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Transcript
Astroparticle physics with high-energy photons I – The physics Alessandro de Angelis Lisboa 2003 http://wwwinfo.cern.ch/~deangeli 2 The starting point Physics constructs models explaining Nature (or better our observations of Nature, or better observations of our interactions with Nature) We know Nature mostly through our eyes, which are sensitive to a narrow band of wavelengths centered on the emission wavelength of the Sun 3 We see only partly what surrounds us We see only a narrow band of colors, from red to purple in the rainbow Also the colors we don’t see have names familiar to us: we listen to the radio, we heat food in the microwave, we take pictures of our bones through X-rays… 4 What about the rest ? What could happen if we would see only, say, green color? The universe we don’t see When we take a picture we capture light (a telescope image comes as well from visible light) In the same way we can map into false colors the image from a “X-ray telescope” Elaborating the information is crucial 5 We know there is something important we don’t see 6 velocity v radius r Gravity: G M(r) / r2 = v2 / r enclosed mass: M(r) = v2 r / G Luminous stars only small fraction of mass of galaxy Many sources radiate over a wide range of wavelengths 7 8 The high-energy spectrum Eg > 30 keV (l ~ 0.4 A, n ~ 7 109 GHz) Although arbitrary, this limit reflects astrophysical and experimental facts: Thermal emission -> nonthermal emission Problems to concentrate photons (-> telescopes radically different from larger wavelengths) Large background from cosmic particles 9 And that things can look different The subject of these lectures… (definition of terms) 10 Detection of high-energy photons from space High-E X/g: probably the most interesting part of the spectrum for astroparticle What are X and gamma rays ? Arbitrary ! (Weekles 1988) X X/low E g 1 keV-1 MeV 1 MeV-10 MeV medium 10-30 MeV HE 30 MeV-30 GeV VHE 30 GeV-30 TeV UHE 30 TeV-30 PeV EHE above 30 PeV No upper limit, apart from low flux (at 30 PeV, we expect ~ 1 g/km2/day) 11 Outline of these lectures 0) Introduction & definition of terms 1) Motivations for the study high-energy photons 2) Historical milestones 3) X/g detection and some of the present & past detectors 4) Future detectors 12 1) Motivations for the study of X/g Probe the most energetic phenomena occurring in nature Nonthermal Nuclear de-excitation/disintegration Electron interactions w/ matter, magnetic & photon fields Matter/antimatter ann. Decay of unstable particles Clear signatures from new physics 13 Motivations (cont’d) Penetrating No deflection from magnetic fields, point ~ to the sources Magnetic field in the galaxy: ~ 1mG R (pc) = 0.01p (TeV) / B (mG) => for p of 300 PeV @ GC the directional information is lost Large mean free path Regions otherwise opaque can be transparent to X/g Good detection efficiency Large mean free path… Transparency of the Universe 14 15 Astronomy Scales Nearest Stars Nearest Galaxies Nearest Galaxy Clusters 4.5 pc 450 kpc 150 Mpc 1 pc= 3 light years ‘GZK cutoff’ 16 HE cosmic rays Interaction with background g ( infrared and 2.7K CMBR) p g N Sources uniform in universe 100 Mpc 10 Mpc HE gamma rays Mrk 501 120Mpc g g e+ e Milky Way Mrk 421 120Mpc 17 Transparency of the atmosphere 18 PHYSICS GOALS Pulsars GRBs AGNs VHM particles Anomalous events Cold Dark Matter SNRs New g-ray Photon propagationInvariance of c Backg. Acceleration mechanisms and the origin of cosmic rays 19 Energetic protons and electrons in the vicinity of astrophysical objects might produce gammas Synchrotron radiation by electrons in magnetic fields could be boosted to TeV energies by inverse Compton scattering If acceleration mechanisms involve hadronic interactions, there are many 0 -> gg (& the g give a clear signature) 20 Active galaxies Many sources, mostly classified according to observational criteria Unified AGN model (Begelman et al. 1984): 10% of the accreted mass is transformed into radiation Different models predict different g spectra But warning : ~300 sources @ the GeV scale, only 15 @ the TeV 21 Pulsars Rapidly rotating neutron stars with T between ~1ms and ~1s Strong magnetic fields (~100 MT) Mass ~ 3 solar masses Crab pulsar R ~ 10 Km (densest stable object known) For the pulsars emitting TeV gammas, such an emission is unpulsed X-ray image (Chandra) 22 g-ray bursts (History, I) An intriguing puzzle of today’s astronomy… A brief history Beginning of the ‘60s: Soviets are ahead in the space war 1959: USSR sends a satellite to impact on the moon 1961: USSR sends in space the 27-years old Yuri Gagarin 1963: the US Air Force launches the 2 Vela satellites to spy if the Soviets are doing nuclear tests in space or on the moon Equipped with NaI (Tl) scintillators 23 g-ray bursts (History, II) 1967 : an anomalous emission of X and g rays is observed. For a few seconds, it outshines all the g sources in the Universe put together. Then it disappears completely. Another in 1969... After careful studies (!), origination from Soviet experiments is ruled out The bursts don’t come from the vicinity of the Earth 1973 (!) : The observation is reported to the world Now we have seen hundreds of gamma ray bursts... 