The peak energy and spectrum from dissipative GRB photospheres

1. The peak energy and spectrum from dissipative GRB photospheres Dimitrios Giannios Physics Department, Purdue GRBs @ Liverpool, June 19, 2012

2. Gamma-ray burst spectrum: a 40+ year mystery Several thousands of bursts observed so far ? fν~ν-1.2 fν~ν0 Nph(t) νfν Epeak~1 MeV Band et al. 1993 E (MeV) • Peak at ~1 MeV consistently • Non-thermal appearance • High radiative efficiency t (sec)

3. Peak energy: a key quantity • Epeak marks where most of the EM energy comes out • Epeak tracks other observables and jet properties (Eiso, L, Γ) Amati 2002; Ghirlanda et al. 2010…

4. Central engine Theoretical Cartoon jet emission synchrotron? Inverse Compton? photospheric? Acceleration Internal dissipation Shocks? B reconnection? something else? optically thin emission? optically thick emission?

5. Internal shock synchrotron as source of GRBs? • Internal shocks Rees & Meszaros 1994 • Unsteady jet composed by shells • A fast shell with γ2>γ1 collides with a slower one dissipating kinetic energy  nonthermal particles • fast particles+ magnetic field Synchrotron radiation • Model cannot explain: • Epeak clustering • spectral slope below peak • high radiative efficiency γ1υ1 γ2υ2 γυ g-rays Observations E*f(E) ✖ ✔ ✖ theory E

6. Central engine Back to the blackboard jet emission Acceleration Internal dissipation Blandford & Znajek 1977 Begelman & Li 1992 Meier et al. 2001 Koide et al. 2001 van Putten 2001 … Barkov & Komissarov 2008 …

7. gamma-ray bursts (GRBs) The strength of the magnetic paradigm: universally produces relativistic outflows jets in galactic centers micro-quasars M87; NASA/Hubble ~3M ~10M MBH~109M Power~1044…49erg/s ~1052erg/s ~1037erg/s

8. Magnetic Fields: critical for jet acceleration and dissipation Magnetic reconnection effective in heating the jet magnetic reconnection region thermal component; energetic particles energy content magnetic component Γ>>1 kinetic component distance r fields may be essential in powering the jet radiation Eichler 1993; Begelman 1998; Drenkhahn & Spruit 2002; Nakamura & Meier 2004; Giannios & Spruit 2006; Moll 2009; McKinney & Blandford 2009; Mignone et al. 2010… Important in understanding jet acceleration Michel 1969; …, Vlahakis & Koenigl 2003; Komissarov et al. 2009; 2010; Tchekhovskoy et al. 2009; 2010; Lyubarsky 2009; 2010; Granot et al. 2011

9. Photospheric emission: a black body? • Deep in the flow τes>>1 thermal energy is trapped • Emission at photosphere • Powerful • Peaking at ~1 MeV Goodman 1986 • Assumed a black body • Detailed radiative transfer required to calculate actual spectrum Giannios 2006; 2008; 2012 photospheric emission ~1012cm ~106cm GRB thermal ✔ optically thin emission energy content ✔ magnetic component ✖✖ kinetic component τ~1 distance r

10. Photospheric spectrum • The simple physics behind the detailed Monte Carlo Comptonization simulations Te>Tph Te~Tph Te>>Tph τ<<1    τ~1 τ<<1 τ>>1 Inverse Compton synchrotron τ~1 E*f(E) τ>>1 E 1 MeV

11. Photospheric emission: not at all thermal-like Fermi Swift η=1000 η=590 Robotic telescopes typically observed η=460 E (MeV) η=350 η=250 Giannios 2006; Giannios & Spruit 2007; Giannios 2008; 2012 extensive theoretical effort: Thompson 1994; Pe’er et al. 2006; Ioka et al. 2007; 2010; Lazzati & Begelman 2010; Beloborodov 2010; Ryde et al. 2011; Vurm et al. 2011; Lazzati et al. 2012…

12. What determines Epeak of the photosphere? • The jet temperature at τ~1 (ignoring additional heating; e.g., Meszaros & Ress 2000) • Emerging spectrum is quasi-thermal: typically Not observed • Dissipation of energy is required for Band spectrum • dissipation affects the location where Epeak forms!

13. Epeak in dissipative photospheres Giannios (2012) • Generic model for dissipative photosphere assuming: • continuous heating of electrons over wide range in distance (including the photosphere) • Compton scattering dominates the e-/photon interactions • Findings: --- Te=Tph, for τ>>>1 (Compton y>>1) --- e- and photons decouple at τ~50 --- Te>Tph, for τ<30-50 (Compton y~1) Epeak forms here !!!

14. Numerical verification Epeak indeed forms at τ~τeq~50 Giannios 2012

15. Key result for photospheric models • Analytic expression for the peak energy • Main prediction: the larger Γ the higher the Epeak already made in Giannios & Spruit 2007 The synchrotron IS model predicts the opposite Epeak~Γ-2 !

16. Observations of GRBs: the brighter, the faster, the higher Epeak Liang et al. 2010 Ghirlanda et al. 2010

17. Other Implications • Prediction: Giannios 2012 • Observations Ghirlanda et al. 2011

18. All photospheric: GRBs, XRFs, ll GRBs? They may all come from the jet photosphere! Epeak~0.1-1 MeV GRBs 103 Epeak~30 keV XRFs Γ 102 10 ll GRBs X-ray flares Epeak~1 keV 1049 1051 1053 L (erg/s)

19. Summary on GRB emission • Magnetic dissipation holds great promise in powering jet radiation • The photosphere of the jet is likely to be the location where GRB prompt emission forms (and maybe XRFs, X-ray flares, ll GRBs) • The peak of the spectrum depends mainly on the bulk Γ of the jet (and forms at optical depth τ~50!) • Key Question: What makes the central engine “the brighter the faster?”