Transcript Slide 1

Gamma-Ray Bursts (GRBs) and
collisionless shocks
Ehud Nakar
Krakow
Oct. 6, 2008
Gamma-Ray Bursts
Flash of g-rays that last several
seconds – the prompt emission
NASA
NASA web site
long lasting decaying radiooptical-X-ray emission – the
afterglow
Fox et. al., 05
Longs & shorts
Kouveliotou et al. 1993
?
Short GRBs
Unknown – possibly NS-NS
or BH-NS coalescence
Long GRBs
collapse of a massive star
Prompt emission - observations
(long GRBs)
 Duration 1-1000s
 1052-1054 erg (isotropic equivalent)
1050-1052 erg/s (isotropic equivalent)
 ~0.01-2 MeV photons
Non-thermal spectrum;
very high energy tail
(at least up to GeV)
 Rapid variability
(less than 10ms)
Prompt emission in the fireball model
G>100
Inner
Engine
g-rays
Relativistic
Internal
dissipation
Wind
(p-e- or p-e+-eor EM)
106cm
1013-1016cm
Prompt emission - theory
(long GRBs)
The emission source is internal dissipation within a
relativistic outflow, G>100!
Radiation process – Unknown
Leading candidates are synchrotron and IC
Outflow composition and magnetization – Unknown
Unmagentized pair plasma is unlikely
Collisionless shocks ? – only if the outflow is baryonic
 mildly relativistic internal shocks
Afterglow - Observations
Peaks in X-ray first (minutes-hour) , then in optical
(hours-days) and finally in radio (days-years)
Fn
B, V, R, I
Temporal & spectral structure: Broken power law
n
t (days)
Stanek et al. 99
Galama et al. 99
The internal-external fireball model
g-rays
Inner
Engine
Relativistic
Internal
dissipation
Wind
Afterglow
External
Shock
(p-e- or p-e+-eor EM)
106cm
1013-1016cm
1016-1018cm
External shock afterglow model
Hydrodynamics:
A relativistic blast-wave that propagates into a perfect
fluid (shocked fluid energy concentrated in – DR ~ R/g2)
g
Shocked
plasma pressure
R/g2
upstream
pressure
•Blast wave decelerates while shoveling mass - gR-3/2
(for a constant external density).
•The burst emission ionize and destruct dust in the
circum-burst medium  upstream is ionized,
unmagnetized, p-e plasma.
Radiation modeling:
•Shock crossing  electron acceleration N(g)  g-p for
g>gm
• ee - fraction of electrons energy out of the internal
energy at the shock crossing
• eB - fraction magnetic field energy out of the internal
energy at any time
 synchrotron + Synchrotron Self-Compton radiation
The model fit for five free parameters:
Ek, n, p, ee and eB
The basic (slow cooling) Synchrotron Afterglow
Spectrum and its time evolution:
t0
t-3/2 t-1/2
Sari et al 1998
Synch self
Absorption
Observations
Model
Fn
Low energy
Fnn-1/3
Galama et al. 99
High Energy Fn  n-p/2
n
The typical parameters that fit the data
ee ~ 0.1
eB ~ 0.01-0.001
p = 2-2.7
Ek,iso = 1052-1054 erg (Comparable to Eg,iso)
n ~ 0.01-10 cm-3 (expected in ISM)
Typical scales
G~100 @ t=100s
G~10 @ t=1 day
G~2 @ t=1 month
(t – observer time since the burst)
shock Lorentz factor -
B Downstream
B Upstream
Bd~ mG-G
Bu~ mG (eB,up~10-9)
Width of shocked plasma ~1012cm @ t=100s
~1016cm @ t=1week
Skin depth
~107 cm
Main microphysical assumptions in the basic
model:
• The shock is thin compared to the emitting region.
• Electrons are coupled to the protons just through
the shock.
