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J. Brit. Astron. Assoc., 112, 6, 2002, p.308
by Nick James
At 04:41 UTC on 2002 October 17 a Russian Proton rocket soared into a cloudless blue sky from its Baikonur, Kazakhstan launch pad. It was carrying a large European observatory into orbit at the start of a five year mission to study gamma-ray sources. Eighty minutes later a tracking station in Belgium received signals from the spacecraft which confirmed that the payload was in excellent health and that the Proton had worked perfectly. The next few weeks were spent testing equipment on board the spacecraft and, at the time of writing (late October), everything was working well.
The INTErnational Gamma Ray Astrophysical Laboratory, INTEGRAL, is a four-tonne spacecraft supporting four important scientific instruments. The IBIS instrument is a large and sensitive gamma ray imaging telescope with a field of view of 9 degrees and 12 arcminute resolution. It is complemented by the SPI spectrometer which will produce detailed spectra of gamma-ray events. INTEGRAL also includes an X-Ray monitor (JEM-X) and a small optical monitor camera. The OMC is basically a 50mm aperture refractor with a V-band filter and a 1K x 1K pixel CCD at its focus. It has a 5 degree field of view and will be used to image fields at visual wavelengths coincident with gamma ray observations.
Since INTEGRAL is an observatory it needs to be in contact with the ground when observations are being made. The orbit has been designed so that contact can be maintained using only two ground stations - the ESA station at Redu, Belgium and the NASA station at Goldstone, California. This orbit has a perigee of 9,000km, an apogee of 153,000km and a period of exactly three sidereal days (just under 72 hours). This means that, for any given place on the Earth, the spacecraft's track across the sky repeats every three days. The inclination is such that from Redu the spacecraft is visible for over 60 hours of each 72 hour orbit.
Data from the instruments on board the spacecraft is processed by the Integral Science Data Centre (ISDC) situated near to Lake Geneva in Switzerland. Software running on computers at the ISDC will identify gamma ray bursts (GRBs) in the data and will rapidly send alerts to the astronomical community. Most alerts will come from the IBIS instrument and a position accuracy of one arc minute or so is expected. Scientists expect that INTEGRAL will detect around 20 GRBs per year and the system is designed to send out alerts to interested observers less than a minute after the event. This will allow ground based observers to study the burst at other wavelengths very soon after it is detected.
Amateur observers with the right equipment and a suitable alert mechanism (e-mail or mobile phone text messages) will also be able to look for the optical transients associated with these mysterious events. This should lead to more observations similar to the recent success with GRB021004.
Image: Operations staff including Nick James (right) wait anxiously in the control room at Redu for the first signal from the spacecraft. Photos by Benoit Demelenne.