ROSAT an International Mission Exploring the High Energy Universe

8/7/2019 ROSAT an International Mission Exploring the High Energy Universe

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MAPPING ANDSTUDYING AUNIVERSE INVISIBLETO THE HUMAN EYEWhether curiosity or fascination firstprompted man to study the heavens,unsatiated, he has continued throughthe centuries to probe cosmic puzzlesusing instruments born of his imagina-tion. One of these instruments, the"Roentgensatellit," known as ROSAT,rode into orbit atop a Delta-II rocketon June 1, 1990. Designed specifi-cally to detect high-energy radiation,ROSAT's telescopes are investigatingX-ray and ultraviolet emissions, regionsof the electromagnetic spectrum thatcannot be seen and that cannot penetratethe Earth's atmosphere.Named for German scientist WilhelmConrad Roentgen, who discovered Xrays in 1895, ROSAT began as ana-

vestigations to be conducted duringROSAT's pointed phase.In the United States, an extensiveGuest Observer program is underway.Through the program, ROSAT's X-rayobserving time will be shared by scien-tists from the United States andthroughout the world. NASA supportsthe Guest Observers with two staffedfacilities and with special software to

, --- aid in the analysis of data. In addi-

__tion, an on-line data base pro-

vides updates on ROSAT'sstatus and information neededto prepare proposals for

additional pointed investi-gations.With ROSAT, man-kind continues its at-tempt to understandthe energetics ofprocesses at work inthe universe. Thediscoveries ofROSAT are ex-pected to raisenew questions to beinvestigated by ob-servations of thenext generation ofX-ray satellites.ional space programin the Federal Republic of

Germany. It grew into an internationaastronomical observatory project withthe involvement of the United Kingdomand the United States.ROSAT's science mission is dividedinto two phases. With its in-orbit check-out period complete, ROSAT has be-gun phase one of its mission, an all-skysurvey to map the heavens. When the6-month mapping survey is complete,the satellite will begin phase two andbe pointed at selected objects, studyingindividual targets, for the remainder ofits mission. All three participatingcountries have invited potential GuestObservers to submit proposals for in-

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A NEW AGEOF ASTRONOMY

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ROSAT'SHIGHLY SENSITIVEINSTRUMENTS............................... page 12

A HISTORY OFX- RAY ASTRONOMY

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PROGRAM FORGUEST OBSERVERS

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SCIENTIFICPOTENTIAL OF ROSAT

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ROSATMILESTONES

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ROSAT'S MISSION INA NEW DECADE OFDISCOVERY.................................. page 9

ORGANIZATION

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Eta Carinae is a massive star in the midst of a nebula of jewel-bright clouds and serpentine dust lanes (on the left as imaged in Xrays, and on the right from an optical telescope). Since it was first observed in 1677, its brightness has waxed to first-magnitudebrilliance, and waned to naked-eye invisibility. Scientists cannot agree on whether Eta Carinae is a dying star preparing to explodein a supernova, an exceptional nova, or an unusual binary.

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AN INTERNATIONALMISSION EXPLORINGTHE HIGH-ENERGY

UNIVERSE

Wilhelm Conrad Roentgen (1845-1923)A scientist with active curiosity,Wilhelm Conrad Roentgen dis-covered X rays by chance. Whiletrying to understand the cause ofluminescence in a Crookes tube(the forerunner of the cathoderay tube), Roentgen covered oneend of the tube to ensure that nolight could escape. When heturned on the tube, a nearbyscreen that had been coated withfluorescent material began toglow. He knew that the glow wasnot caused by cathode-ray elec-trons. Cathode rays couldn'tpenetrate the black cardboard hehad placed at the end of the tube.His curiosity piqued, Roentgeninterrupted his stud)' of cathoderays to learn about the radiationhe called "X."

Roentgen's discovel 3, was seren-dipitous- a matter of good luck--but as Louis Pasteur once said,"Chance favors only the mindthat is prepared." Other scien-tists had noticed the fluorescentglow while using Crookes tubes,but they did not associate it witha new form of radiation. Roent-gen later e._plained, "I didn' t ob-serve, I investigated."Like the scientist for whom it isnamed, ROSAT is prepared forserendipitous discove13'. With itsenhanced observing and all-sl O,survey capabilities, ROSAT iswell positioned to discover theunexpected.

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GAMMA X-RAY ULTRA-VIOLET VISIBLE INFRARED RADIO

Scientists use different means to get above Earth's obscuring atmosphere to study the full spectrum of electromagnetic radiation.A NEW AGEOF ASTRONOMYStargazing has entered a new age -- anage of space astronomy. For centuries,astronomical observations were limitedto what could be seen with the nakedeye and the visible light captured inEarthbound telescopes. Now, by plac-ing instruments above the obscuringatmosphere, astronomers can scan theheavens across the entire electromag-netic spectrum to answerquestions aboutthe cosmos.Light visible to the human eye repre-sents only a fraction of the electromag-netic radiation emitted by objectsthroughout the universe. The new as-

tronomy has scientists investigating oldmysteries with new "eyes" -- instru-ments that view the universe in theelectromagnetic wavelengths outsidethe visible band of the spectrum, emis-sions that do not penetrate to theEarth's surface.Just as visible light passing through aprism is dispersed into a rainbow ofcolors determined by their wavelengths,the invisible part of the electromagneticspectrum can also be separated intodiffering bands of wavelengths. Theserange from very long radio wavelengthsto extremely short gamma-ray wave-lengths. The emission of X rays from

astrophysical objects indicates thepresence of high-energy phenomena inthe universe. The X rays may originatein very hot gases, or plasmas, with tem-peratures of several million degreesKelvin (K). Alternatively, they may beproduced by the interactions of streamsof highly energetic particles with otherparticles or magnetic fields. Ultravioletemissions are produced at somewhatcooler temperatures ranging from10,000 to 100,000 degrees K.When instruments that sense these vari-ous emissions were turned to the heav-ens, scientists discovered a previouslyinvisible aspect of the universe.

