Numerical Studies of Accretion Disks, Jets, and Gamma Ray Bursts

1. Numerical Studies of Accretion Disks, Jets, and Gamma Ray Bursts Christopher C. Lindner1 1 University of Texas at Austin Abstract Jet Creation in Accretion Disk Systems Gamma Ray Bursts Relativistic Jets are streams of plasma moving throughout the universe at speeds up to 99% the speed of light. They produce high-energy X-ray, radio, and visible radiation that are observable from great distances across the universe. (see fig. 1)Many of these jets have been attributed to black hole accretion disk systems. In these systems a disk of material surrounds a black hole and through magnetic processes the material accretes onto the black hole. Strong outflows are collimated into the fantastic jets we observe. Gamma ray bursts are extremely energetic outbursts observable from across the universe. Long duration gamma ray bursts have been found to be associated with the deaths of very large stars. The favored models for these is the “core collapse supernova,” where the massive core of the star collapses into a black hole and black hole accretion powers a luminous jet. Despite numerous observations, many of the details in the underlying physics of these objects are poorly understood. The luminosity of these objects varies greatly at late times, but we do not understand if this is a result of activity near the central engine (e.g. the black hole) or turbulent activity further out as the jet interacts with stellar, interstellar, or intergalactic material. To answer these questions, we aim to perform high resolution hydrodynamic simulations of gamma ray burst progenitors (core collapse supernova). We inject powerful (> 1051 erg/s) jets into a star, and track the evolution of the system under gravitational and hydrodynamic forces. Figure 2: Early jet breakout (left) and late jets (right) in a black hole accretion disk simulation. Contours represent unbound jet material and the disk. Lines show magnetic field lines. A wide variety of fast-moving, high energy streams of plasma have been observed in almost every wavelength of light. Although it is counterintuitive, it is believed that many of these features originate near some of the strongest gravitational sinks in the universe: black holes. The physical processes surrounding these phenomena are quite intriguing, yet much cannot be determined from observational study alone. The production of these jets is intricately linked to the rotation of the black hole and its surrounding accretion disk - the disk of material surrounding the black hole. Previous work has studied these systems where these two rotations were aligned. However, these simplified models cannot account for the precessing jets of objects such as SS433 (see fig. 3), and fail to fully explain which processes set the orientation or power the jet. By breaking this degeneracy in a system where the two rotations are not aligned, we may study these processes in more detail and test a possible explanation for observed precessing systems. We may also learn more about the origin of other black hole accretion disk phenomena, such as gamma ray bursts. Our project studies this work through the use of high resolution, general relativistic magnetohydrodynamic simulations, run in parallel on supercomputing resources such as UT Austin’s / TACC’s Ranger. We are the first group to study these tilted systems in detail (see fig. 2). Figure 1: The neutron star, disk and jet known as the crab nebula (top) and the jet originating near the super massive black hole of M87 (bottom). From NASA. Gamma ray bursts (GRB) are an especially exciting outflow phenomena that are capable of outputting as much energy as our sun will produce in its lifetime in a few seconds. Many of these intriguing objects are also believed to be powered by black hole accretion disks, specifically originating from the death of very massive stars, when the center of the star collapses into a black hole. Although we have much observational data for these events our physical understanding of these systems is lacking and further theoretical study is necessary. Here we present our studies of jets and GRB using high resolution hydrodynamic and magnetohydrodynamic simulations, including investigations of late time GRB evolution and jet production, power, and orientation. 2 x 1011 cm 2 x 1011 cm Figure 4: Logarithmic density plots of a slice of a star 5 s (left) and 25 s (right) after large amounts of energy have been injected to simulate a gamma ray burst. The surface of the star is at 2 x 1011 cm. Notice that much of the star is already disrupted. About the Author Acknowledgements Figure 3: A tilted disk (left). Here the black hole spin axis would be pointed upward. Blue contours show unbound jet material, and red contours show the disk. Lines represent magnetic field lines. In simulation, the disks in these systems precess – the entire system rotates around the spin axis of the black hole (vertical here). These may explain the spiral patterns of jets like that observed in SS433 (right), shown in a 4.85 Ghz intensity map, taken from Blundell and Bowler, 2004. This work was performed in collaboration with Chris Fragile of College of Charleston, and MilosMilosavljevicand Sean Couch of UT Austin. Simulations were performed using the Cosmos++ and FLASH fluid dynamics codes. The software used in this work was in part developed by the DOE-supported ASC/Alliance Center for Astrophysicsal Thermonuclear Flashes at the University of Chicago, using resources at CofC and TACC. We gratefully acknowledge the support of Faculty R&D and SURF and RPG grants from the College of Charleston and a REAP Grant from the South Carolina Space Grant Consortium. Chris Lindner is a first year astronomy graduate student at the University of Texas at Austin. Chris graduated Magna Cum Laude from the College of Charleston with degrees in Astronomy and Physics, and was the first (and currently only) astronomy graduate in the state of South Carolina.