NASA Technical Reports Server (NTRS) 19910013901: ROSAT: An international mission exploring the high energy universe

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by  NASA Technical Reports Server (NTRS)

(NA_A-TM-I04950) ROSAT: AN INTERNATIONAL
MISSION EXPLURING THE HIGH {NERGY UNIVERSF
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vestigations to be conducted during
ROSAT's pointed phase.
In the United States, an extensive
Guest Observer program is underway.
Through the program, ROSAT's X-ray
observing time will be shared by scien-
tists from the United States and
throughout the world. NASA supports
the Guest Observers with two staffed
facilities and with special software to
, --- aid inthe analysis of data. In addi-
vides updates on ROSAT's
status and information needed
to prepare proposals for
__tion, aanddoitni-olninael pdoaitnatedbasienvepstrio--
gations.
With ROSAT, man-
kind continues its at-
tempt to understand
the energetics of
processes atwork in
the universe. The
discoveries of
MAPPING AND ROSAT are ex-
STUDYING A pected to raise
new questions tobe
UNIVERSE INVISIBLE
investigated byob-
TO THEHUMAN EYE servations of the
next generation of
Whether curiosity or fascination first tional space program X-ray satellites.
prompted man to study the heavens, in the Federal Republic of
unsatiated, he has continued through Germany. It grew into an internationa
the centuries to probe cosmic puzzles astronomical observatory project with
using instruments born of his imagina- the involvement of the United Kingdom
tion. One of these instruments, the and the United States.
"Roentgensatellit," known as ROSAT, ROSAT's science mission is divided
rode into orbit atop aDelta-II rocket
into two phases. With itsin-orbit check-
on June 1, 1990. Designed specifi-
out period complete, ROSAT has be-
cally to detect high-energy radiation,
gun phase one of its mission, an all-sky
ROSAT's telescopes are investigating
survey to map the heavens. When the
X-ray and ultraviolet emissions, regions
6-month mapping survey is complete,
of the electromagnetic spectrum that
the satellite will begin phase two and
cannot be seen and that cannot penetrate
be pointed at selected objects, studying
the Earth's atmosphere.
individual targets, for the remainder of
Named for German scientist Wilhelm its mission. All three participating
Conrad Roentgen, who discovered X countries have invited potential Guest
rays in 1895, ROSAT began as ana- Observers to submit proposals for in-
ORIGINAL PAGE
COLOR PHOTOGRAPH
A NEW AGE ROSAT'S
OF ASTRONOMY HIGHLY SENSITIVE
INSTRUMENTS
12
.................................. page 2 ............................... page
A HISTORY OF PROGRAM FOR
X- RAY ASTRONOMY GUEST OBSERVERS
.................................. page 3 ............................... page 16
SCIENTIFIC ROSAT
POTENTIAL OF ROSAT MILESTONES
.................................. page 5 ............................... page 18
ROSAT'S MISSION IN ORGANIZATION
A NEW DECADE OF
DISCOVERY
.................................. page 9 ............................... page 18
Eta Carinae is a massive star in the midst of a nebula ofjewel-bright clouds and serpentine dust lanes (on the left as imaged in X
rays, and on the right from an optical telescope). Since it was first observed in 1677, its brightness has waxed tofirst-magnitude
brilliance, and waned to naked-eye invisibility. Scientists cannot agree on whether Eta Carinae is a dying star preparing to explode
in a supernova, an exceptional nova, or an unusual binary.
ORIGINAL PAGE
COLOR PHOTOGRAPH
AN INTERNATIONAL
MISSION EXPLORING
THE HIGH-ENERGY
UNIVERSE
Wilhelm Conrad Roentgen (1845-1923)
A scientist with active curiosity, Roentgen's discovel 3, was seren-
Wilhelm Conrad Roentgen dis- dipitous- a matter of good luck
covered X rays by chance. While --but as Louis Pasteur once said,
trying to understand the cause of "Chance favors only the mind
luminescence in a Crookes tube that is prepared." Other scien-
(the forerunner of the cathode tists had noticed the fluorescent
ray tube), Roentgen covered one glow while using Crookes tubes,
end of the tube to ensure that no but they did not associate it with
light could escape. When he a new form of radiation. Roent-
turned on the tube, a nearby gen later e._plained, "I didn' tob-
screen that had been coated with serve, I investigated."
fluorescent material began to
Like the scientist for whom it is
glow. He knew that the glow was
named, ROSAT is prepared for
not caused by cathode-ray elec-
serendipitous discove13'. With its
trons. Cathode rays couldn't
enhanced observing and all-sl O,
penetrate the black cardboard he
survey capabilities, ROSAT is
had placed at the end of the tube.
well positioned to discover the
His curiosity piqued, Roentgen
unexpected.
interrupted his stud)' of cathode
rays to learn about the radiation
he called "X."