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A HISTORY OFX- RAY ASTRONOMYThe observation of high-energy radia-tion depends upon the ability to senddetectors above Earth's atmospherebecause it blocks such radiation. Thestudy of celestial objects that emit X-ray, gamma-ray, and ultraviolet radia-tion only became possible with the ad-vent of the space age.In 1962, the science of X-ray astronomywas born with the flight of a smallAerobee rocket launched from WhiteSands, New Mexico. A team of scien-tists sent aloft a payload of three Geigercounters to investigate whether celes-tial sources other than the Sun alsoemitted X rays. The instruments re-corded an unexpected, brilliant sourceof X rays located in the constellationScorpius, later dubbed Sco X- 1.

During the next 8 years, instrumentslaunched on rockets and balloons de-tected several dozen bright X-raysources in the Milky Way Galaxy and afew sources in other galaxies. Theexcitement over X-ray astronomy wasgrowing and, in 1970, NASA launchedthe first satellite devoted to X-ray as-tronomy, the first Small AstronomySatellite (SAS- 1).Also known as "Uhuru" (Swahili forfreedom), SAS-I's task was to performthe first survey of the X-ray sky fromwhich a catalog of X-ray sources couldbe developed. Uhuru discovered sev-eral hundred sources. They includedbinary star systems -- systems in whichtwo stars travel in tandem, revolvingaround one another; supernova rem-

nants -- the remains of stars that haveexploded violently; the nearby An-dromeda Galaxy -- a galaxy similar tothe Milky Way; and several galaxyclusters -- large gravitationally-boundgroupings of galaxies.During the next 7 years, X-ray sourceswere studied by instruments on severalsatellites: among them a small X-raytelescope aboard NASA's Copernicus,two of NASA's Orbiting Solar Ob-servatory satellites, the DefenseDepartment's Vela 5-A, the Astro-nomical Netherlands Satellite, the Brit-ish Ariel 5, and NASA's SAS-3. Inaddition, a vigorous program of rocketand balloon experiments was contin-ued.

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Numerous discoveries are credited tothese early explorations: binary X-raypulsars -- a neutron star orbiting a nor-mal companion and creating an X-rayemission that appears to wink on andoff; X-ray bursters --compact objectsthat suddenly increase in intensity andthen fade; X-ray emission from activestars; and active galaxies where thecentral regions (known as active galac-tic nuclei) emit huge amounts of X rays.Among the latter are "radio" galaxies,known for producing strong radiowaves; "Seyfert" galaxies, named fortheir discoverer Carl Seyfert andknown for intense levels of energyemanating from small central regions;and quasars, the most luminous objectsin the universe, radiating up to a thou-sand times as much energy as the MilkyWay Galaxy from an area no largerthan the solar system.In addition to a wide variety of discretesources, these early experiments de-tected the presence of an isotropic X-ray background radiation arriving fromall directions, the origin of which was asubject of intense speculation. A frac-tion of the observed sources, due to theirX-ray faintness, distance, or the faint-ness of their optical counterparts, re-mained unidentified with any knownastronomical objects.In 1977, NASA launched its first largeorbiting X-ray observatory, HEAO-1,one in a series of three High-EnergyAstronomy Observatory satellites.Weighing 3.5 tons, HEAO-1 carriedinto orbit four experiments that sur-veyed the sky and pinpointed sourcesof X-ray and gamma-ray emission buthad no capability of producing imagesof emitting objects. The observatoryconducted a sky survey, increasing thenumber of cataloged X-ray sources toapproximately 1,500.Accomplishments credited to HEAO-1 are many: the first precise measure-

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(ARCSEC)ROSA T is an evolutionary step along the way to the Advanced X-Ray AstrophysicsFacility (AXAF), NASA's "Great Observatory" for X-ray observations beginning inthe late 1990s. AXAF will provide major advances in spectral and angular resolu-tion, as well as increased sensitivity and energy response.ment of the energy spectrum of thediffuse X-ray background radiation,implying a possible origin in a univer-sal hot plasma; a very large bubble ofhot gas in the constellation Cygnusstretching across more than 1,000 lightyears of space and containing the massof several hundred thousand Suns; anew black hole candidate; and the dis-covery that the class of objects knownas active galactic nuclei are powerfulsources of X rays. HEAO-1 remainedin operation until early 1979.Until the launch of the second HighEnergy Astronomy Observatory in1978, scientists studied X-ray sourcesprimarily by determining their posi-tions, measuring their X-ray spectra,and monitoring changes in their X-raybrightness over time. With HEAO-2(known as the Einstein Observatory),it became possible to routinely pro-duce images of cosmic X-ray sourcesrather than to simply locate their posi-tions. The Einstein Observatory wasthe first imaging X-ray telescope to bedeployed in Earth orbit. With it, as-

tronomers obtained X-ray images ofsuch extended optical objects assupernova remnants, normal galaxies,clusters of galaxies, and active galacticnuclei. Einstein observations revealedthat all classes of objects known toclassical optical astronomy were alsosources of X rays. Among the EinsteinObservatory's most unexpected dis-coveries was that all stars, from thecoolest to the very hottest, emit signifi-cant amounts of X rays.Thousands of cosmic X-ray sourcesbecame known after discoveries fromNASA's Einstein Observatory and theEuropean Space Agency's EXOSATObservatory (launched in 1983) wereadded to the X-ray catalog. Astrono-mers now recognize that a significantfraction of the radiation emitted byvirtually every type of object in thecosmos emerges as X rays. Each suc-ceeding X-ray mission has made dis-coveries at the limit of its capabilityand has tantalized astronomers to pushon to higher capabilities of resolutionand sensitivity.

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SCIENTIFICPOTENTIAL OF ROSATA myriad of unsolved questions awaitsinvestigation by ROSAT. The ob-servatory's unique capabilities willprovide high-resolution imaging of ob-jects with a precision and sensitivitythat match or exceed those of previousobservations.Normal Stars -- Normal stars are ex-cellent candidates for ROSAT obser-vations. While all classes of stars havebeen found to emit X rays at some level,different types of stars apparently emitX rays via several different mechanisms.Cool stars, like the Sun, are knownsources of X rays that originate in alayer above the visible photosphere.The Sun's outermost layer seethes withan intensely hot, low density gas thatcreates a stellar corona, or crown, whichis visible when the bril liant photosphereis masked out, as in an eclipse. X raysare thought to be produced in a stellarcorona by the dynamo action of a star'smagnetic field in which turbulent mo-tion of the field heats gases to a milliondegrees K or more. The Sun will not beobserved with ROSAT because it wouldburn out the sensitive instruments de-signed to observe very faint nonsolarX-ray sources. However, ROSAT willadd to astronomers' knowledge of thestellar corona phenomenon by extend-ing the study of coronae in other coolstars to a very large sample.In hot stars, those which are 5 or 10times hotter than the Sun and 10 to 100times more massive, scientists believestellar winds carry shock-heated blobsof gas that emit X rays. For observa-tions of this emission, the less massivesources should prove the most informa-tive because of the absence of densestellar winds that absorb X rays. Manysuch objects are expected to be detect-able for the first time with ROSAT.

