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COSMIC RAY ACCELERATION and TRANSPORT LECTURE 3: Research TopicsPowerPoint Presentation

COSMIC RAY ACCELERATION and TRANSPORT LECTURE 3: Research Topics

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### COSMIC RAY ACCELERATION andTRANSPORTLECTURE 3: Research Topics

Pasquale Blasi

INAF/Arcetri Astrophysical Observatory

4th School on Cosmic Rays and Astrophysics UFABC - Santo André - São Paulo – Brazil

WHAT NEXT?

- We have found the elegant results of the theory of DIFFUSIVE SHOCK
- ACCELERATION in the TEST PARTICLE approximation and also found
- some of its limitations:
- TOO LARGE EFFICIENCIES TO EXPECT TEST PARTICLES
- THE EXPECTED DIFFUSION COEFFICIENT LEADS TO TOO LOW EMAX
- IF PARTICLES DIFFUSE THEY ALSO LEAD TO WAVE GROWTH

ALL THESE PHENOMENA SUGGEST THAT WE NEED TO MOVE BEYONDTHE TEST PARTICLE APPROXIMATION

NON LINEAR THEORY OF DIFFUSIVE SHOCK ACCELERATION

Dynamical Reaction of Accelerated Particles

PRECURSOR

Velocity Profile

v

SUBSHOCK

Undisturbed Medium

Shock Front

subshock Precursor

Conservation of Momentum

Transport equation

for cosmic rays

WHAT SHOULD BE EXPECT?

UPSTREAM

- PARTICLES WITH HIGH P FEEL
- LARGER COMPRESSION FACTOR
- THEN LOW P PARTICLES
- 2. THE TOTAL COMPRESSION
- BECOMES LARGER THAN 4
- 3. THE SPECTRUM SHOULD BE NO
- LONGER A POWER LAW!
- 4. AT HIGH P THE SLOPE SHOULD
- BE FLATTER THAN 2 !!!
- 5. THE GAS BEHIND THE SHOCK
- SHOULD BE COOLER THAN FOR
- AN ORDINARY SHOCK

High p

Velocity

SUBSHOCK

Rtot Rsub

PRECURSOR

INJECTION

THIS IS THE LEAST UNDERSTOOD ASPECTS OF ALL !

Ideal Collisionless Shock (More) Real Collisionless Shock

rL

WHICH PARTICLES AND HOW MANY PARTICLES

ENTER THE ACCELERATION CYCLE?

INJECTION…cont’d

f(p)

Maxwellian for the Downstream gas

T2

T1

Relation between

Rsub, ξand η

p

Pinj = pth,2

OUTPUT OF THE

CALCULATIONS

Particle Diffusion Wave Growth

Remember that we showed that a particle diffuses along a direction z where

there is an ordered magnetic field B0 because of a small perturbation dB

perpendicular to B0.

In this process the particles lose momentum in the z direction and this p

Is transferred to the dB waves.

This equals the momentum gained by the waves:

And imposing a perfect balance:

In the ISM this is ~10-3 yr-1 but

close to a shock front the grow

can be even larger!!!

dB IS AMPLIFIED BY PARTICLES

MAGNETIC FIELD AMPLIFICATION

Bell 2004, Amato & PB 2009

SMALL PERTURBATIONS IN THE LOCAL

B-FIELD CAN BE AMPLIFIED BY THE

SUPER-ALFVENIC STREAMING OF THE

ACCELERATED PARTICLES

SHOCK

Particles are accelerated because there is

High magnetic field in the acceleration region

High magnetic field is present because particles

Are accelerated efficiently

Without this non-linear process, no acceleration

of cr to High energies (and especially not to the

knee!)

Successes of the SNR paradigm1. Observation of X-ray rims

TYPICAL THICKNESS OF FILAMENTS: ~ 10-2 pc

The synchrotron limited thickness is:

Successes of the SNR paradigm2. Max energy and the knee

107

106

105

104

Magnetic field

amplification leads

to higher values of the

Maximum energy

MAXIMUM MOMENTUM

PROTON

KNEE

pmax/mc

Bohm Diff without

B-amplification

50 100 150 200

M0

PB, Amato & Caprioli, 2007

Observed proton spectrum

Data from Bertaina et al. 2008

Successes of the SNR paradigm3. evidence for a CR precursor ?

PRECURSOR

REGION RIM

Morlino, Amato, PB & Caprioli 2010

Successes of the SNR paradigm4. Balmer dominated shocks

DOWNSTREAM

ION Temperature LOWER because of CR acceleration

NEUTRAL Temperature HIGHER because of charge exchange

+

+

BROAD BALMER LINE IS NARROWER

NARROW BALMER LINE IS WIDER

Observations of Balmer dominated shocks

Helder et al. 2009

INFERRED EFFICIENCY of CR ACCELERATION 50-60% !!!

Observation of Balmer dominated shocksbroader narrow Balmer line

Sollerman et al. 2003

Broadening of the narrow line hints to a mechanism for heating of the neutrals upstream

on scales shorter than ionization scales TURBULENT HEATING

CHARGE EXCHANGE UPSTREAM

Observation of Balmer dominated shocks:Possible evidence for a CR precursor in the narrow Balmer line

A broadened narrow Ha line from upstream shows that the

neutrals and ions have some level of charge exchange

different bulk velocities and/or T’s between the two components

CR precursor

RXJ1713 with fermi data

- e/p Equilibration downstream? (Morlino et al. 2009)
- Very low value of Kep at given time
- Lines from non-equilibrium ionization ? (Ellison et al. 2010)
- What are those Fermi data points telling us?

A Puzzling situation

Courtesy of S. Funk

- Most SNR detected by Fermi have steep spectra (some exceptions, such as
- RXJ1713)
- The predicted spectra would naively require steep diffusion D(E)~E0.7 in conflict
- with anisotropy measurements

A small print in the theory of DSA: a point as important as poorly known

Velocity of scattering centers in the shock frame NOT velocity of the plasma

In general the velocity of scattering centers is small and there is no problem

BUT AMPLIFIED MAGNETIC FIELD high velocity ???

VERY MACROSCOPIC CONSEQUENCES ON SPECTRA!

ROLE OF NUCLEI

Caprioli, PB, Amato 2010

The dynamical reaction of nuclei on the shock structure

turns out to be VERY IMPORTANT

ROLE OF NUCLEI: The knee

Caprioli, PB, Amato 2010

WHERE DO GALACTIC CR end?

- The SNR paradigm hints to a galactic CR spectrum
- ending at ~a few 1017 eV
- Observations of chemical composition also suggest
- the same trend

?

ALTERNATIVE INTERPRETATION OF THE TRANSITION: MIXED COMPOSITION

Allard et al. 2005-2008

UNDERSTANDING THE DIFFERENCE: DIP VS MIXED

From Kampert 2008

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