Simulations of high-energy astrophysical phenomena

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2014-12

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Abstract

Supercomputer technology has revolutionized our studies of the most energetic astrophysical phenomena. Here, I present my simulations of energetic outbursts of gamma rays and the explosions of massive stars, and my efforts to further the computational astrophysics frontier with the development of a radiation hydrodynamics code. First, I present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst (GRB) progenitor star, a "collapsar," onto the central compact object, which we assume is 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 ~ 0.2 [solar mass] s⁻¹. 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 by a sudden and rapid power-law decline in the central accretion rate [mathematical formula], which resembles the L [subscript x] [is 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, 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 will 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. Observational evidence suggests a link between long GRBs and Type Ic supernovae. I propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. I present spherically-symmetric hydrodynamic 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 one spatial dimension and with rotation, explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies ~ 5 x 10⁵⁰ ergs, velocities ~ 3000 km s⁻¹, and unbound material masses > 5 [solar mass] are possible in a rapidly rotating 16 [solar mass] main sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications. In many high-energy astrophysical phenomena, the force of radiation pressure will have a direct effect on the hydrodynamics. Observing radiation is also the primary way we investigate our universe. With this in mind, I present my expansion of the FLASH hydrodynamics code, where I have implemented a gray, flux-limited diffusion (FLD) radiation hydrodynamics (RHD) solver. My solver utilizes the FLASH's diffusion packages that are powered by HYPRE. I have written a new, efficient radiation-matter coupling solver, which exactly integrates the equations for radiation-matter coupling and operates without any time step restrictions. I have also rewritten the unsplit hydrodynamics solver in FLASH to incorporate the changes in PPM characteristic tracing and the Riemann solver required to properly capture the radiation pressure force in regions that are not entirely optically thick. This has required the addition of a new Riemann solver to FLASH, similar to the Riemann solver in the CASTRO RHD code. I then present my validation tests of the code. This code will be made publicly available.

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