INTEGRAL: The International Gamma-Ray Astrophysics Laboratory

C. Winkler

INTEGRAL homepage http://astro.estec.esa.nl/SA-general/Projects/Integral/integral.html

The International Gamma-Ray Astrophysics Laboratory, INTEGRAL, to be launched in 2001, is dedicated to the fine spectroscopy (Delta E=2 keV, Full Width at Half Maximum @ 1 MeV) and fine imaging (angular resolution=12 arcmin FWHM) of celestial gamma-ray sources in the energy range 15 keV to 10 MeV. Concurrent source monitoring will be performed in X-ray (3-35 keV) and optical (550-850 nm).

The scientific topics to be addressed by INTEGRAL include: compact objects, explosive and hydrostatic nucleosynthesis, high energy transients, galactic structure, the Galactic Centre, particle processes and acceleration, transrelativistic pair plasmas, Active Galactic Nuclei, nearby galaxies, cosmic diffuse background, identification of high energy sources, unidentified gamma-ray objects as a class and, of course, unexpected discoveries.

INTEGRAL, with a payload mass of about 2000 kg and a total launch mass of about 4000 kg, will be launched in April 2001 into a geosynchronous highly eccentric orbit with high perigee in order to provide long periods of uninterrupted observation with nearly constant back-ground and away from trapped radiation. The baseline is to launch on a Russian Proton. The parameters for the baseline orbit are: period 48 h, inclination 51.6°, perigee height 46 000 km, apogee height 75 000 km. Alternatively, ESA's Ariane 5 (period 48 h, inclination 65°, perigee height 7000 km, apogee height 114 000 km) can be used. The spacecraft design is compatible with both launchers.

Owing to background radiation effects in the high energy detectors, scientific observations will be carried out while the spacecraft is above an altitude of typically 40 000 km. This means that 100% of the time spent in the orbit provided by Proton can be used for scientific observations (realtime scientific data rate: 86 kbit/s), whereas the spacecraft will stay 85% of its time above 40 000 km if placed in the Ariane 5 orbit.

INTEGRAL will be an observatory-type mission with a nominal lifetime of 2 years, but an extension up to 5 years will be technically possible.

Part of the observing time (i.e. 35% year 1, 30% year 2, 25% year 3+) will be reserved, as guaranteed time, for the institutes (PI collaborations) that have developed the instruments and the INTEGRAL Science Data Centre (ISDC), for Russia and NASA for their contributions to the programme (Proton launch and Deep Space Network ground stations, respectively) and (to a smaller extent) for ESA/INTEGRAL Science Operations Centre (ISOC) and the Mission Scientists from Europe, USA and Russia. This fraction, the Core Programme, will be devoted to: (i) a Galactic plane survey to map the (diffuse) gamma-ray emission, to detect as yet unknown persistent sources (e.g. recent Galactic supernovae), and to facilitate the detection and study of transient sources (Targets of Opportunity, TOOs), and (ii) pointed (deep) observations of selected regions.

However, most of the observing time (65% year 1, 70% year 2, 75% year 3+) will be available to the scientific community at large as the General Programme. Proposals, following a standard Announcement of Opportunity (AO) process, will be selected on their scientific merits only by a single time allocation committee. In principle, observers will receive data from all co- aligned and simultaneously operating instruments on INTEGRAL.

All scientific data will enter the public domain and be made available to the scientific community at large one year after they have been released to the observer. This guarantees the use of the scientific data for different investigations beyond the aim of a single proposal.

Following an ESA AO, issued in 1994, the PIs for the scientific payload complement and the ISDC, together with Mission Scientists, were selected in May 1995. The INTEGRAL payload (Table 3.1.4/1) consists of the SPI spectrometer (using cooled Germanium detectors), the IBIS imager (using CdTe and CsI detector arrays) and two monitor instruments, the JEM-X X-ray monitor and the Optical Monitoring Camera (OMC). All high energy instruments employ coded aperture masks.

Table 3.1.4/1: Key parameters of the INEGRAL scientific payload

An essential factor for achieving the scientific goals, which consequently impacts the payload design, is the capability to achieve - to the maximum extent possible - complementarity in fine spectroscopy and accurate imaging. That is, SPI is optimised to perform fine spectroscopy of narrow lines and study diffuse emission within a large field of view, while IBIS provides accurate point source imaging and good broad line and continuum spectroscopy. The monitors will perform source identification and monitoring at X-ray and optical photon energies. A small particle radiation monitor is also part of the payload, for continuously measuring the particle environment. It is therefore possible to provide essential information to the payload in case high particle back-ground (radiation belts, solar flares) is encountered.

The operational ground segment (ESA/ESOC, ESA and NASA ground stations) will implement the observation plan received from ISOC, performing all classical spacecraft operations.

The science ground segment, managed within the division, is shown in Fig. 3.1.4/1. Using accepted observation proposals, ISOC will establish a timeline of target pointings plus the corresponding instrument configuration. This observation plan will then be forwarded to ESOC for uplink to the spacecraft. Furthermore, ISOC will validate any changes made to parameters describing the onboard instrument configuration and it will keep a copy of the scientific archive produced at ISDC. Finally, the ESA Project Scientist at ISOC will decide on the generation of targets of opportunity triggered by alerts in order to update and reschedule the observing programme.

Figure 3.1.4/1: INTEGRAL Science Ground Segment

ISDC, in Versoix, Switzerland, will receive the complete raw science telemetry plus the relevant ancillary space-craft data from ESOC. Science data will be processed, taking into account the instrument characteristics, and raw data will be converted into physical units. Using incoming science and housekeeping information, ISDC will routinely monitor the instrument science performance and conduct a quick-look science analysis.

A number of key events concerning the hardware programme, supported by the Project Scientist, have taken place in the reporting period, including: the initiation of the Instruments and spacecraft Phase B design phase, the conduct of the first reviews on system and payload levels, and the start of the spacecraft Phase C/D as of November 1996. On the operational side, the overall operational concept for the entire ground segment has been established. The definition phase for the science ground segment (SGS) commenced during the second half of 1995. This included the establishment of high level requirement documents for the overall mission and for the science ground segment, and also their derived implementation plans. ISDC and ISOC have started the definition phase for user requirements and interface negotiations among the elements of the SGS (ISDC, ISOC and ESOC/Mission Operations Centre (MOC) are in progress.

The work of the INTEGRAL Science Working Team, chaired by the Project Scientist, has focused recently on a number of activities, including the share of guaranteed time, a strategy for the Galactic Plane survey, data rights for sources in the instruments' fields of view, gamma-ray burst triggers and their rapid dissemination to the science community.