Solar System Studies

The number of solar system objects accessible to the ROSAT HRI is small, only Jupiter and comets. Nevertheless, these studies have produced some of ROSAT's most stunning results, often at odds with established ideas. The bulk of the scientific work in this area has been performed by US investigators.

With the detection of bright, diffuse, time variable emission from Comet Hyakutake in March, 1996 (Lisse et al. 1996, Science 274, 205), ROSAT opened up an entirely new field of X-ray astronomy and revolutionized our ideas about comets. Follow up studies using the ROSAT All-Sky Survey archive, and new HRI observations of Comets Tabur, Encke, and Hale-Bopp have demonstrated that X-ray emission with properties similar to those observed in Hyakutake is to be found in virtually all comets nearer the sun than 2 AU with V < 12 (Dennerl et al. 1997, Science 277, 1623; Lisse et al. 1998, in preparation ). An explosion of theoretical studies followed the Hyakutake detection. Even though the morphology and temporal variability of the Hyakutake emission eliminates from consideration any model that does not incorporate interaction between the comet and the sun, an abundance of plausible models exists. Among these are thermal Bremsstrahlung of solar wind electrons off cometary neutrals or dust, charge exchange induced emission from solar wind ions, reconnection of solar magnetic field lines, and scattering of solar X-rays attogram dust particles. Although the charge-exchange model comes closest to accounting for all the observed properties, recent HRI/EUVE observations of Encke (plus BeppoSax observations of Hale-Bopp) reveal spectral and morphological inconsistencies with the model predictions. Our understanding of cometary X-ray emission has improved dramatically over the past two years, but continued observations are required to determine the responsible physical processes.

Jupiter has been observed almost annually since the launch of ROSAT. The dramatic X-ray brightening of the north auroral zone following the collision of Comet Levy-Shoemaker with Jupiter has been well documented (Waite et al. 1995, Science 268, 1598); recent studies have concentrated on the detailed morphology and temporal variability of Jupiter's persistent X-ray emission. Waite et al. (1997, Science 276, 104) present evidence for Jovian X-ray emission at low latitudes, in addition to the strong auroral emission concentrated near the poles. This emission is thought to arise from precipitating sulfur and oxygen atoms. The low latitude is generally concentrated in the local noon-to-dusk sector, but sometimes peaks instead in the dawn-to-noon sector. Neither the local time organization of the Jovian X-ray emission nor the variation between morning and afternoon sectors is understood, in part because of the paucity of observations.

Continued observations of these objects using ROSAT clearly have merit. ROSAT's wide field of view is vital for detailed studies of comets passing nearby; the angular extent of most detected comets exceeds AXAF's field of view. (in the meantime, AXAF can be used to detect more distant comets, and provide spectra that may reveal the emission mechanism). For Jovian studies, ROSAT's value is in its baseline - the HRI has now monitored Jupiter over two-thirds of its orbit, and semi-annual to annual monitoring might provide an adequate baseline to allow efforts at modeling the latitude and longitude dependence of the emission.

Stars and Stellar Clusters

ROSAT continues to be an effective and mature observatory for studies of Galactic stars. There are three reasons for ROSAT's continued worth: 1. ROSAT is a stable instrument. The HRI provides a long baseline, going on 9 years, for studies of stellar variability. One of the most important timescales for solar-like stars is the stellar magnetic cycle, with periods of 8-15 years (11 years in the Sun). ROSAT is the first X-ray observatory to afford the baseline and stability to permit us to study variability on these timescales. 2. The soft X-ray response of the HRI is well matched to the coronal temperatures of most stars (except for the highly embedded protostars seen in some regions of star formation). 3. The wide field of view of the HRI is well matched to the typical size of most nearby Galactic clusters. X-ray observations are still the most efficient method of identifying outlying members of young clusters. In the era of AXAF, ROSAT still has a vital role to play, not only as a pathfinder, but also to undertake observations that do not require the full capabilities of AXAF, and for exploring new areas that may not be guaranteed to pay off in X-ray detections.