The corona of the Sun surrounds the solar photosphere like a halo in this photo.Although ROSAT cannot turn its delicate instruments toward the Sun, it will studycoronae in many other stars.Very young stars also exhibit substan-tial X-ray emission, although the originof this radiation remains largely amys-tery. Stars are born in incubators ofcollapsing gas and dust called molecu-lar clouds, which often prevent the es-cape of X rays from their cores. As amolecular cloud collapses, tempera-tures climb and nuclear reactions begin;from this protostar, a star bursts to life.ROSAT's sensitivity may allow obser-vation of these heavily obscured ob-jects.Supernova Remnants -- A pool ofexpanding supernova remnants hasdisrupted and enriched the interstellarmedium since shortly after the birth ofthe Milky Way Galaxy. In the processof a massive star collapsing into a neu-tron star or black hole, rnuch of its massis e_;pelled in a violent explosionknown as a supernova. X-ray studiesof the expanding stellar remnant pro-duced by the explosion tell us muchabout the progenitor star, its evolution,and the nature of the surrounding in-terstellar medium.

Scientists hope the high resolution ofROSAT will be able to reveal structuraldetails of supernova remnants, addingto their understanding of remnant evo-lution.Compact Objects -- Reacting to theexhaustion of its nuclear fuel supplyand the inexorable forces of gravity, astar of mass greater than that of theSun will eventually collapse. De-pending upon the star's exact mass, itwill become either a white dwarf (ap-proximately the size of the Earth), aneutron star (no larger than 10 kilome-ters in radius), or a black hole - amassive object so compact and withgravity so great that not even light canescape it. The X-ray emission from thehot gas surrounding and falling ontosuch compact objects is a key to theirdetection and study.Accurate positions can be obtained byROSAT for several known compact X-ray sources for which positional datahave been poorly defined. Identifica-tion of these sources with optical ob-jects will provide a critical tool in de-

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SNR E0102.2-72.2 in X raysThis supernova remnant in the Small Magellanic Cloud was discovered by theEinstein Observatory. Observations carried out at optical wavelengths soon after thediscovery revealed strong emission lines of oxygen and neon and very little emissionfrom other elements. From an estimated velocity of expansion and its linear size, it isinferred that this remnant is between 1,000 and 2,000 years old. The X-ray imageshows emission from a clumpy ring of ejected gas and dust particles. With ROSA T,scientists hope to study the ring-like nature of the emission in more detail.

Optical image of SNR E0102.2 72Understanding the relationship between the optical and X-ray emission componentsand why they appear so different from one another should reveal interesting aspectsof the physics of shocks in supernova remnants.

termining the nature of their basicphysical parameters.X-Ray Binaries -- A majority of starstravel in pairs, revolving around oneanother in binary systems. In X-raybinaries, where the compact star is at-tracting a flow of X-ray emitting gasfrom its companion, analyses of X-rayflux variations help define the emittingregions. Such analyses are particularlyuseful in identifying an eclipsing binarysystem, where a nonemitting compan-ion acts as a shutter being drawn acrossthe emitting region. Observing theeclipse helps to establish the shape andsize of the region and reveal the physi-cal processes at work.Beyond the Milky Way Galaxy, aseemingly infinite number of other gal-axies, either isolated in space or mem-bers of clusters, are available for studyby ROSAT.Galaxies -- Normal galaxies areknown to be sources of X rays, butbecause they tend to be less X-rayactive than other extragalactic objects,they have been difficult to study. Nor-mal galaxies are generally divided intotwo classes: spiral galaxies, which areflattened disks of gas, dust, and stars,often with bar or spiral-arm patterns;and elliptical galaxies, which arespheroidal systems of stars that areusually more massive than spirals.The predominant X-ray emissionmechanisms differ in spiral and ellipti-cal galaxies. In spirals, the X rays thatare detected represent the combinedemission from many individual sources,such as X-ray binaries and supernovaremnants. ROSAT, with its improvedsensitivity and resolution, will allowdetection of these individual sources inmany galaxies.Dark Matter -- In contrast to theemission from spiral galaxies, X raysfrom elliptical galaxies appear to


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The spiral galaxy M51, al_o known a_ the IVhirlp,ol Galaxy, is actually two interacting gala.vie_ approximately .t5 milli_m lightyear,_' front Earth. The left view of the galaxy is f rom an optical telescope; the right view .shows the _hirlpool as seen in X ruy_sbyinstruments on the Einstein Observatory.originate in a diffuse gas that is heatedto several million degrees K and isgravitationally bound to the galaxy. Thisgas is of particular interest because itprovides information on nonluminousmaterial, the so-called "dark matter,"that may be present in a galaxy. Be-cause the gas is bound by gravity, aknowledge of the gas's density andtemperature will enable scientists toestimate the total mass of the galaxy.The difference between this total massand that fraction observed in the form ofluminous stars and X-ray emitting gas

represents the amount of dark matterassociated with the galaxy.Invisible to optical telescopes, darkmatter therefore contributes a gravita-tional force that cannot be accountedfor by luminous matter. The presenceof dark matter in several galaxies, asimplied by X-ray observations, was ini-tially established by the Einstein Obser-vatory. The greater sensitivity, spatialresolution, and spectral resolution ofROSAT will increase the sample ofgalaxies studied and provide a more

precise determination of the total massand distribution of dark matter in ellip-tical galaxies.Active Galactic Nuclei (AGNs) -- Inaddition to the more common spiral andelliptical galaxies, a small fraction ofgalaxies release very large amounts ofenergy from highly compact regionsinside their nuclei. These so-calledactive galactic nuclei (AGNs) releasemore energy than can be accounted forby the stars contained within the galax-ies. A well-known class of AGN is the

M87, an elliptical galaxy in the Virgo cluster of galaxies, seems unremarkable in the optical view on the left. X rays, however,reveal gases at a temperature of 30 million degrees stretching across half a million light years. In order to retain this high-temperature gas by gravitational attraction, M87 must have a mass billions of times that of the Sun.

