THE ORIGIN OF COSMIC RAYS Implications from and for X and γ-Ray Astronomy
Pasquale BlasiINAF/Osservatorio Astrofisico di Arcetri, Firenze
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
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
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
The case of SNR RXJ1713
NO THERMAL EMISSION FROM THIS REMNANT!!! May be caused by Heating suppression due to efficient Particle Acceleration?
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