1. The prompt phase of GRBs Dimitrios Giannios Lyman Spitzer, Jr. Fellow Princeton, Department of Astrophysical Sciences Raleigh, 3/7/2011

2. Structure of the talk  Main properties of the prompt emission  Models for the GRB flow Fireballs  Internal shocks Poynting-flux dominated flows  Magnetic Reconnection  Radiation region Thomson thin vs photospheric emission for the GRB  Fermi LAT bursts  Correlations: what can we learn for the central engine?

3. E (MeV) Gamma-ray bursts: spectra and variability t (sec) N ph (t) νf ν

4. GRBs: ultrarelativistic jets  Clues The prompt emission has  non-thermal spectral appearance Band et al. 1993; Preece et al. 1998  Rapid variability The GRB-emitting flow is ultrarelativistic (γ>100, 300, 1000?) e.g. Piran 1999…  Big questions Type of central-engine/Jet composition How is the flow accelerated? Which processes result in the observed GRB emission?

5. My focus: why and how do jets radiate? Internal dissipation Central Engine Acceleration External interactions ?

6. (How to tell a millisecond magnetar)  A millisecond neutron star has rotational energy  Extracted on a timescale of ~30 sec for Usov 1992; Thompson 1994; Uzdensky & MacFadyen 2006; Metzger et al. 2007; 2011  After the GRB we are left with a supermagnetar! Contains The magnetic field decays fast (100-1000yr; Thompson & Duncan 1996)  May power SGR superflares ~100 times more powerful than that of SGR 1806-20 in December 2004! SGRs kouveliotou et al. 1998 GRB- magnetar flare galactic rate~10 -3 yr -1 ~10 -5.5 yr -1 Guetta et al. 2005 B Field~10 15 G~10 16 G flaring @~10 4 yr~10 2 -10 3 yr Peak luminosity ~10 47 erg s -1 ~10 49 erg s -1 detectability with BATSE, … ~25 Mpc~250 Mpc DG 2010

7. The driving mechanism: MHD Energy Extraction and/or neutrino annihilation Blandford & Znajek 1977 Koide et al. 2001 van Putten 2001 Lee et al. 2001 Barkov & Komissarov 2008 Neutrino annihilation energy deposition rate (erg cm –3 s -1 ) Ruffert & Janka 1999; Popham et al. 1999; Aloy et al. 2000; Chen & Beloborodov 2007; Zalamea & Beloborodov 2011 B-fields extract rotational energy from the compact object/inner accretion disk at a rate Usov 1992 Uzdensky & McFadyen 2007 Bucciantini et al. 2007 Metzger et al. 2010

8. General considerations: Acceleration  Important quantities of the flow: luminosity L mass flux Efficient acceleration can lead to γ sr ~η  Depending on the energy extraction mechanism, the flow can be dominated by Thermal energy  thermal acceleration (Fireball) Paczynski 1986; Goodman 1986; Sari & Piran 1991 Magnetic energy  MHD acceleration (Poynting-flux dominated flow) Usov 1992; Thompson 1994; Mészáros & Rees 1997; Drenkhahn & Spruit 2002; Lyutikov & Blandford 2003 Baryon loading

9. Fireballs  Parameters: L, η, initial radius r o  Go through fast acceleration Converting thermal energy into kinetic  Saturation takes place when 1. almost all thermal energy is used: γ sr η 2. at the photospheric crossing γ sr < η  Radiation and matter decouple when τ ~ 1 Photospheric emission takes place distance r internal shocks thermal component kinetic component photospheric emission τ~1 energy content

10. Strongly magnetized jets  Recent progress in 2D axisymmetric relativistic MHD simulations & theory Vlahakis & Koenigl 2003; Komissarov et al. 2009; 2010; Tchekhovskoy et al. 2009; 2010; Lyubarsky 2009; 2010 High magnetization flows accelerate to Γ>>1, But most of the energy remains in the B field Shocks are inefficient  Dissipative MHD processes are key to jet emission (and acceleration)  Non-axisymmetric instabilities may develop a large distance leading to dissipation and emission e.g., Lyutikov & Blandford 2003; Narayan & Kumar 2008; Zhang & Yan 2011

11. The reconnection model for GRBs  The field is in general not axisymmetric at the central engine × × ×× × × ×  Model for GRBs: Magnetic field changes polarity on small scales and reconnects v rec =εc Drenkhahn 2002 and Denkhahn & Spruit 2002; see also McKinney & Uzdensky 2011  Dissipation is gradual and leads to acceleration of the flow and heating of plasma  The model predicts a strong photospheric component and optically thin dissipation distance r kinetic component magnetic component thermal photospheric emission τ~1 energy content thin emission

