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Maxim Barkov Space Research Institute, Russia University of Leeds, UK, Serguei Komissarov

Very long GRBs. Maxim Barkov Space Research Institute, Russia University of Leeds, UK, Serguei Komissarov University of Leeds, UK. I. Gamma-Ray-Bursts. Discovery: Vela satellite (Klebesadel et al.1973); Konus satellite (Mazets et al. 1974);

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Maxim Barkov Space Research Institute, Russia University of Leeds, UK, Serguei Komissarov

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  1. HEA-2008, IKI Very long GRBs Maxim Barkov Space Research Institute, Russia University of Leeds, UK, Serguei Komissarov University of Leeds, UK

  2. HEA-2008, IKI I. Gamma-Ray-Bursts Discovery: Vela satellite (Klebesadel et al.1973); Konus satellite (Mazets et al. 1974); Cosmological origin: 1. Beppo-SAX satellite – X-ray afterglows (arc-minute resolution) , optical afterglows – redshift measurements – identification of host galaxies (Kulkarni et al. 1996, Metzger et al. 1997, etc)‏ 2. Compton observatory – isotropic distribution (Meegan et al. 1992); Supernova connection of long duration GRBs: 1. Association with star-forming galaxies/regions of galaxies; 2. Few solid identifications with supernovae, SN 1998bw, SN 2003dh and others... 3. SN bumps in light curves of optical afterglows. 4. High-velocity supernovae ( 30,000km/s) or hypernovae (1052erg).

  3. HEA-2008, IKI Spectral properties: Non-thermal spectrum from 0.1MeV to GeV: Bimodal distribution (two types of GRBs?): long duration GRBs short duration GRBs very long duration GRBs

  4. HEA-2008, IKI Relativistic jet/pancake model of GRBs and afterglows: jet at birth pancake later

  5. HEA-2008, IKI II. Models of central engines (1) Potential of disk accretion onto stellar mass black holes: disk binding energy: thin disk life time: - accreted mass Ultra-dense Hyper-Eddington neutrino cooled disks !!! - disk outer radius - gravitational radius  Shakura–Sunyaev parameter

  6. HEA-2008, IKI GRBs Jet magnetic acceleration: • We get MHD acceleration of relativistic jet up to ≈300 • Conversion of magnetic energy to kinetic one more than 50% • Acceleration have place on long distance req≈2rlc • Jet is very narrow θ<2 • (Astro-ph:0811.1467)

  7. HEA-2008, IKI III. Magnetic Unloading That is criteria of explosion start? How it happens?

  8. HEA-2008, IKI The disk accretion makes easier the explosion conditions. The MF lines shape reduce local accretion rate. MNRAS (2008) 385L, 28 and arxiv:0809.1402

  9. HEA-2008, IKI IV. Realistic initial conditions • Strong magnetic field suppressdifferential rotation in the star (Spruit et. al., 2006). • Magnetic dynamo can’t generate big magnetic flux (???), relict magnetic field is necessarily? • In close binary systems we could expect fast solid body rotation. • The most promising candidate for long GRBs is Wolf-Rayet stars.

  10. HEA-2008, IKI Simple model: If l(r)<lcr then matter falling to BH directly If l(r)>lcr then matter goes to disk and after that to BH Agreement with model Shibata&Shapiro (2002) on level 1% BH

  11. HEA-2008, IKI Power low density distribution model

  12. HEA-2008, IKI Realistic model Heger at el (2004) M=35 Msun, MWR=13 Msun

  13. HEA-2008, IKI Realistic model Realistic model Heger at el (2004) M=20 Msun, MWR=7 Msun M=35 Msun, MWR=13 Msun neutrino limit BZ limit

  14. HEA-2008, IKI V. Numerical simulations: Collapsar model GR MHD 2D Setup black hole M=10 Msun a=0.45-0.6 v Bethe’s free fall model, T=17 s, C1=23 B v v v Dipolar magnetic field v Initially solid body rotation B Uniform magnetization R=150000km B0= 1.4x107-8x107G

  15. HEA-2008, IKI Griped MF and angular momentum

  16. HEA-2008, IKI a=0.6 Ψ=3x1028 a=0.45 Ψ=6x1028

  17. HEA-2008, IKI VI. Conclusions • The Collapsar is promising models for the central engines of GRBs. • Theoretical models are sketchy and numerical simulations are only now beginning to explore them. • Our results suggest that: + Black holes of failed supernovae can drive very powerful GRB jets via Blandford-Znajek mechanism if the progenitor star has strong poloidal magnetic field; + Blandford-Znajek mechanism of GRB has much lower limit on accretion rate to BH then neutrino driven one (excellent for very long GRBs); - Blandford-Znajek mechanism needs very hight magnetic flux or late explosion; ± All Collapsar model need high angular momentum, common envelop stage could help.

  18. HEA-2008, IKI IV. Discussion • Magnetically-driven stellar explosions require combination of • fast rotation of stellar cores and (ii) strong magnetic fields. • Can this be achieved? • Evolutionary models of solitary massive stars show that even much • weaker magnetic fields (Taylor-Spruit dynamo) result in rotation being • too slow for the collapsar model (Heger et al. 2005) • Low metallicity may save the collapsar model with neutrino mechanism • (Woosley & Heger 2006) but magnetic mechanism needs much stronger • magnetic field. • Solitary magnetic stars (Ap and WD) are slow rotators (solid body rotation).

  19. HEA-2008, IKI • The Magnetar model seems OK as the required magnetic field can be • generated after the collapse via a-W dynamo inside the proto-NS • (Thompson & Duncan 1995) • The Collapsar model with magnetic mechanism. Can the required • magnetic field be generated in the accretion disk? - turbulent magnetic field (scale ~ H, disk height) - turbulent velocity of a-disk Application to the neutrino-cooled disk (Popham et al. 1999):

  20. HEA-2008, IKI Inverse-cascade above the disk (Tout & Pringle 1996) may give large-scale field (scale ~ R) This is much smaller than needed to activate the BZ-mechanism! • Possible ways out for the collapsar model with magnetic mechanism. • (i) strong relic magnetic field of progenitor, Y=1027-1028 Gcm-2; • (ii) fast rotation of helium in close binary or as the result of • spiral-in of compact star (NS or BH) during the common • envelope phase (e.g. Zhang & Fryer 2001 ). In both cases • the hydrogen envelope is dispersed leaving a bare helium core.

  21. HEA-2008, IKI • Required magnetic flux , Y=1028 Gcm-2, still exceeds the highest • value observed in magnetic stars. • Accretion rate through the polar region can strongly decline • several seconds after the collapse (Woosley & MacFadyen 1999), • reducing the magnetic flux required for explosion (for solid rotation • factor 3-10, not so effective as we want); • Neutrino heating (excluded in the simulations) may also help to • reduce the required magnetic flux; • Magnetic field of massive stars is difficult to measure due to strong • stellar winds – it can be higher than Y=1027 Gcm-2 ; • Strong relic magnetic field of massive stars may not have enough time • to diffuse to the stellar surface, td ~ 109 yrs << tevol , • (Braithwaite Spruit, 2005)

  22. HEA-2008, IKI log10  (g/cm3), magnetic field lines, and velocity vectors

  23. HEA-2008, IKI log10 

  24. HEA-2008, IKI log10 B/Bp log10 B

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