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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 ?.

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slide1

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
slide2

Cosmic Rays ?

On 7th August 1912, Victor Hess demonstrated that the flux of “ionising radiation” increased above 2 km altitude

slide3

< 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
slide4

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
slide5

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
slide6

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:

slide7

Imaging Atmospheric Cherenkov Telescopes

  • Energy threshold depends on
  • Cherenkov light collection efficiency
  • location - altitude and background light level
  • trigger efficiency for g-rays
slide8

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 ?

slide9

field stars, night sky light

  • moving targets!

Nature’s Challenges

  • humidity
  • unexpected loads!
  • temperature cycle
  • lightning
slide10

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
slide11

The Compton Gamma Ray Observatory 1991 - 2000

EGRET 100 MeV - 30 GeV

  • spark chamber
  • time of flight scintillators
  • NaI calorimeter
slide12

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
slide13

Diffuse background due to p+CR + Hnuclei p0  g g observed but where do we get the p+CR from?

slide14

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

slide15

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.

slide16

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
slide17

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

slide18

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?

slide19

Of 271 discrete sources detected by EGRET above 100 MeV

  • 170 remain unidentified
  • 67 are active galactic nuclei (AGN)
slide20

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
slide21

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
slide22

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
slide23

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

slide24

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
slide25

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

slide26

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 )

slide27

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
slide28

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.

slide29

A hypernova ?

Cosmological distances  require an astronomical energy source!

Invoke shocks in beamed jets!

Merging neutron stars ?

slide30

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
slide31

GLASTlaunch due September 2006

lifetime

> 5 years

Energy range

20 MeV - 300 GeV

Gamma Cygni

slide32

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.

slide33

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
slide34

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
slide35

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
slide36

Flux sensitivity:

bridging the gap between ground-based instruments and satellite data

Mkn 421

slide37

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

slide38

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
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