12. The prompt GRB Prompt GRB Central engine Internal dissipation External interactions Afterglow ~10 6 cm ~10 17 -10 18 cm ~10 11 -10 17 cm

13. Where is the prompt emission produced?  in principle anywhere between the Thomson photosphere r ph (or slightly below) and the deceleration radius r d  Typically r ph ~10 11 cm and r d ~10 17 cm; in this range of radii: density ~12 orders of magnitude optical depth ~6 orders of magnitude Different radiative mechanisms depending on the location of the energy dissipation Case 1: Thomson thin dissipation Case 2: Photospheric dissipation

14. Dissipation in the Thomson thin regime  Shocks accelerate particles and amplify magnetic fields  Big variety of spectra depending on the various parameters:  є diss -fraction of dissipated energy  є B, є e -fraction that goes to B- fields, fast electrons  Fraction ζ of accelerated electrons  Electron power-law index p  Distance of collision  Dominant processes: Synchrotron; synchrotron-self-Compton  Similar for magnetic reconnection at optically thin conditions! Bosnjak, Daigne & Dubus 2008 E*f(E)

15. Photospheric emission  In the fireball the photospheric luminosity is e.g. Mészáros & Rees 2000 Spectrum quasi thermal Goodman 1986 (but not exactly black-body Beloborodov 2011 ) Energy dissipation (shocks, collisional heating) at τ ≥ 1 distorts the spectra Mészáros & Rees 2005; Pe’er et al. 2006  In the reconnection model DG 2006; DG & Spruit 2007

16. Photospheric emission from the reconnection model  If fraction f e ~ 1 of the energy goes into heating the electrons then heating-cooling balance gives the electron temperature everywhere in the flow  Resulting emission spectrum with DG 2006; DG & Spruit 2007; DG 2008 Peak in the sub-MeV range Flat high-energy emission observed low-energy slope  Rather high efficiency L ph ~ 0.03…0.5L, for 100 < η < 1500 ~ ~

17. Dissipative photospheres: reconnection model η=590 η=1000 typically observed Swift Fermi Robotic telescopes DG 2006; DG & Spruit 2007; DG 2008 more models: Pe’er et al. 2006; Ioka 2010; Lazzati & Begelman 2010; Beloborodov 2010; Ryde et al. 2011 τ~1 τ<<1 E (MeV) Compton scattering synchrotron emission η=350 η=460 η=250

18. From the central engine to radiation Millisecond magnetar Spectrum Metzger, Giannios, Thompson, Bucciantini & Quataert 2011 η η typical GRB

19. More dissipative photospheres Pe’er et al. 2006 collisional heating; Beloborodov 2010; Vurm et al. weak shocks ; Lazzati & Begelman 2010 E*f(E) f(E)

20. Recent Developments: GeV emission LAT emission: peaking with (late) MeV but lasts longer! GRB 080916C; Abdo et al. 2009 Ghiselini et al. 2009 Physical origin of GeV emission is (in part?) different from the MeV counts time

21. What to make of Fermi observations?  LAT ‘sees’ two components (physically separated) 1. prompt 2. slow declining  Need to disentangle them before constraining for the prompt emission cite!  cannot assume a single emission cite for MeV and GeV (e.g. Zhang & Pe’er 2009) GBM LAT time L

22. Correlations: what do we see? Involve both time integrated and instantaneous quantities (e.g., ) also Borgonovo & Ryde 2001; Liang et al. 2004; Ghirlanda et al. 2004; Liang & Zhang 2006; Firmani et al. 2006; Collazzi & Schaefer 2008… Yonetoku et al. 2004 Amati 2010 Firmani et al. 2009

23. Correlations: what can we learn? Morsony et al. 2011; Lazzati et al. 2011; see also Thompson et al. 2007 Transparency of fireball emerging from a collapsar? Metzger et al. 2011; see also DG & Spruit 2007 Tendency for brighter bursts to be cleaner? Interpretations within photospheric models for E peak Ave Peak Energy E peak Peak Isotropic Jet Luminosity (erg s -1 )

24. Summary  The prompt emission most likely comes from internal dissipation of energy in the fast flow Internal shocks or Magnetic dissipation or …  Dissipation may take place in Thomson thin or thick conditions Thin case: particle acceleration uncertainties є e, ζ e, p, є B The photospheric interpretation for MeVs is robust  Magnetic reconnection provides a promising process to power a dissipative photosphere

25. Comment The temperature of the flow at the r eq in the observer frame is The E·f(E) spectrum of this component peaks at ~4 times this energy