* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Accretion mechanisms
Survey
Document related concepts
Perseus (constellation) wikipedia , lookup
Hawking radiation wikipedia , lookup
Formation and evolution of the Solar System wikipedia , lookup
History of Solar System formation and evolution hypotheses wikipedia , lookup
Modified Newtonian dynamics wikipedia , lookup
Star of Bethlehem wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Dyson sphere wikipedia , lookup
History of gamma-ray burst research wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Gamma-ray burst wikipedia , lookup
Negative mass wikipedia , lookup
Stellar evolution wikipedia , lookup
Transcript
Accretion Processes in GRBs Andrew King Theoretical Astrophysics Group, University of Leicester, UK Venice 2006 …. a rough guide to accretion mechanisms or …..some glimpses of the obvious • accretion on to a black hole or neutron star yields 10 20 erg/g • this is the most efficient way of extracting energy from normal matter • GRBs are (briefly) the brightest objects in the Universe accretion must power GRBs required mass M 10 20 E 0.1M sun E52 — a successful GRB model must explain why this mass accretes on to a black hole or neutron star on the observed timescale m M ~ stellar mass, so GRBs must involve disruption of a star on a short timescale two possibilities: 1. core collapse of a massive star to BH followed by accretion of significant stellar mass 2. dynamical—timescale disruption of a star by NS or BH companion m M ~ stellar mass, so GRBs must involve disruption of a star on a short timescale two possibilities: 1. core collapse of a massive star to BH followed by accretion of significant stellar mass — long burst 2. dynamical—timescale disruption of a star by NS or BH companion m M ~ stellar mass, so GRBs must involve disruption of a star on a short timescale two possibilities: 1. core collapse of a massive star to BH followed by accretion of significant stellar mass — long burst 2. dynamical—timescale disruption of a star by NS or BH companion — timescale for MS (hours) or WD (minutes) too long, but NS (milliseconds) can explain short bursts long burst differs from usual core—collapse SN because of rapid rotation – standard picture: collapsing core forms torus around black hole `viscosity’ leads to accretion ==> long burst, jets, shocks…… core of massive star Similarly, in compact object mergers, dynamical instability produces a hyperaccreting torus around the more compact star why torus? — angular momentum (it doesn’t take much) Similarly, in compact object mergers, dynamical instability produces hyperaccreting torus around the more compact star why torus? — angular momentum (it doesn’t take much) why hyperaccreting? — good question standard answer — `viscosity’ does the magnetorotational instability work under these conditions? note that `viscosity’ has to form the torus as well as drive accretion H M disc M BH , so self—gravity is important R local physics is extremely complex — nuclear reactions, turbulence, magnetic fields, ….. all in general—relativistic context inherently 3D impossible to capture all of these in one code accretion is complicated accretion is complicated so let’s ignore it Paradigm: model accretion as effectively instantaneous, and just consider its after—effects — fireball this is highly successful but every paradigm has its limitations e.g. some bursts show late, energetic activity simplest possibility: burst `starts again’ since late activity can be comparable to original burst this requires significant mass to accrete at late times — i.e. accretion flow fragments (kinetic energy)/(binding energy) ~ 1/(lengthscale of collapsing object) , so grows during collapse ? analogy with star formation – stars form in clusters since cooling gas clouds fragment (Hoyle 1953) argument: gas pressure cannot resist gravity over lengthscales l ~ cst freefall ~ cs (G ) 1/ 2 so self—gravitating condensations appear, with mass MJ ~ ~ c 3 3 s 1 / 2 as collapse proceeds, density increases. If gas can cool efficiently temperature stays ~ constant (isothermal), so MJ ~ 1 / 2 decreases as collapse proceeds, ==> fragmentation process stops once fragments become opaque, so cooling is slow (adiabatic), ==> cs ~ ( P / ) 1/ 2 so that MJ ~ ~ 3 / 2 ( 4 / 3) ( 1) / 2 now increases as increases Fragmentation cannot occur below a mass M F M chandra(kT / mp c ) 2 1/ 4 (Rees, 1976) where T is temperature when fragment becomes opaque. for likely conditions, thermal neutrino emission is energetically 11 important, limiting temperature to T ~ 10 K M F 0.1 0.5M sun Thus can have BH ( M1 ) + torus + clump (M 2 ) BH + torus makes 1st burst, clump dragged in by GR from radius timescale 4 0 ~a ~ j 8 ~ 10 minutes for 0 j0 ~ 1017 cgs. • clump swallowed whole (no radiation) if does not contact tidal (Roche) lobe before reaching ISCO of BH. Rhorizon M sun • this occurs if M 1 10 RISCO i.e. high BH mass (> 10) or slow spin (a ~ 0) ==> no flare • otherwise mass transfer from clump to BH a0 To make late flare, mass transfer must disrupt clump to make torus i.e. mass transfer in `binary’ must become dynamically unstable Very similar to merger picture for short bursts! Tidal interaction with torus can make orbit wider and eccentric episodic mass transfer Stability ultimately given by comparing Roche lobe radius with clump radius R M as mass is transferred 2 RL 2 RL R2 2M2 5 M2 2 J RL R2 M 2 6 2 M1 J (similar expressions if clump does not corotate). Angular momentum term in J includes GR (slow), plus dynamical—timescale contributions if transferred matter cannot form a disc — occurs when mass ratio clump/BH too large stable mass transfer (no flare) if 0 : M 2 , J 0, (.....) 0 Dynamical instability requires 0 with clump in contact. Inevitable if (……) < 0 Thus flare occurs either when (a) clump is large (large mass ratio) or (b) clump mass drops to mass loss, i.e. ~ 0.2M sun 5 3 and expands strongly on dynamical instability or not depends on equation of state through mass—radius index and tidal angular momentum feedback can have stable accretion followed by instability cf re—energizing followed by flare? all such effects need proper calculation if they are not there, we have learnt something