Presentation on theme: "THE ORIGIN OF COSMIC RAYS Implications from and for X and γ-Ray Astronomy Pasquale Blasi INAF/Osservatorio Astrofisico di Arcetri, Firenze."— Presentation transcript:
THE ORIGIN OF COSMIC RAYS Implications from and for X and γ-Ray Astronomy Pasquale Blasi INAF/Osservatorio Astrofisico di Arcetri, Firenze
Observations Knee 2 nd 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…
maximum energy A CRUCIAL ISSUE: the maximum 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 FRACTIONS OF GeV !!! Similar numbers would be obtained for galactic sources of similar age and in similar conditions. PARTICLE ACCELERATION AT ASTROPHYSICAL SHOCKS IS EFFICIENT ONLY IF PARTICLES GENERATE THEIR OWN SCATTERING CENTERS!
SNR have a chance to explain the bulk Of 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 Undisturbed Medium Shock Front v subshock Precursor Conservation of Mass Conservation of Momentum Equation of Diffusion Convection for the Accelerated Particles
Concave Spectra and Suppression of Gas Heating PB, Gabici & Vannoni (2005) The suppressed heating might have already been detected (Hughes, Rakowski & Decourchelle (2000)) Increasing Pmax Rankine-Hugoniot
Efficiency of Acceleration (PB, Gabici & Vannoni (2005)) Note that only this Flux ends up DOWNSTREAM!!! (CURVED SPECTRA) This escapes out To UPSTREAM (DELTA FUNCTION)
There are Spectra and Spectra … ESCAPINGSPECTRUM p max PB, Gabici and Vannoni 2005 Depends on the temporal evolution of the SNR
Non linear DSA – Magnetic field Amplification Induced by the Accelerated Particles
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) vDvD RATE OF MOMENTUM LOST BY CR BUT THIS MUST EQUAL THE RATE OF MOMENTUM GAIN BY THE WAVES GROWTH RATE OF WAVES BY REQUIRING EQUILIBRIUM: Shock
SELF-CONSISTENT NON-LINEAR DSA WITH MAGNETIC FIELD AMPLIFICATION: THE DIFFUSION COEFFICIENT Amato & PB 2006
Possible Observational Evidence for Amplified Magnetic Fields 240 µG 360 G Volk, Berezhko & Ksenofontov (2005) Rim 1 Rim 2 Lower Fields Some caution is needed in Taking 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?
Figure from Berezhko & Volk 2006 THE PHYSICS IS IN THE CUTOFFS!!! SIMBOL X
THE PHYSICS IS IN THE CUTOFFS 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, Gabici 2007 The importance for the details implies that we have to be good in subtracting the thermal emission
Cutoffs and Pile-ups in the Electron Spectrum Compression 4 Compression 7 Bohm Diffusion Blasi 2007 THE ASSUMPTION OF EXPONENTIAL CUTOFF (α=1) IN THE ELECTRON SPECTRUM IS TYPICALLY UNJUSTIFIED
Maximum Energy in the Proton Spectrum 1 st effect: the magnetic field is amplified thereby leading to higher pmax 2 nd 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 GALACTIC COSMIC RAYS WE ARE SUPPOSED TO 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