Transcript ppt
The prompt phase of GRBs Dimitrios Giannios Lyman Spitzer, Jr. Fellow Princeton, Department of Astrophysical Sciences Raleigh, 3/7/2011 Structure of the talk Main properties of the prompt emission Models for the GRB flow Fireballs Poynting-flux dominated flows Internal shocks Magnetic Reconnection Radiation region Thomson thin vs photospheric emission for the GRB Fermi LAT bursts Correlations: what can we learn for the central engine? νfν Nph (t) Gamma-ray bursts: spectra and variability E (MeV) t (sec) 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? My focus: why and how do jets radiate? ? Central Engine Acceleration Internal dissipation External interactions (How to tell a millisecond magnetar) A millisecond neutron star has rotational energy DG 2010 E rot 3 10 52 Pms-2 erg Extracted on a timescale of ~30 sec for Bs 3 1015 G Usov 1992; Thompson 1994; Uzdensky & MacFadyen 2006; Metzger et al. 2007; 2011 After the GRB we are left with a supermagnetar! Contains E 1049 R 3 B 2 erg B 6 16 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! kouveliotou et al. 1998 GRBmagnetar flare ~10-3 yr-1 ~10-5.5 yr-1 SGRs galactic rate Guetta et al. 2005 B Field ~1015 G ~1016 G flaring @ ~104 yr ~102-103 yr Peak luminosity ~1047 erg s-1 ~1049 erg s-1 detectability ~25 Mpc with BATSE, … ~250 Mpc The driving mechanism: MHD Energy Extraction and/or neutrino annihilation B-fields extract rotational energy from the compact object/inner accretion disk at a rate BR EÝEM B2R2 3 c 2 6 4 Neutrino annihilation energy deposition rate (erg cm –3 s-1) Blandford & Znajek 1977 Koide et al. 2001 van Putten 2001 Lee et al. 2001 Barkov & Komissarov 2008 Usov 1992 Uzdensky & McFadyen 2007 Bucciantini et al. 2007 Metzger et al. 2010 Ruffert & Janka 1999; Popham et al. 1999; Aloy et al. 2000; Chen & Beloborodov 2007; Zalamea & Beloborodov 2011 General considerations: Acceleration Important quantities of the flow: luminosity L L Baryon loading 1 2 mass flux M Mc Efficient acceleration can lead to γsr~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 Fireballs Go through fast acceleration r Converting thermal energy into kinetic Saturation takes place when 1. 2. internal shocks Parameters: L, η, initial radius ro thermal component photospheric emission almost all thermal energy is used: γsr η at the photospheric crossing γsr < η Radiation and matter decouple when τ~1 energy content Photospheric emission takes place kinetic component τ~1 distance r 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 The field is in general not axisymmetric at the central engine Model for GRBs: Magnetic field changes polarity on small scales and reconnects vrec=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 energy content The reconnection model for GRBs thermal magnetic component × × × r B photospheric emission thin emission r kinetic component × τ~1 distance r × × × The prompt GRB Prompt GRB Central engine ~106cm Internal dissipation ~1011-1017cm Afterglow External interactions ~1017-1018cm Where is the prompt emission produced? in principle anywhere between the Thomson photosphere rph (or slightly below) and the deceleration radius rd Typically rph~1011cm and rd ~1017cm; 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 Dissipation in the Thomson thin regime Big variety of spectra depending on the various parameters: єdiss -fraction of dissipated energy єB, єe -fraction that goes to Bfields, 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) Shocks accelerate particles and amplify magnetic fields E*f(E) Photospheric emission In the fireball the photospheric luminosity is e.g. Mészáros & Rees 2000 Lph L 0.05 8 / 3 2/3 2.5 o,7 2/3 52 r L 1/ 4 L52 , 1000 r o,7 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 Lph L 0.16 L1/525 ( )1/3 5 2.5 , 150 L1/525 ( )1/3 5 Photospheric emission from the reconnection model If fraction fe ~ 1 of the energy goes into heating the electrons then Resulting emission spectrum with DG 2006; DG & Spruit 2007; DG 2008 heating-cooling balance gives the electron temperature everywhere in the flow Peak in the sub-MeV range Flat high-energy emission observed low-energy slope Rather high efficiency Lph ~ 0.03…0.5L, for 100 ~ <η< ~ 1500 Dissipative photospheres: reconnection model synchrotron emission Fermi Swift η=1000 τ<<1 η=590 Robotic telescopes η=460 typically observed η=350 L Ý 2 Mc η=250 E (MeV) τ~1 Compton scattering 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 From the central engine to radiation Millisecond magnetar Spectrum η typical GRB EÝ η Metzger, Giannios, Thompson, Bucciantini & Quataert 2011 More dissipative photospheres collisional heating; Beloborodov 2010; f(E) E*f(E) Vurm et al. f(E) Pe’er et al. 2006 weak shocks; Lazzati & Begelman 2010 Recent Developments: GeV emission counts LAT emission: peaking with (late) MeV but lasts longer! time GRB 080916C; Abdo et al. 2009 Ghiselini et al. 2009 Physical origin of GeV emission is (in part?) different from the MeV 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 L time Correlations: what do we see? Involve both time integrated and instantaneous quantities (e.g., 0.5 E peak E iso , E peak (t) L0.5 ) iso(t) Yonetoku et al. 2004 also Borgonovo & Ryde 2001; Liang et al. 2004; Ghirlanda et al. 2004; Liang & Zhang 2006; Firmani et al. 2006; Collazzi & Schaefer 2008… Amati 2010 Firmani et al. 2009 Correlations: what can we learn? Tendency for brighter bursts to be cleaner? Transparency of fireball emerging from a collapsar? Ave Peak Energy Epeak Metzger et al. 2011; see also DG & Spruit 2007 Peak Isotropic Jet Luminosity (erg s-1) Morsony et al. 2011; Lazzati et al. 2011; see also Thompson et al. 2007 Interpretations within photospheric models for Epeak 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 єe, ζe, p, єB The photospheric interpretation for MeVs is robust Magnetic reconnection provides a promising process to power a dissipative photosphere