Neutrino Cooled Accretion Disk as the Central Engine of Gamma Ray Bursts

1. Neutrino Cooled Accretion Disk as the Central Engine of Gamma Ray Bursts N. Kawanaka, S. Mineshige & S. Nagataki (Yukawa Institute for Theoretical Physics) APCTP Winter School on Black Holes Astrophysics 2006, Jan. 17-20 Abstract : It is often theoretically hypothesized that the enormous power released from gamma ray bursts is generated by the accretion of a massive torus with approximately solar mass onto a stellar-mass black hole. Such a torus (or a disk) would become so dense and hot that the main cooling process would be neutrino radiation. We study the steady-state structure of this disk considering the degeneracy of electrons and neutrinos, the opacity of neutrinos, and the equation of state of nuclear matter. The stability of the disk is also discussed. Neutrino reactions §1. Introduction Gamma Ray Burst (GRB)： ・ duration : ～0.01-1000 sec ・ the energy of gamma ray : 　～10keV-1MeV ・ highly time-varying lightcurve ・ relativistic jet? (compactness problem, afterglow etc.) ・ total energy : ～1051 erg §3. Results Piran 1999 solid line, dashed line, dot-dashed line : mass accretion rate = 0.1Msun/s, 1.0Msun/s, 10Msun/s What is the central engine? Neutrino Dominated Accretion Flow （NDAF） density temperature Gravitationally unstable region ・ The remnant of the collapse of a massive star, NS-NS merger etc. ・ ρ～1012 g/cm3 ・T ～1011 K ・ mainly cooled via neutrino radiation Cooling process thick line : neutrino radiation thin line : advection Liberate the energy via neutrino pair annihilation in the baryon-poor region Low accretion rate:advection dominant in outer region, neutrino dominant in inner region High accretion rate:advection dominant because of neutrino trapping FireballFormation？ BH Accretion disk • Previous Studies： • neutrino-thin approximation (Popham, Woosley & Fryer 1999, Narayan, Piran & Kumar 2001, Kohri & Mineshige 2002) • taking into account the opacity of neutrino (Di Matteo, Perna & Narayan 2002, Kohri, Narayan & Piran 2005) Neutrino luminosity thick line:electron neutrino thin line:anti-electron neutrino The higher the mass accretion rate is, the flatter the slope of neutrino luminosity become…neutrino trapping? We want to know the structure of NDAF taking into account the effects of EOS, lepton conservation and neutrino flavors. §4. Discussions §2. The Model Assumptions ・ With the mass accretion rate around 10Msun/s, the outer part of the disk would be gravitaionally unstable→fragmentation→late time activity? (cf. X-ray flare in the early afterglow) ・ The neutrino luminosity from the whole disk is Lν～1053 erg/s. ・ Taking into account the cross section of pair annihilation, the liberated energy would be not enough to explain the observed energy of GRBs → another liberation process? (eg. BZ effect) ・ Future Works: 1. Solve neutrino transfer more precisely, 2. Constitute the transonic solution, 3. Thermal stability, 4. Time-dependent analysis, 5. Take into account the convection, etc. ・ The central black hole…M = 3Msungravity : Newtonian ・ Hydrostatic equilibrium, mass accretion rate=const. (～1Msun /sec) ・ α-viscosity…Trφ = αP ・ Energy balance : Q+vis=Q-ν+Q-rad+Q-adv ・ βequilibrium :μp+μe=μn+μν ・ Equation of State : Lattimer & Swesty (1991) ・ Neutrino transfer : Two stream approximation (Di Matteo et al. 2002)