A number of astronomers have continued their work on open clusters, using ROSAT to identify members through their X-ray emission. For example, Prosser & Randich (1998, ApJ, in press) and Prosser et al. (1998, ApJ, in press) have identified optical counterparts to more than 200 ROSAT X-ray sources in the region of the alpha Persei open cluster. They used follow up optical photometry and spectroscopy to study the full membership of the cluster. Simon & Patten (1997, PASP 110, 238) have undertaken similar work on the IC 2391 cluster. Work is ongoing on the Pleiades, IC 2391, IC 2602, NGC 2232, and Cr 140.

ROSAT continues to provide an excellent platform from which to study star formation, because all young stars are luminous X-ray sources, and most do not have other obvious distinguishing characteristics. Some of these observations challenge our notions of the circumstances of star formation. In the Sco-Cen association, deep HRI images double the number of low mass stars found in PSPC images of the same field (Sciortino et al. 1998, A&A 332, 825). Herbig (1998, ApJ 497, 736) used ROSAT detections of stars in the young cluster IC 348 along with optical spectroscopy to show that weak-line T Tauri stars have a different spatial distribution from classical T Tauri's even though there is no systematic difference in their ages. Brown et al. (1998, 1997 Ringberg Conference: The Orion Complex Revisited, eds. A. Burkert & M. McCaughrean, in press) and Alcala et al. (1998, A&A 330, 1017) show that the low mass stars in the Orion OB association are distributed more widely than previously thought. Even more remarkably, Feigelson et al. (1998, in preparation) have discovered a rich group of about a dozen active young stars in the TW Hya cluster, far removed from any known star formation region or dark cloud.

Observations of individual stars are revealing new information. Coordinated observations show the full range of atmospheric phenomena. For example, Schmitt et al. (1998, ApJ, 500, L25) used simultaneous ORFEUS and ROSAT observations of AB Dor to show that the FUV and X-ray flux variations correlate and thus the emission sources are co-spatial. By means of monitoring observations over the lifetime of ROSAT, Schmitt (1998, A&A 333,199) has discovered apsidal motion in alpha CrB. Piters et al. (1998, A&AS 128, 29) studied the onset of magnetic activity as a function of stellar type using ROSAT data. Fleming (1998, ApJ, in press) has used ROSAT to identify 55 new M dwarfs within 25 pc of the Sun.

And ROSAT observations have continued to lead to stunning new and often unexpected discoveries. Jura et al. (1998, ApJ 192, 1005) used ROSAT HRI to find that the X-ray flux from the HR4796 system comes from HR4796B, a PMS M star. They used this flux to date the system, and find that it is about 30 Myr old. HR4796 has a near-IR excess thought due to a protoplanetary disk. Webb et al. (1998, BAAS 30, 828) used ROSAT data in part to discover 7 nearby T Tauri stars associated with TW Hya. At about 50 pc, these are by far the nearest PMS stars. Their origin is a mystery. Walter, Wolk, & Sherry (1998, in "Cool Stars, Stellar Systems, and the Sun 10", eds. R. Donahue & J. Bookbinder (ASP: San Francisco), in press) discovered a previously-unknown cluster of low mass stars associated with s Ori, near the belt of Orion.

Stellar studies continue to consume a substantial fraction of ROSAT observing time. Investigations currently underway include:

- Deep observations of cool giants. These stars are not known to possess coronae, although they have weak transition region (C IV) emission. ROSAT has the soft response needed to detect the 106 K plasma that some models predict.

- Long term monitoring of stellar cycles. Observations are continuing to establish the coronal cycles in EK Dra and b Hydri, and new observations seek evidence for stellar cycles in Hyades G stars. These observations require long baselines, as the cycle lengths are of order 10-15 years. They are important, because only by observing other stars can we estimate the range of properties of stellar cycles, and thereby, perhaps, predict the excursions of the solar magnetic cycle. As solar activity at the peak of the cycle affects spacecraft operations (on which our society is becoming more and more dependent), and the cycle may affect the terrestrial climate, such long term observations may prove quite valuable for society as a whole.

