The peak energy and spectrum
from dissipative GRB photospheres
Dimitrios Giannios
Physics Department, Purdue
GRBs @ Liverpool, June 19, 2012
Gamma-ray burst spectrum: a 40+ year mystery
νfν
?
Band et al. 1993
Nph (t)
Several thousands of
bursts observed so far
Epeak~1 MeV
E (MeV)
• Peak at ~1 MeV consistently
• Non-thermal appearance
• High radiative efficiency
t (sec)

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…
Peak energy: a key quantity
Theoretical Cartoon
jet emission
synchrotron?
Inverse Compton?
photospheric?
Central
engine
Acceleration
optically thin emission?
optically thick emission?
Internal
dissipation
Shocks?
B reconnection?
something else?
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 
γ2υ2 γ1υ1
Observations
✖
Model cannot explain:

theory
E


✖ ✔
γυ
g-rays
Synchrotron radiation
E*f(E)


Epeak clustering
spectral slope below peak
high radiative efficiency
Back to the blackboard
jet emission
Central
Blandford
& Znajek 1977
engine
Begelman & Li 1992
Meier et al. 2001
Koide et al. 2001
van Putten 2001
…
Barkov & Komissarov 2008
…
Acceleration
Internal
dissipation
The strength of the magnetic paradigm:
universally produces relativistic outflows
jets in galactic centers
gamma-ray bursts
(GRBs)
micro-quasars
~3M
~1052erg/s
~10M
~1037erg/s
M87; NASA/Hubble
MBH~109M
Power~1044…49erg/s
Magnetic Fields: critical for jet acceleration
and dissipation
energy content
magnetic reconnection region
Magnetic reconnection effective in
heating the jet
thermal component;
energetic particles
r
magnetic r 
component B
 
Γ>>1
kinetic
component
distance r
fields
Important
may in
beunderstanding
essential in powering
jet acceleration
the jet radiation
Michel 1969; …,
Eichler
Vlahakis
1993;
& Koenigl
2003; Komissarov
al. 2009; 2010;
Tchekhovskoy
et al. 2009;&2010;
Lyubarsky
2009; 2010;
al. 2011
Begelman
1998; et
Drenkhahn
& Spruit
2002; Nakamura
Meier
2004; Giannios
&Granot
Spruitet2006;
Moll 2009; McKinney & Blandford 2009; Mignone et al. 2010…
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
energy content

thermal
GRB
optically thin emission
magnetic
component
 
r
B
r

kinetic
component
τ~1
distance r
Photospheric spectrum

The simple physics behind the detailed Monte Carlo
Comptonization simulations
Te>>T
~Tphph
>T
τ~1
τ<<1
τ>>1
Inverse Compton
E*f(E)
synchrotron



τ<<1
τ~1
τ>>1
E
1 MeV
Photospheric emission: not at all thermal-like
Fermi
Swift
η=1000
η=590
Robotic
telescopes
η=460
typically
observed
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…
What determines Epeak of the photosphere?

The jet temperature at τ~1 (ignoring additional
heating; e.g., Meszaros & Ress 2000)
E peak
L1/524
 1 1/ 2 MeV,  > *
R6
E peak  1



1/ 4
52
1/ 2
6
L
R
 8 / 3
  MeV,  < *
* 
L52 1/ 4
Where *  2000 
 R6 
Emerging spectrum is quasi-thermal: typically Not
observed

Dissipation of energy
is required for Band spectrum

dissipation affects the location where Epeak forms!
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
Epeak forms here !!!
--- Te>Tph, for τ<30-50 (Compton y~1)

Numerical verification
Epeak indeed forms at τ~τeq~50
Giannios 2012
Key result for photospheric models

Analytic expression for the peak energy
E peak  1.5
4 / 3 1/ 3
1/ 62.5
2.5
1/ 6
53
L
MeV
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 !
Observations of GRBs:
the brighter, the faster, the higher Epeak
Liang et al. 2010
Ghirlanda et al. 2010
Other Implications

Prediction:
E co 
Giannios 2012


Observations
Ghirlanda et al. 2011
E peak

5
1/ 3 1/ 3
1/ 62.5
2.5
1/ 6
53
L
keV
All photospheric: GRBs, XRFs, ll GRBs?
They may all come from the jet photosphere!
Epeak~0.1-1 MeV
Γ
103
102
Epeak~30 keV
10
Epeak~1 keV
1049
1051
L (erg/s)
1053
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?”