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