Gamma-Ray Astronomy Call no Assoc. Prof. Markus Böttcher Clippinger # 339 Phone:

Literature Volker Schönfelder: The Universe in Gamma Rays Springer-Verlag, Berlin, Heidelberg, New York, 2001 Other References: Reinhard Schlickeiser: Cosmic Ray Astrophysics Springer-Verlag, Berlin, Heidelberg, New York, 2002 Malcolm S. Longair: High Energy Astrophysics Cambridge University Press, 1981 G. B. Rybicki & A. P. Lightman: Radiative Processes in Astrophysics John Wiley & Sons, New York, 1979

Preliminary Schedule Sept. 11The gamma-ray sky; detection techniques for gamma-rays Sept. 18Gamma-ray telescopes Sept. 25Gamma-ray emission mechanisms Oct. 2Particle acceleration No class on Oct. 9! Oct. 16Gamma-ray pulsars Oct. 23X-ray/gamma-ray binaries Oct. 30Diffuse emission and unidentified sources Nov. 6Active galactic nuclei Nov. 13Gamma-ray bursts

The Gamma Ray Sky The Electromagnetic Spectrum Need satellites to observe Wavelength Frequency High flying air planes or satellites Gamma Rays: E ph ≥ 100 keV ≥ Hz ≤ 0.1 nm The Atmosphere is opaque to gamma-rays

The Sky in Different Wavelength Bands Radio Waves Infrared Visible light X-rays  -rays

The Gamma-Ray Sky 3C279 (quasar) Plane of the Milky Way (diffuse emission) PSR (pulsar) Vela (pulsar) Geminga (pulsar) Crab (SNR) PKS (quasar) 3C454.3 (quasar) More than half of all gamma-ray sources are still unidentified! EGRET, E > 100 MeV

The Problem of Identifying  -ray Sources EGRET error contours Pulsar Black Hole X- Ray Binary What’s the source of the  -ray emission? Need more information (broadband spectrum; variability)

The Detection of Gamma Rays from Space Interactions of gamma-rays with matter 1.Photoelectric Absorption (E ≤ 300 keV)  abs ~ 3 ~ -3  abs h thr =  ion Gamma Rays are deeply penetrating and do virtually not ionize material → Need to convert the  -ray’s energy to kinetic energy of an electron, and detect / track the electron

3. Electron-Positron Pair Production (E ≥ 8 MeV)  p  → pe + e - h thr = 2 m e c 2 (1 + m e /m p ) ≈ 1022 keV  T = 6.65x cm 2 CC h KN ≈ 511 keV Interactions of gamma-rays with matter (cont.) 2. Compton Scattering (300 keV ≤ E ≤ 8 MeV)

Problems for the Detection of Gamma Rays from Space 1) Low number fluxes Typical fluxes of the brightest  -ray sources in the sky: F E ~ –  -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  (i.e., an excess of n times the standard deviation of the background  -ray flux dF B /dE): F min = n √ where (dF B /dE)  E  A eff (E,  ) T obs  = solid angle over which  -ray flux is impinging on the detector A eff = effective detector area = A*cos  *P det T obs = Integration (observing) time

Problems for the Detection of Gamma Rays from Space (cont.) => Problem, in particular for variable sources: Measured flux Integration time 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.

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: A) Passive Collimation Shield Detector from most directions of the sky, except a narrow cone; virtually no directional information about sources within the cone (typically ~ few o ). DetectorShield

Collimation / Source Localization (cont.) B) Orientation Effects A eff is proportional to cos  Detector Shield  BATSE (Burst and Transient Source Experiment) on the Compton Gamma-Ray Observatory

Collimation / Source Localization (cont.) C) Occultation Techniques For example: Earth Occultation Technique t Flux F source 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  e+e+ e-e- Anti-Coincidence Scintillation Dome Pair conversion layers + closely spaced spark chambers Widely spaced spark chambers; Time-of-flight coincidence system

Detection Techniques 1) Scintillation Techniques 2) Solid State Detectors Gamma-ray produces electron-hole pair; recombination produces a (often UV) photon; registered with optical readout 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  -rays with energies of ~ 1 – 10 MeV, direct scintillation or solid state detection becomes inefficient. Photons interact with matter mainly through Compton scattering  EE E’E’ E  ’ = EE 1 + (E   m e c   – cos  Have the  -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). Need to also measure energy and direction of the recoil electron in layer 1 to uniquely determine  -ray direction. L1L1 L2L2

Detection Techniques (cont.) 4) Spark Chambers 5) Drift Chambers Gamma-ray produces electron-positron pair; pair trajectory is traced by spark chamber technique Gamma-ray produces electron-positron pair; pair trajectory is traced by drift chamber technique

Detection Techniques (cont.) 6) Imaging Atmospheric Cherenkov Telescopes High-energy  -rays (GeV – TeV energies) produce air showers in the atmosphere. Relativistic particles with energy E thr = m e c 2 (n/√n 2 – 1 – 1) (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  -ray. Photons Electrons Positrons

Detection Techniques (cont.) 7) Secondary particle detector arrays; wave front sampling High-energy  -rays (GeV – TeV energies) produce air showers in the atmosphere. Photons Electrons Positrons Measure the time evolution of the wave front of secondary particles (electrons and positrons) to determine primary  -ray’s energy (E > 1 – 10 TeV) and direction. t = const.