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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