Agnieszka Janiuk

1. Agnieszka Janiuk N. Copernicus Astronomical Center, Warsaw Gamma Ray Bursts from Collapsing Massive Stars Collaborations: R. Moderski (CAMK), D. Proga(UNLV), Y. Yuan (ChAS), R. Perna (JILA), T. Di Matteo (CMU), B. Czerny (CAMK), D. Cline (UCLA), S. Otwinowski (CERN), C. Matthey (CERN)

2. To emit fireball, the engine must be very energetic. To produce shocks, the engine must be active and variable for a long time 1014 cm Gamma rays are believed to come from the internal shocks, produced in the relativistic (Γ>100) fireball.

3. Digression: what makes the jet? • Jets are ubiquitous in nature: AGNs, QSOs, XRBs, YSOs, GRBs... • They are not required by any physical law (such as energy conservation). • The 3 proposed mechanisms of jet acceleration: • Radiation pressure • Thermal expansion • Magnetic fields and rotation • Jet is domineted by • Poynting flux (small scales) • Matter (large scales) • Jets are collimated by: • Accretion disk/coronal walls • Pressure gradients in the environment • Surrounding matter-dominated jet • Poynting-jet is able to collimate itself through the toroidal B field Fragile, 2008 (arXiv:0810.0526)

4. The model of a central engine for GRB must answer, which astrophysical process produces the relativistic fireball that emits gamma-rays. • Important constraints (Piran 2005): • Timescales and variability: dt/T ~ 10-3 – 10-4 for 80% of bursts • Short and long GRB dichotomy • Short - hard GRBs (T90<2 s)‏ • Long -soft GRBs (T90 > 2s)‏ • Energy (significant fraction of the binding energy for compact object)‏ • Collimation(1o < θ < 20o)‏ • Rates(about 3 ×10-5 per year per galaxy)‏

5. To emit fireball, the engine must be very energetic. To produce shocks, the engine must be active and variable for a long time. The most popular model invokes the internal shocks in the jet that produce gamma rays and variability (Sari & Piran 1997). Also, variability can be well reproduced with a shot-gun model (Heinz & Begelman 1999). Janiuk, Czerny, Moderski et al. (2006)‏ Kinematic jet model: theoretical lightcurves of long GRBs, depending on the observer’s viewing angle Tested against observations: lightcurves, PDS spectra; Prokopiuk & Janiuk, in prep.

6. Collapse of massive star favored for long GRBs: • - associacion with star forming galaxies (e.g. Fruchter et al. 2006)‏ • - concurrent “SN-like” outbursts (Bloom et al. 1999; Stanek et al. 2003)‏ • - redshift distribution follows the star formation rate (Coward 2007)‏

7. Supernovae • type I: rapid lightcurve evolution • ‏ - type Ia: standard; • - type Ib: He lines produced in the massive ejecta, by non-thermal excitation by fast particles emitted by the (56)Ni -> (56)Co-> (56)Fe decay. • - type Ic: progenitor must be either an extreme WR star, or a binary (Nomoto 1995)‏ • - type II: progenitor is a massive red giant

8. Supernovae observed in associacion with GRBs • - SN1998 bw: GRB 980425 • - SN 2003 dh: GRB 030329 • - SN 2003 lw: GRB 031203 • - SN 2006 aj: XRF 060218 • ‏ • All of these are Type Ic • All have broad line spectra -> • ejection velocities ~ 50,000 km/s • They account for 20% of the • BL SN Ic = 2% of all SN Ic

9. Hypernova: - very high expansion velocity - bright luminosity • - postulated to be an energetic outburst produced by a collapsar (Woosley 1993; Paczyński 1998)‏ • - very strong explosion energy (> few x 1051 ergs)‏ • - strong evidence for assymetry (Nomoto et al. 2005)‏ • - massive star models fit well the observed hypernovae ( Mazzali et al. 2006)‏ • -large uncertainty in modeling due to the initial mass function of massive stars (5-40% core collapse SN form the black hole; Fryer & Kalogera 2001)‏ Eta Carinae: future candidate for hypernova

10. Hypernovae are rare (about 1000 times less frequent than normal SN; (Soderberg et al. 2006) • All hypernovae have been classified as Ib/c SN (no H lines, nor He lines in the spectra); probably a subset of them • Rates of Supernova vs.Hypernova • Rate of all core-collapse SN: 6x10-3 /yr/galaxy( Fryer et al. 2007) • Type Ib/c are 15% of all core collapse • Hypernovae are 5-10% of observed type I b/c • 1-10% of SN Ib/c can be associated with GRBs; this coincides with that of hypernovae • Rate of all core-collapse increases with redshift (no specific data for I b/c or hypernova)‏

