Integral Status Report - October 2003
Following the Crab Nebula "standard candle" observation in February, the sensitivities of the instruments are confirmed to be generally close to expectations. An exception is with SPI, where a higher than expected induced background means the sensitivity is a factor of 2-4 less than stated in the first Announcement of Opportunity (AO). All the other scientific performances such as spatial and energy resolutions and energy coverage are all consistent with expectation. One of the 91 sets of redundant read-out chains of the SPI Anti-Coincidence System (ACS) has failed, but the remaining part of the redundant chain provides full functionality.
Execution of the observing programme continues with a good efficiency with over 93% of the available observing time (when Integral is above the radiation belts) being used for scientific observations. The ground segment is operating well and the problems with data gaps from the Goldstone station have been fixed. The detailed Integral schedule is typically available one month in advance allowing other facilities to coordinate with Integral should they wish.
Operations and Archiving
In order to mitigate the effect of radiation damage on the SPI detectors, it is necessary to periodically anneal or irbakelc them. The first such annealing cycle was successfully performed in February 2003 and the second in July. However, the second annealing did not fully recover the energy resolution of the detectors. Consultation between project staff and the instrument team has indicated that the baking out period was not long enough and an additional annealing interval - with a longer baking period - is being scheduled for November. In consultation with the observers, targets that do not require SPI data will be observed during this interval.
The second Integral AO closed on 5 September 2003. A total of 142 proposals were received resulting in an over-subscription of around a factor 8. Approximately one half the proposals were for compact galactic sources, one quarter for extra-galactic objects and the remainder for nucleosynthesis and miscellaneous objects. The results of the selection process should be communicated to the community in November with observations taking place from mid- December onwards.
Many of the early Integral scientific results are being prepared for a special edition of Astronomy and Astrophysics to appear later this year. Given the complexity of the data analysis techniques required for the coded mask instruments and the low signal-to-noise ratios inherent in gamma-ray astronomy, the wide range of topics amply demonstrate the potential of Integral to contribute significantly in helping solve many of the outstanding issues in high-energy astronomy.
In the meantime, a preliminary analysis of the diffuse emission studies performed during the spring look at the galactic centre region has been performed. These show that the 1809 keV 26Al line intensity is consistent with previous measurements, but is narrower (with a 3.1 keV FWHM) than the 5.4 keV FWHM line reported by the GRIS experiment. Another high priority Integral topic is detailed mapping and spectroscopy of the 511 keV electron-positron annihilation feature, seen from the central Galactic region. An initial analysis of SPI results indicates that the 511 keV line has an intrinsic width of 2.95 keV (FWHM), at the upper range of previous measurements. Early results are consistent with a spherical intensity distribution which may be offset slightly from the Galactic centre. As yet, there is no evidence for a disk component.