JEM-X eager to discover new cosmic 'animals'
3 July 2001Like many of Copenhagen's inhabitants, Dr Niels Lund cycles to work everyday, a 5 km distance from his home to the Danish Space Research Institute. For the past six years, his thoughts whilst peddling have been set on the successful completion of JEM-X, one of the four science instruments aboard ESA's gamma-ray observatory INTEGRAL.
The Joint European Monitor for X-rays, which is being built by a consortium of 7 European laboratories, consists of two identical X-ray telescopes, co-aligned with INTEGRAL's main gamma-ray instruments, IBIS and SPI. Any gamma-ray source seen by them will normally be bright enough to be rapidly observed by JEM-X.
The X-ray monitors will provide complementary data at lower energies (3-35 keV) and with their higher angular resolution (3 arcminutes) they will play a crucial role in identifying sources in crowded fields. During INTEGRAL's observations, JEM-X will be able to alert the scientific community when transient sources - such as gamma-ray bursts - appear and it will itself study their X-ray 'afterglow'. The instrument will also provide information on other sources that may be detected serendipitously.
For principal investigator Niels Lund, JEM-X is the culmination of many years contributing to several major space missions. During this time, in his role as an astrophysicist, his main field of interest has become the detection of gamma-ray bursts.
"We still do not understand these flashes in the sky, sometimes lasting only a fraction of a second, but they must be the result of tremendous explosions far away, releasing incredible amounts of energy," says Neils Lund. "The only way we can better understand them is by identifying and studying probable sources at other wavelengths. To achieve this, gamma-ray bursts have to be precisely located as soon as they occur."
Pinpointing gamma-ray bursts
After working on a cosmic-ray spectrometer on NASA's 'HEAO-3' satellite in the 1970s and 80s, Lund developed an all-sky monitor called 'WATCH' to scan the sky and quickly localise transient sources. This instrument used two rotating detector disks. When hit by photons, these detectors could pinpoint the direction from which the photons arrived.
This 'Rotation Modulation Collimator' was due initially to fly as a piggyback experiment on ESA's EURECA retrievable micro-gravity platform which was to be launched and retrieved by the Space Shuttle. But after the 1986 Challenger accident, WATCH had to find another carrier. The Russian GRANAT gamma-ray satellite, launched in 1989, provided the first opportunity.
"Three of the four WATCH detectors on GRANAT provided very good data until 1995, providing position information on transient sources to within a 1 degree or so," recalls Niels Lund. "However GRANAT did not provide us with this data quickly enough. Optical telescopes also often required a day to look in the right direction. It wasn't the most efficient system to probe such brief phenomena."
But GRANAT did confirm that gamma-ray bursts are uniformly distributed over the sky, and also demonstrated that gamma-ray sources are not just in distant galaxies but also in our Milky Way. In August 1992, WATCH was able to see the sudden appearance of a hard X-ray source that did not disappear. This peculiar source, GRS 1915+105, was the first micro-quasar to be discovered in our Galaxy.
EURECA finally flew in 1992-93 and then the WATCH data could be relayed back much quicker. Yet the 1-degree positioning was not accurate enough for most telescopes. "The breakthrough for the follow-up of gamma-ray bursts came only in 1996 with BeppoSAX. The Italian-Dutch X-ray satellite - still operational - can pinpoint sources five times better, to within 5-10 arcminutes," notes Lund.
Some 50 BeppoSAX alerts have been followed-up until now, and it has become clear that gamma-ray bursts are among the most distant phenomena we can observe in the Universe. "But it still takes hours to alert anybody," says Lund. "With JEM-X and INTEGRAL we hope to reduce the delay to less than one minute. This will give the ground-based telescopes a chance to make unique observations of the source 'in action'."
From WATCH to JEM-X
The historical overview here is not without purpose. Although the JEM-X instruments are different in their design, the experience gained from WATCH and BeppoSAX are embedded in JEM-X.
JEM-X employs coded aperture masks to image the sky, as do IBIS and SPI. Situated 3.4 m below the masks, each detector consists of a chamber filled with a mixture of xenon and methane gas. When a photon enters the chamber through a window, an ionised cloud is produced as it passes through the gas and is amplified (in an electron 'avalanche' of ionisations) by a strong electric field generated above a circular 'MicroStrip plate'.
This plate is a circular piece of glass whose upper surface is covered by microscopic electrode strips, alternating high-voltage anodes and cathodes, spaced only 1 mm apart. Similar strips cover the lower side but at right angles to those on the top layer. The exact point where a photon reaches this grid can be determined by the associated electronics.
"IBIS will certainly be able to detect the most gamma-ray bursts, but JEM-X will provide their most precise position," notes Lund. "Our calculations show that if only a handful of photons from a gamma-ray burst come within our field of view, then JEM-X will be able to pinpoint the source to within a few arcminutes."
These imaging MicroStrip Gas Chambers will be the first of their kind to fly in space. Like all innovative instruments their development has not been without difficulties. Particle accelerator tests on the qualification model and functional tests during assembly of the flight instruments have confirmed the soundness of the design.
However, the conditions required to assemble the microstrips with the electronics have turned out to be extremely critical, in order to ensure that the instruments will be absolutely free of any particles that may cause havoc when the high voltage is applied to the detectors. "We have had many sleepless nights," admits Lund, "but we are now confidently approaching the moment when we can seal the flight models, and deliver them to ESA for integration onto the spacecraft."
JEM-X and its associated data processing software are not the Danish Space Research Institute's sole contribution to INTEGRAL. It has a co-investigator role in INTEGRAL's Science Data Centre, with a particular interest in IBAS, the INTEGRAL Burst Alert System that will warn the worldwide scientific community when a gamma-ray burst flashes in the sky.
Niels Lund is sure that the INTEGRAL mission will provide many surprises: "Nature displays such a diversity of forces. We certainly haven't seen all the animals in this zoo. With their different sensitivities, the instruments on the mission are going to see unimagined behaviours of matter, for example, in high-gravity conditions. New animals which until now have gone unnoticed are surely going to be revealed."
Yes, there is a zoo in Copenhagen. Niels Lund was of course referring to a more Universal one!
The principal institutes and companies participating in JEM-X are: the Danish Space Research Institute, Copenhagen, Denmark; University of Helsinki, Finland; Metorex International Espoo, Finland; Universidad de Valencia, Spain; Instituto Nacional de Ticnica Aerospatial LAEFF, Madrid, Spain; Instituto de Astrophysica, Rome, Italy; University of Ferrara, Italy; Space Research Center, Warsaw, Poland; Copernicus Astronomical Center, Warsaw, Poland; Stockholm Observatory, Sweden.
The Danish Space Research Institute has contributed to many ESA missions, amongst them GEOS, Giotto, Hipparcos, ISO, Cluster, Planck; and to Denmark's first-ever satellite, the Xrsted magnetospheric mission.