Lecture notes - Desy

telescope at Whipple observatory, and about 70 sources of high-energy gamma-rays have been
detected to date (see http://tevcat.uchicago.edu/). Currently operational observatories that
use this technique involve the H.E.S.S. array in Namibia, the MAGIC telescope of the Canary
Islands, and the VERITAS array in southern Arizona. It should be noted that these telescopes
can only detect gamma rays at energies higher than about 100 GeV, for sufficiently many of
the radiating electrons have to be faster than the phase velocity of light in the atmosphere to
produce a detectable amount of Cherenkov light.
With all techniques a very large effective area can be realized: in the case of an airshower
array one observes all showers that have some shower particles hit the detectors, and therefore
the detection probability is not related to the actual size of the scintillation detectors. In the
case of atmospheric Cherenkov telecopes we only need the telescope located within the light
pool on the ground, which has about 200 m radius. This makes for a geometric area of 10 5 m,
which is also roughly the effective area at about 1 TeV, where the light yield is high enough
to in all cases overcome telescope inefficiencies related to, e.g., the mirror reflectivity or the
PMT quantum efficiency. At lower energies around 200 GeV the effective area is a bit smaller.
The technique works only in clear nights, so only about 10% of the time may be used for
observations. Also the field-of-view is usually small with about 4 degrees diameter. On the
other hand, the single-photon localization is good with about 0.1 ◦ .
The real problem for all ground-based gamma-ray detectors is to distinguish between gammaray
showers and cosmic-ray showers, which can be about 10 5 times as numerous. Cosmic-ray
showers usually involve a large number of pions and are therefore muon-rich, whereas gammaray
showers are muon-poor, but do contain some. Cosmic-ray showers also have particles with
larger transverse momenta and therefore appear wider and more fluffy than gamma-ray showers.
Air shower arrays mainly discriminate hadronic showers via the muon content, which provides
about a factor of 10 in gain, so there are only 10 4 hadronic showers per gamma-ray shower. This
relatively poor background rejection largely offsets the gains arising from the large field-of-view
and the high duty cycle. Atmospheric Cherenkov telecopes reject background by imaging the
shower in Cherenkov light, preferentially by three or more telescopes so a full three-dimensional
image can be derived. Cuts are then applied to select gamma-ray-like events based on the shape
of the showers. The background rate after cuts of contemporaneous atmospheric Cherenkov
telecopes is only about 1 Hz/deg 2 . The Crab nebula, a pulsar-wind nebula and the standard
candle of high-energy gamma-ray astrophysics, produces a rate after cuts of about 0.15 Hz in
the same detector, so it is obvious that a very good background rejection is achieved.
The excellent photon localization allows a further background suppression for point sources,
for which one searches for a peak in rate at the location of the source compared with the quasiuniform
background. Essentially all events associated with the Crab nebula should be recorded
within 0.2 ◦ of its true position, and the background rate in that solid angle element is also only
2