The peak energy and spectrum from dissipative GRB photospheres

Dimitrios GianniosPhysics Department, Purdue

GRBs @ Liverpool, June 19, 2012

E (MeV)

Gamma-ray burst spectrum: a 40+ year mystery

νfν

• Peak at ~1 MeV consistently• Non-thermal appearance • High radiative efficiency

Several thousands of bursts observed so far

t (sec)

Nph

(t)?f ν

~ν0

fν ~ν -1.2

Epeak~1 MeV

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)

Am

ati 2

002;

Ghi

rlan

da e

t al.

2010

…

Theoretical Cartoon

Internal dissipation

Centralengine

Acceleration

jet emission

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

Synchrotron radiation

-rays

γ2υ2 γυγ1υ1

E*f

(E)

✔✖✖ theory

Observations

E

Back to the blackboard

Internal dissipation

Centralengine

Acceleration

jet emission

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

M87; NASA/Hubble

MBH~109M~10M~3M

Power~1044…49erg/s ~1052erg/s ~1037erg/s

Magnetic Fields: critical for jet acceleration

€

rΩ

€

rB

distance r

kinetic component

magnetic component

ener

gy c

onte

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

and dissipation

Γ>>1

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

€

rΩ

€

rB

distance r

kinetic component

magnetic component

thermal

photospheric emission

τ~1

ener

gy c

onte

nt

optically thin emission

GRB

~106cm ~1012cm

✖✖

✔✔

Photospheric spectrum The simple physics behind the detailed Monte Carlo

Comptonization simulations

E*f

(E)

τ~1

τ<<1

τ>>1

Inverse Compton

τ>>1τ~1τ<<1

synchrotron

1 MeV

Te~TphTe>TphTe>>Tph

E

Photospheric emission: not at all thermal-like

η=590

η=1000

typically observed

SwiftFermi

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…

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!

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Epeak ≈1L52

1/ 4

R61/ 2 MeV, η > η*

Epeak ≈1L52

1/ 4

R61/ 2

η

η*

⎛

⎝ ⎜

⎞

⎠ ⎟

8 / 3

MeV, η < η*

€

Where η* ≈ 2000L52

R6

⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 4

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 !!!

Numerical verification

Giannios 2012

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

Other Implications

Prediction: Giannios 2012

Observations Ghirlanda et al. 2011

€

Eco =Epeak

Γ= 5

ε1/ 6Γ2.51/ 3η2.5

1/ 3

L531/ 6 keV

All photospheric: GRBs, XRFs, ll GRBs?

L (erg/s)

Γ GRBs

ll GRBs

X-ray fla

resXRFs

105310511049

103

102

10

Epeak~0.1-1 MeV

Epeak~30 keV

Epeak~1 keV

They may all come from the jet photosphere!

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?”