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quasi-stellar object, or "quasar." Themost luminous objects in the universe,quasars are also the most distant objectsever observed. How such objects radi-ate more power than the entire MilkyWay Galaxy from an area smaller thanthe solar system is one of the mostchallenging questions of present-dayastrophysics.The high luminosities of AGNs suggestthat they may be powered by the releaseof gravitational energy as matter isaccreted, or accumulated, onto a com-pact massive central object, such as ablack hole. Current ideas favor theformation of a disk of matter, heated byfriction as material is pulled inward bygravity, accreting onto the central ob-ject. A large fraction of the energyemitted by AGNs is in the soft X-rayband, -- the X-ray band closest to theultraviolet region of the spectrum.ROSAT, with its unprecedented soft X-ray sensitivity, is well equipped to helpscientists understand these energeticobjects.Galaxy Clusters -- Clusters of galax-ies, in which many galaxies are gravita-tionally bound together, represent an-other area of study for ROSAT. EarlyX-ray astronomy experiments discov-ered these clusters to be copious sourcesof X-ray emission, now known to origi-nate in hot (multimillion degree K) gaspermeating each cluster. The mass ofthis gas is usually comparable to orgreater than that of the galaxies that canbe seen in visible light. The total massof a cluster -- including member gal-axies, the X-ray emitting gas, and any"dark matter" -- can be estimated byusing X-ray observations in the sameway as for elliptical galaxies. ROSATwill be especially effective for observ-ing the lower-temperature clusters thatradiate predominantly in the soft X-rayregion.Diffuse X-Ray Background -- Inaddition to discrete sources of X rays,8

Clusters of galaxies as observed with the Einstein Observatorythe existence of an apparently uniformand isotropic X-ray glow, called thediffuse X-ray background, has beenknown since the earliest rocket experi-ments. Although this radiation has beenextensively studied, its source remainsa subject of debate.Two possible origins for the X-raybackground have been proposed: anintergalactic hot gas, more or lesssmoothly distributed throughout theuniverse; orthe combined emission froma large number of discrete sources toonumerous and weak to be individuallydetected by past instruments. A strongconstraint on a possible diffuse sourceorigin for the background was recentlyprovided by results from experimentson NASA's Cosmic Background Ex-plorer (COBE), which indicate that anysuch hot gas would have to be highlyclumped and not uniformly distributed.A number of candidates for the underly-ing source population in the discrete-

source theory of the X-ray backgroundhave been proposed, including suchpossibilities as starburst galaxies, ac-tive galactic nuclei, quasars, or a classof objects not yet known.ROSAT's enhanced sensitivity andspatial resolution can be used to helpdetermine the origin of the diffuse X-ray background, by making deep expo-sures of selected sky regions otherwisedevoid of known sources. ROSAT willattempt to detect and resolve the indi-vidual objects that may be contributingto the diffuse background.By virtue of its enhanced capabilitiesfor observing the X-ray characteristicsof a wide range of astrophysical objectsand processes, ROSAT offers astrono-mers a new window on the universe.Each new observation holds the poten-tial for discovery. Each new discoveryholds the promise of solving a cosmicmystery and providing a clearer pictureof the universe.

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ROSAT FAIRING

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ROSAT'SMISSIONIN A NEWDECADE OFDISCOVERY

evolution of the objects within itis expected to increase greatly

during the decade of the 1990s.Scientists will be studying celestialobjects across the entire electromag-

netic spectrum with several major spaceobservatories scheduled for launchduring this decade: the Hubble SpaceTelescope (HST), already in orbit, forvisible, infrared, and ultraviolet wave-lengths; the Gamma Ray Observatory(GRO) for gamma rays; the AdvancedX-Ray Astrophysics Facility (AXAF)for X rays; and the Space Infrared Tele-scope Facility (SIRTF) for infrared ra-diation.In the investigation of X-ray sources,ROSAT will follow the path set byHEAO-2 (the Einstein Observatory)and will be a key link in preparing forAXAF observations. ROSAT contrib-utes to this evolution in instrument ca-pability with its enhanced sensitivity,resolution, and completeness of skycoverage. ROSAT has a sensitivityfive times greater and angular resolu-tion (capability to separate adjacentsources) three times greater thanHEAO-2, which was the most sensi-tive X-ray observatory previously flown.For the United States, ROSAT's spe-cific mission is to advance the scienceof astrophysics through the study of X-ray emission from nonsolar celestialobjects. This will be realized primarilythrough the pointed phase studies ofselected sources and, to a lesser extent,through limited participation in the X-ray all-sky survey.

ROSAT also carries a Wide-Field Cam-era, which will extend the satellite'scoverage of celestial phenomena to ex-treme ultraviolet wavelengths, 300 to60 angstroms (0.042 to 0.21 kilo elec-tron Volts, or keV). This camera,developed and supplied by the UnitedKingdom, will provide the first surveyof the sky in this little-studied region ofthe electromagnetic spectrum.Objects to be studied during ROSAT'spointed phase are being selected by theinternational astrophysics communitythrough proposals to a Guest Observerprogram. Proposals for the first 6months of pointed observations wereinvited in 1989. Additional calls forproposals will take place during thelifetime of ROSAT.ROSAT was launched into orbit aboarda two-stage Delta-lI launch vehicle fromthe Cape Canaveral Air Force Station inFlorida by the US Air Force for NASA.The Delta II was augmented with a spe-cially designed fairing to accommodatethe ROSAT spacecraft. NASA assistedROSAT operations by providing pre-launch testing support, Deep SpaceNetwork (DSN) support in the firstweeks after spacecraft separation fromthe launch vehicle, and backup DSNsupport of the German ground trackingand data system, if needed, throughoutthe mission.ROSAT's orbit is nearly circular, at analtitude of approximately 580 km and atan inclination to the Earth's equator of53 , with an orbital period of 96.2 min-utes. Designed to observe X rays in therange from 0.1 keV to 2 keV, com-monly called the low-energy or soft X-ray band, the ROSAT telescope is sosensitive that it can detect and record Xrays from all known classes of celestialsources.During the all-sky survey, the X-raytelescope scans a band 2 wide duringeach revolution around the Earth, thus

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ROSAT traveled into orbit aboard aDelta-H launch vehicle on June 1, 1990.Inset: ROSAT during pre-launchpreparations.

