Transcript ppt

Bulk Comptonization GRB Model and its
Relation to the Fermi GRB Spectra
Demosthenes Kazanas
NASA/GSFC
Apostolos Mastichiadis
Un. Of Athens
References:
DK, M. Georganopoulos, A. Mastichiadis 2002
A. Mastichiadis, DK 2006
DK, A. Mastichiadis, M. Georaganopoulos 2007
A. Mastichiadis, DK 2009

There are (at least) two outstanding issues with the
prompt GRB emission (Piran 2004):
 A. Dissipation of the RBW free energy. Energy stored
in relativistic p’s or B-field. Sweeping of ambient protons
stores significant amount of energy in p’s anyway.
Necessary to store energy in non-radiant form, but hard
to extract when needed.
 B. The
presence of Epeak ~0.1 – 1.0 MeV.
If
prompt emission is synchrotron by relativistic electrons
of Lorentz factor (LF) same as shock Ep ~G4, much too
strong to account for the observations.

We have proposed a model that can resolve both
these issues simultaneously. The model relies:
On a radiative instability of a relativistic
proton plasma with B-fields due to the
internally produced sychrotron radiation (Kirk &
Mastichiadis 1992 ).
 1.
On the amplification of the instability by
relativistic motion and scattering of the
internally produced radiation by upstream
located matter, a ‘mirror’ (Kazanas & Mastichiadis 1999).
 2.
D
pg
e+e-
eB
Bg
In the blast frame the
width of the shock D ~
R/G is comparable to
its observed lateral width
thereby considering all
processes taking place in
a spherical volume of
radius D

The instability involves 2 thresholds:
(All particles behind the shock are assumed to have Lorentz factors
equal to the shock one G, i.e. no accelerated particle populations!)
1. A kinematic threshold for the reaction p g  e- e+
The photon at the proton rest frame must have energy
greater than 2 me c2 . The synchrotron photon has
an energy ES ~ bG2 at the plasma frame and ~ bG3
on the proton frame. So the condition reads
b G3 > 2 me c2 or G > (2/b)1/3
2. A dynamic threshold :
At least one of the synchrotron photons must be able to
produce an e- e+ - pair before escaping the plasma
volume. Because each electron produces Ng ~
G/bG2 ~ 1/bG photons, we obtain the following
condition for the plasma column density (note
similarity with atomic bombs!)
nspgR > bG
or nspgR G2 > bG3 ~ 2
Similarity of GRB/Nuclear Piles-Bombs


The similarity of GRB to a “Nuclear Pile” is more
than incidental:
1. They both contain lots of free energy stored in:
Nuclear Binding Energy (nuclear pile)
 Relativistic Protons or Magnetic Field (GRB)

2. The energy can be released explosively once
certain condition on the fuel column density (and
not mass) is fulfilled (Note: no particle acceleration required!!).

The instability requirements are greatly reduced if
the radiation produced at the shock is mirrored by
upstream located matter (because the energy of each
photon increases by G2; DK, A. Mastichiadis 1999) to the
following conditions:
b G5 > 2
n spg R G4 > bG5 ~ 2
For n ~ 1, R ~ 1017R17, G ~ 211 (n R17)1/4, T ~ 40 sec
THE SPECTRA
Min(mec2 G1+1, bG6+1) ~ 100 GeV photons
BC, SSC
bG4+1 ~ 1 MeV photons (GBM photons are due to BC)
BC RS
bG2+1 ~ 10 eV O-UV photons
Syn.
RBW
R/G2
Mirror
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We have modeled this process numerically. We
assume the presence of scattering medium at R
~1016 cm and of finite radial extent.
We follow the evolution of the proton, electron
and photon distribution by solving the
corresponding kinetic equations.
We obtain the spectra as a function of time for the
prompt GRB emission.
The time scales are given in units of the comoving
blob crossing time Dco/c ~ R / G2c ~ 2 R16/G2.6 sec.
Prompt GRB Spectra (Mastichiadis & Kazanas 2006)
BC: b G5
S : b G3
SSC : meG2
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We have also modeled the propagation of a
relativistic blast wave through the wind of a WR
star (that presumably collapses to produce the
relativistic outflow that produces the GRB).
In this case we also follow the development of
the blast wave LF and the radiative feedback on
it.
In this scenario the “scattering screen” necessary
for the model to work is provided by the presupernova star wind, the length scales smaller,
R0~1013 cm and the GRB is a “short GRB”.
Evolution of the LF and the luminosity
Evolution of Spectra with Time (eB~1)
Evolution of Luminosity with Time (eB~0.01)
Ep decreases
with decreasing
Luminosity. The
BC component
Decreases faster
Than the SSC leavin
The LAT flux after
The GBM one is
Gone.
Aftreglow, GRB, XRF, Unification

