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What is a Gamma-Ray Burst?

Short g -ray flashes E > 100 keV 0.01 < t 90 < 1000s Diverse lightcurves BATSE detected 1/day = 1000 /year/universe Energy ~ 10 52 f g -1 f W/0.1 erg. Near star forming regions 2 SN Ibc associations Supernova component in lightcurves. What is a Gamma-Ray Burst?.

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What is a Gamma-Ray Burst?

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  1. Short g-ray flashes E > 100 keV 0.01 < t90 < 1000s Diverse lightcurves BATSE detected 1/day = 1000 /year/universe Energy ~ 1052 fg-1 fW/0.1erg Near star forming regions 2 SN Ibc associations Supernova component in lightcurves What is a Gamma-Ray Burst?

  2. Superbowl Burst GRB Light Curve M = E/ G c2 ~ 10-6 Msun ms variability + non-thermal spectrum Compactness G > 100

  3. 3. Palomar < 1 day 2. BeppoSAX (X-ray) Keck spectrum z=1.60 Eiso = 3x1054 erg ~ Msunc2 9th mag flash 6-33 hrs GRB 990123 34-54 hrs 4. HST 17 days ~ 1’ 1. CGRO ~1o

  4. 135 models (1993) Note: most are Galactic and are ruled out for long bursts

  5. Hyper-accreting black hole or high field neutron star (rotating) GRB photons are made far away from engine. Can’t observe engine directly in light. (neutrinos, gravitational waves?) Electromagnetic process or neutrino annihilation to tap power of central compact object.

  6. Well-localized bursts are all “long-soft” “short-hard” bursts ? hardness Duration (s) Kulkarni et al

  7. SN 1998bw/GRB 980425 NTT image (May 1, 1998) of SN 1998bw in the barred spiral galaxy ESO 184-G82 [Galama et al, A&AS, 138, 465, (1999)] WFC error box (8') for GRB 980425 and two NFI x-ray sources. The IPN error arc is also shown. 1) Were the two events the same thing? 2) Was GRB 980425 an "ordinary" GRB seen off-axis?

  8. GRB991121 Bloom et al (ApJL,2002)

  9. GRB030329/SN2003DH see also: Hjorth et al , Fox et al Nature (2003) extremely close = 800 Mpc

  10. SN 1998bw/GRB 980425 The supernova - a Type Ic - was very unusual. Large mass of 56Ni 0.3 - 0.9 solar masses;(note: jets acting alone do not make 56Ni)Sollerman et al, ApJL, 537, 127 (2000) McKinzie & Schaefer, PASP, 111, 964, (1999)Extreme energy and mass > 1052 erg > 10 MsunIwamoto et al., Nature, 395, 672 (1998) Woosley, Eastman, & Schmidt, ApJ, 516, 788 (1999) Mazzali et al, ApJ, 559, 1047 (2001) Exceptionally strong radio sourceLi & Chevalier, ApJ, 526, 716, (1999) Relativistic matter was ejected 1050 - 1051 ergWieringa, Kulkarni, & Frail, A&AS, 138, 467 (1999) Frail et al, ApJL (2001), astroph-0102282 Probability favors the GRB-SN associationPian et al ApJ, 536, 778 (2000)

  11. Merging neutron star - black hole pairs Strengths: a) Known event b) Plenty of angular momentum c) Rapid time scale d) High energy e) Well developed numerical models Ruffert & Janka, Rosswog et al, Lee et al, Aloy et al Weaknesses: a) Outside star forming regions b) Beaming and energy may be inadequate for long bursts But this model may still be good for a class of bursts called the “short hard” bursts for which we have no counterpart information yet (SWIFT).