• All Electrons are accelerated – relaxing this
assumption can change the best fit parameters by a
factor f<mp/me (Eichler & Waxman 05)
• ee and eB are constant in time and space – eB
cannot drop significantly far in the downstream
(Rossi & Rees 02)
Afterglow observations strongly suggest that weakly
magnetized relativistic collisionless shocks:
•Generate magnetic field with ~10-4-10-2 of equipartition.
•This magnetic field survives long after crossing the shock
(>107 skin depths).
•Polarization indicate that the magnetic field is anisotropic
on large scales with ratio ~2:1
•Efficiently accelerate electrons (in equipartitoin with
protons energy) at least up to TeV
Note: External shock is the most popular and successful
afterglow model. But, it is not the only model and it cannot
explain all afterglow observations in all bursts.
Short GRBs
• Prompt emission is similar to long GRBs
• About dozen observed afterglows (mostly in X-ray)
suggest a similar mechanism and physical properties as
in long GRB afterglows
• The progenitor is an old stellar system and therefore
the expected circum burst medium is the interstellar
medium – unaffected by massive stellar wind.
The ability of collisionless shocks to generate field an
accelerate particles is not unique to upstream which is
shaped by stellar wind (e.g., with high density clumps)
Nakar 07
Magnetic field generation in GRB
external shocks
Equipartion field on a skin depth scale is thought to be
generated in unmagnetized shocks by the Weibel
instability (Moiseev & Sagdeev 63; Kazimura et al 98; Medvedev & Loeb
99, …)
But … without sustaining process it is expected to decay
over a similar scale (Gruzinov 01; Chang et al., 08)
How can the shock generate strong magnetic field
that survives over ~109 skin depths?
Suggested processes:
• Interaction of the thermal plasma (upstream and/or
downstream) with accelerated particles via kinetic
instabilities (e.g., recent numerical results by Keshet et al. 08
and Spitkovsky 08)
• Amplification of the downstream field via
downstream vorticity generated by
• Density inhomogeneity in the upstream (e.g., Sironi &
Goodman 07)
• Angular energy anisotropy of decelerating blast
wave (Milosavljevic, Nakar & Zhang 07)
• Interaction between streaming protons and the
upstream plasma via nonresonant streaming instability
(e.g., Bell 2004, Milosavljevic & Nakar 06, Reville et al 06, …)
Generation of upstream density inhomogeneies
by streaming protons (Couch, Milosavljevic & Nakar 2008) `
assumption: protons are accelerated in the shock by Fermi process
shock frame
Upstream frame
~R/G2
~R/G
G
e
upstream
p
p
e
p
upstream
IC cooling grantee that if protons are accelerated to gp>103
then protons stream farther upstream then electrons
• Nonresonant streaming instability amplifies the
magnetic field and produces density inhomogeneities
(e.g., Bell 04)
• Even if the field is not amplified by orders of
magnitude (e.g., Pelletier et al 08), density contrast of order
unity is generated. Such contrast is enough in order to
amplify the downstream field to the observed levels by
generating downstream vorticity.
• In GRB external shocks there is enough time to
generate order unity density contrast even if the seed
field is the pre-existing mG field
Summary
• GRB prompt emission may be a result of mildly relativistic
collisionless shocks (if GRB jets are baryonic).
• GRB external shocks are unmagnetized ultra-relativistic
collisionless shocks and are the prime candidates to be the
source of the observed afterglows, in which case these shocks:
• Generate long lasting magnetic field to sub-equipartition
level
• Efficiently accelerate electrons at least to Tev energies
• Several processes were suggested as the source of the
generated magnetic field in these shocks. None of which is
confirmed yet.
Thank!
Are all the electrons need to be accelerated?
g dn/dg
ee fee
eB feB
E E/f
n n/f
g
If only a fraction me/mp<f<1 is accelerated the above
mapping results in similar fit to f=1 (Eichler & Waxman 05).
Can the magnetic field decay after the shock?
No decay
Decay after
crossing 1%
No decay
Decay after
crossing 1%
A decay of the magnetic field after the plasma crosses
much less than 1% of the shocked shell is hard to
explain by the observations (Rossi & Rees 02)