The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), is an astronomical satellite for observing the gamma-ray sky. It was selected by the science program committee of the European Space Agency (ESA) on June 3rd, 1993 as a medium size mission.

After satellites such as the Compton Gamma-Ray Observatory (CGRO) launched by the Americans and the Russian-European satellite GRANAT, INTEGRAL is the first gamma-ray mission conceived from the beginning as an observatory, with observing time open to everyone.

The INTEGRAL satellite is about 5 meters high and more than 4 tonnes in weight. It was launched on October 17, 2002 by a Russian PROTON launcher from Baikonur in Kazakstan. The image on the right shows the nominal orbit of the INTEGRAL satellite. It is a highly eccentric orbit with a revolution period around the Earth (blue sphere) of 3 sideral days (slightly less than 72 hours). The closest point to Earth, the perigee, is at an altitude of about 10'000 km and the most distant point of the orbit, the apogee, is at about 150'000 km, which is roughly one half of the Earth-Moon distance. The inclination of the orbit with respect to the equatorial plane was 51.6 degrees at launch, this angle increased slowly with time towards the almost polar current orbit. The highly excentric orbit was chosen to minimise the periods with highly unstable instrument background due to electrons and protons trapped in the radiation belts (in yellow) and to allow for long periods of uninterrupted observation.

INTEGRAL was designed for a nominal lifetime of 5 years. The spacecraft and instruments are working extremely well. In October 2009 it was decided to extend the mission at least up to the end of 2012.

INTEGRAL's Instruments

The image on the right shows the INTEGRAL spacecraft with its four scientific instruments:

• The gamma-ray spectrometer SPI (foreground cylinder)
• The gamma-ray imager IBIS (big square in the back)
• The two X-ray monitors JEM-X (1 & 2) (the two circles between IBIS and SPI)
• The optical monitoring camera OMC (small conical tube at the left of JEM-X)

Other visible parts of the spacecraft include the solar panels (in blue) producing the needed energy, the star trackers (two tubes at the right of JEM-X) used for the positioning of the spacecraft and the antennae for receiving and sending informations to the Earth. More details can be seen on the exploded view (left image).

The INTEGRAL science payload consists of two main instruments, the spectrometer SPI and the imager IBIS supplemented by two subsidiary instruments, the X-ray monitor JEM-X and the optical monitoring camera OMC. The two subsidiary instruments are of great importance as they allow INTEGRAL to observe for the first time a single astronomical source simultaneously in the optical, X-rays and gamma-rays. Thanks to the technological improvements since the CGRO and GRANAT satellites the measurements combine an angular resolution and a sensitivity never reached before.

Observing gamma-rays implies the use of a very special method, due to the fact that gamma-rays, unlike visible light, cannot be deflected with a glass lens or a mirror. INTEGRAL's instruments for the high-energy range (spectrometer, imager and X-ray monitor) resort to the use of the coded-mask technique.

The gamma-ray spectrometer SPI

The Spectrometer on INTEGRAL (SPI) measure the energy of gamma-rays with an extraordinary accuracy over an energy range between 20 keV and 8 MeV (electron-volt). This is achieved thanks to an array of 19 hexagonal high-purity germanium detectors. A cooling system allows the detectors to reach a temperature of 85 Kelvin (-188 degrees Celsius). Cosmic protons and neutrons slowly damage the germanium crystals resulting in a loss of resolution and efficiency of the detector. To counteract this phenomena and restore the crystals, the detector is occasionally heated up to a temperature of 100°C for 24 hours. This process takes place every 6 months or so.

The coded mask, made of 3 cm thick tungsten, is composed of 127 hexagonal elements (63 opaque and 64 transparent elements). It is placed 1.7 meter above the 19 germanium detectors (image on the left). The field of view is 16 degrees with an angular resolution of 2 degrees. Narrow emission lines from radioactive nuclei, such as ²⁶Al, or from the annihilation of positrons are some of the main observational subjects of the spectrometer SPI. These observations allow the study of the formation of chemical elements and the detection of anti-matter in the Universe.

The gamma-ray imager IBIS

The Imager on Board the INTEGRAL Satellite (IBIS) has been optimised for fine imaging and precise detection of radiation sources. The coded mask of IBIS, made of tungsten, is placed 3.2 meters above the detector to optimise the angular resolution. The angular resolution that can be achieved with a mask is limited by the spatial resolution of the detector. The spatial resolution is dependent on the number of small sensitive elements of the detector called pixels (picture elements). The imager has a detector with a large number of pixels, all physically distinct and with their own electronics.

The detector uses two parallel planes of pixels located one on top of the other, thus allowing detection of both low and high energy photons. The top layer, ISGRI, is made of 16,384 CdTe (Cadmium-Tellure) pixels and the bottom layer, PICsIT, is made of 4096 CsI (Cesium-Iode) pixels. In practice IBIS achieves an angular resolution of 12 arcmin over an energy range between 15 keV and 1 MeV (electron-volt). This instrument mainly observes compact objects, such as neutron stars and black holes.

The two X-ray monitors JEM-X

There are two identical Joint European X-ray Monitors, JEM-X 1 & 2, which complement the two previously mentioned instruments and which play a decisive role in the detection and identification of gamma-ray sources. JEM-X works simultaneously with the other instruments in the energy range between 3 and 35 keV and with an angular resolution of one arcmin. Like the two previous instruments, the X-ray monitor uses a coded mask, in this case, placed 3.4 meters above the detector. The detector consists of two identical chambers filled with Xenon at a pressure of 1.5 bar (the normal atmospheric pressure at sea level is 1 bar). Usually only one of the two JEM-X units is used to optimize the life time of the instrument.

The optical monitoring camera OMC

The optical monitoring camera (OMC) is a CCD detector of 1024 x 1024 pixels located in the focal plane of a 50 mm lens with a V (visible) filter. Each pixel has a size of 13 x 13 micrometers covering a field of 17.6 x 17.6 arcsec. The total field of view of the OMC camera is of 5 x 5 degrees. The camera is placed on top of the satellite and is sensitive to sources up to an apparent magnitude of about 18.

The space environment monitor IREM

The INTEGRAL radiation environment monitor (IREM) is a small additional instrument aboard INTEGRAL. It is a particle (protons and electrons) counter, which performs a wide range of radiation monitoring in-orbit. The results are downloaded to ground as part of the spacecraft telemetry and made available by the ISDC with the data from the scientific instruments. When a strong solar flare occurs, with a very much increased particle background measured by IREM, the high electrical tension powering some of the instrument detectors could be switched off to avoid any damage.