- The ROSAT HRI, both in its wide field of view and its soft response, is still the premier instrument for observing young stellar clusters. Among the targets for AO8 are the r Oph, NGC 2264, Mon R1, Barnard 35, the Gum nebula, R CrA, Chamaeleon I, and Jewel Box clusters.

It is clear that there has been no letup in excellent science derived from ROSAT stellar observations, and that there is no expectation for a diminution from future observations.

Compact Galactic Objects

The HRI is well suited for many studies related to compact galactic objects, be they neutron stars, cataclysmic variables, X-ray binaries, or soft g-ray repeaters. Its most useful contributions come in providing source locations for follow up observations in other bands, and temporal variability studies. The HRI is of particular use when the source is embedded in diffuse emission, such as a supernova remnant. Also, now that an observation baseline of several years exists for many source, it becomes possible for the first time in many sources to investigate long term soft X-ray temporal variability. Below we present a sampling of the broad variety of HRI results in this area.

SGRs

Neutron Stars in SNRs

Localizing Compact X-ray Sources

Supernova Remnants, Planetary Nebulae, and Diffuse Emission

SNRs

In the evolved SNR IC 443, HRI observations have been combined with ASCA data to investigate the morphology of a region of anomalously hard X-ray emission, characterized by a nonthermal spectrum (Keohane et al. 1997, ApJ 484, 350). The hard emission comes entirely from a region from which the SNR shock interacts strongly with a molecular cloud. About half the emission arises from a single concentration, shown by the HRI to have a spatial extent of < 1'. The hard emission is thought to be synchrotron radiation from TeV particles whose density is enhanced by the cloud-shock collision; the concentration is a manifestation of enhanced particle acceleration from a surrounding concave shock, as predicted by Jones & Kang (1993, ApJ 402, 560).

One of the most spectacular recent results comes from one of the most spectacular celestial objects, the Crab Nebula. By comparing the five Crab HRI images taken over the duration of the mission, Greiveldinger et al. (1998, A&A in press) have discovered dramatic spatial variations throughout the Nebula, but particularly around the ring of emission defining the equatorial plane of the pulsar.

ROSAT HRI observations are particularly useful in studying SNRs in the LMC. HRI observations have been used to uncover a new SNR projected at less than 2 arc minutes from the luminous X-ray binary LMC X-1 (Chu et al. 1997, PASP 109, 554), a possible synchrotron nebula in the Crab-type SNR N157B (Wang & Gotthelf 1998, ApJ 494, 623), and the colliding SNR DEM L316 (Williams et al. 1997, ApJ 480, 618). The HRI image of the LMC SNR N63A has been compared to optical and radio images to demonstrate that N63A is a remnant in a cloudy interstellar medium (Chu 1997, AJ 113, 1815). The HRI and PSPC observations of the LMC H II complex N11 show clearly the distribution of hot gas heated by SNRs (Mac Low et al. 1998, ApJ 493, 260). ROSAT HRI observations are also useful in studying SNRs in more distant galaxies; a small number of SNRs have been detected in M31 (Magnier et al. 1997, ApJ 490, 649).

Planetary Nebulae

Diffuse Emission

The HRI survey of the LMC (PSPC image) continues. The goal of this survey is to map a 2.5 degree x 2.5 degree area centered on the giant H II region 30 Doradus and a 1 degree x 2 degree area along the LMC bar, with an average integration time of 20 ksec. In AO5-AO8, 1.2 million sec for 60 fields have been awarded. While this survey will produce an excellent catalog of point sources, it has also detected diffuse X-ray emission from scales of a few tens of pc to over a thousand pc. The smaller sources are associated with SNRs and superbubbles. Some of the larger diffuse sources are confined within supergiant shells, but some do not seem to be associated with any visible interstellar structures (Chu & Snowden 1998, AN, 319, 101). This HRI survey of the LMC allows us to examine the diffuse X-ray emission with a linear spatial resolution of ~2 pc over an area of ~6.7 kpc^2 The HRI survey of the Small Magellanic Cloud (SMC) also continues. Preliminary results indicate that one of the primary purposes of the high-resolution mapping, to be able to distinguish between truly diffuse emission and unresolved point sources in the 3/4 keV PSPC mosaic, is being fulfilled, supporting a diffuse origin for a significant fraction of the observed flux (Snowden, private communication).

