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announcement: next week only : office hours Tuesday 2-4

announcement: next week only : office hours Tuesday 2-4. X-ray astronomy. - with X-rays one normally uses energies instead of wavelengths eg 1 < E x < 100 keV use E = h n = hc/ l to convert - a photon with energy 1 keV has a wavelength of 1.3 nm high energy astrophysics.

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announcement: next week only : office hours Tuesday 2-4

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  1. announcement: next week only : office hours Tuesday 2-4

  2. X-ray astronomy • - with X-rays one normally uses energies instead of wavelengths • eg 1 < Ex< 100 keV use E = hn = hc/l to convert • - a photon with energy 1 keV has a wavelength of 1.3 nm • high energy astrophysics sources of X-ray emission • - fast moving electrons, deflected by ions or in a magnetic field • high energy atomic transitions not normally • found at temperatures generated by nuclear burning of stars

  3. the moon in X-rays diffuse X-ray background light from many distant sources reflection and absorption by the moon

  4. detectors originally detected with Geiger counters or proportional counters (gas volume with a high-voltage wire inside) - X-rays easily ionize gas atoms - ionization electrons attracted to the wire by the electric field - pulse generated by electron avalanche near the wire can also usescintillators coupled to photomultiplier tubes (PMTs) - X-ray scatters off atomic electron - recoil electron produces ionization or excitation along its path - scintillator converts this to light detected by the PMTs now one also uses semiconductors (eg CCDs)

  5. History X-rays do not penetrate the atmosphere – need to observe from space - field began in US in late 1940’s using rockets – observed X-rays from the sun - in 1960’s the center of the galaxy was detected Scorpius X-1 first satellite (SAS-I, also called Uhuru) launched Dec 12, 1970 off Kenya discovered pulsating X-ray sources Hercules X-1 - 1.24 sec period  compact source Cygnus X-1 – leading black hole candidate Leader of Uhuru team Riccardo Giacconi was awarded 2002 Nobel Prize in Physics

  6. Imaging X-ray Telescopes • - X-rays can’t be focused using conventional optics but they can • be deflected by grazing incidence reflections • - like skipping stones on water • with focussing one has an imaging detector technique was pioneered on Einstein observatory 1978-81 - field of view 25 arc-minutes (size of moon) - resolution for central 10 arc-min was 2 arc-sec (similar to earth-based optical)

  7. XMM Newton launched by ESA 1999 better for spectroscopy and faint sources (larger area) Chandra X-ray Telescope launched by NASA 1999 better for imaging (superior optics)

  8. Chandra Images (huge improvement over previous detectors) Crab nebula and pulsar at different times over one year smallest ring = 1 light year ~ 70 000 Earth-Sun distance pulsar at the centre of a supernova remnant (V Kaspi – McGill)

  9. Gamma-ray Astronomy gamma-rays are photons with energies in excess of about 1 MeV TeV (106 MeV) photons are routinely detected these days they originate in very violent astrophysical phenomena and are comparatively rare sources of gamma-rays: cosmic ray (high-energy particle) interactions with interstellar gas supernova explosions (the death of a star) electrons in magnetic fields like X-rays, gamma-rays do not pass through the atmosphere -- need satellites History: predicted that astronomical objects would emit gamma-rays in 1950’s but not detected until 1960’s - solar flares, gamma-ray backgroun since then numerous satellites:

  10. EGRET on the Compton Gamma-Ray Observatory launched in 1991 works by converting the photon (E > 1 MeV) into an electron-positron pair (e+e-) tracking devices follow the e+ and e- and their directions are used to get the incident direction of the gamma-ray energy of the gamma-ray is measured by scintillators and phototubes at the bottom of the detector two neutron stars

  11. Very High Energy (VHE) gamma-ray astronomy high energy gamma rays make showers of electrons and positrons when they hit the atmosphere these particles travel near the speed of light in vacuum and are therefore superluminal (ie they go faster than the speed of light in air) the “shock-wave” that results results in the emission of UV and blue light called Cherenkov Radiation v > c shock wave v < c no shock wave

  12. - gamma-rays cause particle showers which cause Cherenkov radiation • radiation is collected by large (10 m) mirrors and used to measure the • energy and direction of the incident gamma-ray

  13. During 1990s a few single-dish detectors (and one array of small dishes) were operated and explored the high energy sky Now there are arrays of large dishes being built to improve sensitivity first two telescopes of the 4 element HESS array in Namibia – reflectors are 12 m in diameter HESS ‘camera’ comprising 960 photomultiplier tubes

  14. Neutrino Astronomy - newest branch of astronomy - only kind of astronomy that does not use photons • - neutrinos are sub-atomic particles that have no electric charge • and have almost zero mass • only interact via the weak force • four basic forces in nature – strong (nuclear), electromagnetic, weak and gravitational • neutrinos are hard to detect because they can go through almost anything • postulated in 1933, by Wolfgang Pauli, to explain features of beta (neutron) decay • - named by Enrico Fermi (little neutral one) • only experimentally verified in 1956 by Reines and Cowan • useful messengers from astrophysical sources • they escape from dense environments that could block photons • (eg the centres of stars – solar neutrinos, supernova neutrinos)

  15. Neutrino Detectors • radiochemical: let neutrinos interact by inverse beta decay n + Z  e + Z ’ and look for traces of Z ‘ in a pure sample of Z eg the Homestake detector looked for radioactive argon in a huge tank of chlorine water Cherenkov: use a giant underground tank of water instrumented with photomultiplier tubes (sensitive light detectors) to detect the light from different particle reactions

  16. Super-Kamiokande (Japan) • originally built in 1980s to look for proton decay • (3000 tonne detector) • detected neutrino burst from supernova 1987a • measured neutrinos from the sun and earth’s • atmosphere • 50000 tonne detector built to continue neutrino work 50 cm diameter phototubes (largest made) partially complete detector partially filled detector

  17. Sudbury Neutrino Observatory (SNO) located in an INCO nickel mine in Sudbury, Ontario deepest neutrino experiment in the world uses 1000 tonnes of heavy water (D2) (heavy water on loan from CANDU nuclear reactor program) unique in that it can detect neutrino flavour (and solve the solar neutrino problem) acrylic vessel to contain heavy water under construction underground fish-eye photo of the acrylic vessel equipped with phototubes artist’s conception of the SNO detector

  18. very large detectors expected fluxes from very distant (extra-galactic) sources are small need very large detectors to hope to see anything large detectors exist or are under construction at the South Pole and in the Mediterranean Sea to detect Cherenkov light from very high energy neutrino interactions in ice or water ANTARES detector near Marseille

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