ORIGINAL PAGE
BLACK AND WHITE PHOTO_RAPN
GAMMA X-RAY ULTRA-VIOLET VISIBLE INFRARED RADIO
Scientists use different means toget above Earth's obscuring atmosphere to study thefull spectrum of electromagnetic radiation.
A NEW AGE
OF ASTRONOMY
tronomy has scientists investigating old astrophysical objects indicates the
Stargazing has entered anew age -- an mysteries with new "eyes" -- instru- presence of high-energy phenomena in
age of space astronomy. For centuries, ments that view the universe in the the universe. The X rays may originate
astronomical observations were limited electromagnetic wavelengths outside invery hot gases, or plasmas, with tem-
to what could be seen with the naked the visible band of the spectrum, emis- peratures of several million degrees
eye and the visible light captured in sions that do not penetrate to the Kelvin (K). Alternatively, they may be
Earthbound telescopes. Now, by plac- Earth's surface. produced by the interactions of streams
ing instruments above the obscuring of highly energetic particles with other
Just as visible light passing through a
atmosphere, astronomers can scan the particles or magnetic fields. Ultraviolet
prism is dispersed into a rainbow of
heavens across the entire electromag- emissions are produced at somewhat
colors determined bytheir wavelengths,
netic spectrum toanswerquestions about cooler temperatures ranging from
the invisible part of the electromagnetic
the cosmos. 10,000 to 100,000 degrees K.
spectrum can also be separated into
Light visible to the human eye repre- differing bands of wavelengths. These When instruments that sense these vari-
sents only afraction of the electromag- range from very long radio wavelengths ous emissions were turned to the heav-
netic radiation emitted by objects to extremely short gamma-ray wave- ens, scientists discovered a previously
throughout the universe. The new as- lengths. The emission of X rays from invisible aspect of the universe.
2
ORIGINAL PAGE
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A HISTORY OF
X- RAY ASTRONOMY
The observation of high-energy radia- During the next 8 years, instruments nants -- the remains of stars that have
tion depends upon the ability to send launched on rockets and balloons de- exploded violently; the nearby An-
detectors above Earth's atmosphere tected several dozen bright X-ray dromeda Galaxy -- a galaxy similar to
because it blocks such radiation. The sources in the Milky Way Galaxy and a the Milky Way; and several galaxy
study of celestial objects that emit X- few sources in other galaxies. The clusters -- large gravitationally-bound
ray, gamma-ray, and ultraviolet radia- excitement over X-ray astronomy was groupings of galaxies.
tion only became possible with the ad- growing and, in 1970, NASA launched
During the next 7years, X-ray sources
vent of the space age. the first satellite devoted to X-ray as-
were studied by instruments on several
tronomy, the first Small Astronomy
In 1962, the science of X-ray astronomy satellites: among them a small X-ray
Satellite (SAS- 1).
was born with the flight of a small telescope aboard NASA's Copernicus,
Aerobee rocket launched from White Also known as "Uhuru" (Swahili for two of NASA's Orbiting Solar Ob-
Sands, New Mexico. A team of scien- freedom), SAS-I's task was toperform servatory satellites, the Defense
tists sent aloft apayload of three Geiger the first survey of the X-ray sky from Department's Vela 5-A, the Astro-
counters to investigate whether celes- which acatalog of X-ray sources could nomical Netherlands Satellite, the Brit-
tial sources other than the Sun also be developed. Uhuru discovered sev- ish Ariel 5, and NASA's SAS-3. In
emitted X rays. The instruments re- eral hundred sources. They included addition, a vigorous program of rocket
corded an unexpected, brilliant source binary star systems -- systems inwhich and balloon experiments was contin-
of X rays located in the constellation two stars travel in tandem, revolving ued.
Scorpius, later dubbed Sco X- 1. around one another; supernova rem-
3
O_'"'':' "tI'J'AL PAGE
COLOR PHOTOGRAPH
1000
Numerous discoveries are credited to Z
___loo
these early explorations: binary X-ray
pulsars -- aneutron star orbiting a nor-
mal companion and creating an X-ray
emission that appears to wink on and
_ 10
off; X-ray bursters --compact objects
._J
that suddenly increase in intensity and
then fade; X-ray emission from active
0
stars; and active galaxies where the
I.&l
central regions (known as active galac-
c_
tic nuclei) emit huge amounts of Xrays.
