INTEGRAL Status Report - February 2004
The Time Allocation Committee (TAC) for the second INTEGRAL Announcement of Opportunity met in October and selected the observations to be performed from 2003 December 17 onwards. This was no easy task given the high (factor ~8 in time) over-subscription. All the AO-2 proposers have been notified of the TAC decisions and a long-term plan until February 2005 is available under http://astro.estec.esa.nl/Integral.
Operations and Archiving
Besides the interruption due to the enhanced level of solar activity, the observing programme continued 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 with the detailed INTEGRAL schedule being typically available between two weeks and one month in advance allowing other facilities to coordinate with INTEGRAL, if desired.
In order to mitigate the effect of radiation damage on the SPI detectors, it is necessary to periodically anneal or .bake. them. The first such annealing cycle was successfully performed in 2003 February and the second in 2003 July. However, the second annealing did not fully recover the energy resolution of the detectors indicating that the baking out period was too short. An additional, longer duration, annealing was performed in November, after the solar activity had subsided. An analysis of SPI spectra showed that the annealing was fully successful with the energy resolution being fully recovered to its pre-launch values.
On 2003 December 6 one of the 19 SPI detectors suddenly stopped operating. Various recovery attempts have been unsuccessful. Analysis indicates that fault is most likely a component failure in the detector.s pre-amplifier. The SPI team, together with ex-INTEGRAL project team members and SCI-A personnel are continuing to investigate the failure and may propose additional recovery actions. A preliminary analysis of the reduction in SPI scientific performance indicates that this is limited to the ~5% reduction in effective area.
After the closure of the autumn galactic centre visibility window, INTEGRAL continued making scans of the central radian of the galaxy and along the galactic plane searching for new transients and other interesting phenomena. In addition, the accreting low-mass X-ray binaries XB 1916-053 and 4U 0614+09 were observed together with the galactic micro-quasar GRS 1915+105. Taking advantage of the large INTEGRAL field of view, the blazar 0716+714 has been observed simultaneously with the Seyfert galaxies Mkn 3 and Mkn 6 which lie in the same region of sky. The observation of the active galactic nucleus MR 2251-178, which was started in 2002 December, was completed. Deep (1 to 2 x 106 sec) observations of the Vela region, and the Cas A (together with Tycho) and IC 433 supernova remnants have been performed, primarily to study nucleosynthesis by characterizing the gamma-ray line emission.
The third release of the Integral Science Data Centre (ISDC) public off-line scientific analysis (OSA) software occurred in 2003 December. The new release includes increased functionality, particularly for the low-energy imager (ISGRI) where the uncertainties associated with image reconstruction have been sharply reduced and spectral extraction algorithms improved. For the X-ray monitor (JEM-X) improved background models and data extraction algorithms are included. The ISDC continues to routinely dispatch data products to observers within 6 weeks of their observation. Using results obtained with OSA-3, the ISDC and instrument teams are evaluating the in-flight instrument sensitivities in order to verify or update those published in the AO-2 documentation.
A special edition of Astronomy & Astrophysics dedicated to INTEGRAL (volume 411, 2003 November) has appeared containing 73 papers covering mission and instrument descriptions and performance and early science results.
A paper based on INTEGRAL results has been submitted to Nature (Lebrun et al.). This addresses the nature of the 20 to 200 keV Galactic gamma-ray emission. Below and above this energy range diffuse emission processes dominate, but the nature of the emission in this energy range was, until now, highly uncertain. Deep ISGRI observations of the central portion of the Galaxy have revealed 43 point-like sources. The integrated flux from these faint sources fully accounts for the shape and intensity of the 20 to 200 keV spectrum observed by previous missions. This means that the soft gamma-ray spectrum of the Galaxy is dominated by emission from compact objects, such as black holes and neutron stars in binary systems.
After observing one gamma-ray burst a month in the INTEGRAL field of view for the first 6 months of the mission, astronomers had to wait another 6 months to observe the seventh! This (GRB 031203) was detected and positioned automatically by the burst alert software (IBAS) running at the ISDC and an alert sent out within 20 sec of the burst occurring with an uncertainty radius of only 2.7 arc minutes. This allowed XMM-Newton to observe the field within 6 hours of the burst . currently their fastest Target of Opportunity response to date. A fading X-ray afterglow was detected surrounded by an expanding ring of emission. While predicted, such an expanding ring has never been seen before and is most likely due to X-rays scattered off dust grains in our own Galaxy.