  12. Requirements on the Central Engine and its Immediate Surroundings (long-soft bursts) • Provide adequate energy at high Lorentz factor • Collimate the emergent beam to approximately 0.1 radians • In the internal shock model, provide a beam with rapidly variable Lorentz factor • Allow for the observed diverse GRB light curves • Last approximately 10 s, but much longer in some cases • Explain diverse events like GRB 980425 • Produce a (Type Ib/c) supernova in some cases • Make bursts in star forming regions

  13. GRB central engine: • Relativity (SR & GR) • Magnetic Fields • Rotation (progenitors) • Nuclear Physics • Neutrinos • EOS • Turbulence • 3D • Range of Lengthscales

  14. “Delayed” SN Explosion Accretion vs. Neutrino heating Burrows (2001) ac Muller (1999)

  15. Pre-Supernova Density Structure Bigger stars: Higher entropy Shallower density gradients Woosley & Weaver (1995)

  16. Failure of delayed mechanism Bigger stars: 1. Accrete faster & longer 2. Larger binding energy & smaller explosion energy explosion binding Fryer, ApJ, 522, 413 (1999), Burrows (1999)

  17. Stellar Rotation Fukuda (1982) no mass loss Mass loss Heger (2000) No B fields

  18. Collapsars A rotating massive star whose core collapses to a black hole and produces an accretion disk. Type Mass/sun BH Time Scale Distance Comment I 15-40 He prompt 20 s all z neutrino-dominated disk II 10-40 He delayed 20 s – 1 hr all z black hole by fall back III >130 He prompt ~20 s z>10? time dilated, redshifted *(1+z) very energetic, pair instability, low Z Type I is what we are usually talking about. The 40 solar mass limit comes from assuming that all stars above 100 solar masses on the main sequence are unstable (except Pop III).

  19. IF Two plausible conditions occur: 1. Failure of neutrino powered SN explosion a. complete b. partial (fallback) 2. Rotating stellar cores j > 3 x 1016 cm2/s THEN Rapidly accreting black hole, (M~0.1 M/s) fed by collapsing star (tdyn ~ 446 s/ ½ ~ 10 s) Disk formation COLLAPSAR

  20. Collapsar Simulations: • pre-SN 15 Msun Helium star • Newtonian Hydrodynamics (PPM) • alpha viscosity • rotation • photodisintegration (NSE alpha, n, p) • neutrino cooling, thermal + URCA optically thin • Ideal nucleons, radiation, relativistic degenerate electrons, positions • 2D axisymmetric, spherical grid • self gravity, pseudo-Newtonian (PW) • Rin = 9 Rs Rout = 9000 Rs MacFadyen & Woosley (1999):

  21. Collapsar Disk Animation PPM hydrodynamics, Paczynski-Witta potential, EOS, neutrino cooling, nuclear reactions, a viscosity Stellar collapse w/ rotation. Density structure. No disk, no wind. Note: Accretion shock, funnel clearing, pole to equator density contrast, fluctuating polar density Initial model: 15 Msun Helium (Wolf-Rayet) star evolved with mass loss. R= 8 x 108 cm Show inner 1% in radius disk mass = .001 M_sun Low viscosity a=.001

  22. Disk Formation Movie

  23. Accretion Shock, Disk formation t = .75 s neutrino coolong allows accretion no cooling=> dynamically unstable CDAF? Could emit GWs but maybe no GRB Photodisintegration Si,O,C -> free neutrons And protons Enhanced neutrino cooling

  24. a = 0.1 <M> = 0.07 Msun /s = 1.3 x 1053 erg/s

  25. Use 1D neutrino cooled “slim” disk models from Popham et al (1999). spin mass

  26. Collapsar results • Sustained accretion >10s • Sufficient energy • Time scale set by He core collapse • Disk-feeding time scale not disk-draining • Neutrino cooling allows accretion • Neutrino annihilation energetically possible • calculable in any case

  27. Funnel geometry channels any fireball. Density contrasts can be huge.

  28. Jet Birth Thermal energy deposition focused by toroidal funnel structure T = 5.7 ms E = 5 x 1050 erg/s Edep = 2.8 x 1048 erg . . Ejet = f Maccc2 fmax ~ .06 - .4 MHDnn