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During the all-sky survey, ROSAT's telescopes will scan the celestial sphere in great circles as the satellite orbits the Earth.completing the survey in 6 months.Scientists expect to locate more thani00,000 X-ray sources with a positionalaccuracy of approximately 30 arc sec-onds during the ROSAT sky survey.In its second phase, ROSAT will bepointed at selected individual X-raysources. Many X-ray sources are faint,and a typical ROSAT observation willrequire approximately 10,000 seconds(about 3 hours) to record an X-ray sig-nal of adequate strength.The German Space Operations Center(GSOC), located in Oberpfaffenhofennear Munich, operates the spacecraft

using the 15-meter antenna at the DeepSpace Station near Weilheim, Ger-many. The spacecraft contacts theground station on six consecutive orbitsdaily, for 6 to 8 minutes per contact.During periods when no communica-tions are possible, commands are storedon the spacecraft and data are stored onone of two tape recorders. The taperecorders can hold 21 hours of data.After telemetry capture at Weilheim,data are sent to the GSOC for a qualitycheck and initial processing. Data arereformatted as necessary and transmit-ted for evaluation to the German ROSAT

Science Data Center at the Max PlanckInstitute for Extraterrestrial Physics(MPE) in Garching. The GSOC dis-tributes Guest Observer data tapes tothe ROSAT Science Data Centers in thethree participating nations.MPE processes and analyzes X-raydata acquired during the survey modeand is responsible for compiling an X-ray source catalog. The processing,distribution, analysis, and archiving ofthe data from the ultraviolet camera arethe joint responsibility of the UnitedKingdom and the Federal Republic ofGermany.

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ROSAT'SHIGHLY SENSITIVEINSTRUMENTSTHE SATELLITEThe design of the ROSAT spacecraftwas driven by the structure of its X-raytelescope and Wide-Field Camera.Spacecraft support systems were builtaround the telescope assembly, whichis nearly 4 meters (13 feet) long. Thespacecraft, which weighs 2429 kilo-grams (5,354 pounds), has a squarebody with an adapter for the Delta-IIrocket. An array of three solar panelsprovides 1,000 Watts of power to thespacecraft and science payload. Thelarge, unobstructed rear surface of thearray dissipates excess heat into space.During launch, two of the solar panelswere folded over the central body of thespacecraft. These were deployed onorbit, along with antenna masts. Pro-tective telescope "doors" were thenopened to permit the first observations.Orbiting the Earth at 17,000 miles perhour, the satellite locates and locks ontotargets using gyroscopes, Sun sensors,and magnetometers for coarse orienta-tion information, and two star trackersfor a highly accurate sky reference basedon known star positions. A system ofgyroscopes, reaction wheels (angularmomentum flywheels), and magnetictorquing devices are used to maintainstable pointing at a selected target andto re-orient the spacecraft to point at anew target.

THE X-RAY TELESCOPEin X-ray astronomy, each new projectand advance in technology has led tonew discoveries. ROSAT carries thefinest high-resolution X-ray mirrors evermade. The ROSAT X-ray telescope'sprincipal subsystems are its mirror as-

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Flight model of ROSAT

sembly and its focal-plane detectors.Two Position-Sensitive ProportionalCounters and a High-Resolution Imagerare mounted on the turret in the focal-plane instrument section, where theycan be used one at a time.Because X rays interact more stronglywith metallic surfaces than does visiblelight, a critical angle exists for the re-flection of X-ray photons (particles ofelectromagnetic radiation). If the X raystrikes a mirror at an angle greater thanthe critical angle, it is absorbed andlost. To be reflected, it must strike themirror surface at ag razing angle, hencethe name "grazing incidence mirror."ROSAT uses four pairs of nested graz-ing incidence mirrors to provide thetotal reflecting area required for the

specified energy range. ROSAT's mir-rors, known as a Wolter Type I con-figuration, consist of tubelike shellsnested inside one another. Each shellcontains a pair of hyperbolic and para-bolic grazing incidence mirrors sup-ported at one end by a central flange.All of the mirror shells are made ofZerodur, a glass ceramic, and coatedwith a thin layer of gold to increase X-ray reflectivi(y.The ROSAT mirrors yield higher angu-lar resolution and produce less scatter-ing than any previous X-ray mirrors,thereby permitting greater image con-trast. The X-ray mirror assembly is theproduct of a joint endeavor betweenGermany's Max Planck Institute forExtraterrestrial Physics (MPE) and theCarl Zeiss Corporation.

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The High-Resolution Imager was provided by the Harvard-Smithsonian Center for Astrophysics.HIGH-RESOLUTION IMAGERThe ROSAT High-Resolution Imager(HRI), which was developed for NASAby the Harvard-Smithsonian Center forAstrophysics, is based upon a designflown successfully on the Einstein Ob-servatory. Several modifications havebeen made to enhance the HRI's per-formance, including an increase inquantum efficiency and a reduction inthe level of internal background.While the HRI has spatial resolutionsuperior to that of the Position-SensitiveProportional Counters, it has very lim-ited energy resolution and covers a

smaller field of view. Consequently,the HRI is better suited for preciselylocating X-ray sources, for separatingsources in regions where they are tooclose together for study by the propor-tional counters, and for resolving small-scale features of extended objects.The detector consists of twomicrochannel plates in a cascade con-figuration, with a grid of crossed wiresfor electronic readout (see figure above).Microchannel plates absorb incident Xrays and amplify the signal for positiondetermination via the crossed-wire gridbelow the plate. Each microchannel

plate is an array of small hollow tubes orchannels. An X-ray photon striking thesurface of a channel frees an electron.The electric field produced by a highvoltage applied across the microchannelplate accelerates this electron, whichthen collides with the wall of the tube toproduce more electrons. A series ofelectrons thus cascades down the tube,multiplying in number until a sufficientsignal is produced to be recorded elec-tronically, revealing the location of theincident X-ray photon. The array ofsuch events is used to produce the X-ray image of a given field.