Nearby Normal Galaxies

One of the great strengths of the ROSAT HRI is its ability to detect and monitor discrete sources in nearby galaxies. Ironically, this is one of the areas in which the HRI has been most underutilized, and therefore for which its pathfinding for AXAF is least complete. There are dozens of nearby galaxies which have never been imaged in the X-ray band and for which a 20 ks HRI "snapshot" would be immensely valuable. Studies of local galaxies have shown that the Milky Way (and the Local Group) might be unusual in the absence of high luminosity (>10^39 erg s^-1) sources. Indeed, HRI studies of Local Group galaxies, some of which we discuss below, show very few of any type of X-ray source. The local universe has not been sufficiently carefully studied for us to understand the meaning of the observed source population differences. This is one area in which the HRI could make an immediate, significant contribution.

HRI observations have now been performed of all the gas-rich, and many of the gas-poor Local Group galaxies. The small number of source detections is puzzling. There are a number of recently published papers on our nearest neighbors. Dubus et al. (1997, ApJ 490, L47) have used observations taken over a 6-year interval to find a periodicity of ~10^6 days in M33 X-8, the most luminous X-ray source in the Local Group (Lx~10^39 erg s^-1), located at the center of M33. Their result suggests the emission from the center of M33 is dominated by single object, perhaps a 10 solar mass black hole. In contrast, the only detected source in M32, a satellite of M31 and the nearest elliptical galaxy, is offset from the nucleus (Loewenstein et al., 1998, ApJ 497, 681). This source is likely to be an X-ray binary. Brandt et al. (1997, MNRAS 291, 709) report on observations of three Local Group galaxies, IC 10, NGC 147, and NGC 185. Only IC 10 contains a detectable source. This source was found to be variable, and positionally coincident with a Wolf-Rayet star; the authors suggest it is a Wolf-Rayet+Black Hole binary.

Revealing HRI studies have been made of other nearby galaxies as well. An HRI image of the Antennae (NGC 4038/4039) reveals complex and intricate X-ray emission associated with both galaxies, coincident with star forming regions and H II regions (Fabbiano et al. 1997, ApJ 478, 542). In addition three unresolved sources and prominent nuclear emission (Lx~1040 erg s-1) were detected. NGC 1316 (Fornax A) shows an anticorrelation in the inner galaxy between X-ray surface brightness and optical dust patches, suggesting intrinsic X-ray absorption (Kim et al. 1998, ApJ 497, 699). Buote & Canizares (1996, ApJ 468, 184) use an HRI image of NGC 720 to show that the X-ray isophotes rotate with increasing radius. This observation cannot be reconciled with a spherically symmetric, nearly isothermal hot gas. Moreover, it indicates that the dark and visible matter must have different radial distributions. Bregman & Houck (1997, ApJ 485, 159) used the HRI to understand the contribution of confusing sources to the extended emission detected by the PSPC. They found four such sources, removal of the flux of which led to a better diffuse flux estimate, which in turn led to a more accurate interpretation of the PSPC/ASCA spectral data. One of these sources is the X-ray bright SN 1986J. Houck et al. (1998, ApJ 493,431) use ROSAT HRI and ASCA SIS data to produce a spectrum and light curve of SN 1986J. They find that flux drops as t^-2 over a 5-year period. This can be contrasted to the flat light curve over the same interval for SN 1978k (Schlegel et al. 1996, ApJ 456, 777).

