ROSAT AS AN AXAF PATHFINDER FOR SNR AND SNE
NASA/Goddard Space Flight Center, email@example.com
While ROSAT HRI observations have already done much to pave the way for AXAF observations, observations remain to be performed in a number of areas. Among these are establishing high quality baseline images for proper motion studies, identifying new SNR out of the All Sky Survey list of candidates, locating scientifically interesting regions meriting high resolution spatially-resolved spectroscopy, searching for neutron star counterparts, observing new SNe as they are discovered, and tracking the light curves of known SNe.
Over the past six years, ROSAT has produced a substantial number of important discoveries about SNR and SNe by virtue of its sensitive imaging capability. Among these are these discoveries are: approximately 450 new SNR candidates in the All Sky Survey, bullets of ejecta from the Vela progenitor, neutron star remnants in Puppis A and others, and emission from a number of supernovae both new and old. The ROSAT HRI has carried out high resolution observations of nearly all the bright Galactic remnants, and most in the Magellanic Clouds. The images from these provide a rich data base from which interesting regions can by chosen for further study with AXAF. Despite this rich data base, a number of important pathfinding activities remain to be done.
This pathfinding role is critical. Full utilization of AXAF's sub-arc-second imaging capability will require extremely long exposures, even for the brightest remnants and regions within remnants. Cas A, the brightest remnant, can serve as an example here. If we assume it has a uniform surface brightness, then just to obtain 10 counts per square arc second pixel requires an AXAF exposure of approximately 10,000 s. Imaging to a third of an arc second (AXAF's practical limit) will require an exposure of 100,000 s. High resolution spatially resolved spectroscopy will require still more time. And Cas A is the brightest remnant: Tycho, with the second highest integrated flux, has an order of magnitude lower surface brightness. Thus as complete a catalog as possible of high resolution ROSAT images will be necessary for judicious selection of AXAF targets.
In the sections below we briefly describe some of the areas in which additional ROSAT observations of SNR can help make the most scientifically productive use of AXAF. It should be noted that each observation described below has high inherent scientific value as well.
An supernova remnant at a distance of 1 kpc, with a radial expansion velocity of 1,000 km s increases in angular diameter by 0.4 arc seconds per year. Many Galactic remnants are sufficiently nearby (3 kpc) and have fast enough shock velocities (500 km s) so that proper motion can be detected using high resolution X-ray imagers over a baseline of a decade or less. This has already been done for Tycho and Cas A using ROSAT and Einstein HRI data (Hughes 1996; Gotthelf & Keohane 1996). Deep ROSAT HRI observations of other bright remnants will provide a baseline to which AXAF images can be compared to detect proper motion more sensitively. Among the remnants for which this should be possible are Kepler, SN1006, G290.1+1.8, and Puppis A.
There are two ways by which X-rays are produced by an isolated neutron star: synchrotron emission associated with magnetospheric processes around a pulsar and thermal emission from the cooling surface. Examples of both have been detected using ROSAT. Recent ROSAT results include the discovery of candidate thermally emitting neutron star remnants in Puppis A (Petre et al. 1996) and near CTB 1 (Hailey & Craig 1995), and the ``rediscovery'' of the candidate in RCW 103 (Becker 1996). There are a number of proposed associations between SNR and young radio pulsars for which the corresponding X-ray emission has not been detected (Caraveo 1993; Frail et al. 1994). Deep HRI observations of these pulsars may reveal unresolved X-ray emission, or possibly small synchrotron nebulae (as seen around PSR B1823-13 - Finley et al. 1996). Any candidate neutron star or compact synchrotron nebula within a SNR will be an excellent target for AXAF imaging and nondispersive spectroscopy, while any relatively isolated such object can be observed by the AXAF gratings.
The nearly complete ROSAT HRI mapping of the Cygnus Loop will provide a catalog of regions in that SNR worthy of deep imaging and (nondispersive) spectroscopic study using AXAF. While a few of the brighter Galactic remnants have been completely mapped using the ROSAT HRI (e.g., Puppis A) most have not. Using the ROSAT HRI to map as many remnants as possible will reveal additional regions worthy of further study. One prominent example of a remnant worthy of a complete mapping is W28, the archetype of the ``mixed-morphology'' SNR, with a shell-like radio morphology, and a centrally-filled X-ray morphology. The PSPC observations reveal strong X-ray emission arising from regions where OH masers have been detected in the radio, a certain indication of strong interaction between the SNR shock and a molecular cloud. Deep ROSAT HRI imaging can identify radio/X-ray/optical correlations at the 5'' level, but cannot trace the details to the arc-second level possible in the optical or radio. Thus ROSAT can identify regions worthy of AXAF observations.
The ROSAT All-Sky Survey has revealed approximately 450 uncataloged, extended X-ray sources at low Galactic latitude, some of which are probably new SNR (Aschenbach 1996). ROSAT HRI follow up observations of these can help confirm whether any are in fact SNR. They can also tell us which are the most interesting, and thus worth observing with AXAF (or XMM or Astro-E).
Additionally, many of the sources the HRI will detect in nearby galaxies will be SNR. Compiling a catalog of these for spectroscopic studies using AXAF will is an vital task to be done by ROSAT.
One of ROSAT's most significant scientific contribution has been its studies of supernovae: observing X-rays from SNe much earlier after optical maximum than anticipated by theoretical models, and observing a class of highly luminous X-ray SNe, with L erg s. Any extragalactic SN detected by ROSAT will be an ideal target for AXAF imaging and spectroscopy. The CCD spectrometer can search for evidence of enhanced metal abundances from explosive nucleosynthesis and for evidence of a hidden pulsar, while the imaging capability can be used to measure the extent of the rapidly expanding shell.
ROSAT should continue to follow the light curves of known SNe, such as SN1987a, SN1993j, SN1986j, and SN1978k. As these SNe encounter different layers in the circumstellar medium, the X-ray luminosity and spectrum should vary. For instance, within the next few years the shock front of SN1987a should encounter dense circumstellar material, and therefore become far more X-ray luminous. In addition, as the ROSAT HRI carries out detailed observations of nearby galaxies, it may detect X-ray emission from other young SNe. More importantly, as new, bright SNe are discovered, the HRI can be used as a Target of Opportunity instrument to search for X-rays. This should be done as a matter of course for SNe showing strong radio emission, as this is an indicator of interaction with a dense circumstellar medium.
From the shopping list above, it should be abundantly clear that there are a substantial number of scientifically worthwhile observations of SNR and SNe that remain to be done using the ROSAT HRI. It is the author's hope that we will have the use of ROSAT as an astronomical resource for many years to come.
I would like to acknowledge the contribution of Jack Hughes to the list of ideas presented here.
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Becker, W. 1996, ``Röntgenstrahlung from the Universe,'' eds. H.-U. Zimmermann, J.E. Trümper & H. Yorke, (Garching: MPE), 103
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Finley, J.P., Srivivasan, R., & Park, S. 1996, ApJ, in press
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Hughes, J.P. 1996, ``X-Ray Imaging and Spectroscopy of Cosmic Hot Plasmas,'' in preparation
Petre, R., Becker, C.M., & Winkler, P.F. 1996, ApJL, in press
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