Note from the Top
June 1, 1995 marked a major milestone in the ROSAT mission, the fifth anniversary of its launch. Such an occasion offers reason to commemorate ROSAT's major scientific achievements as well as consider what is in store for the remainder of the mission. ROSAT's major scientific accomplishments are many and growing; an enormous number of papers is published monthly in the refereed literature (see the recent publication list on page 17) Despite being restricted to a single detector, ROSAT is still a powerful mission. Judging from the response to the AO6 solicitation, there is no reason to think that there are limitations on the scientific usefulness of the HRI. In the U.S., a total of 176 proposals were submitted, about the same number submitted in response to AO5. The total amount of time requested was a record 30 million seconds. This represents a factor of 4-5 oversubscription of the U.S. time share, and is about eight million seconds more than has ever been requested. Part of this large oversubscription is the result of an onincreased under- standing by the community of the best ways to utilize ROSAT. For instance, over a dozen proposals were received with requests for over 500 ks. As the mission continues, we expect that an ever increasing share of time will be devoted to large projects.
The future also looks bright from the viewpoint of mission performance. While the loss of a gyroscope in late 1993 resulted in a new set of operational constraints, the net consequence is only a ~10 percent reduction in overall observing efficiency (though some observations, especially time constrained ones, become difficult to accomplish). The satellite has performed reliably over the past year, and there is no apparent degradation of any subsystem that might endanger the mission. On the political front, both the German and the U.S. programs have been promised funding at least through fiscal 1997 (though at reduced levels compared with previous years). Barring the unforeseen, we anticipate releasing AO7 next spring. And we hope to continue the mission beyond AO7, for as long as the program is receiving scientifically worthwhile proposals.
In late September, the ROSAT project participated in a dual celebration. In honor of the centennial of the discovery of X-rays by Wilhelm Conrad Röntgen in Würzburg on November 8, 1895, and the fifth anniversary of the launch of ROSAT, MPE and the Astronomy Institute of the University of Würzburg hosted a conference entitled "Röntgenstrahlung from the Universe." Over 300 astronomers from around the world attended the meeting. While ROSAT talks formed the core of the scientific program, results from other missions, like ASCA and Granat, were also presented, making this one of the most exciting X-ray astronomy conferences in recent years.
Otherwise, despite significant cutbacks over the past two years, the RSDC continues to make progress in making ROSAT data accessible to the general community. Progress has been made in understanding the PSPC. The time and spatial dependence of the gain are being carefully scrutinized, and correction methods are being developed. Also, a substantial number of new software tools have been made available. Most notable is the software for performing background subtraction, exposure correction, and mosaicking of both PSPC and HRI images.
In November 1994, the first two source catalogs based on PSPC pointed data were released. One of these was produced by the ROSAT team, led by MPE, and is a screened compilation of sources detected by SASS. The other, an independent effort by Nick White and Lorella Angelini of GSFC, and Paolo Giommi of ESRIN, relied on the detection algorithm in XIMAGE to find sources. There are about 70,000 sources in each catalog, comparable with the number found in the All Sky Survey. The deeper coverage of a smaller fraction of the sky (~10 percent) provides a complimentary view to the RASS of the soft X-ray source population. Both of these catalogs will soon be replaced by the ROSAT Results Archive (RRA), which will be the definitive catalog from pointed ROSAT observations. The RRA is being produced collaboratively by the US, UK, and German data center teams.
Finally, one major personnel change within the USRSDC should be noted. Rick Harnden, who for the past several years has managed the SAO team, is spending two years at NASA Headquarters as a Visiting Scientist. He has been replaced by Dan Harris, a ROSAT team member since the founding of the USRSDC. On behalf of the entire USRSDC team, I want to publicly express our gratitude to Rick for his many significant contributions to the ROSAT program.
In November 1994, a panel of scientists was convened by NASA Headquarters to review the scientific productivity of its astrophysics missions , and based on this, to recommend funding levels for future years. A total of thirteen projects were included in the review, among them HST, CGRO, ROSAT, EUVE, and XTE. This review was especially crucial to the future of the U.S. participation in ROSAT, as support for the mission was scheduled to be terminated on September 30, 1995. As part of the review, overall rankings of "science per dollar" were compiled for the near term (fiscal years 1995 and 1996) and long term (fiscal years 1997 and 1998). ROSAT was ranked third (behind HST and CGRO) and sixth in the two categories. The primary science strengths noted by the panel report are its cost effectiveness, the capability of the HRI for large-scale, high-resolution mapping, and the use of ROSAT as a pathfinder for AXAF. The panel recommended that funding for ROSAT continue through 1997, and that guest observer support be reinstated. The panel also directed the USRSDC to place its priority on finishing the current work on the PSPC calibration and ensuring that the data analysis software is in a stable state to support the use of the long-term archive.
In response to the senior review recommendations, the USRSDC has done the following.
1.) We have reinstated funding for the guest observer program. We anticipate making available a modest amount of funding, about $700k, to support AO6 observations. This funding is a combination of some of the budget augmentation ROSAT received from the Senior Review plus some uncommitted funds from the ASCA guest observer program.
2.) We have initiated work on the ROSAT Results Archive (RRA). While this effort consumes only a modest amount of manpower and utilizes GSFC and SAO personnel already associated with ROSAT, it would not have been possible without the infusion of additional funds to support these individuals.
3.) We continue our collaborative efforts with MPE to complete the PSPC calibrations. Unfortunately, this effort has been hampered by the discovery of time dependencies in the PSPC performance. In addition, we have made substantial headway in our detailed understanding of the HRI properties, and will continue these efforts as well.
4.) We are seeking means by which we can continue the U.S. participation in ROSAT beyond fiscal 1997.
5.) Implicit in the recommendations was strong support for large projects such as those initiated in AO5 (e.g., mapping the Cygnus Loop and the LMC). While the ROSAT project will not dictate what time fraction should be spent on these, we are encouraging the community to submit proposals for such projects. We anticipate that a growing fraction of the U.S. time share will be spent on these in future observing cycles.