Active Galactic Nuclei

AGN Variability. Virtually all AGN show variable emission, and monitoring the variability is well established as a powerful tool for unraveling the origin of the continuum and line emission in quasars and BL Lacs. Leighly & OÕBrien (1997, ApJ 481, L15) carried out a comprehensive survey of a radio-loud quasar, 3C 390.3, with the ROSAT HRI. They found evidence for flares and non-linear variability, which is difficult to explain in the standard model for the X-ray emission in AGN -- a hot Comptonized corona on the accretion disk (e.g. Haardt & Maraschi 1993). The key was to get a well sampled X-ray light curve -- 3C 390.3 was observed every 3 days for about 9 months. Long term variability was also established for PG quasars by Fiore et al. (1997 Astronomical Time Series, eds. D. Maoz et al. (Dordrecht:Kluwer), p. 277) where it had been long suspected anecdotally that large amplitude X-ray variability was associated with unusually narrow emission lines. They were able to argue from the HRI variability that steep X-ray spectrum, narrow emission line quasars are emitting close to the Eddington Luminosity. Studies of low luminosity but prototypical objects such as M87 and its jet (Harris et al. 1997, MNRAS 284, L21 ) revealed important constraints on the jet emission mechanisms and geometry.

All such studies carried out to date can be characterized as pilot studies -- just scratching the surface of what can be done with this powerful probe of the AGN phenomenon. Well sampled light curves over many months are crucial to avoid aliasing. Much can be done with bright objects, so AXAF is not required, and the large allocations of time required are not likely to be available at any rate.

Follow-up of large deep surveys. Large area surveys are important for establishing the nature of many astrophysical populations, since they can reveal most clearly the unavoidable censoring of observations by selection effects. X-ray surveys also give a direct look at the nature of the X-ray background. The superior spatial resolution of the HRI can play an important role in the identification of sources and ultimate interpretation of surveys carried out with other instruments. For example, large area X-ray surveys have been carried out with both ASCA and the ROSAT PSPC, and HRI positions have been crucial for making identifications (e.g., Pietsch et al. 1998, AA 333, 48); even Einstein deep surveys have recently used ROSAT positions to complete the interpretation (Danziger & Gilmozzi 1997, A&A 323, 47).

In addition, there are several surveys currently underway or planned in the near future at other wavebands where it is of great interest to have near-contemporaneous X-ray flux measurements of the same part of the sky. Mid- and near-IR surveys by ISO, WIRE, SIRTF, and 2MASS are expected to yield obscured AGN which are missed by optical/UV surveys. AGN will be easily distinguished from the thousands of other sources in these surveys by their X-ray emission. Large area optical redshift surveys are underway at several telescopes, in addition to the FIRST VLA survey in the radio. There are hints that a large fraction of the AGN population is simply missed by the traditional optical/UV surveys. The comparison of AGN discovered by all techniques will reveal once and for all the importance of such selection effects. Large area surveys are crucial for the study of AGN, because AGN are rare, and too few are found in deep pencil beam surveys such as the Hubble Deep Field to give meaningful insight into the AGN population as a whole. Many pointings with ACIS are required to cover the area of the IR, radio and optical surveys, and so the total exposure times with AXAF are prohibitive given the oversubscription. Slightly shallower large area surveys of well-studied regions carried out with the ROSAT HRI will nicely complement AXAF deep surveys of small areas, and serendipitous surveys of random parts of the sky.

Pathfinder surveys of AGN classes. The X-ray to optical flux ratio in the nuclear component of quasar emission has a scatter (at a particular Lx and Lopt) of about a factor of 400. Therefore, before detailed spectroscopy of individual objects can be planned with AXAF or XMM, it is essential to have a reliable, recent X-ray flux measurement. Objects bright enough to carry out high resolution spectroscopy with AXAF or XMM are easily detectable with the HRI. HRI surveys of large numbers of AGN will allow not only study of the X-ray properties of various subclasses, but are essential to identify individual objects which are bright enough for follow-up spectroscopic work.

