ROSAT A NEW LOOK AT THE SKY J.E. Trümper Max-Planck-Institut für Extraterrestrische Physik Garching, Germany

X-ray astronomy, which in the last In the following, we shall summarise three decades has become one of the some of the early scientific highlights of main tools for exploring the Universe, is ROSAT's mission. presently receiving fresh stimulus from the ROSAT satellite observatory laun­ Sun, Moon and Stars ched on 1 June 1990. The list of known Lunar scattering and bremsstrahlung cosmic X-ray sources includes almost all The birth of cosmic X-ray astronomy in kinds of astronomical objects, ranging 1962 was accidental. The rocket experi­ from the Sun and nearby stars out to qua­ ment of Giacconi et al. aimed to detect sars at the edge of the known Universe. In X-rays from the Moon which were expec­ many sources, the X-rays are emitted by ted from the scattering of solar X-rays. plasmas with temperatures of millions to Instead, the group found something much billions of degrees — conditions found in more interesting, namely Sco X-1, the stellar corona, in shock regions produced strongest X-ray source in the sky, and a by strong stellar winds or supernova ex­ diffuse sky background of X-rays. plosions, in accretion disks around com­ Detection of soft lunar X-rays from an pact objects, or in the intergalactic gas in Fig. 1 — The ROSAT in orbit (artist's im­ Earth-orbiting satellite was in fact accom­ clusters of galaxies. pression). The X-ray telescope comprises a plished nearly 28 years later in an early Another important emission mecha­ 84 mm in diameter 4-fold nested Wolter ROSAT observation. Fig. 3 shows the sun­ nism is synchrotron radiation by ultra-re­ 1 system with two position-sensitive pro­ lit crescent on the Moon, thus demonstra­ lativistic electrons as seen in the Crab ne­ portional counters (0.1-2.4 keV, 2° field of ting that the lunar X-ray luminosity arises bula and other supernova remnants con­ view, 25 arcsec resolution, 42% FWHM taining active pulsars. Inverse Compton energy resolution) and a high resolution scattering of relativistic electrons with multichannel plate detector (0.1-2.4 keV, photons plays a rôle in other sources. 0.6°, 4 arcsec). The XUV telescope com­ Whatever the mechanism, the emission of prises a 57 mm in diameter 3-fold nested X-rays requires extreme conditions in the Wolter-Schwarzschild system with 2 micro- source region and is often connected with channel plate detectors (40-200 eV, 5°, explosive events. 1 arcmin) with filter spectroscopy in two ROSAT (Fig. 1) carries a large imaging bands. X-ray telescope working in the 0.1-2.4 keV energy range and providing a 3-10 fold improvement in angular resolution and sensitivity in the pointing mode compared All-Sky Survey and to predecessors, namely NASA's Einstein Detailed Observations Observatory and ESA's EXOSAT. A novel A major and novel task ROSAT perfor­ feature is its capability to make images in med was the first all-sky survey with an 4-5 colour bands over the 0.1-2.4 keV imaging telescope. The hitherto most sen­ energy range. There is also a smaller XUV sitive survey of this type, made with the telescope working at 40-200 eV. The sa­ HEA0-1 satellite in 1977/8, resulted in 840 sources. About 60000 sources have Fig. 2 — A section of the ROSAT all-sky tellite involves an international collabora­ survey close to the galactic centre. tion In which Leicester University, leading being found In the ROSAT survey (Fig. 2), a UK consortium, suppled the wide-field which has been completed but is not yet camera for the XUV telescope. The satel­ fully analysed. This represents a wealth of lite was launched by NASA and the Smith­ new information on individual sources, sonian Astrophysical Observatory sup­ and opens up the possibility of source/ plied the high resolution imager for the class studies based on large, unbiased X-ray telescope. Germany supplied the samples. satellite, overall project management, the Following the sky survey which took six main ground station, the X-ray mirrors, months, ROSAT is now being used to and the remaining hardware items. carry out detailed pointing observations of individual sources — a programme which is open to guest observers. A large num­ Professor J.E. Trümper has been Director of the MPI für Extraterrestrische Physik, W-8046 ber (1500) of proposals were submitted Garching bei München, Germany, since 1975. for the first year, exceeding the available Fig. 3 — The first X-ray image of the Moon He studied at Universities in Halle, Hamburg, time by a factor of six and demonstrating taken in the soft X-ray band shows the and Kiel where he held academic posts before the strong interest of a widely distributed crescent as well as a distinct X-ray