COSMIC RAY ORIGINS Stella Bradbury, University of Leeds, U.K. • g-ray sources • the cosmic ray connection • detection technique • galactic and extragalactic accelerators • future instruments and new targets • Ultra High Energies
Cosmic Rays ? On 7th August 1912, Victor Hess demonstrated that the flux of “ionising radiation” increased above 2 km altitude
< 0.1 % of the “cosmic rays” are actually g-rays • collection area of satellite detector ~ 0.8 m2 • collection area of Cherenkov Telescope ~ 40,000 m2 • typically number of g-rays per m 2 above energy E E-1.5
cosmic ray nuclei should produce g-raysin collisions with interstellar material diffuse g-raybackground along galactic plane? • high energy g-rays indicate extreme environments and particle acceleration processes • g-rays which are not deflected by galactic magnetic fields may point to localized cosmic sources g-ray astronomy
TECHNIQUE • < 50 GeV e-e+ pairs produced in satellite volume and trapped • > 250 GeV sample the Cherenkov light pool at ground calorimetric measurement • ~ 0.01% of primary energy Cherenkov light
g-ray proton • Background Rejection • g-ray generates “airshower” through e+e- pair production & bremsstrahlung • cosmic ray and air nuclei collide p0 g, p m + n (simulations rely on extrapolation from accelerator data) Simulated Cherenkov lateral distribution at ground:
Imaging Atmospheric Cherenkov Telescopes • Energy threshold depends on • Cherenkov light collection efficiency • location - altitude and background light level • trigger efficiency for g-rays
a single 12.5 mm Ø photomultiplier pixel of the Whipple Telescope camera subtends 0.12º • width of a typical g-ray Cherenkov image is 0.3º • use cluster trigger g-ray ? nucleon? local muon ?
field stars, night sky light • moving targets! Nature’s Challenges • humidity • unexpected loads! • temperature cycle • lightning
Analogue Optical Fibre Signal Transmission • 120 prototype channels based on VCSELs in the Whipple Telescope camera • 150 MHz bandwidth • low pulse-dispersion allows a short ADC gate less background light included • lightweight - 50 kg for a 1000 pixel camera vs. 400 kg for co-axial cable • attractiveness to rats - similar to co-ax
The Compton Gamma Ray Observatory 1991 - 2000 EGRET 100 MeV - 30 GeV • spark chamber • time of flight scintillators • NaI calorimeter
g-ray sources • Likely g-ray production mechanisms : • p+ + p+ p+ + p+ +p+ +p- +p0 then p0 g + g • thermal photon + p+ X then p0 g + g • e- bremsstrahlung or synchrotron g below a few 100 MeV • inverse Compton scattering of thermal photons g by relativistic e- Solar flare EGRET solar flare spectrum : • evidence for p+ collisions p0 decayg-rays of mean Eg ~ mpc2 ~ 67 MeV • excess g-rays < 100 MeV require e- bremsstrahlung
Diffuse background due to p+CR + Hnuclei p0 g g observed but where do we get the p+CR from?
The Crab Nebula, standard candle of TeV astronomy. • The Crab pulsar wind shock injects relativistic particles into its surrounding supernova remnant. Chandra X-ray image VLT optical image • 1965: TeV g-rays from Crab Nebula predicted • 1989: 9s detection above 700 GeV published from 82 hours of data TeV g-rays - point source
Spectral energy distribution g-ray production mechanism? TeV spectrum consistent with e- synchrotron self-Compton emission magnetic field ~16 nT within 0.4 pc of the pulsar.
Where docosmic ray nucleons come from? • Shell-type supernova remnants? • outer layers of dead star bounce off collapsing core (in which e- + p+ n + ne) • huge release of energy + O, N… Fe present • shock front propagates, sweeping up gas from interstellar medium 1st order Fermi acceleration • compressed B fields act as scattering centres for relativistic charged particles • particles gain momentum as they cross the shock front repeatedly
Detection of TeV g-rays from Cassiopeia A by HEGRA can still be explained as e- inverse Compton without e.g. a po decay component Chandra X-ray image of Cas A Still no conclusive evidence for acceleration of relativistic nuclei
Giant molecular clouds could act as a target for p+CR + H p0 g + g if bathed in uniform cosmic rays or as a cosmic beam dump for a neighboring particle accelerator such as a black hole binary: Cosmic ray production must be high in starburst galaxies where there is a high supernova rate and strong stellar winds?