Examples of pathfinder surveys with ROSAT include the following: --Mini-AGN and Liners. The ubiquity of low luminosity activity in the nuclei of nearby galaxies is hinted at by optical searches for weak emission lines and dynamical studies of the cores of nearby bulges with HST. However, the most direct way to demonstrate the presence of an active nucleus is to detect a point X-ray source at the center of the galaxy. The spatial resolution of the HRI is crucial to resolve any extended emission associated with the AGN outflow (e.g., Colbert et al. 1997, BAAS 191, 130.09 ) and to resolve other point sources in the bulge. Several hundred galaxies could easily be surveyed with the HRI. Only half a dozen or so are scheduled to be observed with AXAF, and those are the subject of detailed study of previously well-studied objects. The statistical question will be better addressed with the HRI. --High redshift quasars. These objects are extreme in terms of both their luminosity and redshift, and give the best leverage on studies aimed at getting a physical model to explain the origin of the cosmological evolution of the quasar population. High redshift are rare and the ROSAT all-sky survey was not deep enough to detect more than a handful. Pointed observations at objects discovered by optical or radio surveys are the best way to study their X-ray properties. Flux measurements with the HRI can identify bright objects for follow-up spectroscopy with other missions.

Quasar hosts and environment. There is a clear correlation between radio loudness and quasar cluster environment (steep spectrum radio loud quasars live in rich cluster environments compared to other types of quasars) and a clear correlation between radio loudness and X-ray loudness in quasars. However, we lack any physical explanation for the underlying connection between quasar properties and the galactic environment. The ROSAT HRI is capable of surveying a large number of quasars to search for associated cluster X-ray emission (Hall et al. 1997, AJ 113, 1179). The cluster luminosity reflects the depth of the gravitational potential well, and indicates whether or not the quasar is in a relaxed or collapsing system. Such dynamical information complements that obtained by optical imaging and galaxy redshift surveys.

In the quasar host galaxies themselves, extended emission in Seyferts, probably from a hot outflow, is one of the major discoveries of the ROSAT HRI. Surveys of larger numbers of objects, to search for revealing correlations with radio source structure, narrow line emission properties, galaxy inclination, and associated enhanced star-formation are necessary to understand the phenomenon.

Detailed studies of jet physics and the interaction of jets with ambient IGM. The most spectacular manifestation of the quasar phenomenon is the production of relativistic jets, seen so beautifully in the cm-wave radio VLA images of radio loud AGN. The origin of jets, what collimates them, and how they propagate to large distances from the central engine are still poorly understood. Very few knots and hot spots in radio jets are detected in the optical, and fewer still in the X-ray. The classical cases (from the Einstein era) are knot A in the M87 jet and an emission patch in the 3C 273 jet. The importance of X-ray detections is that physical constraints can be found which are more restrictive than those available from the radio data alone. Harris et al. have recently detected X-ray emission from two other sources: hot spot B in the radio jet of 3C 390.3 (Harris et al. 1998 ApJL, in press), and a radio knot in the 3C 120 jet (in preparation). In both cases, synchrotron self-Compton emission does not provide a viable model (as it does for Cygnus A hotspots, Harris et al. 1994 Nature 367, 713), and the synchrotron model is thus preferred.

In the case of 3C 390.3 (Fig. 4), it appears that the radio jet is impinging on an external galaxy, and the resulting shock provides the X-ray emitting, high energy electrons. For 3C 120, there is no optical emission detected and the cause of the steep gradient in radio brightness on the outside edge of the knot is unknown. Note that for both of these sources as well as for knot A in M87, the X-rays appear to come from a region which has a high gradient in radio brightness. An X-ray survey of known radio jets with the HRI will find other examples amenable to further study, and is too time-consuming to be attempted with AXAF initially.

Clusters of Galaxies

The long exposures available during the HRI-only phase of the ROSAT mission are enabling qualitatively new studies of clusters of galaxies to be made. The higher background of the HRI compared to the PSPC (or the Einstein IPC before it) typically meant that previously most observers sacrificed the improved angular resolution for increased sensitivity to the low surface brightnesses characteristic of clusters. Now, there are several situations where the HRI is being used to advantage for cluster observations.