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A cross section of the ROSAT X-Ray telescope

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Flight model of ROSAT at DornierGmbH

OR_G!NAL PAGECOLOR PHOTOGRAPH


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This image of the Cassiopeia A supernova remnant was recorded during ROSA T'scheek-out period by one of the Position-Sensitive Proportional Counters.

the WFC extends ROSAT's spectralcoverage into the extreme ultravioletregion, 0.042 to 0.21 keV.The WFC functions as an autonomousinstrument, with its own star tracker(for position information), thermalcontrol system, and command and datahandling system. Power, on-board datastorage, command reception, and tel-emetry are provided by the spacecraft.Coaligned with the X-ray tele_ope, theWFC has a wider field of view (5 circular diameter). The optics consistof a nested set of three grazing - inci-dence mirrors, known as Wolter-Schwarzschild Type I, fabricated fromnickel-plated 'aluminum and coated with

POSITION -SENSITIVEPROPORTIONAL COUNTERSThe two Position-Sensitive Propor-tional Counters (PSPCs) on ROSATare improved versions of those flownon sounding rockets by MPE. ThePSPCs are a type of gas counter inwhich X rays are photoelectrically ab-sorbed.X rays enter the detector through itsentrance window and interact with thegas inside. The photoelectrons pro-duced by the interaction are acceler-ated; as they move through the gas,they produce more electrons. Planes ofwires locate the electrical signals, re-cording the position and amplitude ofeach incoming X-ray photon event.The strength of the electronic signal isproportional to the energy of the inci-dent X ray. The collection of all of theevents from a given source provides itsposition and energy spectrum.While these detectors do not resolvesources in space as accurately as theHRI, they cover a wider field of viewand provide photon energy measure-ments not possible with the HRI.

WIDE-FIELD CAMERAThe Wide-Field Camera (WFC) wasdeveloped and supplied by a consor-tium of institutions in the United King-dom led by the University of Leicester.Complementing the X-ray telescope,

gold for optimum reflectance.Two identical detector assemblies aremounted on a focal-plane turntable sothat either one can be ,selected for u_. Afilter-wheel assembly containing eightspectral filters is located in front of thedetectors. Any one of the filters may bechosen to select a specific energy band,depending on the target to be studied.

THE ROSAT OBSERVATORYX-RAYTELESCOPEIPOSITION-SENSITIVE HIGH-PROPORTIONAL RESOLUTIONCOUNTERS IMAGER

ENERGY RANGE 0.1 - 2 keV 0.1 - 2 keV

WIDE-FIELD CAMERAI I I

0.04 - 0.21 keV

ANGULAR 10 arcsec (FWHM, 5 arcsec (halfRESOLUTION at 1 keV) energy width)

ENERGY 40% at I keVRESOLUTION none

1.7arcmin (halfenergywidth at0.04 keV)

FIELD OF VIEW circular, 32 arcsec circular,2diameter 5diameter

OR!G!NAL PAGECOLOR PHOTOGRAPH

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PROGRAM FORGUEST OBSERVERSScientists around the world were invitedto submit proposals for the objects to bestudied by ROSAT during the first 6months of the pointed phase. Half of thepointed observation time with the X-raytelescope will be devoted to observa-tions conducted under the US GuestObserver Program, with the remaining50 percent allotted to the correspond-ing programs of the Federal Republic ofGermany and the United Kingdom.Proposals for participation in the USGuest Observer Program are submittedto NASA Headquarters, where a two-stage process is followed:

A scientific and technicalevaluation directed by NASA isconducted by peer-review panelsand by the US ROSAT ScienceData Center (RSDC) staffto assessfeasibility. All feasible proposalsare prioritized according to scien-

tific merit. Final selection of USproposals is made by the Directorof the NASA Astrophysics Divi-sion. The International ROSATUsers' Committee, made up of rep-resentatives from the three partici-pating countries and chaired by theFRG's Project Scientist, meets toresolve duplication among recom-mended proposals and to assigneach proposal an observationalpriority rank.

Observations selected from the first callfor proposals in 1989 are scheduled forexecution during ROSAT's first pointedphase. A second call for proposals willbe announced at a later date. While thenominal ROSAT mission lifetime is 2years, the satellite is expected to remainoperational for a much longer period.New observing proposals will be soughtperiodically.ROSAT will be pointed at selectedindividual X-Ray sources for varying

lengths of time, depending upon theintensity of each source. An hour ormore of observing time may be requiredto obtain sufficient data for analysis ofaparticular X-ray source. Observationsof the faintest sources will require sus-tained pointing of the spacecraft at agiven target over several orbits.Data are processed initially at the Ger-man Space Operations Center, inOberpfaffenhofen, Federal Republic ofGermany. Magnetic tapes containingmaster data records are shipped to theUS ROSAT Science Data Center(RSDC) at NASA's Goddard SpaceFlight Center in Greenbelt, Maryland.The ROSAT Standard Analysis Soft-ware System (SASS), developed byMPE and the Harvard-SmithsonianCenter for Astrophysics (CfA), is usedto yield a standard data product for eachobservation. Following verification andSASS processing at CfA, the data arereleased to original investigators andarchived in the Goddard RSDC. Dataare treated as proprietary for the origi-

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The above plots were made using the PROS computer software. The plot on the left shows the pulsation frequency of the pulsar insupernova remnant CTBI09 as determined by the timing analysis package. Spectral modeling is also available with the software.On the right, the raw X-ray counts for the remnant are compared to a theoretical model.