11. The collapsar • Woosley (1993): SN Type Ib 'failed' because of a fast rotation of the Wolf-Rayet star • Paczyński (1998): some GRBs must be linked to the cataclysmic deaths of massive stars -> hypernovae • MacFadyen & Woosley (1999) and follow up works: hydrodynamical computations of the relativistic jet propagation through the stellar envelope • MacFadyen & Woosley (1999)‏ • Two reasosns for SN to fail (Fryer 1999): • Large ram pressure at the top of the convective zone • Large binding energy for the most massive stars • GRB progenitors: the most massive stars, that fail to produce an explosion under the standard core-collapse supernova

12. The collapsar engine of a GRB Must form a black hole in the center of the star Must produce sufficient angular momentum to form a disk around black hole Must eject the hydrogen envelope, so that the jet can punch out of the star

13. Progenitors of the I b/c Supernovae: single or binary stars? • Most massive stars (Mass > 20 solar masses; Hirshi et al. 2004)‏ • Wolf-Rayet stars: have lost the H envelope due to strong winds • Single stars: only fast rotating stars above solar metallicity produce strong shocks and eject lots of nickel (Heger at al. 2003)‏ • Fast rotating stars can mix their envelopes, burning effectively H into He (Yoon & Langer 2005)‏ • Binary stars: mass transfer can eject matter and lead to He star formation. • Possibly, >75% of all massive stars are in close binaries (Kobulnicky et al. 2006)‏ SN 2008D

14. Single stars: only fast rotating stars above solar metallicity produce strong shocks and eject lots of nickel (Heger at al. 2003)‏ This GRB rate must be lower by a factor Indicating the fraction of stars that retain large angular momentum

15. Metallicity measurements • Wind mass loss sensitive to metallicity • At lower metallicities, weaker winds allow more massive cores => GRBs probably will not occur above solar metallicity • Metallicity measurements: • Absorption lines in the GRB afterglows • Emission lines of HII regions in the GRB host galaxy • Interstellar extinction in the host galaxy • Morphology of the host galaxy, e.g. Compared to SMC/LMC Nebula NGC 2359 There is no consistent picture: direct measurements argue for higher Z, while indirect measurements indicate lower Z.

16. Progenitors of Hypernovae • Most of the currently discussed progenitors do not distinguish between fallback and direct collapse black holes • GRBs probably will not occur at solar metallicity, if we need a direct collapse to black hole. At lower metallicities, weaker winds allow more massive cores. • Below ~0.4 ZSun, the stars cannot loose the He envelope (Heger 2003). • Star is either born rotating rapidly, or is spun up by interaction (tidal forces, merger). In binaries, the companion is used to strip off the hydrogen envelope without the angular momentum loss. • Single stars can also loose the H envelope because of mixing and burning to He(Yoon & Langer 2002). But if the He envelope is also lost, these models are ruled out. WR124 „Constraints are more restrictive for single-star models, but without better understanding of winds we cannot say more” (Fryer et al. 2007).

17. (Janiuk, Proga, Moderski. 2008a, 2008b) How long is a long GRB? Chemical composition and density distribution in the pre-SN star (Woosley & Weaver 1995)

18. How the pre-collapse star rotates? • The distribution of specific angular momentum in the pre-SN star unknown. • Stellar evolution models: • Neglect centrifugal forces • Do not accurately treat the angular momentum transport through magnetic fields • Sensitive to the loss of ang. momentum through wind • Some assumptions we have made: • Polar angle dependence (differential rotation)‏ • Radius dependence (rigid rotation, with a possible cut-off on lspec)‏ • Constant ratio of centrifugal to gravitational forces lspec = l0 (1-cos θ)‏ lspec = l0 (r/rin)sin2θ (Janiuk et al. 2008a, 2008b)

19. Conditions for torus existence • The rotation must prevent the envelope material from the radial infall onto BH. • Specific angular momentum • lspec > lcrit = 2GMBH/c (2-A+2(1-A)1/2)1/2 • BUT: • Black hole mass is growing fast (accretion rate of 0.01-1 Msun/s) • Spin can be changing • => the GRB is emitted only until l>lcrit is satisfied.