  29. Relativistic Jet Movie

  30. Collapsar stages • Iron core collapse, disk formation T~1010K, r~108gcm-3, photodisintegration, n cooling, pair capture, disk is free nucleons (2 s) • Polar density declines to allow jet birth ½rv3 Edep(2-5 s) • Jet tunnels out of star (5 s) Wolf-Rayet • Jet powered for ~10 more seconds. Evacuates polar channel and reaches asymptotic speed. (10 s) T_GRB  T_collapse

  31. Type Ib or Ic Supernova Red Supergiant R~1013 cm Wolf-Rayet Star R~1011 cm Blue Supergiant R~1012 cm

  32. Supernovae Type I No Hydrogen Type II Hydrogen Ia WD cosmology Ib, Ic exploding WR core collapse massive stars thermonuclear old pop. E galaxies

  33. “Nickel Wind” Nickel Wind Movie T > 5 x 109 K

  34. Fallback in weak SN explosions Shock reaches surface of star but parts of star are not ejected to infinity.

  35. Fallback accretion Mms ~ 25 Msun Same star exploded with a range of explosion energies. Significant accretion for thousands of seconds – days.

  36. If fallback fuels a jet with power fmc2 May power “hypernova” or long duration GRB Weak supernova shock

  37. Shock breakout X-Ray transient

  38. What made SN1998bw+GRB980425? 1. Accretion powered hypernova w/ Nickel windMacFadyen (2002) E~ 1052 erg, M(Ni)~0.5 M 2. “Brief” jet tengine tjet Engine dies before jet breakout. Mildly relativistic shock breakout GRB from G~3 shock breakout (Tan et al 2001, Perna & Vietri 2002) MacFadyen (1999)

  39. Collapsars • Can make “long” GRBs in H stripped (WR) stars. tengine > tescape • Short bursts may be compact binary mergers. • Need SN failure & angular momentum • Low metallicity, binary can help • Star can explode -> SN if nickel is made. Predicts GRB/SN association. Type Ibc. • SN/GRB ratio may depend on angular momentum. • “Nickel wind” can explode star -> hypernova • H env. Type II (no GRB), no H Type I + GRB

  40. GRB/GW • Long GRBs • not brighter than SN in GW? • very far Gpc • very rare < 1% SN • Short GBs • merging ns-bh binaries? • maybe closer than long bursts • short delay between event and GRB? • good for SWIFT/LIGO

  41. Rates • SN: 1/s = 100,000 /day • GRB: 1/day (BATSE) = 1,000/day • GRB rate = 1% of SN rate • maybe more collapsars than GRBs • => more rapidly rotating SN • SN with collapsar engine • look for bright Type Ic (w/ broad lines)

  42. SN GW • SN1998bw/GRB980425 • 40 Mpc • maybe dominant GRB • rapid rotaters • SNAP/ROTSE look for 1998bw 2003dh like SN • many light curves -> better t_explode

  43. Implications • Probe engine directly • collapse duration vs. GRB duration • collapse/GRB delay (internal vs. external?) • disk properties – low viscosity? big disks?

  44. Issues • too much j => no GRB? but bright GW? • may need low metallicity for GRB • prefer high redshift • don’t know nearby rate • but 980425 may imply rate is high • look for weak GRBs like 980425

  45. Principle Results • Sustained accretion .1 Msun/s for>10s • Jet formation and collimation • Sufficient energy for cosmo. GRB • Neutrino cooling & photodissociation allows accretion • Massive bi-conical outflows develop • Time-scale set by He core collapse • Fallback -> v. long GRB in WR star or asymmetric SN in SG

  46. Black hole formation may be unavoidable for low metallicity Solar metallicity Low metallicity With decreasing metallicity, the binding energy of the core and the size of the silicon core both increase, making black hole formation more likely at low metallicity. Woosley, Heger, & Weaver, RMP, (2002)

  47. The more difficult problem is the angular momentum. This is a problem shared by all current GRB models that invoke massive stars... In the absence of mass loss and magnetic fields, there would be abundant progenitors. Unfortunately nature has both. 15 solar mass helium core born rotating rigidly at f times break up

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