IBIS is a state-of-the-art orbiting gamma ray imaging telescope, on board the INTEGRAL satellite, that is providing for the first time, images of the Universe in gamma rays, with an unprecedented angular accuracy—better than 1 arc-minute—and a sensitivity 10 times better than what has previously been achieved. It has a total weight of one ton. It is built with the same philosophy as that of nuclear physics instruments—made out of 20,000 independent solid state detectors, each with its own dedicated spectroscopic electronics channel. The paper also outlines the initial outstanding astrophysical results obtained in the first eight months of operation. A summary of the basic performance of the telescope after an accurate in-flight calibration campaign is also assessed in the paper.

On the technical side, the solid-state detectors used for the imaging detection plane are made out of single Cadmium-Telluride and Cesium-Iodine elements—the same elements that are going to be widely used in the near future in medical applications for X-Ray diagnostics, 3D tomography, etc. It is the first time a space-borne high-energy detector was built that uses a pixellized-array capable of producing real images of the sky in the gamma ray range (15 keV to 10 MeV). The capacity to produce images of large parts of the sky—over 30x30 squared degrees—is achieved with a novel technique based on a Tungsten "coded mask" as an imaging device to modulate gamma rays that are impossible to focus with traditional optical or X-ray type mirrors. The so-called "coded mask" projects a shadowgram onto the pixellized detector-planes and images of the sky are thereby reconstructed by decoding the detector shadowgram utilizing the mask pattern.

IBIS has just provided the first images of the Milky Way plane in gamma rays, disentangling cosmic gamma ray sources from within this crowded field. It has detected more than 130 gamma ray single-sources—most of them black holes and neutron stars—including 30 completely new and unknown objects. The nature of those high-energy sources is still a mystery, hopefully to be solved within the next few months using a combination of observations gathered using X-Rays, optical, IR, and radio wavelengths. The IBIS/INTEGRAL observatory is open to the wide international community and any scientist can apply for a dedicated observation, pending the approval of an international peer-review committee. IBIS is serving the scientific community at large in providing a unique combination of unprecedented gamma-ray wide-field imaging capability coupled with a broad-band spectroscopy and high-resolution timing over the energy range from X to gamma rays. To date, the IBIS telescope has been working nominally in orbit for more than eighteen months.

I started my work in X-Ray and Gamma-Ray Astrophysics in 1973. At the end of the 1980s, an international group of gamma ray astronomers, mainly from Europe and the USA, planned to build an innovative imaging and spectroscopic telescope which was to become the basic component of a newly-generated Space Observatory. This idea materialized in 1993 when ESA—the European Space Agency—approved the launch of an observatory-class mission named Integral. The satellite had an anticipated weight of almost 4,000 kg to be placed in orbit with a Russian Proton launcher. As the Principal Investigator, I proposed the IBIS that was selected as the on-board gamma ray imaging telescope. The telescope was built with the financial contributions received from 11 institutions among eight different countries.

Professor Pietro Ubertini
Istituto di Astrofisica e Fisica Cosmica
Rome, Italy