24 g-ray bursts: why they are important They might represent objects near the edge of the observable Universe The energy could be 1015 times larger than the energy from a supernova E ~ 1045 J They could be a new observational tool for cosmologist g-ray bursts: what we know and what we’d like to know They come from every direction in the sky Frequently no optical emission (BeppoSAX 1997) Far away from the galaxy A puzzle… Time duration is wildly variable Mostly extragalactic Afterglows after > 1h… Several mechanisms proposed, enormous energies: a great chance that they’re so far... 25 Importance of the multiwavelength approach 26 27 A recent consensus Many sources can be related to SN remnants Mechanism accounting for repeated shocks (Dar, De Rujula) Matter of precise poninting: Work for GLAST Synergy with gravitational wave detectors Work for LIGO But: Maybe different kinds of bursts… 28 Probability of bursts Present estimate: 1 GRB/100My/Milky Way Galaxy => Already ~ 100 GRB in our galaxy Energy ~ 1045 J According to Dar, it is not unlikely that a GRB has already interacted with the atmosphere… 29 Diffuse background radiation Is it really diffuse (<- produced at a very early epoch) or a flux from unresolved sources ? Angular resolution is the key Physics in extreme conditions: photon propagation Due to gg -> e+e-, CMB and visible light absorb g at the PeV and at the TeV At the GKZ cutoff (1020 eV) the Universe regains transparency to g The transparency of the Universe gives insights on the infrared/ optical diffuse background Quantum gravity (Amelino-Camelia et al., Ellis et al.) V = c (1 - e E/EQG) Effects on GRB could be O(100 ms) 30 31 => Intergalactic g absorption Photons interact with the IR background => relationship source distance / maximum observed photon energy Measurement from the distortion of AGN spectra Data in the range 50 GeV - 300 GeV would be crucial And an important byproduct: the best constraints on Lorentz violation, photon oscillations etc. 32 Particle physics at high energies Today’s accelerator physics limited & many early discoveries in particle physics came from the study of cosmic rays Motivation for particle physicists to join 33 Particle Physics Particle Astrophysics Energy of accelerated particles Active Galactic Nuclei Binary Systems SuperNova Remnant LHC CERN, Geneva, 2007 Cyclotron Berkeley 1937 34 DM Candidates M > ~ 40 GeV if SUSY (LEP) 35 Probing dark matter: WIMPs Some dark matter candidates (e.g. SUSY particles) would lead to monoenergetic g lines through annihilation X q X q or gg or Zg 36 Anomalous events Anomalous showers at UHE (> 7 PeV) from Cygnus X-3 (Samorski & al. 1983): almost no photons… Increasing total photon X-section due to virtual gluons Increasing neutrino X-section New particles Anomalous events (highly penetrating hadrons) Normally killed as “irreproducible results”, but… Study of exotic objects: other phenomena Top-Down : Decay of massive cosmic strings (1015 GeV, Kolb & Turner 1990) Unknown transients Time resolution is the key 37 38 2) Historical milestones 1952 Prediction of He X/g high energy emission (Hayakawa) 1957 Sputnik 1 1958 Inventory of cosmic sites expected to radiate in the X/g (Morrison) 1968 (11 years after the Sputnik): X emission of the galaxy 1972 g from Crab Nebula 1973 First report on gamma ray bursts 1978 Gamma-ray spectroscopy : e+e- annihilations @ the GC 1983 Nuclear processes at the GC 39 Some selected results 40 X/g Satellites in the ’90s GRANAT (SIGMA), 1990/97 Accreting black holes Jets CGRO, 1991/2000 BATSE, thousands of GRB EGRET, hundreds of GRB in the HE region BEPPO Sax, 1996/2002 SN remnants 41 Gamma satellites EGRET [+BATSE] Diffuse g emissions dominate the g-ray sky. After removing the identified point sources, ~ mass distribution Moreover, isotropic emission at high latitude going like E-2.07+-0.03 Pulsars, all observed also in the radio (apart from Geminga) Most point sources unidentified Gamma-Ray Bursts, not expected in any model. No apparent E cutoff, E as high as 18 GeV The pulsar spectrum depends on the wavelength => Different energies produced in different regions Results from ground-based 42 43 VHE sources Observations in the ‘90s confirm earlier detection of VHE emissions from Crab nebula and discover new VHE sources in pulsars (PSR 1706-44, Vela) No pulsed emission TeV emission from AGN, with flares Mkr 421 Mkr 501 Models differ in the kind of particles emitted & E spectrum Synchrotron model => 2 humps, one from synchrotron and one from inverse Compton Variability over a large range of timescales Observational hole upper limit from EGRET 44 UHE (and EHE ?) No sources of UHE g (only diffuse emission) No signal from established VHE g sources No signals from hypothetical new sources (primordial black holes, black holes accreting from a nearby star…) Although the GRB spectrum from BATSE/EGRET is hard (E-2), no UHE g seen (and they would be expected…) Absorption in the em field ? Detection problems ? 45 Comment on VHE and UHE gammas Ground-based astronomy operates in regimes of large background => results are matter of discussions VHE emissions from Crab and Vela are accepted as genuine No episodic emission widely accepted yet Many astronomical models of AGN suffer from lack of information in the ~50 GeV region… Fill the hole No relevant information for particle physics, yet Relevant is what should have been observed, but has not TeV gammas from SN shocks should have been seen Correlation between EGRET objects, TeV emissions and SNR ? 46 The progress at a glance 47 Sensitivity 48 Summary High energy photons (often traveling through large distances) are a great probe of physics under extreme conditions Observation of X/g rays gives an exciting view of the HE universe Many sources, often unknown Diffuse emission Gamma Ray Bursts No clear sources above ~ 30 TeV What better than a crash test to break a theory ? Do they exist or is this just a technological limit ? We are just starting… Next lecture: many new detectors being built or planned Future detectors: have observational capabilities to give SURPRISES !