Particle acceleration in the universe

Transcription

1 Particle acceleration in the universe Some issues and challenges Etienne Parizot (APC Université Paris Diderot - France)

2 Astrophysics 2 Everything we know about the universe comes from the observation of light from the cosmos! (+ the knowledge of the laws of Physics, of course!) Masses, distances, temperatures, magnetic fields, chemical compositions, energy densities, mechanical power, nuclear reactions, ages, velocities, gravitational fields everything! central notion of astrophysical sources : relatively well-defined, localized objects, which can be isolated from the environment or background observed through their emissions at various wavelengths spectrum made of continuum + lines sources contrast with the so-called diffuse emission (NB: this may depend on the instrument resolution!)

3 Astrophysics 3 In passing, let s note that astrophysics is not specifically an experimental science, but an observational science there are experiments, with detectors and very precise measuring instruments, but you cannot prepare your own set up, change the conditions of what you observe, run a given sequence of events with your own parameters, etc. observations of classes of objects with similar behavior or aspects, with different parameters or environmental conditions + numerical experiments : simulations of processes with chosen ingredients trying to fit (reproduce) observations + actual experiments relevant to astrophysics: exciting and important new area of activities: see e.g. talk by Pisin Chen

4 High-energy astrophysics 4 First identified astrophysical sources: stars! identified in a loose sense: means pin-pointed and distinguished as individual entities in the sky But stars are identified in the modern sense only since the 20 th century! This means that the following is known: - emission process of the light, - origin of the power, - general physical description of the object - basic physical parameters - distribution of matter and processes at work - past and future, etc. i.e. know what it is and understand what s going on in physical terms

5 High-energy astrophysics 5 The development of spectroscopy and spectro-imagery has brought powerful tools to understand astrophysical sources The development of non visible astronomy led to the discovery of new types of sources (radio waves, IR, UV, X-rays, gamma-rays ) Necessity to understand the new sources model them, identify the emission processes, describe their structure and dynamics, etc. Various classes of sources

6 High-E astrophysics sources 6 Supernova remnants

7 High-E astrophysics sources 7 Active Galactic Nuclei (AGNs)

8 High-E astrophysics sources 8 Supernova remnants Active Galactic Nuclei Pulsars Pulsar wind nebulæ Novæ Micro-quasars Magnetars X-ray binaries Gamma-ray bursts Hot spots Superbubbles Relativistic jets

9 Non thermal sources 9 Common feature to all high-energy astrophysical sources: non-thermal emission! Non-thermal radiation spectrum => produced by a population of energetic particles which are not thermalized ~> out-of-equilibrium Non-thermal particles => Energetic particles accelerated in situ by some dynamical process, through an electromagnetic mechanism Non-thermal processes Synchrotron emission, Bremsstrahlung, Inverse Compton, nuclear de-excitation, fluorescence, π 0 decay, X-ray lines, etc.

10 Energetic particles 10 Energetic particles are ubiquitous in the universe The modeling of high-energy astrophysics sources always involves the description of energetic particle acceleration What are the acceleration mechanisms in the universe?

11 Energetic particles 11 Energetic particles are ubiquitous in the universe The modeling of high-energy astrophysics sources always involves the description of energetic particle acceleration What are the acceleration mechanisms in the universe? Energetic particles are also observed directly (i.e. not just through the induced radiation in the sources where they are accelerated) COSMIC-RAYS Everything we know about the universe is inferred from light coming from the cosmos, but also from these cosmic rays: additional messengers!

12 Cosmic rays 12 Energetic particles, which fill and diffuse through the Galaxy and (at the highest energies) the entire universe! Discovered on Earth through their ionizing power (Coulomb!) Hess (1912) demonstrates that they come from space! ~> the cosmic rays at sea level mostly consist of secondary particles produced by the interaction of the primary cosmic rays with the molecules in the upper atmosphere 1938: Pierre Auger discovers the atmospheric showers induced by cosmic rays, and deduces the existence of cosmicrays with unbelievable energies: E ~10 15 ev! ~ : cosmic rays are used to study high-energy physics and give birth to particle physics! (discovery of antimatter, muons, pions, strange particles, etc.)

13 Key discovery: atmospheric showers : Coincident detection of secondary particles over large areas from the cascade induced by a single cosmic-ray event (Pierre Auger) 1 very energetic particle particle shower atmospheric shower many secondary particles

14 32 orders of magnitude A wonder of the Physical world! 14 CR flux The cosmic-ray spectrum! ev 100 MeV 12 orders of magnitude Energy

15 32 orders of magnitude The cosmic-ray energy spectrum 15 Flux ~ 1 particle / m 2 / second Out of equilibrium!!! ~ 1 particle / m 2 / yr 100 MeV ev Energy ~ 1 particle / m 2 / billion years!