16 PACEC ,"_, 1 +" ".) PHOTOGRAPH


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10 30 50 70CTB 109. Einstein IPC sequence8102

demo_center_smo, imh (20:120.20:120)ContourLevels:

FieldCenter: 111.o4976.79122 h58m56s 53.101+58036,47, 36.72025.39217.55812.142X Scale: 0.63 pix/m 8.396Y Scale: 0.63 pix/m 5.8064.0152.776

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The contour plot on the left, a smoothed image of the supernova remnant CTB 109, was produced using the PROS spatial analysispackage. The software also allows a guest observer to create a gray-scale image, like this one on the right of CTB 109, to whichcolor has been added.nal investigators for 1 year from thedate of receipt, after which they becomegenerally accessible.The United States is providing exten-sive assistance to its ROSAT GuestObservers. Two Guest Observer facili-ties have been developed: at the NASAGoddard Laboratory for High-EnergyAstrophysics in Greenbelt, Maryland,and at the Harvard-Smithsonian Centerfor Astrophysics in Cambridge, Massa-chusetts.As part of the RSDC activities, the CfAhas developed a standard set of soft-ware packages for scientific analysis.They are transportable and run underthe Image Reduction and Analysis Fa-cility (IRAF*). IRAF is aproduct of theNational Optical Astronomy Observa-

tories and is already a familiar tool tomany astronomers. Using this newpackage, called "PROS" (for Post Re-duction Off-Line Software), observerscan extract and display photon counts,smooth their data, perform analysis ofX-ray spectra and light curves (graphsshowing a source's changes in bright-ness over time), as well as perform othermodeling. Because PROS is compat-ible with the widely used IRAF, it facili-tates spectral studies and comparisonsof X-ray data with data collected atother wavelengths for the same object.An on-line computer service for infor-mation retrieval is also being offered bythe US ROSAT Science Data Center.The Mission Information and PlanningSystem (MIPS) will provide readily

ORi(I!NAL PACECr,_r,,_ PHOTOGRAPH

accessible data to help potential ROSATusers plan their observing proposals.With it, a prospective observer can cal-culate observing time and viewing win-dows, and can access a technical database providing performance specifica-tions of the ROSAT X-ray instrumentsand existing information on the source.The system also contains a bulletin boardand mail facility where present observ-ers and prospective proposers will findinformation on the Guest Observer pro-gram, the status of observations anddata processing, and items of generalinterest.* IRAF is distributed by the NationalOptical Astronomy Observatories,operated by the Association of Universi-ties for Research in Astronomy, lnc.under contract to the National ScienceFoundation. ]7


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ROSAT MILESTONES1975: Max Planck Institute forExtraterrestrial Physics (MPE)proposed ROSAT as a Germannational space program to the FederalMinistry for Research and Technol-ogy (Bundesministerium fiJrForschung und Technologie, BMFT)1982: NASA and BMFT signedmemorandum of understandingestablishing cooperation betweenUnited States and Federal Republic ofGermany (FRG) on ROSAT, includ-ing launch from Space Shuttle in 19871983: BMFT and British Science andEngineering Research Council(SERC) signed memorandum of

understanding establishing FRG-United Kingdom (UK) cooperation onROSAT1987: NASA/BMFT decision tolaunch ROSAT on a Delta-II launchvehicle, rather than the Space ShuttleFeb 1989: First Research Announce-ment released soliciting proposals forpointed observationsMay 1989: US, FRG, and the UKreceived a total of 717 proposalsOct 1989: Telescope and flightinstruments calibratedOct 1989: ROSAT pre-ship reviewheld in FRG

Oct 1989: Meeting of InternationalUsers Committee to resolve conflictsin the recommended national proposalselectionsFeb 1990: ROSAT shipped to CapeCanaveral Air Force Station, FloridaJune 1, 1990: ROSAT launchedJuly 29, 1990: Observatory checkoutcompleted; all-sky survey begunLaunch + 8 months: Begin pointedphase of mission. Pointed observa-tions will continue throughout themission, which is expected to last atleast until January 1992

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Technical Glossarya rc s ec on d60 arc seconds = I a rc minute, 60 arc minutes = 1on the circumference of a circlecascade configurationan arrangement of devices connected in a series sothat they multiply the effect of each deviceeleclron Volt (eVJa general unit of energy for fundamental particlesand electromagnetic radiationextragalacticbeyond the Milky Way Galaxyfluxquantity flowing across a given areaisotropicquality of having the same intensity in all direc-tionsKelvinthe standard international unit of absolute tem-peratureluminositythe intrinsic energy output of a starmagnetometerinstrument for measuring intensity of a magneticfieldmicrochannel platesplates that consist of extremely small cylinder-shaped electron multipliers mounted side by sideIn provide image intensificati on

plasmaa high-temperature ionized gasprogenitor starthe star responsible for an outburst or supernovaprotoslara star in the process of formingspatial resolutioncapability to distinguish separate radiation sourcesthat appear close togetherspec tra l r eso lu tionthe capability to resolve detailed features in thespectrum of a sourcestarburst galaxygalaxy with a high rate of new star formationtelemetrytransmission of instrument readings to a remotelocationtorquing deviceon ROSAT, a device that uses the Earth's mag-netic field to maintain slabilitySmall Magellanic Cloudone of two small irregular galaxies close to theMilky Way Galaxy, known as the Large andSmall Magellanic Clouds; visible in the Southernskies.X-ray bursterobject in space repeatedly producing sudden, in-tense bursts of X-rays, typically lasting only a f ewseconds.

The US ROSATTeamDr. Lennard A. Fisk, Associate Administrator, Officeof Space Science and Applications, NASAHeadquartersAlphonso V, Diaz, Deputy Associate Administrator,Office of Space Science and Applications, NASAHeadquartersDr. Char les J. Pellerin, Jr., Dir ect or , Astr ophysicsDivi sion, Of fice of Space Science and Appli cat ions,NASA HeadquartersJohn A. Lintott, ROSAT Program DevelopmentManager, Astruphysics Division, NASA HeadquartersDr. Guenter Riegler, ROSAT Program OperationsManager, Astr ophysics Divi sion, NASA Headquart ersDr. Alan N. Bunner, ROSAT Program Scientist,Astrophysi cs Division, NASA I l eadquar ter sDr. Louis Kaluzienski, Deputy Program Scientist,Ast ro physi cs D iv is ion , NASA Headqua rt er sDr. John W. Townsend, Jr., formerly Dir ector , NASAGoddard Space Fl ight Cent erGil bert W. Ousley, St ., ROSAT Pr oj ect Manager,NASA Goddard Space Flight CenterDr. Stephen S. Holt, ROSAT Project Scien ti st , NASAGoddard Space Fl ight Cent erDr. Robert Petre, Deputy ROSAT Pr_._j ect Sci entist,NASA Goddard Space Flight CenterDr. Robert Price, Director, ROSAT Scicnce DataCenter, NASA Godda rd Spa ce Flight CenterJohn Gerdes, HRI Project Manager, Harvard-Smithsonian Center for AstrophysicsDr. Martin Zombeck, HRI Project Scientist, Harvard-Smithsonian Center for AstrophysicsDr. Stephen S. Murray, Data Analysis CenterManager, Harvard-Smithsonian Center forAstrophysics