shadow moving to direct the University of Tubingen's international community in X-ray tele­ in the diffuse X-ray background cast by the Astronomical Inst. in 1971 as Full Professor. scope observations. dark side of the moon. 210 Europhys. News 22 (1991) One of the great puzzles concerning stel­ lar X-rays has been the lack of X-ray emis­ sion from red giants. ROSAT observations deep into space have pushed down the upper flux limits considerably. For Arctu­ rus, the coronal flux density at the stellar surface must be less than 10-4 times smaller than that for the Sun [Aires T.R. et al., Astrophys. J., submitted]. Many more accreting objects ROSAT has also increased the number of known binary systems containing mat­ ter accreting white dwarfs, neutron stars or black holes. These systems are extre­ mely bright and their observation has Fig. 4 — A ROSAT image of the Cygnus been a notable feature of X-ray astronomy Fig. 5 — The coloured diagram shows a loop, an ancient supernova remnant seen in since the early days. ROSAT will give more shadow in the diffuse X-ray flux distribution detail for the first time. The colour codings detailed information about the soft emis­ cast by photoelectric absorption of the denote the average photon energy per pixel sion which is often radiation which has cloud (cloud contour lines measured by the (red : low ; blue : high). been reprocessed (scattering; heating and IR satellite IRAS at 100 µm). re-emission). The high sensitivity of from the backscattering of solar coronal ROSAT also allows the detection of ac­ Supernova 1987A : disappointing X-rays. The Moon casts a distinct shadow creting objects in nearby galaxies such as Disappointing, but nevertheless inte­ in the diffuse cosmic X-ray background. A the Large and Small Magellanic Clouds. resting, were the observations of the real surprise was the discovery of a low For example, about 180 sources have been Supernova 1987A explosion. We conside­ level X-ray flux from the dark side of the found in the Andromeda nebula, of which red this object to be important enough to Moon : this may be due to bremsstrahlung about 25 could be identified with X-ray make it ROSAT's "first light" target. The from magnetic solar wind electrons strik­ binaries, most of them residing in globular first and all subsequent observations ing the lunar surface [1]. clusters. yielded a series of decreasing upper limits Supernova Remnants and showing that the thermal emission from Orbiting white dwarfs Diffuse Galactic Emission the supernova's blast wave is not yet Cool stars possess hot X-ray emitting The remnants of the giant stellar explo­ strong enough to generate X-rays, and coronae which are very likely structured that the view to the central (probably hot) sions marking the end of massive stars are region is still obscured by the expanding and heated by magnetic fields. Hot stars the most beautiful objects in the X-ray sky. are thought to produce X-rays by means ROSAT in its sky survey detected almost cloud [4]. of their strong, radiatively driven winds all of the approximately 50 known X-ray The first shadows from cool clouds which become unstable and dissipate pro­ supernova remnants. Fig. 4 shows the The hot phase of the interstellar me­ digious amounts of energy in shocks. With Cygnus loop as an example which could dium has a temperature of about 106 K I5-20000 stellar X-ray sources, the be imaged completely and in detail for the and is believed to be the result of many old ROSAT survey provides a very large statis­ first time owing to the "unlimited field of supernova remnants which have been dis­ tical basis for correlations of X-ray fluxes view" of the all-sky survey. solved, leading to a patchy, diffuse X-ray and temperatures with spectral types and emission of low surface brightness. This classes, magnetic fields and rotational Pressure variations found diffuse emission could be imaged directly velocities. The other new aspect is that ROSAT for the first time owing to the low intrinsic Unexpectedly, ROSAT detected X-ray obtains colour images. In the absence of background of the ROSAT detector. The emission from a number of A-stars which interstellar absorption, the colour relates sky survey thus contains much informa­ were supposed to be X-ray quiet, being directly to the temperature T of the emit­ tion about the distribution of hot plasma located between coronal- and stellar-wind ting gas ("colour temperature"). At the and of the cool gas which absorbs X-rays. type sources. In the meantime, there is same time, the density n of the radiating Of special interest in this context is the strong evidence that, at least in some of plasma can be estimated from the obser­ discovery of the first X-ray "shadows" these systems (e.g. Beta Crateris [2]), the ved brightness (~n2), leading to the pos­ cast by cool interstellar clouds [Snowden emission comes from a white dwarf orbit­ sibility of estimating the thermal pressure S. et al., Science, in press ; Burrows D.N. et ing the A-star. The detection of such (n X T) distribution over the remnant. al., Nature, submitted]. This allows one Sirius-type systems is of great interest Surprisingly, pressure differences of up to to separate "foreground" and "back­ because it should be possible to estimate a factor of 50 are found within one super­ ground" emissions originating in front of the mass of the white dwarf, thus impro­ nova remnant whereas one expected con­ and behind the absorbing cloud. Fig. 5 ving our understanding of the small num­ ditions equilibrium : the reasons for such shows one of these shadows. ber of such objects for which the mass is large deviations from equilibrium are not However, a large number of such mea­ known. yet understood [Aschenbach B., private surements is necessary in order to derive communication]. a coherent picture of the large scale distri­ Red giants absorb The sky survey also brought a rich har­ bution of the hot plasma in our Galaxy. Another surprise was that the number vest of new supernova remnants (about The important open question is whether a of single white dwarfs detected in the 50 to 60) which had not been seen before static galactic "halo" or an outflow of hot survey is an order of magnitude smaller in the radio and optical bands. The first plasma exists. than the pre-ROSAT estimate. This is pro­ example was a shell source of almost 2° Diffuse emissions and absorption pat­ bably due to photoelectric absorption in diameter in Auriga which probably ex­ terns are also found in the X-ray images of by heavier elements in the photospheres ploded in a low density region of the inter­ nearby galaxies, in particular the Large about which nothing was known before. stellar medium [3]. and Small Magellanic Clouds [4]. Europhys. News 22 (1991) 211 Interstellar dust structures in the diffuse sky background Another important aspect is the mea­ near the North Ecliptic Pole [8]. Optical surement of X-ray halos which surround observations show that these structures point sources because of scattering on are due to a supercluster — the first to be interstellar dust grains. Similar observa­ discovered by X-ray observations [Burg R. tions to those performed on bright sour­ et al., Nature, submitted]. ces by previous instruments can be car­ All-in-all, it Is evident that ROSAT will ried out with improved sensitivity using tell us much more about the large scale ROSAT because of the large collecting distribution of matter in our Universe. power and the extremely low microrough­ REFERENCES ness (3.5 Å RMS) of the mirror system. First results [5] show that the grain size [1] Schmitt J.H.M.M. et al., Nature 349 (1991) 583. distribution in the range 0.04-0.4 µm is [2] Fleming T.A. et al., Astron. & Astrophys. measured much more accurately. Lett. 246 (1991) L47. [3] Pfeffermann E. et al., Astron. & Astro­ Galaxies, Clusters of Galaxis phys. Lett. 246 (1991) L28. and Quasars Fig. 6 — A ROSAT image of the Abell 2256 [4] Trümper J. et al., Nature 349 (1991) 579. It is well known that galaxies are not galaxy cluster reveals a clear double peak [5] Predehl P. et al., Astron. & Astrophys. structure indicating the merging of two Lett. 246 (1991) L21. distributed randomly in space but show clusters. distinct clustering effects. Clusters of [6] Briel U. et al., Astron. & Astrophys. Lett. galaxies are strong X-ray emitters and Feeding active galactic nuclei 246 (1991) L10. their radiation comes from a hot plasma or reprocessing [7] Shanks T. et al., Nature 353 (1991) 315. Quasars and other active galactic nuclei [8] Hasinger G. et al., Astron. & Astrophys. produced by galaxy-galaxy collisions. Lett. 246 (1991) L2. While not too much happens to the stars (AGN), which represent the largest class in such events, it would appear that the of ROSAT survey sources (numbering interstellar media of the colliding galaxies about 20000), are found at even larger Complementary EC Rôle are shock heated and swept out into inter- distances and redshifts. The few hundred A Communication by the Commission of galactic space. of these sources which have been identi­ the European Community (CEC) to the EC Estimates show that the total number fied so far show redshifts of up to z = 3.8. Council and Parliament in 1998 sought to of clusters of galaxies expected to be The ROSAT data confirm that the power develop a systematic approach to EC space found in the ROSAT survey is 4000-6000 law spectra of these objects generally activities. For