Among the latter are "radio" galaxies,
known for producing strong radio
waves; "Seyfert" galaxies, named for
their discoverer Carl Seyfert and 5,0 2.0 1,0 0.5
known for intense levels of energy ANGULAR RESOLUTION
emanating from small central regions;
(ARCSEC)
and quasars, the most luminous objects
in the universe, radiating up to a thou- ROSA T is an evolutionary step along the way to the Advanced X-Ray Astrophysics
sand times asmuch energy as the Milky Facility (AXAF), NASA's "Great Observatory" for X-ray observations beginning in
the late 1990s. AXAF will provide major advances in spectral and angular resolu-
Way Galaxy from an area no larger
tion, as well as increased sensitivity and energy response.
than the solar system.
ment of the energy spectrum of the tronomers obtained X-ray images of
In addition toa wide variety of discrete
diffuse X-ray background radiation, such extended optical objects as
sources, these early experiments de-
implying a possible origin in auniver- supernova remnants, normal galaxies,
tected the presence of an isotropic X-
sal hot plasma; a very large bubble of clusters of galaxies, and active galactic
ray background radiation arriving from hot gas in the constellation Cygnus nuclei. Einstein observations revealed
all directions, the origin of which was a
stretching across more than 1,000 light that all classes of objects known to
subject of intense speculation. A frac-
years of space and containing the mass classical optical astronomy were also
tion of the observed sources, due to their
of several hundred thousand Suns; a sources of X rays. Among the Einstein
X-ray faintness, distance, or the faint-
new black hole candidate; and the dis- Observatory's most unexpected dis-
ness of their optical counterparts, re- covery that the class of objects known coveries was that all stars, from the
mained unidentified with any known
as active galactic nuclei are powerful coolest to the very hottest, emit signifi-
astronomical objects.
sources of X rays. HEAO-1 remained cant amounts of X rays.
In 1977, NASA launched its first large in operation until early 1979.
Thousands of cosmic X-ray sources
orbiting X-ray observatory, HEAO-1, Until the launch of the second High became known after discoveries from
one in a series of three High-Energy
Energy Astronomy Observatory in NASA's Einstein Observatory and the
Astronomy Observatory satellites.
1978, scientists studied X-ray sources European Space Agency's EXOSAT
Weighing 3.5 tons, HEAO-1 carried primarily by determining their posi- Observatory (launched in 1983) were
into orbit four experiments that sur- tions, measuring their X-ray spectra, added to the X-ray catalog. Astrono-
veyed the sky and pinpointed sources and monitoring changes in their X-ray mers now recognize that a significant
of X-ray and gamma-ray emission but brightness over time. With HEAO-2 fraction of the radiation emitted by
had no capability of producing images
(known as the Einstein Observatory), virtually every type of object in the
of emitting objects. The observatory it became possible to routinely pro- cosmos emerges as X rays. Each suc-
conducted asky survey, increasing the
duce images of cosmic X-ray sources ceeding X-ray mission has made dis-
number of cataloged X-ray sources to rather than to simply locate their posi- coveries at the limit of its capability
approximately 1,500. tions. The Einstein Observatory was and has tantalized astronomers to push
Accomplishments credited to HEAO- the first imaging X-ray telescope tobe on to higher capabilities of resolution
1are many: the first precise measure- deployed in Earth orbit. With it, as- and sensitivity.
4
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SCIENTIFIC
POTENTIAL OF ROSAT
A myriad of unsolved questions awaits
investigation by ROSAT. The ob-
servatory's unique capabilities will
provide high-resolution imaging of ob-
jects with a precision and sensitivity
that match or exceed those of previous
observations.
Normal Stars -- Normal stars are ex-
cellent candidates for ROSAT obser-
vations. While all classes of stars have
been found to emit X rays at some level,
different types of stars apparently emit
X rays via several different mechanisms.
Cool stars, like the Sun, are known
The corona of theSun surrounds thesolar photosphere like ahalo in thisphoto.
sources of X rays that originate in a Although ROSAT cannot turn itsdelicate instruments toward the Sun, it willstudy
layer above the visible photosphere. coronae in many other stars.
