Transcript ppt

Gamma-ray bursts from
magnetized collisionally heated
jets
Indrek Vurm
(Hebrew University of Jerusalem)
in collaboration with
Andrei Beloborodov (Columbia University)
Juri Poutanen (University of Oulu)
Raleigh 2011
Dissipation in compound flows
(Beloborodov 2010)
τγγ=1
R*~1012 cm
τT=1
R0
Rs
Rn
D
τn=1
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I
S
S
Γp~500
Γn< Γp
I
P
A
T
I
O
N
= MeV
Protons and neutron flows decouple at Rn
= GeV
Proton flow accelerates until Rs at the expense of radiation
Γn< Γp  n-p collisions  dissipation of bulk kinetic energy
Two branches
 Elastic: heats the proton component
 Inelastic: pion production  muons  electron-positron pairs
Numerical method: kinetic equations
Mihalas (1980), Beloborodov (2011)
Radiative transfer equation
in the flow frame:
I
- specific intensity
j
- emissivity

ν
  cos 

- opacity
- photon frequency
- angle relative to radial direction
- bulk Lorenz factor
Kinetic equation for pairs:
Processes: Compton, synchrotron,
pair-production/annihilation,
Coulomb collisions
N
- pair density

- electron Lorentz factor
t
- proper time
- heating/cooling rate
Simulation setup
τT=1
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Rs
R
Simulations run in the comoving
n
R0
frame, starting at Rn
D I S S I P
RTE and pair kinetic equations
evolved in comoving time
τn=1
Γn< Γp
Initial conditions at Rn from
relativistic fluid-dynamics
Evolution of particle and photon distributions followed selfconsistently until τT«1, τγγ « 1.
Model parameters:
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Lp, Ln – kinetic luminosities of the proton and neutron flows
Γp, Γn – corresponding Lorentz factors
εB – magnetization (fraction of flow kinetic energy in B-field)
R0 – radius at the base of the flow
τγγ=1
A T I O N
Spectra: non-magnetized flows
red - Monte Carlo (Beloborodov 2010)
blue - kinetic
MeV
pairs
GeV
Heating-cooling
balance
cooling,
pair cascades
injection
Thermal
Thermal
Compton
Annihilation
line
Non-thermal
Compton
γγ - absorption
Lp=1052 erg/s
Ln=2x1051 erg/s
Γp=600, Γn=100
r0=107 erg/s
Magnetized flows
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εB ≠ 0 ⇒ synchrotron emission
from non-thermal pairs
Magnetization:
Synchrotron peak:
Magnetized flows
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εB ≠ 0 ⇒ synchrotron emission
from non-thermal pairs
εB << 1
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softer low-energy slopes
soft excess below ~50 keV
Magnetization:
Synchrotron peak:
Magnetized flows


εB ≠ 0 ⇒ synchrotron emission
from non-thermal pairs
εB << 1
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
softer low-energy slopes
soft excess below ~50 keV
Magnetization:
Synchrotron peak:
Magnetized flows


εB ≠ 0 ⇒ synchrotron emission
from non-thermal pairs
εB << 1
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

softer low-energy slopes
soft excess below ~50 keV
εB ≈ 1
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suppression of pair cascades
steep high-energy slopes
distinct GeV component
Magnetization:
Synchrotron peak:
Magnetized flows


εB ≠ 0 ⇒ synchrotron emission
from non-thermal pairs
εB << 1



softer low-energy slopes
soft excess below ~50 keV
εB ≈ 1



suppression of pair cascades
steep high-energy slopes
distinct GeV component
Magnetization:
Synchrotron peak:
Magnetized flows
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red - GRB 090902B
black - simulation
εB ≠ 0: synchrotron emission
from non-thermal pairs
εB << 1
softer low-energy slopes
soft excess below ~50 keV
εB ≈ 1
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GRB 090902B
suppression of pair cascades
steep high-energy slopes
distinct GeV component
Abdo et al. (2009)
Magnetization:
Synchrotron peak:
Low-energy slope
Photon index vs magnetization
Low-energy photon indices in the
commonly observed range for wide
range of magnetizations
Nava et al. 2011
Soft excess

Significant excess below ~15 keV
in 14% of bright BATSE bursts
86 bright bursts (BATSE)
Excesses relative to PL
50/(1+z) keV
15 keV
Preece et al. 1996
Low-energy emission
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Partially self-absorbed
synchrotron emission predicts
a universal power-law α = -1
Can extend to the optical
band, typical delay ~1 sec
Es  r 1
- SSA energy
J s ( Es )  r 3 - emissivity near Es
J s r 3  Ls ( E )  const.
  1
Radiative efficiency
Collisional dissipation
retains its efficiency in
magnetized flows
Heated flows
ϵ = Lγ/L ~ 0.5
Lγ – radiative luminosity
L – kinetic luminosity
flow
Summary
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Collisional dissipation in magnetized flows:
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Band shape preserved
Low-energy photon indices in the commonly observed range over
several orders in magnetization
Soft excess, distinct high-energy emission component
Robust prediction of low-energy emission with α = -1
High radiative efficiency maintained