THE ORIGIN OF COSMIC RAYS Implications from and for X and γ-Ray Astronomy

Pasquale BlasiINAF/Osservatorio Astrofisico di Arcetri, Firenze

Observations

Knee 2nd knee Dip/Ankle GZK?

Proton

knee

Helium

knee

Iron

?

Basics of the standard model…

CRs are accelerated in SN remnants up to some badly defined max energy, through DSA

The propagation in the Galaxy is diffusive on some unspecified background of waves

At some high energy CRs start being mainly of extra-galactic origin

The knee in the spectrum is either due to propagation effects or to acceleration inside the sources

The ankle might be the transition site from galactic to extragalactic CRs, or may be not…

A CRUCIAL ISSUE: the maximum energymaximum energy of accelerated particles

For the ISM, the diffusion coefficient derived from propagation is roughly

For a typical SNR the maximum energy comes out as For a typical SNR the maximum energy comes out as FRACTIONS OF GeV !!!FRACTIONS OF GeV !!!

Similar numbers would be obtained for galactic sources of similar age and in Similar numbers would be obtained for galactic sources of similar age and in

similar conditions. similar conditions.

0.60.310 3 29 αE=ED αGeV

2

2

1

1

21

3

u

ED+

u

ED

uu=Eτacc

PARTICLE ACCELERATION AT ASTROPHYSICAL SHOCKS IS EFFICIENT ONLY IF PARTICLES GENERATE THEIR OWN SCATTERING CENTERS!

age) time,(LossMin on timeAccelerati such that max E

SNR have a chance to explain the bulkOf CRs ONLY if

1.Efficient particle acceleration takes place (>10%)

2. The magnetic field close to the shock is strongly amplified and made highly turbulent

NON LINEAR THEORY OF

DIFFUSIVE SHOCK ACCELERATION

Non linear DSA –

I. Dynamical reaction of the Accelerated particles

The Basic Physics of Modified Shocks

Un

dis

turb

ed

Med

ium

Sh

ock F

ron

t v

subshock Precursor

00 uρ=xuxρ

xP+xP+xuxρ=P+uρ CRgg,2

02

00

tp,x,Q+p

fp

dx

du+

x

fu

x

fD

x=

t

f

3

1

Conservation of Mass

Conservation of Momentum

Equation of DiffusionConvection for theAccelerated Particles

Concave Spectra and Suppression of Gas Heating

PB, Gabici & Vannoni (2005)

10050,10,0 =M

The suppressed heating might have already been

detected (Hughes, Rakowski & Decourchelle (2000))

cm=p max 10 3

cm=p max 10 5 10

Increasing

Pmax

Ranki

ne-H

ugon

iot

Efficiency of Acceleration (PB, Gabici & Vannoni

(2005))

Note that only thisFlux ends up DOWNSTREAM!!!(CURVED SPECTRA)

This escapes out To UPSTREAM(DELTA FUNCTION)

There are Spectra and

Spectra…

ESCAPINGESCAPINGSPECTRUMSPECTRUM

pmax

PB, Gabici and Vannoni 2005

Depends on the temporal evolution of the SNR

Non linear DSA –

Magnetic field Amplification Induced by the AcceleratedParticles

MAGNETIC FIELD AMPLIFICATION Resonant Streaming Instability

(Achterberg 1983, Zweibel 1978, Bell 1978)

Non-resonant Streaming Instability (Bell 2004)

Firehose Instability

…

Cosmic Ray self-generated diffusion (a simple view)

)v(v ADCR mn

dtdP CRCR

A

W

vB

WdtdP 1

8

2

vDRATE OF MOMENTUM LOST BY CR

BUT THIS MUST EQUAL THE RATEOF MOMENTUM GAIN BY THE WAVES

GROWTHRATE OFWAVES

BY REQUIRING EQUILIBRIUM:

A

AD

gas

CR

vvv

nn cycW

Shock

SELF-CONSISTENT NON-LINEAR DSA WITH MAGNETIC FIELD AMPLIFICATION: THE DIFFUSION COEFFICIENT

Amato & PB 2006

Possible Observational Evidence for Amplified Magnetic FieldsPossible Observational Evidence for Amplified Magnetic Fields

240 µG

360 G

Volk, Berezhko & Ksenofontov (2005)

Rim 1

Rim 2

Lower

Fields

Some caution is needed inTaking into account the Damping

4-6 keV

Interpretation of the observations

Cutoff

Volk, Berezhko & Ksenofontov (2005)

Tycho SNR

Curv

ed s

pec

trum

Cu

toff

The case of SNR RXJ1713

NO THERMAL EMISSION FROM THIS REMNANT!!! May be caused by Heating suppression due to efficient Particle Acceleration?

Figure from Berezhko & Volk 2006

THE PHYSICS IS IN THE CUTOFFS!!!

SIM

BO

L X

THE PHYSICS IS IN THE THE PHYSICS IS IN THE CUTOFFSCUTOFFS The position of the X-ray cutoff is

independent of the strength of B (for Bohm diffusion)

The way the flux goes to zero tells us about the spectrum of accelerated electrons

The cutoff in the gammas tells us about the cutoff in the proton spectrum or in the electron spectrum

Ellison,Patnaude,Slane, PB, Gabici2007

The importance for the details implies that wehave to be goodin subtracting thethermal emission

Cutoffs and Pile-ups in the Electron Spectrum

Compression 4 Compression 7Bohm Diffusion Bohm Diffusion

Blasi 2007

THE ASSUMPTION OF EXPONENTIAL CUTOFF(α=1) IN THE ELECTRON SPECTRUM IS TYPICALLY UNJUSTIFIED

])exp[-(E/E )( max 0 ENEN

Maximum Energy in the Proton Spectrum

1st effect: the magnetic field is amplified thereby leading to higher pmax

2nd effect: the precursor slows down the upstream plasma thereby reducing the pmax

PB, Amato & Caprioli 2007

Self-Generated

Bohm modified

Bohm unmodified

knee

IF SNR ACCELERATE GALACTICCOSMIC RAYS WE ARE SUPPOSEDTO DETECT GAMMA RAY SPECTRA WITH CUTOFFS AROUND 10-100 TeV

CONCLUSIONS X-ray measurements seem to show magnetic field

amplification in SNR shocks…this could be a breakthrough in the investigation of the origin of CRs

The most important effects are to searched for in the position and shape of the cutoffs

X-ray Astronomy can play a crucial role in several ways: Imaging of the X-ray emitting region behind and

possible ahead of the shock (precursor) Precision measurement of the decaying region in

the synchrotron spectrum (aimed at identifying the thermal component and possible contaminations from secondary electrons)

Determination of the temperature downstream, related to the CR acceleration efficiency