Supercritical phenomena

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Supercritical phenomena in high energy
astrophysics
The area 1. Gamma-ray bursts? 2. Soft gamma
repeaters? 3. Jets of active galactic nuclei
(blazars, BL Lacs)! 4. Microquasars? Many
high energy particles in a limited volume
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What is supercriticality?
A runaway behaviour. Our definition an
exponential regime of the energy release.
Nuclear explosion, nuclear pile dN/dt N
average_multiplicity(1-sink) 1/to
m gt 1 supercriticality
N exp(t/to)
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Possible supercritical phenomena I. Proton
bomb a cloud of ultrarelativistic protons --gt
ee-, photons Stern Svensson (1991).
Monte-Carlo, general effect (only conf.
proc.) Kirk Mastichiadis (1992). Analitics,
general effect Kazanaz Georganopoulos (2001).
Analitics, GRBs Kazanaz Mastichadis
Georganopoulos (2006) II. Photon avalanche
relativistic fluid --gt photons Derishev et al.
(2003) general idea Stern (2003) relativistic
shocks --gt electromagnetic catastrophe (MC) Stern
Poutanen (2006) jets of AGNs, MC,1D Stern
Poutanan (2007) jets of AGNs, MC, 2D
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A cloud of relativistic protons column density
N/cm2, Lorentz factor g p g --gt D --gt p,n, e,
g, n s(pg)x 10-28 cm2 1 photon of energy
eo 500/g proton gives - g photons above eo
if there is no optically thick (gg -gt ee-) soft
radiation - g2 photons if there is optically
thick radiation (a more natural case) The
supercriticality condition s(pg) x N g2 gt 1,
or Ng mpc2 gt 1025/g erg/cm2
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When this condition can be the case? 1. An
accreting black hole. For example a corona with
relativistic protons. Total energy of protons
R2 1025/g erg For 108 Msol black hole if R
10 Rg, then E 1053/g erg This is reasonable,
but how to construct a magnetic trap for these
protons?
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2. Gamma ray bursts. E4p 1053 erg. At R
1014 cm (photosphere) the column density of
protons in the shock can reach 1024 erg/cm2 in
the rest frame or 1024erg/cm2 /G in the shock
frame. We need 1025 erg/cm2/G. An amplifying
mirror can help.
The shock viewed from comoving frame
G
ejecta
External medium
Protons g G
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Reflection from external matter (Compton
only) shock frame amplification t G2 t
10-4, G 300 There is some stress to arrange
the supercriticality condition for GRBs. A
more serious objection Shock at such
conditions will not survive another supercritical
process electromagnetic catastrophe. Is the
proton bomb just a curious theoretical
construction?
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Photon avalanche a supercritical dissipation of
relativistic flows Fermi acceleration a charged
particle crosses a shock front back and forth
gaining factor G2 in energy at each round
trip. It is not efficient in the case of
relativistic shocks. Derishev et al. (2003)
and Stern (2003) independently suggested a
powerful alternative to Fermi acceleration partic
le conversion mechanism. Works excellent for
relativistic shocks in the presence of a dense
radiation field.
conversion
Fermi
Charge conversion pg --gt n... gg --gt
ee- Inverse Compton
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Stern (2003) electromagnetic catastrophe in
ultrarelativistic shocks (GRBs) A shock with G
gt10 propagating in environment with density
above 104 cm2 explodes in electromagnetic radiati
on and the shock front disappears.
G
Contact discontinuity
Shocked stellar wind
Ejecta (piston)
Stellar wind
x G2
Problems - too soft spectrum - magnetic field
should be below equipartition - can the external
shock reproduce the time variability?
Shock 1/3 c
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Stern Poutanen (2006, 2007) Dissipation of
AGN jets into gamma-rays via the photon avalanche
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M87
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Cen A
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Cygnus A
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Blazar spectra
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Blazars, some facts The luminosity in
gamma-rays up to 1049 erg/s (isotropic, EGRET
data). After correction for the beaming (103) it
becomes 1046 while maximal observed bolometric
luminosity of quasars is 1047 (accretion disk).
Typical Lorentz-factor 10 20, maximal 40.
(VLBI observation of superluminal motion). The
shortest variability time scale 20 min the
distance scale 3 1013 G2 1016 cm. Maximal
detected energy 100 TeV (BL Lacs).
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What kind of the energy source we deal with -
the jet internal energy, or - the total
energy of the jet respectively to the external
environment? Only the first scenario is
intensively studied so far (internal shocks,
magnetic reconnection) The second scenario is
much more efficient, but a mechanism of a strong
friction between the jet and external
environment is necessary. Can high energy photons
provide such friction?
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Arav, Begelman (1992), ApJ, 401, 125 Dissipation
of the jet energy dissipation through photon
exchange between the jet and external environment
due to Compton scattering . Requires a dense
environment. Could be the case for SS433. In
the case of AGNs we have no sufficient density to
provide Compton reflection But we have
sufficient density of soft photons from the
accretion disk to provide conversion of high
energy photons into ee- pairs We have
magnetic field to bind pairs to the relativistic
jet and to the external environment. We also
have some amount of isotropic photons. This is
enough to launch the supercritical dissipation.
How it works?
