This document describes the X-ray Timing Explorer (XTE), a mission sponsored by the Office of Space Science and Applications of NASA and managed by NASA's Goddard Space Flight Center. This X-ray astronomy observatory is scheduled to be launched in 1996 on a Delta II rocket. The primary objective of the mission is the study of temporal and broad-band spectral phenomena associated with stellar and galactic systems containing compact objects. These systems include white dwarfs, neutron stars, and possibly black holes, and they involve a variety of physical processes. The scientific instruments span the energy range 2-200 keV, and time scales from microseconds to years can be studied. The instruments are provided by science and engineering teams at Goddard Space Flight Center, the University of California at San Diego, and the Massachusetts Institute of Technology. All of the observing time will be available to the international community through the peer-review process. The capabilities of the instruments and spacecraft described herein reflect current designs. This brochure was prepared by the XTE Science Working Group for distribution at the January 1992 meeting of the American Astronomical Society and for distribution to interested members of the general public.

The Cosmos, X-Rays, and XTE

High-Energy Astrophysics

The celestial regions of highest temperature and strongest gravitational fields typically emit intense high-energy radiation, e.g. X-rays and gamma rays. X-radiation arises from gases accreting onto compact stellar objects such as white dwarfs, neutron stars, and black holes. Similarly, the hot plasmas from the innermost regions of active galactic nuclei (possibly massive black holes) at the center of many galaxies emit intense high-energy radiation.

Astronomers have long studied the sky in order to determine the nature of celestial objects. In the past century, the use of powerful telescopes and advanced instrumentation have led to great strides in our understanding of the cosmos. We have learned that the Sun is only one of 100 billion stars that make up the Milky Way Galaxy and that this galaxy is only one of billions of galaxies in an expanding Universe. Moreover, astronomers have learned much about the life histories of stars, galaxies, and even of the universe itself.

Stars are known to be spheres of gas held together by gravity and powered by thermonuclear burning in their centers. When the nuclear fuel is exhausted, the star changes size and color several times and finally collapses to become a compact white dwarf (about the size of the earth), or a very compact neutron star (about the size of Manhattan Island), or possibly a black hole. A neutron star is about 100 trillion times more dense than the earth; it is like packing almost 1 million earths into a sphere the size of Manhattan Island. Magnetic fields on the surface of a neutron star can be 1 trillion times stronger than the earth's magnetic field. The gravitational field of a black hole is so strong that gravity prevents even a light beam from escaping its interior.

Some of these compact objects are accompanied by a normal gaseous star in a binary system. If the two stars of the binary are sufficiently close to one another, gas from the surface of the normal star is attracted by gravity toward the compact object (Fig. C). As the gas falls toward the compact object (called accretion), it gains so much energy that it becomes hot enough (~10 million degrees) to emit X-rays (Fig. D). This radiation provides astronomers with detailed information about conditions in the environment of incredibly strong gravitational and magnetic fields near a compact object. Such studies fall in the domain of high-energy astrophysics.

High-energy radiation is also seen from distant galaxies (collections of billions of stars) that lie well beyond our own Milky-Way Galaxy. Some of these exhibit an intense point of light at the center, known as the active galactic nucleus (AGN). In some cases, the nucleus is so bright that it can be seen at great distances when the light from the surrounding stars is nearly undetectable; quasars are this type of AGN. These nuclear regions can emit more light than all the stars in the galaxy. They often exhibit rapid variations of light indicating they are very small in size, possibly of size comparable to our solar system. Also, these galaxies often show immense jets of particles and radiant energy emanating from the nucleus.

The most likely explanation of active galactic nuclei is that the nucleus is a black hole 10 to 100 million times more massive than the sun. Surrounding matter is pulled by gravity into its deep potential well with the attendant emission of radiation at many wavelengths
( Fig. F). The X-rays and gamma rays will come from the innermost and hottest regions close to the massive black hole. A full understanding of these objects is greatly aided by concurrent observations from the radio to the X-ray and gamma-ray.

Astronomy with X-rays

Many discoveries have come from X-ray astronomy, including accreting neutron-star binaries, hot gases in clusters of galaxies, stellar coronae, and black-hole candidates. Variations in intensity are a typical characteristic of emission from compact sources and provide crucial dimensional information. Spectral information provides diagnostics of the underlying emission processes.

In the last half century, many new spectral windows on the cosmos have opened: radio, infrared, ultraviolet, X-ray and gamma-ray. With the exception of the radio band and parts of the infrared band, these measurements have been made from orbiting satellites because the radiation is absorbed in the earth's atmosphere.

X-rays are one of the several types of radiation that comprise the electromagnetic spectrum (Fig. 5). They are characterized by a short wavelength and a high photon energy of 0.2 to about 200 thousand electron volts (keV). These photons can be emitted by gases of very high temperature (~10 million degrees Kelvin) or by very energetic non-thermal particles. The high penetrating power of X-rays allows them to escape from the hot gaseous environs of an object and to travel through the diffuse gases of interstellar space so that astronomers on earth may observe them directly. X rays are an ideal probe of the innermost regions near compact objects.

