COSMIC RAY ORIGINSStella Bradbury, University of Leeds, U.K.
• -ray sources
• the cosmic ray connection
• detection technique
• galactic and extragalactic accelerators
• future instruments and new targets
• Ultra High Energies
On 7th August 1912, Victor Hess demonstrated that the flux of “ionising radiation” increased above 2 km altitude
Cosmic Rays ?
• collection area of satellite detector ~ 0.8 m2
• collection area of Cherenkov Telescope ~ 40,000 m2
• typically number of -rays per m 2 above energy E E-1.5
< 0.1 % of the “cosmic rays” are actually -rays
• cosmic ray nuclei should produce -raysin collisions with interstellar material diffuse -raybackground along galactic plane?
• high energy -rays indicate extreme environments and particle acceleration processes
• -rays which are not deflected by galactic magnetic fields may point to localized cosmic sources -ray astronomy
• < 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
TECHNIQUE
Background Rejection
• -ray generates “airshower” through e+e- pair production & bremsstrahlung
• cosmic ray and air nuclei collide 0 + simulations rely on extrapolation from accelerator data)
Simulated Cherenkov lateral distribution at ground:
-ray proton
Imaging Atmospheric Cherenkov Telescopes
Energy threshold depends on
• Cherenkov light collection efficiency
• location - altitude and background light level
• trigger efficiency for -rays
• a single 12.5 mm Ø photomultiplier pixel of the Whipple Telescope camera subtends 0.12º
• width of a typical -ray Cherenkov image is 0.3º
• use cluster trigger
-ray ? nucleon? local muon ?
• humidity• unexpected loads!• temperature cycle• lightning
Nature’s Challenges• field stars, night sky light• moving targets!
• attractiveness to rats - similar to co-ax
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
EGRET 100 MeV - 30 GeV
The Compton Gamma Ray Observatory 1991 - 2000
• spark chamber
• time of flight scintillators
• NaI calorimeter
Solar flare
• evidence for p+ collisions decay-rays of mean E ~ mc2 ~ 67 MeV
• excess -rays < 100 MeV require e- bremsstrahlung
-ray sourcesLikely -ray production mechanisms : • p+ + p+ p+ + p+ + + + - + 0 then 0 • thermal photon p+ X then 0 • e- bremsstrahlung or synchrotron below a few 100 MeV• inverse Compton scattering of thermal photons by relativistic e-
EGRET solar flare spectrum :
Diffuse background due to p+CR + Hnuclei 0 observed
but where do we get the p+CR from?
The Crab Nebula, standard candle of TeV astronomy.
•1965: TeV -rays from Crab Nebula predicted
• 1989: 9 detection above 700 GeV published from 82 hours of data
Chandra X-ray image
TeV -rays - point source
VLT optical image
• The Crab pulsar wind shock injects relativistic particles into its surrounding supernova remnant.
Spectral energy distribution -ray production mechanism?
TeV spectrum consistent with e- synchrotron self-Compton emission magnetic field ~16 nT within 0.4 pc of the pulsar.
Where do cosmic ray nucleons come from?
Shell-type supernova remnants?
• outer layers of dead star bounce off collapsing core (in which e- + p+ n + e)
• huge release of energy + O, N… Fe present
• shock front propagates, sweeping up gas from interstellar medium
• compressed B fields act as scattering centres for relativistic charged particles
• particles gain momentum as they cross the shock front repeatedly
1st order Fermi acceleration
Chandra X-ray image of Cas A
Detection of TeV -rays from Cassiopeia A by HEGRA can still be explained as e- inverse Compton without e.g. a o decay component
Still no conclusive evidence for acceleration of relativistic nuclei
Giant molecular clouds could act as a target for p+
CR + H + 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)
• photon flux forward beamed and Doppler shifted
• -ray emission region must be > a light day from AGN core to escape absorption via pair production - probably moving along jet
• rapid optical variability and lack of thermal emission lines in EGRET’s AGN suggest we are looking almost straight down the jet
~ 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
Active Galactic Nuclei
• optical depth for TeV + UV/optical e± must be less than 1 limits ratio of rest frame luminosity to size of emission region
• a Doppler beaming factor of 9 was derived from flare on right
Rapid TeV -ray flares emission region only ~ size of solar system!
Whipple Telescope - Mkn 421
-ray Production Mechanism?
• synchrotron self-Compton e- + synch e- + -ray
• external inverse Compton e- + external e- + -ray
• photo-meson production p+ + 0, ± -rays,
e ± , n,
Assume emission region is associated with shock accelerated particles, then pick any combination of :
Markarian 501 April ‘97
Multiwavelength Observations
• might expect simultaneous TeV -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 -ray & optical flares could come from different sites time lag?
• proton induced cascade outbursts?
4.2
2.6
1.7
1.1
Markarian 501 Spectral Energy Distribution
• Power in X-rays & -rays very similar - both much greater in 1997 • Synchrotron peak shifted from 1 keV to 100 keV during outburst
TeV -ray Energy Spectra of Mkn 421 & Mkn 501
There are only 6 established TeV -ray emitting AGN; the most recent flared to a detectable level on 17/05/02
003.014.2 E
E
eEdE
dN
007.095.1 E
E
eEdE
dN
Mkn 421
Mkn 501
Common feature is a cut-off at E0 ~ 4 - 6 TeV. Is this intrinsic to such objects - limit of accelerator?
Extragalactic Infrared Background :
may cut-off -ray flux from distant AGN as -ray + IR e- + e+
( cross-section peaks at -ray target
~ 2 (mec2)2 )
TeV -ray detection of AGN 600 million light years away limits on IR background density 10 more restrictive than direct satellite measurement in 4 - 50m range plagued by foreground starlight
Possible IR contributors:
• early star formation
• Very Massive Objects (dark matter candidates)
• heavy light + IR
for 0.05 eV < m< 1 eV
-ray Horizon
In 1969-70 the Vela 5 nuclear test detection satellites discovered -ray bursts.
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?)
A hypernova ?
Merging neutron stars ?
Cosmological distances require an astronomical energy source!
Invoke shocks in beamed jets!
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
Future Instruments
GLAST launch due September 2006
lifetime > 5 years
Energy range
20 MeV - 300 GeVGamma Cygni
CELESTE, Solar II & GRAAL use the same principle.
Lowering the energy threshold of ground-based -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.
The MAGIC Telescope on La Palma
Imaging telescope with a single 17m diameter dish.
Energy threshold < 15 GeV with future hybrid photodetectors or APDs
operational late 2002 ?
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
• energy threshold ~100 GeV
• first telescope now in place at the Gamsberg
H.E.S.S. - an array of 4 ( 16 ?) 12 m diameter telescopes
Flux sensitivity:
bridging the gap between ground-based instruments and satellite data
Mkn 421
-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
UN-conventional TARGETS• neutralino search or q q e.g. 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 condensates e.g. coherent bunch of 100 GeV photons could mimic an airshower due 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