The first is to resolve fine details in low redshift objects. One of the most interesting areas in this regard is the interaction between X-ray emitting cluster gas and the radio lobes of the brightest cluster galaxy when it happens to be a radio galaxy. Earlier work has been reported by Boehringer et al. (1993, MNRAS 264, L25 for NGC 1275) and Carilli et al. (1994, MNRAS 270, 173 for Cygnus A). These observations provided strong support for the idea that jets of material emanating from the nucleus power the radio galaxies. The effects of the jets are clearly seen in the displaced X-ray gas. The latest observations of this kind are of M87, the central galaxy in the Virgo cluster, by Harris & Owen (1998, in preparation). These data are shown in Fig. 5. Here, although there is a relation between the X-ray gas and the radio plasma, what that relation is not as obvious as it was in the previous cases. Particularly interesting is the very sharp boundary to the X-ray emission at the northern edge of the southwest X-ray arm.

Another situation where large amounts of observing time are essential is survey work. To date cluster surveys have relied primarily on PSPC observations because of its large field of view and low background. However, with the growing number of archival HRI observations and the unambiguous selection of clusters from their extent in the HRI, several groups are conducting new cluster surveys. Ebeling is searching for nearby clusters in the direction of the galactic plane. Many very large scale structure formation studies are hampered by the zone of avoidance. X-ray selection seems an ideal way to bridge the gap between the northern and southern galactic hemispheres because the plane of the galaxy is transparent to X-rays. The soft response of ROSAT is not ideal for this work, but it is adequate as is demonstrated by Fig. 6 that shows a new cluster found by Ebeling at a galactic latitude of -17 degrees. Forman and Vikhlinin are exploring the HRI database for distant clusters. This survey has the potential to constrain cosmological parameters.

A third situation where the HRI is used to advantage for cluster studies is the study of distant objects which the PSPC is incapable of resolving. The only published sample of distant X- ray selected clusters of galaxies with complete redshift information is still the Einstein Medium Sensitivity Survey from the early 1990's. There are 25 sources with redshift greater than 0.3 in Gioia & Luppino (1994, ApJS 94, 583). Twenty-one of these 25 now have deep HRI images, mostly through the work of Donahue and collaborators (e.g., 1996, ApJ 468, 79) and Henry and collaborators (e.g., 1997, ApJ 489, L1). All but two of the 21 observed objects are in fact extended X-ray sources, thereby confirming the EMSS identifications. A second incomplete sample of HRI observations of 11 Abell clusters with z ~ 0.25 has been analyzed by Rizza et al. (1998, MNRAS, submitted).

These data provide the first look at the morphologies of high redshift objects. Given the high percentage of substructure in low redshift clusters, ~50 percent, nearly every object in the distant sample should have such structure if the density parameter were contain the usual suspects: some with central peaks indicating cooling flows with no substructure and some with evidence for mergers. These two types are in about the same proportions as at low redshift providing little support for high density universes.

However, at the very highest redshifts (~0.8) there are intriguing indications that substructure may be more prevalent. There are only two EMSS clusters at these distances (Donahue et al. 1998, ApJ, in press and in preparation). More recent cluster samples compiled from the PSPC pointings provide a few more, although only one of which has HRI data so far (Gioia et al. in preparation). Two of these three very distant clusters have the filamentary morphologies that are expected from large scale structure formation theories. Obviously these statistics are only suggestive, but this is an area of research that the ROSAT HRI can address. There are about half a dozen additional examples known at z ~ 0.8 and it would be a wise investment of time to observe them first with ROSAT before using AXAF because of the extremely long exposures required with either observatory.

As the above examples show, studies of distant clusters with the ROSAT HRI have reached a level comparable to that of early work on their nearby counterparts. Combined with ASCA temperatures, these data are already being used to measure cosmological parameters such as the amplitude of the fluctuation spectrum and the density parameter with accuracies around 10 percent. What is needed now is an investigation into possible biases in the samples which is only possible with a large number of objects from different surveys for cross comparison.