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INTEGRAL U.S. Guest Observer Facility

INTEGRAL: The INTErnational Gamma-Ray Astrophysics Laboratory. A project of the European Space Agency


artist's conception of INTEGRAL in space The ESA scientific mission INTEGRAL (The INTErnational Gamma-Ray Astrophysics Laboratory) was selected by ESA in 1993 as the next ESA medium-size scientific mission (M2) of the Horizon 2000 program. Launched in October of 2002, the mission was conceived as an observatory led by ESA with contributions from Russia (PROTON launcher) and NASA (Deep Space Network ground station).

INTEGRAL is dedicated to the fine spectroscopy (E/E = 500) and fine imaging (angular resolution: 12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV with concurrent source monitoring in the X-ray (3-35 keV) and optical (V-band, 550 nm) energy ranges. The four experiment packages are described below.

photo of SPI in cleanroom The spectrometer (SPI) (SPectrometer on INTEGRAL) will perform spectral analysis of gamma-ray point sources and extended regions in the 20 keV - 8 MeV energy range with an energy resolution of 2 keV (FWHM) at 1 MeV. This will be accomplished using an array of 19 hexagonal high purity Germanium detectors cooled by a Stirling cooler system to an operating temperature of 85 K. A hexagonal coded aperture mask is located 1.7 m above the detection plane in order to image large regions of the sky (fully coded field of view = 16 degrees) with an angular resolution of 2 degrees. In order to reduce background radiation, the detector assembly is shielded by a veto (anticoincidence) system which extends around the bottom and side of the detector almost completely up to the coded mask. The aperture (and hence contribution by cosmic diffuse radiation) is limited to ~ 30 degr. A plastic veto is provided below the mask to further reduce the 511 keV background. The GSFC Astrophysics Science Divison scientists Drs. Steve Sturner and Chris Shrader are responsible for generating and testing updated SPI instrument response functions which are required when any of the 19 detectors fail.

photo of IBIS instrument The Imager (IBIS) (Imager on Board the Integral Satellite) provides diagnostic capabilities of fine imaging (12 arcmin FWHM), source identification and spectral sensitivity to both continuum and broad lines over a broad (15 keV - 10 MeV) energy range. The Imager will exploit simultaneously with the other instruments on Integral celestial objects of all classes ranging from the most compact galactic systems to extragalactic objects. A tungsten coded-aperture mask (located at 3.2 m above the detection plane) is optimized for high angular resolution. As diffraction is negligible at gamma-ray wavelengths, the angular resolution obtainable with a coded mask telescope is limited by the spatial resolution of the detector array. The Imager design takes advantage of this by utilizing a detector with a large number of spatially resolved pixels, implemented as physically distinct elements. The detector uses two planes, one 2600 cm^2 front layer of CdTe pixels, each (4x4x2) mm (width x depth x height), and a 3100 cm^2 layer of CsI pixels, each (9x9x30) mm. The CdTe array (ISGRI) and the CsI array (PICsIT) are separated by 90 mm. The detector provides the wide energy range and high sensitivity continuum spectroscopy required for Integral.

photo of JEM-X instrument The Joint European X-Ray Monitor (JEM-X) supplements the main Integral instruments (Spectrometer and Imager) and plays a crucial role in the detection and identification of the gamma-ray sources and in the analysis and scientific interpretation of Integral gamma-ray data. JEM-X will make observations simultaneously with the main gamma-ray instruments and provides images with arcminute angular resolution in the 3 - 35 keV prime energy band. The baseline photon detection system consists of two identical high pressure imaging microstrip gas chambers (5 bar, Xenon). Each detector unit views the sky through its coded aperture mask located at a distance of apx 3.2 m above the detection plane.

photo of OMC The Optical Monitoring Camera (OMC) consists of a passively cooled CCD (2048 x 1024 pixels, imaging area: 1024 x 1024 pixels) working in frame transfer mode. The CCD is located in the focal plane of a 50 mm (diameter) lens including a Johnson V-filter to cover the 500 - 850 nm wavelength range. The OMC will be mounted close to the top of the payload module structure.

Participation by U.S. astronomers in the INTEGRAL guest-observer program has been substantial. US-based INTEGRAL Guest Observers are supported by a Guest Observer Facility (GOF) at the NASA/Goddard Space Flight Center (GSFC). The GOF provides technical (including proposal) information and analysis software developed by ESA and the ISDC, as well as user support on the US side of the Atlantic.