INTRODUCTION
R. Petre
U.S. ROSAT Project Scientist
NASA/Goddard Space Flight Center,
petre@lheavx.gsfc.nasa.gov
This special issue of the ROSAT Newsletter represents the proceedings of a workshop sponsored by the U.S. ROSAT Science Data Center (USRSDC) on the afternoon of Monday, April 27, in San Diego, CA. The goal of this workshop was to provide input from the U.S. ROSAT community regarding the utilization of the next 2+ years of the U.S. share of ROSAT observing time. This input will be used in formulating the USRSDC proposal to the biennial NASA Senior Review of space astrophysics missions.
The workshop concentrated on two major topics, identified by the most recent (1994) Senior Review as key U.S. ROSAT activities: the so-called ``large observing programs,'' and the use of ROSAT as a ``pathfinder'' mission for AXAF. Brief presentations about ongoing large observing programs were made by the PI's, and a series of presentations were made about how existing and future ROSAT observations can help maximize the scientific efficiency of AXAF. One of the key points made by virtually all the speakers who addressed the latter issue is the strong complementarity between ROSAT and AXAF: there are many scientific investigations that the ROSAT HRI will carry out that will not be performed by AXAF.
The workshop was well attended (at least 100 participants), and as documented below, there is a vast amount of science remaining to be carried out by the ROSAT HRI, more than enough to justify the continuation of U.S. participation in the mission not only over the next two years, but until the demise of the satellite.
This introduction is meant to place the subsequent contributions into context. I first discuss the heritage of the large observing programs, and then offer a few general thoughts about ROSAT's role as a pathfinder for AXAF.
Large Observing Programs
A large observing program is an integrated program requiring a substantial fraction of ROSAT observing time, typically more than 300 ks. While such programs were carried out piecemeal since the beginning of the mission, the first explicit attempt to encourage such programs occurred in 1994 during AO-5, the first HRI-only observing cycle. In each of the past three proposal rounds (AO's 5-7) we have received at least a dozen proposals for large observing programs. Many of these have requested on the order of 10 seconds, a quarter of the total U.S. time for one year. No time shares of this magnitude have been awarded to a proposal in a single observing cycle, but it has been possible to carry out large programs over the course of several years. Below I list the large observing programs currently underway, and the total time required to complete them. One encouraging aspect of large proposals is that the PI's, in recognizing that their observations constitute a substantial fraction of a valuable resource, generally voluntarily waive the one-year proprietary period. Thus the entire astronomical community can share the wealth of the large programs.
The PI's of most of these projects discuss them below. There are a substantial number of other large observing programs suggested here as well, only some of which have been formally proposed. Moreover, there are other large programs absent from these proceedings, such as a deep, systematic mapping of M31, deep monitoring of other nearby galaxies (e.g., M33), and a systematic study of gravitational lenses to find the lensing clusters in the X-ray band (e.g., see Hattori et al., in ``X-ray Imaging and Spectroscopy of Astrophysical Plasmas''). The summed observing time of the programs suggested below would easily keep ROSAT productively occupied through the turn of the millenium.
ROSAT as a ``Pathfinder'' for AXAF
ROSAT serves its role as a pathfinder by carrying out observations that ultimately can make AXAF more scientifically productive. Basically this means observing potential AXAF targets in a way that allows a determination of whether AXAF observations of them are necessary. It is clear in this context that since ROSAT's primary usage is for high resolution imaging, the ROSAT images can be used to determine whether AXAF's sub-arc-second imaging power will yield important additional information about a source. This is not a frivolous point. The effective area of the ACIS is approximately twice that of the ROSAT PSPC, or six times that of the HRI. With an angular resolution approximately an order of magnitude higher than that of the ROSAT HRI, and thus a factor of 100 more effective resolution elements in an image, AXAF is severely surface brightness limited. Observations that make use of AXAF's full imaging capability must be long; many or most of those that do not can be performed by ROSAT.
The contributions below describe the many ways in which new HRI observations can continue ROSAT's pathfinding role. They also stress the complementary nature of many of the observing programs ROSAT can perform.
Acknowledgements
I consider the ROSAT Workshop in San Diego an enormous success. I was exposed to many new ideas for large observing programs and pathfinder observations. More importantly, it became clear that the high energy astrophysics community is still enthusiastic about the high resolution imaging that ROSAT can carry out. I thank most wholeheartedly all who attended the workshop, especially the contributors, both for their insightful presentations and their extremely prompt submission of written material. These proceedings go to press less than five weeks after the workshop.
I would like to extend special gratitude to Harvey Tananbaum and those members of the AXAF Science Center who participated in the workshop. Interestingly, the speakers from the ASC were the individuals who most strongly stressed the complementarity between ROSAT and AXAF. We thank the ASC for its continuing enthusiasm for ROSAT.
Finally, I would like to thank Steve Snowden for assembling the proceedings.
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