Browsing by Subject "Black hole physics"
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Item Collapsar accretion and the gamma-ray burst X-ray light curve(2010-05) Lindner, Christopher Carl; Milosavljević, Miloš; Bromm, Volker; Kumar, Pawan; Sneden, Christopher A.; Wheeler, John C.We present axisymmetric hydrodynamical 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 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 [scientific symbols]. 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 with a sudden and rapid power-law decline in the central accretion rate Ṁ [proportional to] t⁻²̇⁸, which resembles the L[subscript x] [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 down, 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 can 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.Item Multidimensional multiscale dynamics of high-energy astrophysical flows(2010-05) Couch, Sean Michael; Wheeler, J. Craig; Milosavljević, Miloš; Bromm, Volker; Hoeflich, Peter; Jaffe, Daniel; Kumar, PawanAstrophysical flows have an enormous dynamic range of relevant length scales. The physics occurring on the smallest scales often influences the physics of the largest scales, and vice versa. I present a detailed study of the multiscale and multidimensional behavior of three high-energy astrophysical flows: jet-driven supernovae, massive black hole accretion, and current-driven instabilities in gamma-ray burst external shocks. Both theory and observations of core-collapse supernovae indicate these events are not spherically-symmetric; however, the observations are often modeled assuming a spherically-symmetric explosion. I present an in-depth exploration of the effects of aspherical explosions on the observational characteristics of supernovae. This is accomplished in large part by high-resolution, multidimensional numerical simulations of jet-driven supernovae. The existence of supermassive black holes in the centers of most large galaxies is a well-established fact in observational astronomy. How such black holes came to be so massive, however, is not well established. In this work, I discuss the implications of radiative feedback and multidimensional behavior on black hole accretion. I show that the accretion rate is drastically reduced relative to the Eddington rate, making it unlikely that stellar mass black holes could grow to supermassive black holes in less than a Hubble time. Finally, I discuss a mechanism by which magnetic field strength could be enhanced behind a gamma-ray burst external shock. This mechanism relies on a current-driven instability that would cause reorganization of the pre-shock plasma into clumps. Once shocked, these clumps generate vorticity in the post-shock plasma and ultimately enhance the magnetic energy via a relativistic dynamo process.Item Probing the nature of dark matter with the first stars and black holes(2022-06-14) Liu, Boyuan (Ph. D. in astronomy); Bromm, Volker; Ferrara, Andrea; Boylan-Kolchin, Michael; Offner, Stella S. R.; Finkelstein, Steven L.We use semi-analytical models and cosmological hydrodynamic simulations to derive the imprints of dark matter physics on the first generation of stars, black holes and galaxies, which, combined with observational data, place constraints on the properties of dark matter. Inspired by the recent detection of gravitational waves, we further investigate the gravitational waves of the first star remnants as a promising probe for early star/structure formation and dark matter physics. Our results indicate that although the first stars only make up a tiny fraction (~ 10⁻⁵) of all stars (ever formed) in the Universe, a much higher fraction (~ 10⁻³ -0.1) of massive compact object mergers can originate from the first stars, which carry valuable information of the early Universe. Finally, we explore the effects of primordial black holes, which can make up all or part of dark matter, on first star formation, and discuss their implications on using gravitational waves as a direct probe of dark matter.