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Gamma-Ray Astronomy Call no. 07483 Assoc. Prof. Markus Böttcher Clippinger # 339 Phone: 593-1714 E-mail: [email protected] Literature Volker Schönfelder: The Universe in Gamma Rays Springer-Verlag, Berlin, Heidelberg, New York, 2001 Other References: • G. B. Rybicki & A. P. Lightman: Radiative Processes in Astrophysics John Wiley & Sons, New York, 1979 • Malcolm S. Longair: High Energy Astrophysics Cambridge University Press, 1981 • Reinhard Schlickeiser: Cosmic Ray Astrophysics Springer-Verlag, Berlin, Heidelberg, New York, 2002 Preliminary Schedule Sept. 11 The gamma-ray sky; detection techniques for gamma-rays Sept. 18 Gamma-ray telescopes Sept. 25 Gamma-ray emission mechanisms Oct. 2 Particle acceleration No class on Oct. 9! Oct. 16 Gamma-ray pulsars Oct. 23 X-ray/gamma-ray binaries Oct. 30 Diffuse emission and unidentified sources Nov. 6 Active galactic nuclei Nov. 13 Gamma-ray bursts The Gamma Ray Sky The Electromagnetic Spectrum Gamma Rays: Wavelength Frequency Need satellites to observe High flying air planes or satellites Eph ≥ 100 keV n ≥ 1019 Hz l ≤ 0.1 nm The Atmosphere is opaque to gamma-rays The Sky in Different Wavelength Bands Radio Waves Visible light g-rays Infrared X-rays The Gamma-Ray Sky Plane of the Milky Way (diffuse emission) 3C279 (quasar) Geminga (pulsar) Crab (SNR) PSR 1951+32 EGRET, E > 100 MeV (pulsar) 3C454.3 (quasar) Vela (pulsar) PKS 0528+134 (quasar) More than half of all gamma-ray sources are still unidentified! The Problem of Identifying g-ray Sources EGRET error contours Pulsar Black Hole XRay Binary What’s the source of the g-ray emission? Need more information (broadband spectrum; variability) The Detection of Gamma Rays from Space Gamma Rays are deeply penetrating and do virtually not ionize material → Need to convert the g-ray’s energy to kinetic energy of an electron, and detect / track the electron Interactions of gamma-rays with matter 1. Photoelectric Absorption (E ≤ 300 keV) sabs hnthr = cion sabs ~ l3 ~ n-3 n Interactions of gamma-rays with matter (cont.) 2. Compton Scattering (300 keV ≤ E ≤ 8 MeV) sC sT= 6.65x10-25 cm2 n hnKN≈ 511 keV spg → pe+e- 3. Electron-Positron Pair Production (E ≥ 8 MeV) n hnthr= 2 mec2 (1 + me/mp) ≈ 1022 keV Problems for the Detection of Gamma Rays from Space 1) Low number fluxes Typical fluxes of the brightest g-ray sources in the sky: FE ~ 10-2 – 10-3 g-rays cm-2 s-1 MeV-1 2) High background from cosmic rays Background of high-energy particles (protons and electrons) constantly irradiating the detector Problems for the Detection of Gamma Rays from Space (cont.) => Need long integration times to measure a significant signal Sensitivity limit for detection of a source at a confidence level of n s (i.e., an excess of n times the standard deviation of the background g-ray flux dFB/dE): Fmin = n √ (dFB/dE) DE DW Aeff (E, Q, F) Tobs where DW = solid angle over which g-ray flux is impinging on the detector Aeff = effective detector area = A*cosQ*Pdet Tobs = Integration (observing) time Problems for the Detection of Gamma Rays from Space (cont.) => Problem, in particular for variable sources: Low duty cycle: Source signal may extend only over a small fraction of the integration time! Measured flux is only an average over the integration time. Measured flux Integration time Problems for the Detection of Gamma Rays from Space (cont.) 3) Collimation / Source Localization Gamma-rays are highly penetrating Can not be collimated, e.g., by mirrors or lenses! Solutions: Shield Shield Detector from most directions of the sky, except a narrow cone; virtually no directional information about sources within the cone (typically ~ few o). Shield A) Passive Collimation Detector Collimation / Source Localization (cont.) B) Orientation Effects Aeff is proportional to cosQ Q BATSE (Burst and Transient Source Experiment) on the Compton Gamma-Ray Observatory Detector Shield C) Occultation Techniques Flux Collimation / Source Localization (cont.) Fsource For example: Earth Occultation Technique t Source Collimation / Source Localization (cont.) D) Coded Masks Coded mask casts a shadow pattern on the detector, which can be unfolded to calculate the distribution of sources in the field of view. Detector Collimation / Source Localization (cont.) E) Tracking the trajectory of secondary particles For example: In pair conversion telescopes Trajectory of secondary electron/positron pair is tracked by imaging (optical readout) or spark chamber technique Anti-Coincidence Scintillation Dome g Pair conversion layers + closely spaced spark chambers e+ e- Widely spaced spark chambers; Time-of-flight coincidence system Detection Techniques 1) Scintillation Techniques Gamma-ray produces electron-hole pair; recombination produces a (often UV) photon; registered with optical readout 2) Solid State Detectors Gamma-ray produces multiple electron-hole pairs in doped semiconductors; recombination produces an optical photon; registered with optical readout Detection Techniques (cont.) 3) Compton Telescopes For g-rays with energies of ~ 1 – 10 MeV, direct scintillation or solid state detection becomes inefficient. Photons interact with matter mainly through Compton scattering Have the g-ray undergo Compton scattering event in an upper detector layer (1); determine direction of motion and energy of the down-scattered photon in a second, lower detector layer (2). Eg Eg’ = 1 + (Eg/mec2)(1 – cosf) Need to also measure energy and direction of the recoil electron in layer 1 to uniquely determine g-ray direction. Eg f L1 Eg’ L2 Detection Techniques (cont.) 4) Spark Chambers Gamma-ray produces electron-positron pair; pair trajectory is traced by spark chamber technique 5) Drift Chambers Gamma-ray produces electron-positron pair; pair trajectory is traced by drift chamber technique Detection Techniques (cont.) 6) Imaging Atmospheric Cherenkov Telescopes High-energy g-rays (GeV – TeV energies) produce air showers in the atmosphere. Relativistic particles with energy Ethr = mec2 (n/√n2 – 1 – 1) Photons Electrons Positrons (n = index of refraction) produce nano-second flashes of Cherenkov radiation. Imaging the shape and extent of Cherenkov light pattern gives energy and arrival direction of primary g-ray. Detection Techniques (cont.) 7) Secondary particle detector arrays; wave front sampling High-energy g-rays (GeV – TeV energies) produce air showers in the atmosphere. Measure the time evolution of the wave front of secondary particles (electrons and positrons) to determine primary g-ray’s energy (E > 1 – 10 TeV) and direction. Photons Electrons Positrons