Integral was designed to focus on celestial objects that radiate in the energy band between 10,000 and 10 million electron volts (eV). They are 10 000 to 10 million times more energetic than the photons that reach our eyes from the Sun and stars. The gamma rays are generated by the most energetic and violent events in the Universe. Integral has two main instruments; a spectrometer and a camera, besides monitors that cover the soft x-ray range below 10000 eV and the optical wavelength band.
The celestial objects of interest scanned by Integral include black holes in our Galaxy, neutron stars, X-ray binary stars, pulsars, remnants of supernova explosions and gamma ray bursts, and high energy cosmic background.
The operation of Integral is far from routine. The high-energy sky is dynamic; a large majority of the gamma ray sources vary with time. Sudden outbursts appear where nothing was detected and vanish in no time or linger on unpredictably. Many gamma ray sources are in binary stellar systems, where material from the companion star is accreted onto the other object, releasing enormous energy. Many outbursts provide plenty of photons which are rather scarce in gamma rays.
Observers work round the clock. Any unexpected event is called ‘Targets of Opportunities’. One such event occurred on 5 December 2004, when a scientist unexpectedly telephoned the Controllers to redirect the satellite towards a powerful new source. It was a Sunday but all concerned rushed in and within 24 hours, a new set of 4000 commands was loaded to the satellite after verification of the request. The source happened to be in the constellation Cassiopeia in the outer reaches of the Milky Way. It had suddenly flared two days earlier.
It was soon designated as IGR J00 291+5934. The source turned out to be the faster accreting X-ray millisecond pulsar (rotating neutron stars). It had been observed, based on Integral’s data, by NASA’s Rossi X-ray Timing Explorer. The rotational period was only 1.67 millisecond or 37,500 rpm. This could be the missing link between the slow accretion pulsars in binary systems and the very fast isolated pulsars.
The newly found pulsar had a companion. The two stars orbit one another in only 1.5 hours. The much smaller star is expected to fully eat up the companion. Neutron stars in binary systems can accelerate (slow down when they are alone) and can cannibalize its companion. The speeding up is over a long period: about 100000 years (!) from 1.67 millisecond to 1.66 millisecond!
Integral is in a highly eccentric orbit, with its apogee at 153,000 km and perigee at 10,000 km from the Earth, with an inclination of 51.6 degrees to the equator. It means Integral spends 84 per cent of the time above an altitude of 60,000 km and is thus completely outside the Earth’s potentially very damaging radiation belts. Fine imaging of celestial gamma ray sources is done in the energy range of 15KeV to 10 MeV, and spectroscopy in the range of 20 KeV to 8 MeV.
GLAST (since called Fermi) has a formidable agenda to study gamma rays (in the relatively unexplored range of 10-100 GeV), neutron stars, cosmic rays and supernova remnants, the Milky Way galaxy, the early Universe, our solar system and the search for dark matter. Fermi has already discovered 12 new gamma ray-only pulsars.