Of 271 discrete sources detected by EGRET above 100 MeV • 170 remain unidentified • 67 are active galactic nuclei (AGN)
Active Galactic Nuclei ~ 1 % of galaxies have a bright central nucleus that outshines the billions of stars around it Radio and X-ray observations reveal relativistic jets presumed to be powered by a central supermassive black hole • rapid optical variability and lack of thermal emission lines in EGRET’s AGN suggest we are looking almost straight down the jet • g-ray emission region must be > a light day from AGN core to escape absorption via pair production - probably moving along jet • photon flux forward beamed and Doppler shifted
Rapid TeV g-ray flares emission region only ~ size of solar system! Whipple Telescope - Mkn 421 • optical depth for gTeV + gUV/optical e± must be less than 1 limits ratio of rest frame luminosity to size of emission region • a Doppler beaming factor of d 9 was derived from flare on right
g-ray Production Mechanism? Assume emission region is associated with shock accelerated particles, then pick any combination of : • synchrotron self-Compton e- + gsynch e- + g-ray • external inverse Compton e- + gexternal e- + g-ray • photo-meson production p+ + g p0, p± g-rays, e ± , n, n
Markarian 501 April ‘97 • Multiwavelength Observations • might expect simultaneous TeV g-ray and X-ray flares if due to the same e- population (self-Compton) • increase in e- density increase in ratio of self-Compton to synchrotron emission? • in external IC model g-ray & optical flares could come from different sites time lag? • proton induced cascade n outbursts? 4.2 2.6 1.7 1.1
Markarian 501 Spectral Energy Distribution • Synchrotron peak shifted from 1 keV to 100 keV during outburst • Power in X-rays & g-rays very similar - both much greater in 1997
TeV g-ray Energy Spectra of Mkn 421 & Mkn 501 Mkn 501 Mkn 421 Common feature is a cut-off at E0 ~ 4 - 6 TeV. Is this intrinsic to such objects - limit of accelerator? There are only 6 established TeV g-ray emitting AGN; the most recent flared to a detectable level on 17/05/02
Extragalactic Infrared Background : may cut-off g-ray flux from distant AGN as gg-ray + gIR e- + e+ ( cross-section peaks ateg-ray etarget~ 2 (mec2)2 )
TeV g-ray detection of AGN 600 million light years away limits on IR background density 10 more restrictive than direct satellite measurement in 4 - 50 mm range plagued by foreground starlight g-ray Horizon • Possible IR contributors: • early star formation • Very Massive Objects (dark matter candidates) • nheavy nlight + gIR for 0.05 eV < mn< 1 eV
A whole new class of objects? 20 keV - 1 MeV VLT optical afterglow of GRB000131 - at redshift 4.5 1013 light years distant (epoch of galaxy formation?) In 1969-70 the Vela 5 nuclear test detection satellites discovered g-ray bursts.
A hypernova ? Cosmological distances require an astronomical energy source! Invoke shocks in beamed jets! Merging neutron stars ?
Future Instruments • Swift • NASA Gamma Ray Bursts mission • hard X-ray, UV & optical instruments • launch autumn 2003 • INTEGRAL • ESA mission for spectroscopy & imaging at 15 keV - 10 MeV • launch 17th October 2002 • AGILE • Italian Space Agency mission optimised for fast timing & simultaneous coverage at 10 keV - 40 keV & 30 MeV - 30 GeV • launch beginning of 2004
GLASTlaunch due September 2006 lifetime > 5 years Energy range 20 MeV - 300 GeV Gamma Cygni
Lowering the energy threshold of ground-based g-ray detection Solar arrays: very large mirror area but small field of view. STACEE (2001 - ) 50 GeV - 250 GeV > 2000 m2 of heliostats reflect Cherenkov light via a secondary mirror onto a photomultiplier camera in the tower. CELESTE, Solar II & GRAAL use the same principle.
The MAGIC Telescope on La Palma Imaging telescope with a single 17m diameter dish. • operational late 2002 ? • Energy threshold < 15 GeV with future hybrid photodetectors or APDs
VERITAS array of 12m telescopes in Arizona: • 1st telescope on-line 2003 • 7 by end of 2006 • uses stereoscopic technique - viewing Cherenkov flash from different angles to improve background rejection
H.E.S.S. - an array of 4 ( 16 ?) 12 m diameter telescopes • energy threshold ~100 GeV • first telescope now in place at the Gamsberg
Flux sensitivity: bridging the gap between ground-based instruments and satellite data Mkn 421
g-rays from Cold Dark Matter? • CDM candidate neutralinos may be collected at the galactic centre • accelerator experiments restrict particle mass to 30 GeV - 3 TeV • an annihilation line may be observable with GLAST or next generation Atmospheric Cherenkov Observatories Simulated GLAST detection above diffuse background c c g g
UN-conventional TARGETS • neutralinosearchc c g g or c c q q e.g. p decays • primordial black holes - TeV photons emitted during final 1 - 0.1 s of evaporation ? • quantum gravity E dependent time dispersion of AGN flares ?? • Bose Einstein condensatese.g. coherentbunch of 100 GeV photons could mimic anairshowerdue to a single 1TeV photon • EGRET unidentified sources - position location to 0.02 should reduce number of possible counterparts by 10 • TeV all-sky surveys • cosmic ray composition studies - Cherenkov light emitted before primary interaction Z2 , independent of energy, arrives 3-6 ns after main image