Fig. 5 (G): Frequency Bands

X-rays have been detected from essentially every type of astronomical object: black holes, accreting white dwarfs and neutron stars, supernovae, coronal active stars, active galactic nuclei (e.g. quasars), and clusters of galaxies. (Although the existence of black holes remains in some doubt, there is strong evidence that they reside in some X-ray binary systems and in active galactic nuclei.) There is also a diffuse background of X-rays from the entire sky.

A major characteristic of X-radiation from compact objects is the variability of the intensity. Examples are pulses of X-rays at regular intervals from magnetized spinning neutron stars (X-ray pulsars; Fig. 6), X-ray bursts from thermonuclear explosions on neutron-star surfaces, the sudden emergence of X-ray novae, and rapid (~100 second) aperiodic variability and flares in AGN and distant quasars.

Fig. 6 (H): X-ray Pulsar

As with all types of astronomy, spectral information is very important for the diagnosis of the underlying processes. For example, an excess of power at a specific frequency (a spectral line) can indicate the presence of iron atoms in the object. Simultaneous observations from different observatories (radio to the gamma ray), known as multifrequency studies, are now recognized as an important tool in astronomy.

Studies of the character of X-ray sources by past space missions have led to a broad understanding of the emitting systems. Temporal and broad-band spectral studies in X-ray astronomy in the medium to hard X-ray energy band (1 - 200 keV) have developed through a series of satellites including the U.S. HEAO-1, the European EXOSAT, the Japanese Ginga, and the Soviet/European Granat missions. Imaging and high-resolution spectral studies at lower X-ray energies (0.1-4 keV) have been carried out with the U.S. Einstein and the German ROSAT missions. Each of these missions has made vital new contributions to our knowledge of the emitting systems.

X-ray Timing Explorer

Major problems remain unsolved, e.g. the internal structure of neutron stars, the existence of black holes, and the origin of the cosmic X-ray background. XTE will address these and other questions through timing studies that range from a microsecond to years, spectral studies from 2-200 keV, and a design that facilitates multifrequency observations.

Much more remains to be learned about X-ray emitting systems. We do not know how the plasmas behave on time scales comparable to their free-fall time (~100 microseconds) near a neutron star or black hole, nor do we understand the internal structure of neutron stars. Neither do we know the origins of the cosmic X-ray background, nor whether or not black holes truly exist. We do not know if there is a fast rotating pulsar left over from the recent supernova event SN1987A, and the interior regions of AGN are still an enigma.

XTE will address these and other fundamental questions about the nature of the cosmos. The large effective area (~0.8 m2 total) and broad band of sensitivity (2-200 keV) of its three instruments make it especially valuable for timing of intensity variations and for the determination of broad-band spectra from high-energy sources. For the first time, studies of variability ranging from about 1 microsecond to several years will be carried out. XTE's design and flexibility of operations will allow it to respond rapidly to changes in the X-ray sky (within hours) and will facilitate multifrequency observations.

There are two NASA missions in X-ray astronomy planned for this decade, the large imaging X-ray telescope mission known as the Advanced X-ray Astrophysics Facility (AXAF) and the smaller temporal/spectral XTE described herein. In addition there are forthcoming European, Russian, Italian and Japanese missions. Among planned future missions, XTE is the only one that will cover the entire 2-200 keV region with a large collecting area, an especially low background, high temporal resolution, and rapid response to changes in the X-ray sky.

XTE can study more than a thousand of the brightest X-ray sources in the sky (Fig. 7). These represent a diverse set of objects and physical processes.

Fig. 7 (I): LASS Map

XTE Instruments and Spacecraft

'State of the Art' and Synergism of the XTE Instruments

XTE's 3 instruments constitute a powerful integrated observatory. Its advanced features and its synergistic approach to its objectives will greatly enhance its productivity.

XTE will carry out its studies with the 3 instruments listed in Table 1. The count rates expected from a few notable astrophysical objects are given in Table 2. A Proportional Counter Array (PCA) of large area (6250 cm2) will be sensitive to X-rays from 2 to 60 keV. This instrument is supported by a powerful microprocessor-driven flight data system with multiple analysis channels capable of processing high rates (up to about 500,000 X-rays per second) with a minimum loss of information. The PCA will work in tandem with the High Energy X-ray Timing Experiment (HEXTE) which consists of crystal scintillator detectors (1600 cm2) that extend the XTE energy sensitivity up to 200 keV.

Together these two instruments are a single powerful "telescope". The large areas and low backgrounds provide high sensitivity to weak sources. They can view a single source in their common 1 degree field of view. The third instrument is an All Sky Monitor (ASM) that scans most of the sky every 1.5 hours in order to monitor the intensities and spectra of the brightest ~75 sources in the sky. It will explore the long-term behavior of these sources and will provide timely information on any large changes of intensity and spectral shape. This allows the powerful main instruments (PCA and HEXTE) to be pointed rapidly (within a few hours) at the object for studies with great sensitivity.