The Sun's outermost layer seethes with
an intensely hot, low density gas that Very young stars also exhibit substan- Scientists hope the high resolution of
creates astellar corona, or crown, which tial X-ray emission, although the origin ROSAT will be able toreveal structural
isvisible when the brilliant photosphere of this radiation remains largely amys- details of supernova remnants, adding
is masked out, as in an eclipse. X rays tery. Stars are born in incubators of to their understanding of remnant evo-
are thought to be produced in a stellar collapsing gas and dust called molecu- lution.
corona by the dynamo action of a star's lar clouds, which often prevent the es-
Compact Objects -- Reacting to the
magnetic field in which turbulent mo- cape of X rays from their cores. As a
exhaustion of its nuclear fuel supply
tion of the field heats gases to amillion molecular cloud collapses, tempera-
and the inexorable forces of gravity, a
degrees K or more. The Sun will not be tures climb and nuclear reactions begin;
observed with ROSAT because itwould from this protostar, astar bursts to life. star of mass greater than that of the
burn out the sensitive instruments de- ROSAT's sensitivity may allow obser- Sun will eventually collapse. De-
pending upon the star's exact mass, it
signed to observe very faint nonsolar vation of these heavily obscured ob-
will become either a white dwarf (ap-
X-ray sources. However, ROSAT will jects.
proximately the size of the Earth), a
add to astronomers' knowledge of the
Supernova Remnants -- A pool of neutron star (no larger than 10 kilome-
stellar corona phenomenon by extend-
expanding supernova remnants has ters in radius), or a black hole - a
ing the study of coronae in other cool
disrupted and enriched the interstellar massive object so compact and with
stars to a very large sample.
medium since shortly after the birth of gravity so great that not even light can
In hot stars, those which are 5 or 10 the Milky Way Galaxy. In the process escape it. The X-ray emission from the
times hotter than the Sun and 10to 100 of amassive star collapsing into a neu- hot gas surrounding and falling onto
times more massive, scientists believe tron star or black hole, rnuch of its mass such compact objects is a key to their
stellar winds carry shock-heated blobs is e_;pelled in a violent explosion detection and study.
of gas that emit X rays. For observa- known as a supernova. X-ray studies
Accurate positions can be obtained by
tions of this emission, the less massive of the expanding stellar remnant pro-
ROSAT for several known compact X-
sources should prove the most informa- duced by the explosion tell us much
tive because of the absence of dense about the progenitor star, its evolution, ray sources for which positional data
have been poorly defined. Identifica-
stellar winds that absorb X rays. Many and the nature of the surrounding in-
such objects are expected to be detect- terstellar medium. tion of these sources with optical ob-
able for the first time with ROSAT. jects will provide a critical tool in de-
O_!_INAL PAGE 5
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wT - " _-
termining the nature of their basic
physical parameters.
X-Ray Binaries -- A majority of stars
travel in pairs, revolving around one
another in binary systems. In X-ray
binaries, where the compact star is at-
tracting a flow of X-ray emitting gas
from its companion, analyses of X-ray
flux variations help define the emitting
regions. Such analyses are particularly
useful in identifying an eclipsing binary
system, where a nonemitting compan-
ion acts as a shutter being drawn across
the emitting region. Observing the
eclipse helps to establish the shape and
size of the region and reveal the physi-
SNR E0102.2-72.2 in Xrays
cal processes at work.
This supernova remnant in theSmall Magellanic Cloud wasdiscovered by the Beyond the Milky Way Galaxy, a
Einstein Observatory. Observations carried out atoptical wavelengths soon after the
seemingly infinite number of other gal-
discovery revealed strong emission lines of oxygen and neon and very little emission
from other elements. From an estimated velocity of expansion and its linear size, itis axies, either isolated in space or mem-
inferred that this remnant is between 1,000 and 2,000 years old. The X-ray image bers of clusters, are available for study
shows emission from aclumpy ring of ejected gas and dust particles. WithROSA T, by ROSAT.
scientists hope tostudy the ring-like nature ofthe emission inmore detail.
Galaxies -- Normal galaxies are
known to be sources of X rays, but
because they tend to be less X-ray
active than other extragalactic objects,
they have been difficult to study. Nor-
mal galaxies are generally divided into
two classes: spiral galaxies, which are
flattened disks of gas, dust, and stars,
often with bar or spiral-arm patterns;
and elliptical galaxies, which are
spheroidal systems of stars that are
usually more massive than spirals.
The predominant X-ray emission
mechanisms differ in spiral and ellipti-
cal galaxies. In spirals, the X rays that
are detected represent the combined
emission from many individual sources,
such as X-ray binaries and supernova
remnants. ROSAT, with its improved
sensitivity and resolution, will allow
detection of these individual sources in
many galaxies.
Optical image of SNR E0102.2 72
Dark Matter -- In contrast to the
Understanding the relationship between the optical and X-ray emission components
emission from spiral galaxies, X rays
and why they appear sodifferent from oneanother should reveal interesting aspects
from elliptical galaxies appear to
ofthe physics ofshocks insupernova remnants.