ROSAT Guest Observer Facility


Harvey Tananbaum
Harvard-Smithsonian Center for Astrophysics,


The previous NASA Senior Review identified large observing programs and the use of ROSAT as a ``pathfinder'' for AXAF as two primary goals for the continuation of U.S. participation in the ROSAT mission. As part of the ROSAT Science Workshop at the HEAD meeting, the AXAF Science Center was invited to comment on the ways in which the continued use of the ROSAT HRI will help us prepare for the AXAF mission.


In view of the exciting and important ongoing observations with ROSAT (many summarized in the preceding presentations), it is clear that Science Quality should be the primary criterion for ongoing HRI observations with ROSAT. Until AXAF is launched, ROSAT provides the only X-ray imaging capability at the level; this is a high quality resource, and we must continue to peer review for the assignment of the U.S. observing time to ensure effective and fair use of this precious capability. Moreover, there will be important science that ROSAT HRI can do, such as high resolution mosaic images of very large, extended X-ray sources, which AXAF likely will not do. To facilitate this entire process, we can identify examples of types of observations which would be high quality science and potential ``pathfinders'' for AXAF via newsletter articles such as these and encourage people to write such proposals

We have grouped examples of pathfinder information into four categories for this presentation. (Undoubtedly, there are many other worthwhile examples besides those identified here.)

Flux Measurements

Such observations are useful for individual objects not previously observed (e.g., newly discovered, high redshift QSOs), for classes of objects not yet systematically studied (e.g., moderate redshift narrow line galaxies), and for potentially (or known) variable X-ray sources. These measurements will help AXAF to establish exposure times for imaging observations and grating spectroscopy. They will discover and highlight objects and classes which need AXAF data, and will allow AXAF to project required exposure times and to observe fainter (less luminous/more distant) sources early in the mission.

Extent/Structure Observations

Such HRI observations can be carried out on known extended objects to look for ``compact'' bright structures (e.g., knots in SNRs) and to measure full X-ray extent (e.g., halos around elliptical galaxies and very extended emission from clusters of galaxies). These data will help AXAF to select nearly point-like features for grating spectroscopy and will provide essential information for establishing observing strategies (exposure times & patterns) for faint, extended objects. Strategy will also include choice of appropriate AXAF imager.

The ROSAT HRI can also be used to search for ``new'' types of extended emission (e.g., around point-like sources at the center of AGNs - already known to exist in 20% of Seyfert galaxies). Such data will help AXAF to establish the importance and value of such observations, provide guidance on exposure times, and provide an initial set of targets

The ROSAT HRI also can provide X-ray maps for multiple source objects (e.g., star clusters). This will help AXAF to optimize pointing direction and roll orientation for grating observations of multiple object fields


With ``deep'' HRI exposures, we can find significant numbers of new sources (e.g., X-ray selected clusters) for which there are presently no adequate samples for science studies or for selection as AXAF targets. Such surveys will provide AXAF with more information on the systematic behavior of various classes (e.g., through evolution and luminosity functions), as well as provide valuable targets for more sensitive, detailed AXAF observations.

Software - Algorithms/Techniques

Additional high-quality HRI science investigations with ROSAT have a significant ``discovery'' potential - with unanticipated structure, variability, or other new features. In addition to suggesting targets and scientific investigations for AXAF, such discoveries may indicate a need for advanced software (e.g., to examine very fine, extended sub-structure or very faint, extended halo emission) which can influence the development of the ASC Data System, especially algorithms and analysis tools.

ROSAT All-Sky Survey

An additional ROSAT item of great relevance for AXAF is the schedule for full release of ROSAT All-Sky Survey data. The RASS contains essential information (including upper limits) on flux for a large number of potential AXAF targets. For optimal use of AXAF, these data should be made available to all scientists to help them determine relevance for AXAF observing and realistic observing times. Ideally, this information should be made available to all scientists interested in writing AXAF proposals before the release of the first AXAF NRA, scheduled for 1 September 1997.

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