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The jet
X G2
The external environment
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How the mechanism works Very roughly it can be
represented as a 5-step cycle with energy
transmission Ci at each step 1. An external
photon produces a pair in the jet. (C1 1 for a
100 GeV photon). 2. The pair gains energy
respectively the ambient medium frame when it
turns around in the jet magnetic field. (C2 gtgt1
G2 on average) 3. Comptonization of soft
external photons by the pair (C3 lt1 but can
reach 0.5 - 0.7). 4. A photon leaves the jet and
interacts in the ambient medium (0.1 lt C4
lt0.5) 5. Reflection pair production
Comptonization back to the jet (C5lt 0.3)
Runaway condition (criticality) C1 x C2 x C3 x
C4 x C5 gt 1 Then photons breed
exponentially. (the average energy does not rise
this is a breeding, not an acceleration)
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What we need to launch the mechanism 1.
Transversal magnetic field in the jet Hj to
provide step 2 (energy gain). 2. Transversal
magnetic field He in the external environment to
provide step 5 (reflection).
3. Soft background radiation to provide steps 1,
4 via pair production and steps 3,5 via
Comptonization. There are two components -
direct radiation of the disk (multicolor
blackbody) - scattered, nearly isotropic
component from the broad line region and
surrounding dust. First, longitudinal,
component is sufficient at steps 1 and 5, the
second is necessary at steps 3 and
4. Compactness l L sT/R c mec2 for L 1044
erg/s at R 1017 cm l 10-4 4. A relatively
thin (D ltlt Rjet) transition layer between
relativistic jet and ambient medium. 5. A seed
high energy photon.
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Model of a quasar
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Our models A. Jet propagation the broad line
region. R 1017 cm. Source of the isotropic
light broad line emission. B. Jet at a parsec
scale (0.6 1019 cm). The source of the isotropic
IR radiation is the dust. C. Jet at 300 Rg
(1016 cm) The source of the large angle radiation
is the disk.
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The numerical model Jet the cylinder split
into 64 X 20 cells which can independently
decelerate, (two dimensional dust
approximation) Start a small amount of seed
high energy photons soft photon
background Simulation Large particle nonlinear
Monte-Carlo code (Stern 1988, Stern et al. 1995).
Compton scattering, pair production, synchrotron
radiation, fine tracking in magnetic field. 219
221 Large particles. We solve a time dependent
problem a transition process from a
non-radiating initial state to a steady-state.
Boundary conditions a flow with fixed Lorentz
factor G at the inlet. Output emitted
synchrotron and Comptonized photons of evolving
spectrum and intensity, differential deceleration
of the jet
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7 simulation runs 1. A, Ld 1045 erg/s, Lj
1045 erg/s G20 efficiency 0.30 2. A,
1044 1044 20
0.53 3. A, 0.5 1044
1043 20
0.56 4. A, 1045
1045 10
0.12 5. A, 1044 1044
10 0.31 6. B,
1045 lt 1045
20 0.21 7. C 1043
1043 20
0.19
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Results
In all cases we observe an exponential rise of
the energy release.
1045, BLR
1045, 2 parsec
1044, BLR
1045, BLR, G 10
Total energy release into photons vs. time
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Lightcurves Fluctuations sometimes exceed the
numerical noise. This is a natural behaviour of
a near-critical dynamical system.
Self-organized criticality a full analogy with
a sand pile. The role of the pile slope plays the
velocity gradient.
BLR, G20
3 1043
1045
BLR, G10
1044
1045
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Beam-on photon spectra at infinity. An
important detail is missing the synchrotron
peak
Higher compactness
Lower compactness
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Side-on spectra. There is a hint on the
synchrotron peak, but less prominent than
observed.
A higher compactness
A lower compactness
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Electron spectra. They share up to 4 of the jet
energy.
Slowly cooling pairs
!!! If these pairs were slightly reheated, this
would solve the problem of the synchrotron peak
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2 parsec
1045
1045
0.5 1044
G
G
BLR, 0.5 1044
z20
Radius
Dz 2
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Beamed photons
BRL,1045
Isotropic photons
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Summary I. The proton bomb Hardly possible
at accreting black holes Pulsars?
Possible in external shocks of GRBs, more likely
as an additional amplifying mechanism. II. The
photon avalanche. 2.1 GRBs. Should work if
the external shock does exist at R lt1016cm. Can
not explain all GRBs because of disagreement
in the hardness of the low energy part of
spectra. There are some problems with time
variability. Can be responsible for a fraction of
GRBs.
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2.2. The photon avalanche in relativistic jets. -
In the case of typical jets of AGNs it is
difficult to avoid this phenomenon. - It is
well motivated by the requirement of a high
efficiency of Gamma-ray emission. - It when
taken alone can not explain the entire spectrum
of a blazar. The photon avalanche should be
followed by a moderate pair reheating,which is
quite natural. - Then the synchrotron-Compton
model used to fit blazsar spectra is a
misinterpretation. Yes, the low energy peak has
the synchrotron origin, but is emitted further
downstream the jet by cooled pairs.
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- Very likely the decelerating jet behaves as a
dynamical system in the near-critical regime.
This can explain the time variability of
blazars. - There should be a number of bright
effects which we cannot reproduce with dust
approximation generation of internal
shocks, turbulence Future (M?)HD massive
parallel computations