20. The black hole grows due to accretion • The time evolution of the collapsar => iterative procedure • 1. BH mass = iron core mass • 2. Envelope schells accrete • 3. Check for conditions given by the changing BH mass and spin • Various possible accretion scenarios

21. GRB requires: large accretion rate and spinning BH Schwarzschild and Kerr BH case: Janiuk & Proga, 2008, ApJ, 675, 519; Janiuk, Moderski & Proga, 2008, ApJ, in press;

22. Hyperaccretion: neutrino-cooled disk • - Cooling mechanisms: neutrino emission, • advection, Helium photodisintegration, • radiation - Neutrinos can be absorbed and scattered - Equation of state should treat the species under the condition of reactions equilibrium and supplemented by the charge neyutrality condition • Electron-positron capture and beta-decay • p + e- → n + νe • n + e+→ p + νe • n → p + e- + νe • Thermal emission • e+ + e-→νi + νi • n + n → n + n + νi + νi • γ→νe + νe - - - - - - ν p e- n (Popham, Woosley & Fryer 1999; Di Matteo, Perna & Narayan 2002; Kohri & Mineshige 2002; Janiuk et al. 2004; Kohri, Narayan & Piran 2005; Janiuk et al. 2007; Chen & Beloborodov 2007)‏ e+ α

24. The disk accreting at rates > 0.1 MSun s-1 is so hot and dense (T~1010-1011 K, ρ~1010-1012 g cm-3) that the plasma is totally opaque to photons, and neutrinos can also be trapped • Energy from the disk can be extracted by neutrino annihilation • Alternatively, energy can be extracted by the magnetic field and spinning black hole (the Blandford-Znajek mechanism) Chen & Beloborodov (2007)‏

25. Efficiency of neutrinos depends on initial accretion rate, and decreases in time Janiuk et al. (2004)‏ Chen & Beloborodov (2007)‏ • Neutrino annihilation inefficient when accretion rate < ~0.01 Msun/s. • This will slightly depend on viscosity and black hole spin.

26. We end up with three kinds of jets from the collapsar: 0-1.5 s 0-430 s 0-130 s • Precursor jet, • powered by v-v • large m, small A - • First jet, • powered by • both v-v and BH rotation • large m, large A - • Second jet, powered by BH rotation • small m, large A .

27. Precursors found in ~20% of BATSE sample Lazzati (2005) GRB 0803319B Brightest optical counterpart: mv = 5.3 Empirical model of a 2-component jet fitted with two opening angles (Racusin et al. 2008)‏ Image from “Pi of the sky”, http://grb.fuw.edu.pl

28. Instabilities in the accreting torus: possible mechanism of causing a long time gap between the precursor and the burst, or the short-term variability seen in the prompt phase (Wang & Meszaros 2007). • Precursor phase in the prompt emission seen in some GRBs, might be produced by the jet breaking through envelope (Paczyński 1998; Ramirez-Ruiz et al. 2002) • Recent hydro Simulations by Morsony, Lazzati & Begelman (2007) found three distinct phases during the jet propagation: precurosr jet, shocked phase and unshocked phase. • Density, pressure and gamma_inf at time 30 s. • Cocoon: high ρ, high P, low Γ; • Precursor: high Γ, off-axis; • Shocked jet: low ρ, high P, high ρ; • Unshocked jet: low ρ, low P, high Γ

29. Instabilities in the accretion disk: - May be related to the late-time activity of the GRB (such as X-ray flares; e.g. Perna et al. 2006)‏ - proposed as the sources of gravitational waves, that may probe the angular momentum of the collapsing star (Fryer et al. 2002)‏ - Black hole spin can be coupled to the disk, enhancing the strength of the instability, then possibly detectable by LIGO (van Putten 2005)‏

30. Thermal instability: the local density and pressure drops, while the temperature increases. Our solution is based on the detailed treatment of the EOS, coupling the beta-equilibrium and the neutrino trapping effects, as well as including the information of the chemical composition in the process of Helium photodisintegration. Janiuk et al. (2007)‏

31. Summary • GRB long durations may provide constraints for the rotation law in the pre-SN star. • The minimum accretion rate limit for the neutrino-powered jets, in the Schwarzschild black hole models, results in GRB durations up to 40-100 s. • The minimum accretion rate and BH spin limit, for jets powered by both neutrinos and black hole spin, results in GRB durations up to 50-130 s. • The above values will be smaller if the H/He envelope was already stripped • In the Kerr black hole models, we find the solutions corresponding to three kinds of jets: precursor jet, early jet and late jet, powered by different mechanisms. Possibly, the opening angle of these jets is changing, which would have some observational consequences. • The instabilities in the accreting torus play important role for the observed emission

32. Constraints on the GRB progenitor from observations and SN models • Type Ic SN => progenitor must loose the H and most of He envelope • Occur in the brightest parts of galaxies => come from the most massive stars • Occur in metallicities from 0.01 to 1 => single star models strongly constrained • Single star models may require mixing to burn H into He effectively • Binary star models fit better to the observational constraints

33. Thank you