16 The cosmic-ray energy spectrum 16

17 The cosmic-ray energy spectrum 17 Galactic extragalactic

18 Galactic cosmic rays 18 GCRs play a ley role in the Galactic ecology! Heating and ionization of the interstellar medium (ISM) Regulation of star formation Direct influence on astrochemistry Production/amplification of turbulent magnetic field Nucleosynthesis of Li, Be and B Dynamical equilibrium of the different phases of the ISM Energy density (>~ 1 ev/cm 3 ), comparable to light and magnetic field

19 Ultra-high energy cosmic rays 19 Existence of cosmic rays with macroscopic energies! 1962: a cosmic ray with E ev!!! = several joules! Lorentz factor of v 0, c 1 second 3500 years 1.5 m d(earth,sun) 15 october 1993: ev!!! ~ 50 joules! UHECRs are very interesting for high-energy astrophysics and astroparticle physics What are their sources? How are they accelerated? What can they teach us about high-energy physics? Can we use them as new (non photon) messengers from high-energy sources?

20 Origin of cosmic rays? 20 Cosmic rays sources are still not clearly identified As charged particles, cosmic rays are deflected by magnetic fields Larmor radius: r L = E/qBc Proton E = ev B = 3 µg r L ~ 1/3 parsec r L << size of the Galaxy isotropization UHECRs should not be deflected much, and thus show the direction of their source brand new astronomy! (cosmic-ray astronomy!) However, UHECRs are extremely rare! Unknown sources!

21 Particle acceleration in the universe 21 Ubiquitous acceleration mechanism: Diffusive Shock Acceleration Multiple diffusions of the particles across a shock wave Example: expanding shock of a supernova remnant! Magnetic reconnection Example: solar flare Unipolar induction Example: pulsars Still unknown/unexplored mechanisms? Example: currently unidentified sources? Unnoticed sources? UHECR sources?

22 Particle acceleration in the universe 22 Magnetic reconnection Example: solar flare Plasma flow with frozen magnetic field Change of magnetic field configuration energy release with transitory electric field (db/dt) particle acceleration NB: many open questions!

23 Particle acceleration in the universe 23 Unipolar induction Fast rotating permanent magnet Example: pulsar Huge magnetic fields: up to T Very large angular velocities: millisecond periods! Huge induced potential drops and E fields Efficient acceleration NB: huge E fields in vacuum and in plasmas IZEST!

24 Diffusive shock acceleration Supernova explosion (~ 3/century) supersonic ejecta: V = 10 4 km/s super-alfvénic flow collisionless shock wave Chandra (satellite X) Tycho (1572) 24 Red kev, Green kev, Blue kev Key aspect of the shock wave = discontinuity in velocity! V shock

25 Diffusive shock acceleration Supernova explosion (~ 3/century) supersonic ejecta: V = 10 4 km/s super-alfvénic flow collisionless shock wave Chandra (satellite X) Tycho (1572) 25 Red kev, Green kev, Blue kev Key aspect of the shock wave = discontinuity in velocity! + magnetic turbulence! resonant interaction between energetic particles V shock and plasma waves

26 Diffusive shock acceleration 26 Reflection off magnetic walls No energy gain, because a B field does not produce any work

27 Diffusive shock acceleration 27 Simple analogy Tennis ball bouncing off a standing wall v v elastic bounce unchanged velocity

28 Diffusive shock acceleration 28 Simple analogy Tennis ball bouncing off a standing wall v v elastic bounce unchanged velocity v V v + 2V unchanged velocity with respect to the racket elastic bounce ball acceleration

29 Diffusive shock acceleration 29 Reflection off magnetic walls No energy gain, because a B field does not produce any work moving magnetic structure energy gain! V

30 Diffusive shock acceleration 30 Reflection off magnetic walls No energy gain, because a B field does not produce any work moving magnetic structure or energy loss! ( drop shot at tennis!) V

31 Diffusive shock acceleration 31 Reflection off magnetic walls No energy gain, because a B field does not produce any work moving magnetic structure energy change [equivalent to the work of the induced E field ] V

32 Diffusive shock acceleration 32 Always head-on interactions across a shock wave! shock front n 2, p 2, T 2 n 1, p 1, T 1 v 2 downstream medium v 1 upstream medium velocity discontinuity: Dv/c In the downstream rest frame, the upstream medium is coming towards the particles that cross the shock In the upstream rest frame, the downstream medium is coming towards the particles that cross the shock

33 Diffusive shock acceleration 33 Always head-on interactions across a shock wave! shock front n 2, p 2, T 2 v 2 downstream medium n 1, p 1, T 1 v 1 upstream medium velocity discontinuity: Dv/c Energy gain at each shock crossing! compression ratio Balance between exponential energy growth and constant probability of escaping away from the shock (due to the global drift along the flow in the shock rest frame) universal power law spectrum in E -2!!

