and Cosmology

1. Introduction and Overview
32
Fig. 1.32. Left: ROSAT, a German–US–British cooperation,
was in orbit from 1990 to 1999 and observed in the energy
range between 0.1 and2.5 keV (soft X-ray). Upper right:
Chandra was launched in July 1999. The energy range of
its instruments lies between 0.1 and 10 keV. Its highly elliptical
orbit permits long uninterrupted exposures. Lower
right: XMM-Newton was launched in December 1999 and
is planned to be used for 10 years. Observations are carried
out with three telescopes at energies between 0.1 and
15 keV
1.3.6 Gamma-Ray Telescopes
The existence of gamma radiation was first postulated
in the 1950s. This radiation is absorbed by the atmosphere,
which is fortunate for the lifeforms on Earth.
The first observations, carried out from balloons, rockets,
and satellites, have yielded flux levels of less than
100 photons. Those gamma photons had energies in the
GeV range and above.
Detailed observations became possible with the satellites
SAS-2 and COS-B. They compiled a map of
the galaxy, confirmed the existence of a gamma background
radiation, and for the first time observed pulsars
in the gamma range. The first Gamma Ray Bursts
(GRB), extremely bright and short-duration flashes
on the gamma-ray sky, were detected in the 1970s
by military satellites. Only the Italian-Dutch satellite
Beppo-SAX (1996 to 2002) managed to localize a GRB
with sufficient accuracy to allow an identification of the
source in other wavebands, and thus to reveal its physical
nature; we will come back to this subject later, in
Sect. 9.7.
An enormous advance in high-energy astronomy was
made with the launch of the Compton Gamma Ray Observatory
(CGRO; Fig. 1.33) in 1991; the observatory
was operational for nine years. It carried four different
instruments, among them the Burst And Transient
Source Experiment (BATSE) and the Energetic Gamma
Ray Experiment Telescope (EGRET). During its lifetime
BATSE discovered more than 2000 GRBs and
contributed substantially to the understanding of the
nature of these mysterious gamma-ray flashes. EGRET