Edison P. Liang

Similarity Criteria for the Laboratory Simulation of Supernova Hydrodynamics

The conditions for validity and the limitations of experiments intended to simulate astrophysical hydrodynamics are discussed, with application to some ongoing experiments. For systems adequately described by the Euler equations, similarity criteria required for properly scaled experiments are identified. The conditions for the applicability of the Euler equations are formulated, based on the analysis of localization, heat conduction, viscosity, and radiation. Other considerations involved in such a scaling, including its limitations at small spatial scales, are discussed. The results are applied to experiments aimed at simulating three-dimensional hydrodynamics during supernova explosions and hydrodynamic instabilities in young supernova remnants. In addition, hydrodynamic situations with significant radiative effects are discussed.

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

Search for Relativistic Curvature Effects in Gamma-Ray Burst Pulses

We analyze the time profiles of individual gamma-ray burst (GRB) pulses that are longer than 2 s by modeling them with analytical functions that are based on physical first principles and well-established empirical descriptions of GRB spectral evolution. These analytical profiles are independent of the emission mechanism and can be used to model both the rise and decay profiles, allowing for the study of the entire pulse light curve. Using this method, we have studied a sample of 77 individual GRB pulses, allowing us to examine the fluence, pulse width, asymmetry, and rise and decay power-law distributions. We find that the rise phase is best modeled with a power law of average index r = 1.48 ± 0.07 and that the average decay phase has an index of d = 2.44 ± 0.12. We also find that the ratio between the rise and decay times (the pulse asymmetry) exhibited by the GRB pulse shape has an average value of 0.47, which varies little from pulse to pulse and is independent of pulse duration or intensity. Although this asymmetry is largely uncorrelated to other pulse properties, a statistically significant trend is observed between the pulse asymmetry and the decay power-law index, possibly hinting at the underlying physics. We compare these parameters with those predicted to occur if individual pulse shapes are created purely by relativistic curvature effects in the context of the fireball model, a process that makes specific predictions about the shape of GRB pulses. The decay index distribution obtained from our sample shows that the average GRB pulse fades faster than the value predicted by curvature effects, with only 39% of our sample being consistent with the curvature model. We discuss several refinements of the relativistic curvature scenario that could naturally account for these observed deviations, such as symmetry breaking and varying relative timescales within individual pulses.

Supernova hydrodynamics experiments on the Nova laser

In studying complex astrophysical phenomena such as supernovae, one does not have the luxury of setting up clean, well-controlled experiments in the universe to test the physics of current models and theories. Consequently, creating a surrogate environment to serve as an experimental astrophysics testbed would be highly beneficial. The existence of highly sophisticated, modern research lasers, developed largely as a result of the world-wide effort in inertial confinement fusion, opens a new potential for creating just such an experimental testbed utilizing well-controlled, well-diagnosed laser-produced plasmas. Two areas of physics critical to an understanding of supernovae are discussed that are amenable to supporting research on large lasers: (1) compressible nonlinear hydrodynamic mixing and (2) radiative shock hydrodynamics.

Potential Vorticity Evolution of a Protoplanetary Disk with an Embedded Protoplanet

We present two-dimensional inviscid hydrodynamic simulations of a protoplanetary disk with an embedded planet, emphasizing the evolution of potential vorticity (the ratio of vorticity to density) and its dependence on numerical resolutions. By analyzing the structure of spiral shocks made by the planet, we show that progressive changes of the potential vorticity caused by spiral shocks ultimately lead to the excitation of a secondary instability. We also demonstrate that very high numerical resolution is required to both follow the potential vorticity changes and identify the location where the secondary instability is first excited. Low-resolution results are shown to give the wrong location. We establish the robustness of a secondary instability and its impact on the planets torque. After the saturation of the instability, the disk shows large-scale nonaxisymmetry, causing the torque on the planet to oscillate with large amplitude. The impact of the oscillating torque on the protoplanets migration remains to be investigated.

Characterizing counter-streaming interpenetrating plasmas relevant to astrophysical collisionless shocks

A series of Omega experiments have produced and characterized high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks. Single and double CH2 foils have been irradiated with a laser intensity of ∼ 1016 W/cm2. The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2000 km/s, an electron temperature of ∼ 110 eV, an ion temperature of ∼ 30 eV, and a density of ∼ 1018 cm−3 were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured single foil plasma conditions were used to calculate the ion skin depth, c/ωpi∼0.16 mm, the interaction length, lint, of ∼ 8 mm, and the Coulomb mean free path, λmfp∼27mm. With c/ωpi≪lint≪λmfp, we are in a regime where collisionless shock formation is possible.

Exploring Broadband GRB Behavior during γ-Ray Emission

S. A. Yost, H. F. Swan, E. S. Rykoff, F. Aharonian, C. W. Akerlof, A. Alday, M. C. B. Ashley, S. Barthelmy, D. Burrows, D. L. Depoy, R. J. Dufour, J. D. Eastman, R. D. Forgey, N. Gehrels, E. Gogus, T. Guver, J. P. Halpern, L. C. Hardin, D. Horns, U. Kizilolu, H. A. Krimm, S. Lepine, E. P. Liang, J. L. Marshall, T. A. McKay, T. Mineo, N. Mirabal, M. Ozel, A. Phillips, J. L. Prieto, R. M. Quimby, P. Romano, G. Rowell, W. Rujopakarn, B. E. Schaefer, J. M. Silverman, R. Siverd, M. Skinner, D. A. Smith, I. A. Smith, S. Tonnesen, E. Troja, W. T. Vestrand, J. C. Wheeler, J. Wren, F. Yuan, and B. Zhang

The Connection between Spectral Evolution and Gamma-Ray Burst Lag

The observed delay between the arrival times of high- and low-energy photons in gamma-ray bursts (GRBs) has been shown by Norris et al. to be correlated to the absolute luminosity of a burst. Despite the apparent importance of this distance indicator, there has yet to be a full explanation to its origin. Any attempt at explaining this relation must take into consideration that the observed lag is the direct result of spectral evolution. In particular, as the energy at which the GRBs νFν spectra is a maximum (Epk) decays through the four BATSE channels, the photon flux peak in each individual channel will inevitably be offset, producing what we measure as lag. In order to produce a robust relationship between the observed spectral evolution and spectral lag, we measure the rate of Epk decay (Φ0) for a sample of clean single-peaked bursts with measured lag. We use this data to provide an empirical relation that expresses the GRB lag as a function of the bursts spectral evolution rate. This implies that the luminosity of a GRB is directly related to the bursts rate of spectral evolution, which we believe begins to reveal the underlying physics behind the lag-luminosity correlation. We discuss several possible mechanisms that could cause the observed evolution and examine their connections to the bursts luminosity.

A New Model for the Hard Time Lags in Black Hole X-Ray Binaries

The time-dependent Comptonized output of a cool soft X-ray source drifting inward through an inhomogeneous hot inner disk or corona is numerically simulated. We propose that this scenario can explain from first principles the observed trends in the hard time lags and power spectra of the rapid aperiodic variability of the X-ray emission of Galactic black hole candidates.

Luminosity enhancement factor for thermal Comptonization and the electron energy balance

Comptonization of soft photons by hot thermal electrons is an important cooling mechanism governing the electron energy balance in astrophysical plasmas. Thermal Comptonization is treated in terms of the luminosity enhancement factor η, which gives the average change in energy of a photon between injection into and escape from a scattering medium. The region in which the photon scatters is characterized by the dimensionless electron temperature θ = kT/m e c 2 , Thomson depth τ T , and geometry.