Astronomers scramble to use INTEGRAL
28 February 2001The Call for Proposals to use guest observer time on ESA's forthcoming gamma-ray observatory has resulted in 291 individual proposals. These represent approximately 323 million seconds - more than 10 years - of observing time! This is 19 times greater than all the open time available during the first half of the nominal two-year mission.
Computers at the INTEGRAL Science Operations Centre coped with an electronic stampede as the closing deadline on 16 February approached. The Call for Proposals was issued on 1 November, but almost all the submissions arrived during the last week, 85% arriving in the final 48 hours, and 250 submissions were registered during the final 15 hours - one every four minutes!
The response to a previous Call for Letters of Intent - issued to pinpoint areas of scientific interest - had indicated that demand would be heavy. But the actual oversubscription level is exceptional. It is also high when compared to other high-energy missions. XMM-Newton's AO-1 was oversubscribed 6.8 times, the Comptel and Egret instruments on Compton-GRO respectively 2.5 and 5, and BeppoSAX 5 times.
This level of response reflects the increasing awareness that gamma-ray observatories, and more generally high-energy space platforms, are of immense value to the astronomical community. Many gamma-ray astronomers were frustrated when NASA's Compton mission was switched off last year and INTEGRAL is today the only purpose-built gamma-ray observatory on the horizon.
"We must be cautious, however, not to over-interpret this high oversubscription" explains Chris Winkler, INTEGRAL's Project Scientist. "XMM-Newton was overwhelmed by the sheer number of observation requests, with average durations of 20-30 kiloseconds. In our case, exposures will be much longer. For instance, all the observations that will use INTEGRAL's SPI spectrograph to investigate nucleosynthesis necessarily have long integration times, of at least a million seconds. Just twenty of these proposals and we are full for the whole year. There is also another explanation: INTEGRAL extends down to the hard X-ray range, around 100 keV down to ~ 10 keV, and this ability has prompted many proposals from the X-ray astronomy community."
A first statistical analysis of the 291 proposals shows that compact objects within our own galaxy is the biggest category (121), followed by extragalactic objects (91), nucleosynthesis investigations (46) and miscellaneous, including gamma-ray bursts (33).
In terms of the amount of requested observing time, the nucleosynthesis category is significantly more important, mainly due to the fact that this category contains a relatively large number of proposals to use SPI.
The 291 proposals have now been passed to the INTEGRAL Time Allocation Committee (TAC) which will probably have some hard choices to make. "If there are three proposals on the same target and it's the same science" says Chris Winkler, "then the best one will win".
The 28 TAC members will be meeting 14-18 May to compare their evaluations and make a choice. Their recommendations will then be submitted to ESA's Director of Science and the final list of approved observations published in June. After the submission rush, a long wait is in store for all the candidates!