34 Limitations of shock acceleration 34 Magnetic turbulence and waves must be present on both sides of the shock shock front V shock ~ easy downstream (shocked medium) waves resonantly produced upstream by energetic particles themselves tricky! It works: we do see particle acceleration at collisionless shocks! (supernovæ, extragalactic, interplanetary, etc.) important problem for relativistic shocks! Challenging for ultra-high-energy cosmic rays (UHECR)

35 Limitations of shock acceleration 35 Keep the particle inside the accelerator! Shocks fronts are not infinite planes! Key limitation, due to the size of the accelerator The Larmor radius of the particle must be smaller than the size of the accelerator In fact, diffusion-advection at the shock implies: ( work of an effective induced E field ) so-called Hillas criterion

36 Hillas plot 36

37 Hillas plot 37

38 Limitations of shock acceleration 38 Hillas criterion not so many candidates for ultra-highenergy cosmic rays (UHECRs)! Optimistic view : sources are among the few candidates the particle acceleration process works at its maximum possible efficiency we roughly see the end of the acceleration spectrum Pessimistic view : Adding refinements and taking into account actual conditions will significantly reduce the maximum energy and make the process simply fail for UHECRs Optimistic in another way! it just requires other ideas for particle acceleration in the universe!

39 Limitations of shock acceleration 39 Acceleration (energy gain) competes with energy losses! The longer the particle stays in the accelerator, the higher its probability to interact with ambient fields or particles energy losses - synchrotron radiation - Inverse Compton scattering - photo-pion production - photo-dissociation Problem for large shocks Problem for high-power regions Can severely challenge the Hillas criterion!

40 New ideas for particle acceleration? 40 What about wake-field acceleration? See the works of Tajima, Takahashi, Chen, Hillmann, Ebisuzaki Application to Active Galactic Nuclei? see the talk by T. Ebisuzaki this afternoon! Any role in gamma-ray bursts? (Hugely powerful events!) They emit in a few seconds the total energy radiated by the Sun in 10 billion years! Ultra-relativistic outflows and huge amount of high-energy photons in a small volume (10 46 J in a few tens of km?) Short timescale of acceleration can we avoid losses? In any case, one should investigate non linear effects A new field within astrophysics, very little explored (if at all!) obvious connections with the izest community

41 Other possible connections 41 Exploration of high-energy physics Hadronic physics from UHECR interactions in the atmosphere (shower physics, cross sections, etc.) Exploring fundamental physics at ev Highest-energy particles in the universe can we use them as the cosmic rays were used in the first half of the 20 th century to discover new structures and new physics? Exploring space-time structure UHECRs propagate in space-time at an unexplored energy scale may feel small-scale structures Lorentz Invariance Violation constraints from UHECRs and gamma-ray astronomy!

42 Attacking the UHECR puzzle 42 Go into space to increase the statistics at UHE energy JEM-EUSO! UV telescope with large fieldof-view, high sensitivity and high frequency (400 khz) on the International Space Station Observe km 2 at once! Momentum is building up! Stay tuned!

43 EUSO-Balloon: successful pathfinder! 43 Pathfinder and fully operational prototype of the JEM-EUSO technology (with all subsystems) Successful flight on 24 th 25 th of August (Timmins, Ontario) [Flight campaign funded by CNES]

44 Astrophysics and IZEST 44 The acceleration of particles in the universe is challenging and not fully understood new ideas are welcome! There are extreme astrophysical environments (e.g. huge E fields) where non linear electromagnetic effects and relativistic optics could be very important must be studied! UHECRs offer a way to explore high-energy physics and fundamental physics at the highest energies known Being so challenging for astroparticle physics also makes them particularly precious: they may guide us to key discoveries! let s intensify the search! Unidentified (either unknown or not understood) sources wait for major advances: may require cross-disciplinary insight! let s push our interactions further!