AcknowledgmentsWithin NASA, ROSAT is managed by the AstrophysicsDivision of the Office of Space Science and Applica-tions. The detai led implementat ion of the US ROSATProgram is under the management of the Goddard SpaceFlight Center. The Goddard Fl igh t Proj ect s D irectorat e,ROSAT Project Office. provides overall United Statesproject management and the Space and Earth SciencesDirectorate is providing the Project Scientist and ROS ATScience Data Centers. The Flight Projects Directorate+Or bit al Launch Services Office, a lso provided NASAfield center management of the Deha-II launch vehicle.The ROSAT lelescope was designed and developed byGermany's Max Planck Ins ti tu te for Extra te rres tr ia lPhysics (MPE) and the Carl Zeiss Corporation, under thcdir ection of Pr ofessor Joachim Tr iimper. The spacecr aftwas built at Domier GmbH, also of Germany. ThePosition-Sensitive Proportional Counter s we re p rov id edby MPE. From the United States, the High-Resolutionlmager was provided by the Ilarvard-Smithsonian Cen-ter for Astrophysics, under the direction of Drs. HarveyTananbaum, Stephen S. Murray, and Marlin Zombeck.From the United Kingdmn, the Wide-Field Camera wasprovided by a conso rt ium o f the Univer si ty of Leicester,Rutherford-Appleton Laboratories, and Mullard SpaceScience Laboratory under the d irec tion of ProfessorKenneth Pounds.

Brochure PreparationManage r, P at ri ci a Peogra, BDM Int ernat ional, Inc. ;Editor, Marilyn Finley, BDM International, Inc.;Graphic Artist, left Lilly, BDM [nternafional, Inc.Contributors:Dr , Kei tb Arnaud, Goddard Space Flight Center/University of Maryland; Dr. Alan Bunner, NASAROSAT Program Scient ist ; Dr . Cynthia Cheung,BDM International , Inc .; Dr. Jim lleppner, BDMloternafional, Inc.; Dr, Jack Hughes, Harvard+Smithsonian Center for Astrophysics; Dr. LouisKaluzienski, NASA ROSAT Deputy ProgramSci entist ; Mr . John Linlott, NASA ROSAT ProgramManager; Dr. Albert Opp, BDM, I nternati onal, Inc.;Dr, Eric Schlegel, Goddard Space Flight Center./Universities Space Research Association; Dr. T. JaneTurne r, Godda rd Spac e Fl igh t Cen ter /Univer si ti esSpace Research Association.CreditsCover: Puppis A, Einstein Observa tory , courtesy of Harvard-Smithsonian Center for Astrophysics..Overv iew: ROSAT, Artist's illustration.Table of Concerns: Eta Co rinne in X rays , Einst ein Observa-tory, courtesy of Har vard- Smit hsoni an Center for Ast ro-physi cs; Eta Carinae opt ical+ cour tesy tff National Opt icalAstronomy Observatories.Page l: Wilhelrn Conrad R,.:,,entgen, Deutsches Museum,Munich.Page 2: Artlsl's illustration of the electromagnetic spectrum.Page 3: HEAO- Ia l l- sky map, cour tes y of US Naval Re,_earchLaboratory.Page 4: Ar list's i llustr ati on of ROSAT as an evolutionary,st ep to NASA's "Gr eat Observatory" AXAF.Page 5: So la r co ron a, cour tesy of Harvard-Smithsonian Cen-t er for Astroph:,rsics.Page 6: SNR E102.2- 72 i n X r ays, Ei nstein Observat ory,cour tes y o f Har','ard-Smithsonian Center for Astrophysics;op ti cal image , t ake n at Cerro Tololo Inter American Obser-va lo r:_, i n Chi le, cou rt esy of Ihe As so ci at io n o f Univer si ti esfor Research i n Astr onomy, IncPage 7:M51 in i :_ pt ica l l igh t, o ff ici al US Naval Ob se rva to ryphotograph; MS/ in X rays, Einstein Observatory, courtesyof Harvard-Smi thsonian Center for Ast rophysics; Mg7, op-t ical , courtesy of Nationa l Optical Astronomy Observatories;M87+ in X ray, Einstein Observatory. Harvard SmithsonianCenter fc,r Astrophysics.P_ge , q:Clus te rs o f galax ies in X rays, Einstein Observatory,courtesy of Harvard-Smithxonlan Center for Astrophysics.Page 9: Expl oded view of Delta-It l aunch vehi cle.Page 10: ROSAT amp a D elta-It launch vehicle, Iw, th phoIog ra ph s cour les y o f NASA.Page I 1 : Arl ist 's ill ust rat ion of ROSAT orbit.Page 12: Exp lod ed "*, Jewof ROSAT key elemen ts, c ou rt esyof Domi er GmbHPage 1] : High-Resol uli on lmager, courtesy of Har_'ard-Smithsonian Center for Ast ro physi cs (CfA) ; tIRI diagram,Goddard Spa ce F light Center; cross section of X-ray tele-scope , b as ed on d ia gram cour tes y o fMax Planck Inst it ut e forExtraterrestrial Physics,Page 14: Flight model of ROSAT, courlesy of DomierGmhHPage 15: Imager y f ront ROSAT, cour tes y o f BMFT antiMPEPages 16 a nd 17: Compu te r-gen eral ed g ra ph s, courte:,y tffNASA Goddard Spa ce Flight Center.

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ROSAT an International Mission Exploring the High Energy Universe

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