effects was a slow cooling model. As a result, GRB models must confront the fact that observed pulses do not evolve in the manner which curvature demands.

The Unique Signature of Shell Curvature in Gamma-Ray Bursts

Abstract: As a result of spherical kinematics, temporal evolution of received gamma-ray emission should demonstrate signatures of curvature from the emitting shell. Specifically, the shape of the pulse decay must bear a strict dependence on the degree of curvature of the gamma-ray emitting surface. We compare the spectral evolution of the decay of individual GRB pulses to the evolution as expected from curvature. In particular, we examine the relationship between photon flux intensity (I) and the peak of the u F u distribution (E_{peak}) as predicted by colliding shells. Kinematics necessitate that E_{peak} demonstrate a power-law relationship with I described roughly as: I=E_{peak}^{(1-\zeta)} where \zeta represents a weighted average of the low and high energy spectral indices. Data analyses of 24 BATSE gamma-ray burst pulses provide evidence that there exists a robust relationship between E_{peak} and I in the decay phase. Simulation results, however, show that a sizable fraction of observed pulses evolve faster than kinematics allow. Regardless of kinematic parameters, we found that the existence of curvature demands that the I - E_{peak} function decay be defined by \sim (1-\zeta). Efforts were employed to break this curvature dependency within simulations through a number of scenarios such as anisotropic emission (jets) with angular dependencies, thickness values for the colliding shells, and various cooling mechanisms. Of these, the only method successful in dominating curvature effects was a slow cooling model. As a result, GRB models must confront the fact that observed pulses do not evolve in the manner which curvature demands.

Cosmic γ-ray bursts as a probe of star formation history

Abstract: The cosmic γ-ray burst (GRB) formation rate, as derived from the variability-luminosity relation for long-duration GRBs, is compared with the cosmic star formation rate. If GRBs are related to the collapse of massive stars, one expects the GRB rate to be approximately proportional to the star formation rate. We found that these two rates have similar slopes at low redshift. This suggests that GRBs do indeed track the star formation rate of the Universe, which in turn implies that the formation rate of massive stars that produce GRBs is proportional to the total star formation rate. It also implies that we can use GRBs as a probe of the cosmic star formation rate at high redshift. We find that the cosmic star formation rate increases steeply with redshift at z>2.5. This is in apparent contrast to what is derived from measurements of the cosmic star formation rate at high redshift from optical observations of field galaxies, suggesting that much high-z star formation is being missed in the optical surveys, ...

High-energy transient explorer-2

Abstract: We describe the scientific goals, the spacecraft and instrumentation, and the operation of the High-Energy Transient Explorer-2 (HETE-2), which is currently scheduled to be launched in early 2000.

The duration errors of gamma-ray bursts

Abstract: We have recalculated the durations of GRBs using a method which calculates the parabolical background fit using the standard background and burst intervals. This analysis indicates that the errors associated with the durations of the short bursts are larger than previously published in gamma-ray burst catalogs. Also the error bars are asymmetric, i.e. non-gaussian.