A Note from the Top

--Rob Petre

Over the past few months, two key events marked a fundamental change in the ROSAT mission. First, on December 20, 1993, the HRI replaced the PSPC as the primary focal- plane instrument. After a six-month "vacation," the HRI is working nominally, and it should be capable of a long lifetime. While the official usage of the PSPC has now ended, sufficient detector gas remains to allow five days of operations per month. That time will be used to fill out incomplete observations from AO1 through AO4, perform end-of-life calibrations, and carry out Target of Opportunity observations. The PSPC has fulfilled its promise as a workhorse detector, and while some questions invariably remain about calibration issues, it is clear that it provided us an spectacular view of the soft Xray sky. We now look forward eagerly to the first opportunity ever to carry out a substantial amount of high resolution imaging X-ray astronomy (24 million seconds of exposure time through the end of AO5).

The second event was the breaking of a string of approximately 21 months of flawless operations when the z-axis gyro failed in November. It took until early March to regain full operations. And while ROSAT's capability for carrying out a full program of HRI observations has been restored, the unexpected difficulties encountered in recovering from the gyro loss suggests that the next attitude control system failure will leave ROSAT with reduced operational capabilities. We can only hope that the next failure will not occur for a long time.

It is clear from the number of presentations at scientific meetings that an enormous amount of work is being done on ROSAT data by the worldwide guest observer community, and that the most interesting and substantive results are just beginning to find their way into the literature. The scientific highlight of the past few months was the U. S. ROSAT Science Symposium and Data Analysis Workshop, held for three days in November. This meeting attracted nearly 200 participants, and offered over 130 talks and posters. Over 90 percent of these presentations will be represented in the symposium proceedings, which is nearly ready for submission to the publisher. Additionally, there was a two-day ROSAT workshop in November hosted by the MPE team, which featured about 30 talks from European ROSAT observers. At the January APS meeting in Washington, D. C., there was an invited ROSAT session, featuring talks on dark matter (by Richard Mushotzky), nearby galaxies (by Knox Long), and the X-ray background (by Richard Griffiths). ROSAT observations of SN1993j were detailed in an invited HEAD talk (by Walter Lewin), and there were at least two dozen contributed talks and posters featuring ROSAT results. Finally, ROSAT was well represented at the recent meeting in Tokyo entitled, "New Horizon of X-Ray Astronomy." While this meeting was primarily dedicated to highlighting the initial ASCA results (as indicated by the subtitle "First Results from ASCA"), one clear conclusion that could be drawn from the results presented is the strong complementarity between the two missions, and the necessity of having both high resolution, broad band spectroscopy, and high resolution, large-field-of-view imaging.

By the time this newletter is circulated, the deadline for the AO5 proposal submission will have passed. While the situation regarding funding of guest observer grants is still unsatisfactory (i. e., no funding has been made available for AO5 investigations), the fact that AO5 was released in the U. S., and that the U. S. ROSAT Science Data Center shall remain open to process and distribute data through AO5 (at least), are clear indications that NASA is still committed to the ROSAT program in the U.S.