Inclusion of non-thermal particle populations
and repeating the same arguments as above one
obtains the evolution of Epeak with G or with
time. A simple calculation gives (DK, AM, MG
2007)
Ep ~ 4 10-2 [G(t)/50]2 ~ t-3/4
(talk/poster by R. Margutti on Monday)
Conclusions
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The “nuclear pile” GRB model provides an over all satisfactory
description of several GRB features, including the dissipation
process, Ep and the Fermi observations.
Provides an operational definition of the GRB prompt phase.
No particle acceleration necessary to account for most of
prompt observations (but it is not forbidden!).
GBM photons due to bulk Comptonization ( => possibility of
high polarization ~100%).
It can produce “short GRB” even in situations that do not
involve neutron star mergers.
Exploration of the parameter space and attempt to systematize
GRB phenomenology within this model is currently at work.
Distribution of LE indices
a
The spectra of doubly scattered component (Mastichiadis &
DK (2005))
S=1, a=-1
S=1/2, a=-3/2
S=2, a=0
120 G
12 G
1.2 G
0.12 G
Fig. 6.| Plot of t he phot on luminosity evolut ion as t his is measured in t he comoving frame
in t he EE case where np = ne = 104 cm¡ 3 while B varies (bot t om to t op) from 0.12 G t o 120
G by increment s of a fact or of 10. The rest of t he paramet ers are as st at ed in t he t ext . Time
is measured in blob crossing t imes and t he value t = 0 has been set at t he inst ant when t he
Eiso of the three different spectral components as a function of B for G=400 and
np=105 cm-3. x 103 denotes the relative g-ray – O-UV normalization of GRB 990123,
041219a.
1 MeV
100 GeV
X
103
O-UV
Epeak as a function of the magnetic field B
Variations
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If the “mirror” is in relative motion to the RBW then the
kinematic threshold is modified to b G3 G2rel ~ 2; Grel is the
relative LF between the RBW and the “mirror”.
The value of Epeak is again ~ 1 MeV, however the
synchrotron and IC peaks are higher and lower by G2rel than
G2 .
In the presence of accelerated particles the threshold
condition is satisfied even for G< (2/b)1/5. This may explain
the time evolution of GRB941017 (Gonzalez et al. 04)
GRB flux is likely to be highly polarized (GRB 031206,
Coburn & Boggs 03).
This model applicable to internal shock model (photons
from downstream shell instead of “mirror”).
{ 23 {
Fig. 6.| Plot of t he phot on luminosity evolut ion as t his is measured in t he comoving frame
in t he EE case where np = ne = 104 cm¡ 3 while B varies (bot t om to t op) from 0.12 G t o 120
G by increment s of a fact or of 10. The rest of t he paramet ers are as st at ed in t he t ext . Time
is measured in blob crossing t imes and t he value t = 0 has been set at t he inst ant when t he
RBW ent ers t he re° ect ion zone.
Then ....
to
4 >
>
¾
p° R n » b¡ or ¾p° R n ¡ » 2
the source. Hence, the
¡
+
e p° ! e e react ion
2
re N ' ° c =b° c = 1=b° c
tron photons produced
Shock – Mirror Geometry
formation region, generally not much different than the
reshold condit ion
µ ¶ 1=5
2
5 >
>
¡ b » 2 or ¡ »
:
b