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

2. E (MeV) Gamma-ray burst spectrum: a 40+ year mystery νf ν Peak at ~1 MeV consistently Non-thermal appearance High radiative efficiency Several thousands of bursts observed so far t (sec) N ph (t) ? f ν ~ν 0 f ν ~ν -1.2 E peak ~1 MeV Band et al. 1993

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

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

5. Internal shock synchrotron as source of GRBs?  Model cannot explain: E peak clustering spectral slope below peak high radiative efficiency  Internal shocks Rees & Meszaros 1994 Unsteady jet composed by shells A fast shell with γ 2 >γ 1 collides with a slower one dissipating kinetic energy  nonthermal particles fast particles+ magnetic field  Synchrotron radiation  -rays γ2υ2γ2υ2 γυ γ1υ1γ1υ1 E*f(E) ✔✖ ✖ theory Observations E

6. Back to the blackboard Internal dissipation Central engine Acceleration jet emission 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 M BH ~10 9 M  ~10M  ~3M  Power~10 44…49 erg/s ~10 52 erg/s ~10 37 erg/s

8. Magnetic Fields: critical for jet acceleration distance r kinetic component magnetic component energy content thermal component; energetic particles magnetic reconnection region 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… Magnetic reconnection effective in heating the jet 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 and dissipation Γ>>1

9. Photospheric emission: a black body?  Deep in the flow τ es >>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 distance r kinetic component magnetic component thermal photospheric emission τ~1 energy content optically thin emission GRB ~10 6 cm ~10 12 cm ✖✖ ✔ ✔

10. Photospheric spectrum  The simple physics behind the detailed Monte Carlo Comptonization simulations E*f(E) τ~1 τ<<1 τ>>1 Inverse Compton τ>>1 τ~1τ<<1 synchrotron  1 MeV T e ~T ph T e >T ph T e >>T ph E

11. Photospheric emission: not at all thermal-like η=590 η=1000 typically observed Swift Fermi Robotic telescopes Giannios 2006; Giannios & Spruit 2007; Giannios 2008; 2012 E (MeV) η=350 η=460 η=250 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 E peak 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 E peak forms!

13. E peak in dissipative photospheres Giannios (2012)  Generic model for dissipative photosphere assuming: 1. continuous heating of electrons over wide range in distance (including the photosphere) 2. Compton scattering dominates the e - /photon interactions  Findings: --- T e =T ph, for τ>>>1 (Compton y>>1) --- e - and photons decouple at τ~50 --- T e >T ph, for τ<30-50 (Compton y~1) E peak forms here !!!

14. Numerical verification Giannios 2012 E peak indeed forms at τ~τ eq ~50

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

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

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

18. All photospheric: GRBs, XRFs, ll GRBs? L (erg/s) Γ GRBs ll GRBs X-ray flares XRFs 10 53 10 51 10 49 10 3 10 2 10 E peak ~0.1-1 MeV E peak ~30 keV E peak ~1 keV They may all come from the jet photosphere!

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?”