science, there resulted a steepens towards low energies, strongly series of initiatives such as an opening up of — many more than have been identified EC programmes to microgravity experi­ up to now using X-rays [Böhringer H., pri­ reinforcing the evidence that these soft "X-ray excesses" and the UV excesses ments from 1990, studies of earth obser­ vate communication]. Optical spectrosco­ vation systems, and the development of pic redshift measurements are scheduled ("UV bumps") in AGN spectra have the a working relationship with ESA via joint and these will permit measurements of same origin, i.e., they both result from a working groups. the cluster distribution out to redshifts z of broad maximum in the XUV whose main Technical and political changes called for 0.3 or more — much further than by part is obscured by interstellar photoelec­ a fresh appraisal so Roy Gibson, ESA's for­ simply observing the distribution of ga­ tric absorption. This XUV bump could be mer Director-General who heads a task laxies (z < 0.1). We note also that gene­ due to either emission from an accreting force to set up the EC's European Environ­ rally well-know large scale structures disk feeding a central supermassive black mental Agency, led a review of EC space hole or to reprocessing. activities by an expert panel. The panel's such as the "Great Wall" and the "Great report The European Community : Cross- Attractor" are both within the reach of The galactic background: a decreasing roads in Space published in October called ROSAT's spectroscopic capabilities. quasar density and a supercluster for a long-term strategy with the environ­ The nature of the extragalatic X-ray ment as a priority in order to intensify invol­ Clusters grow by merging vement in space-related areas. Such a stra­ background is still unresolved 29 years tegy may emerge once an EC Parliament The main unanswered question con­ after its discovery. Is it made up from the report updating the 1987 Toksvig Report, a cerning the distribution of space objects is superposition of mainly faint sources, or is second CEC Communication due in March how the initially homogeneous Universe it partly due to truly diffuse emission from 1992 and the results of CEC internal studies evolved into the present highly structured intergalactic space? One needs to per­ are presented. state containing galaxies, clusters and form very deep observations to answer In seeking a truly collaborative European superclusters. While the analysis of the these questions. The longest ROSAT tele­ space policy of maximum benefit building large ROSAT samples will take some time, scope pointing extends to flux levels on some impressive recent achievements, clues to the evolution of individual clusters almost a factor of 10 smaller than those of the panel felt long-term EC goals should be of galaxies can be found by studying their to foster: a competitive space industry; an the Einstein Observatory: about 45 % of independent launch capability; a frame- morphology. the total extragalactic flux can be resolved work for exploiting space technology espe­ As a matter of fact, the ROSAT observa­ into point sources [Hasinger G. et al., in cially in telecommunications and earth tions show that a number of bright clus­ preparation] which are mainly quasars at observation; basic knowledge and further ters contain more internal structure than large redshifts [7]. In the deepest ROSAT technologies essential for a vigorous space previously thought. For instance, in Abell field, the density of objects is 200 per effort; and international collaboration. 2256 one sees (Fig. 6) a clear double peak square degree, i.e., more than the quasar Space science was seen as a "pathfinder" distribution suggesting the infall of a smal­ density found in deep optical fields with wide implications and challenging but ler cluster into a larger one [6]. Further­ [Hasinger G. et al., in preparation]. stimulating technical requirements, so it is essential to study the Earth and the Uni­ more, the well-known Coma and Perseus The number - flux relationship (log N - verse in a complementary way to ESA and clusters exhibit interesting sub-structures. log S) at these weak fluxes shows a clear large multinational programmes. Space It appears that one is seeing here some flattening, indicating that the space den­ science should not be "put Into a cup­ indications of how clusters grow, namely sity of X-ray emitting quasars decreases board" and space scientists ought to be by "merging" or the infall of small groups beyond a redshift of z = 2. Also of great further encouraged to turn to EC program­ of galaxies. interest is the discovery of some patchy mes for support.

212 Europhys. News 22 (1991)