Investigating Nucleosynthesis During Formation of Black Holes and Extracting Their Properties

Abstract

Measurement of black hole spins is of great interest to physicists in general and to astrophysicists in particular, as this provides a direct proof of the theory of general relativity for rotating black holes. It involves probing the innermost region of the accretion disk around the black hole where general relativistic effects play a dominant role. Apart from that, measuring spins of supermassive black holes in active galactic nuclei (AGNs) is further important because it throws light on the relative role of gas accretion versus mergers in recent eras of the life of the host galaxy and its AGN. Recent developments in theory and observation have enlightened the spin for some supermassive black holes, although this science is still in its infancy. We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, the core collapse of massive stars first leads to the formation of a proto-neutron star. After that, an outward-moving shock triggers a successful supernova. However, the supernova ejecta lack momentum, and within a few seconds, the newly formed neutron star gets transformed into a stellar-mass black hole via massive fallback. The hydrodynamics of such an accretion disk formed from the fallback material of the supernova ejecta has been studied extensively in the past. We use these well-established hydrodynamic models for our accretion disk in order to understand nucleosynthesis, which is mainly advection-dominated in the outer regions. Neutrino cooling becomes important in the inner disk where the temperature and density are higher. The higher the accretion rate (M?\dot{M}M?) is, the higher the density and temperature are in the disks. We deal with accretion disks with relatively low accretion rates: 0.001M??s?1<M?<0.01M??s?10.001 M_\odot \, s^{-1} < \dot{M} < 0.01 M_\odot \, s^{-1}0.001M??s?1<M?<0.01M??s?1, and hence these disks are predominantly advection-dominated. We use He-rich and Si-rich abundances as the initial condition of nucleosynthesis at the outer disk, and being equipped with the disk hydrodynamics and the nuclear network code, we study the abundance evolution as matter inflows and falls into the central object. We investigate the variation in the nucleosynthesis products in the disk with the change in the initial abundance at the outer disk and also with the change in the mass accretion rate. We report the synthesis of several unusual nuclei like 31P^{31}\text{P}31P, 39K^{39}\text{K}39K, 43Se^{43}\text{Se}43Se, 35Cl^{35}\text{Cl}35Cl, and various isotopes of titanium, vanadium, chromium, manganese, and copper. We also confirm that isotopes of iron, cobalt, nickel, argon, calcium, sulphur, and silicon get synthesized in the disk, as shown by previous authors. Much of these heavy elements thus synthesized are ejected from the disk via outflows and hence they should leave their signature in observed data. We investigate nucleosynthesis inside the outflows from gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, massive stars undergo core collapse to form a proto-neutron star initially, and a mild supernova (SN) explosion is driven. The SN ejecta lack momentum, and subsequently, this newly formed neutron star gets transformed into a stellar-mass black hole via massive fallback. The hydrodynamics and the nucleosynthesis in these accretion disks have been studied extensively in the past. Several heavy elements are synthesized in the disk, and much of these heavy elements are ejected from the disk via winds and outflows. We study nucleosynthesis in the outflows launched from these disks by using an adiabatic, spherically expanding outflow model, to understand which of these elements thus synthesized in the disk survive in the outflow. While studying this, we find that many new elements like isotopes of titanium, copper, zinc, etc., are present in the outflows. 56Ni^{56}\text{Ni}56Ni is abundantly synthesized in most of the cases in the outflow, which implies that the outflows from these disks in a majority of cases will lead to an observable SN explosion. It is mainly present when outflow is considered from the He-rich, 56Ni/54Fe^{56}\text{Ni}/^{54}\text{Fe}56Ni/54Fe-rich zones of the disks. However, outflow from the Si-rich zone of the disk remains rich in silicon. Although emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs by Chandra, BeppoSAX, XMM-Newton, etc., Swift seems to have not yet detected these lines. Stellar-mass black holes, forming by the core collapse of very massive, rapidly rotating stars, are expected to exhibit a high-density accretion disk around them developed from the spinning mantle of the collapsing star. A wide class of such disks, due to their high density and temperature, are effective emitters of neutrinos and hence called neutrino-cooled disks. Tracking the physics relating the observed (neutrino) luminosity to the mass, spin of black holes, and the accretion rate (M?\dot{M}M?) of such disks, here we establish a correlation between the measurement of black hole spins and the disk properties. Measurement of black hole spins is of great interest to physicists in general and to astrophysicists in particular, as this provides a direct proof of the theory of general relativity for rotating black holes. It involves probing the innermost region of the accretion disk around the black hole where general relativistic effects play a dominant role. Apart from that, measuring spins of supermassive black holes in active galactic nuclei (AGNs) is further important because it throws light on the relative role of gas accretion versus mergers in recent eras of the life of the host galaxy and its AGN. Recent developments in theory and observation have enlightened the spin for some supermassive black holes, although this science is still in its infancy. In this scenario, we present a unique method based on the proposal described in the previous chapter, to predict the spins of several supermassive black holes in the Palomar-Green Quasar sample, which further helps us to predict which of these systems will harbor jets and outflows.