The Swift Gamma-Ray Burst Mission Italian site U.K. site

Swift Mission Director's Status Report Log - June 2005

June 16, 2005

Swift has kept up its pace of scientific discovery over the past month, but we've also had some downtime for maintenance and recovery from anomalies. Key highlights include the discovery of two bursts which had bright optical afterglows which were followed by the UVOT - GRB050525a (GCN 3475) and GRB050603 (GCN 3516), and the discovery of a very soft GRB - GRB 050528 (GCN 3496). Please refer to the GCN archive or the Swift project pages for a complete list of Swift bursts.

Since May 13 Swift discovered, observed and, in some cases, continues to observe the following bursts: May 20, May 22, May 25a and b, May 28, June 3 and June 7. Three of these bursts were initially discovered by INTEGRAL, but Swift was able to follow them up with detections by the BAT, XRT or UVOT. Of these seven bursts, six were positively identified by the XRT with an X-ray counterpart, and the other (May 28) found three faint possible counterparts. Important ground based follow-ups have provided redshifts for three more Swift discovered GRBs: April 16a, z=0.6535; May 25a, z=0.606; and June 3, z=2.821. GRB050525a was a superb laboratory for the UVOT, in which light curve measurements were made of the optical counterpart for up to ten days after the burst occurred.

We continue to conduct Target of Opportunity observations of exceptional targets, which include a young, bright supernova pre-peak (SN2005cf) and Comet Tempel 1 (which will be observed next month by Swift when the Deep Impact probe strikes the comet). The BAT detected an outburst of A0535+26, with an outburst flux greater than three times that of the Crab Nebula. This high flux state has been sustained for over a month, and Swift has conducted ToO observations to monitor this source.

Over the past month each of the three instruments have conducted code uploads to improve the performance of the instruments, based on our growing in-flight experience. The BAT code load aimed to improve the instrument sensitivity, speed the return to operations in case of a reboot and reduce the possibility of false GRB triggers. The XRT code load aimed to mitigate the effect of warmer operating temperatures, and keep the instrument in the most effective observing modes. The UVOT code load aimed at avoiding unnecessary triggering of the self-protection when bright stars are crossed during maneuvers.

While the long term effect of these code load operations is to improve observatory efficiency and reliability, the effect during the load and testing was to temporarily have the instruments off-line or in off- nominal modes for GRB detection for a total of about four days.

In addition, the spacecraft or instruments experienced several anomalies which resulted in loss of observing time. On Day 142 the BAT processor had a reboot, resulting in about 36 hours with no GRB detection. On Day 147 the XRT CCD was apparently struck by a micrometeoroid which generated a very bright pixel. This forced the XRT to use engineering modes for about 48 hours to diagnose and adjust the software to handle these effects. Since we made this change to disable bright pixels, XRT instrument performance has returned to normal.

On Day 148 the Star Tracker lost lock on stars, and Swift had to be manually restored to GRB detection condition, losing about 5 hours of observing time. On Day 150 the Star Tracker generated a bad quaternion, resulting in an ACS safehold. About 60 hours of observing time were lost. On Day 162 the observatory conducted an exceptionally long planned slew, during which the Earth constraint moved enough to cause a safe hold violation. About 66 hours of observing time were lost.

We have investigated each of these anomalies, and we can see no causal link between them. The total effect has been to reduce the Swift operational efficiency to approximately 70% over the past month (as compared to more than 90% over the previous ten weeks.) We are closely monitoring all of these systems, and we hope to return to the previous levels of efficiency. We are exploring means to speed the recovery process to also reduce the impact of any future anomalies. The specific anomalies seen this month have their root causes understood and can be largely avoided in the future.