The peak energy and spectrum from dissipative GRB photospheres Dimitrios Giannios Physics...
Embed Size (px)
Transcript of The peak energy and spectrum from dissipative GRB photospheres Dimitrios Giannios Physics...
The peak energy and spectrum from dissipative GRB photospheres
Dimitrios GianniosPhysics Department, Purdue
GRBs @ Liverpool, June 19, 2012
Gamma-ray burst spectrum: a 40+ year mystery
• Peak at ~1 MeV consistently• Non-thermal appearance • High radiative efficiency
Several thousands of bursts observed so far
fν ~ν -1.2
Band et al. 1993
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)
Shocks? B reconnection? something else?
synchrotron?Inverse Compton? photospheric?
optically thin emission?optically thick emission?
Internal shock synchrotron as source of GRBs?
Model cannot explain: Epeak clustering
spectral slope below peak high radiative efficiency
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
Back to the blackboard
Blandford & Znajek 1977Begelman & Li 1992Meier et al. 2001Koide et al. 2001van Putten 2001…Barkov & Komissarov 2008…
gamma-ray bursts (GRBs)
The strength of the magnetic paradigm: universally produces relativistic outflows
jets in galactic centers micro-quasars
Power~1044…49erg/s ~1052erg/s ~1037erg/s
Magnetic Fields: critical for jet acceleration
nt 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
Photospheric emission: a black body? Deep in the flow τes>>1
thermal energy is trapped
Emission at photosphere Powerful Peaking at ~1 MeV
Assumed a black body
Detailed radiative transfer required to calculate actual spectrum Giannios 2006; 2008; 2012
optically thin emission
Photospheric spectrum The simple physics behind the detailed Monte Carlo
Photospheric emission: not at all thermal-like
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…
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!
R61/ 2 MeV, η > η*
8 / 3
MeV, η < η*
Where η* ≈ 2000L52
Epeak 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:
--- 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 !!!
Epeak indeed forms at τ~τeq~50
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 !
Epeak =1.5ε1/ 6Γ2.5
4 / 3η2.51/ 3
L531/ 6 MeV
Observations of GRBs: the brighter, the faster, the higher Epeak
Liang et al. 2010 Ghirlanda et al. 2010
Prediction: Giannios 2012
Observations Ghirlanda et al. 2011
ε1/ 6Γ2.51/ 3η2.5
L531/ 6 keV
All photospheric: GRBs, XRFs, ll GRBs?
They may all come from the jet photosphere!
Summary on GRB emission Magnetic dissipation holds great promise in powering jet
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!)
What makes the central engine “the brighter the faster?”