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announcement:

next week only : office hours Tuesday 2-4

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X-ray astronomy

- with X-rays one normally uses energies instead of wavelengths

eg 1 < Ex < 100 keV use E = h= hc/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

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the moon in X-rays

diffuse X-ray backgroundlight from many distantsources

reflection and absorptionby the moon

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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 use scintillators 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)

detectors

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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 sourcesHercules X-1 - 1.24 sec period compact sourceCygnus X-1 – leading black hole candidate

Leader of Uhuru teamRiccardo Giacconi wasawarded 2002 NobelPrize in Physics

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Imaging X-ray Telescopes

- X-rays can’t be focused using conventional optics but they canbe 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)

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Chandra X-ray Telescope

launched by NASA 1999

better for imaging(superior optics)

XMM Newton

launched by ESA 1999

better for spectroscopyand faint sources(larger area)

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Chandra Images

Crab nebula and pulsar at different times over one year

pulsar at the centre of a supernova remnant(V Kaspi – McGill)

(huge improvement overprevious detectors)

smallest ring = 1 light year ~ 70 000 Earth-Sun distance

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QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

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Gamma-ray Astronomy

gamma-rays are photons with energies in excess of about 1 MeVTeV (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 gassupernova 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’sbut not detected until 1960’s - solar flares, gamma-ray backgrounsince then numerous satellites:

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EGRET on the Compton Gamma-Ray Observatorylaunched 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 incidentdirection of the gamma-ray

energy of the gamma-ray is measured by scintillatorsand phototubes at the bottom of the detector

two neutronstars

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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

no shock wavev > cshock wave

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

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During 1990s a few single-dish detectors (and one arrayof 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 elementHESS array in Namibia – reflectors are12 m in diameter

HESS ‘camera’ comprising 960 photomultiplier tubes

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Neutrino Astronomy

- newest branch of astronomy- only kind of astronomy that does not use photons

- neutrinos are sub-atomic particles that have no electric chargeand 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)

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Neutrino Detectors

radiochemical: let neutrinos interact by inverse beta decay + Z e + Z ’ and look for traces of Z ‘ in a pure sample of Z

eg the Homestake detector looked for radioactive argon in ahuge 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

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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’satmosphere - 50000 tonne detector built to continue neutrino work

50 cm diameter phototubes(largest made)

partially complete detector partially filled detector

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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)

artist’s conceptionof the SNO detector

acrylic vessel to contain heavy water under construction underground fish-eye photo of the

acrylic vessel equipped with phototubes

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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 underconstruction at the South Pole and in the Mediterranean Seato detect Cherenkov light from very high energy neutrino interactions in ice or water

ANTARES detector near Marseille