Table 1: Instruments
Table 2: Counts

The power and uniqueness of XTE comes in large part from the natural synergism of the 3 instruments and the versatile spacecraft (Fig. 8). These address a single well-defined objective: the timing and broad-band spectra of X-ray sources from 2 - 200 keV. The spacecraft permits rapid pointing to almost any point on the sky. The PCA/HEXTE measures short-term variability to microsecond levels while the ASM measures long-term (hours to months) light curves of bright sources. Long-term variability of faint sources may be monitored with repeated brief PCA/HEXTE observations.

Fig. 8 (J): Synergism

The PCA and HEXTE detectors provide optimum efficiency and low background over the 2 - 200 keV range with proven technologies. They are supported with new, advanced features that markedly enhance the performance over that of previous missions, e.g.:

(1) the 'chopping' (offset pointing) of the HEXTE detectors and automatic gain control for background control and subtraction,

(2) the conceptually new ASM design, a one-dimensional scanning 'Dicke camera' that yields positions to O(<,~)3', while at the same time requiring only a modest data rate,

(3) an advanced on-board microprocessor-driven data system permits very high throughput (e.g. 5 x 105 c/s), microsecond timing and fast-Fourier transforms, high time resolution burst searches, pulsar folds, and simultaneous use of different data modes, and

(4) the high performance spacecraft, especially high telemetry rate, accessibility to 93% of the sky on any day of the year, and rapid maneuvers.

For the purposes of temporal and broad-band spectral studies, these features are fully the equivalent of upgrading a standard optical telescope with new high-sensitivity focal-plane instruments, fast readout capability, computer-controlled precise pointing, and on-line data analysis. The improvement in science productivity should be enormous.

                             Table 1.  XTE Instruments                                           
          Instrument        Detector    Net     Band-wi  Field     Time   Tele-  Sensi-   
                                        Area    dth      of View   Reso-  metry  tivity   
                                        Geom.   (keV)    (FWHM)    lutio  rates  (milli-  
                                        (cm2)                      n      (kb/s  Crab)    
PCA      Proportional     5 Xe        6250    2 - 60   1o x 1o   1 us   18 ;   0.1      
         Counter Array    Prop.                                         256    (10      
                          Counters                                             min)     
HEXTE    High-Energy      NaI/CsI     1600    20 -     1o x 1o   10 us    5    1        
         X-ray Timing     (2                  200      (Rocking                (105 s)  
         Experiment       clusters)                    )                                
ASM      All-Sky Monitor  1-dim       90      2-10     0.2 x     1.5      3    30       
                          PSPC +                       1o a      h             (1.5     
                          Mask                                                 h)b      
a Effective beam of crossed fields; positions at  O(>,~)5[[sigma]]  are obtained                       
to O(<,~) 3' x 15'.  Gross FOV of each SSC  is 6o x 90o FWHM  b 10 mCrab in 1 day                      

                   Table 2:  Expected Counts*                                  
                              2-10 keV    10-30 keV     >30 keV       
Proportional Counter Array (PCA)**                                          
Background  (1s):           20 cts        24 cts        16 cts        
Crab Nebula  (1s):          8700          1205          80            
Her X-1 (1.24s):            1390          850           15            
X1728-34(1s; burst):         10,625       3750          2             
Sco X-1 (1s)                160,000       4600          4             
Cyg X-1 (1ms flare)(2.5     23            9             1             
SS Cyg  (5 mCrab; 1s):      44            4             -             
AGN  (1.3 mCrab; 10 s):     113           42            4             
1/2 High-Energy Experiment  (HEXTE)***                                      
Background  (1s)                 -        6 cts         29 cts        
Crab Nebula  (1s)                -        170           130           
Her X-1 (1.24s)                  -        40            2             
X1728-34 (1s; burst)             -        80            8             
Sco X-1 (1s)                     -        1670          40            
Cyg X-1 (1ms flare)(2.5          -        1             3             
SS Cyg  (5 mCrab; 1s)            -        -             -             
AGN (1.3 mCrab at 5 keV;         -        9             10            
10 s)                                                                 
1/3 All Sky Monitor (ASM)****                                               
Diffuse background  (1s):   40                                        
Crab Nebula  (1s):          90                                        
Sco X-1 (1s)                1400                                      
*        The assumed integration time for each source is given.             
**       "1 mCrab" nebula flux ~ 1.06 uJy at 1.25 x 1018 Hz                 
(5.2 keV). ***    Nominally, only 1/2 the HEXTE is on a source              
at a given time due to the                'rocking' collimators.            
****   Counts are for one of the 3 shadow cameras with 30 cm2               
net effective area                (60 cm2 gross) for a given                
source.  The mask occults 1/2 of the                60-cm2                  
sensitive area.                                                             

Proportional Counter Array (PCA)

The PCA instrument consists of 5 large proportional counters with anticoincidence features which provide a very low background. A mechanical hexagonal collimator provides 1deg. (FWHM) collimation. Sources as faint as 1/1000 of the Crab nebula can be detected in a few seconds.

The PCA consists of 5 large detectors with total net area of 6250 cm2 (Fig. 9). Each detector is a version (50% larger) of the HEAO-1 A2 HED sealed detector. They are filled with xenon gas and achieve low background through efficient anti-coincidence schemes including side and rear chambers and a propane top layer. The xenon of the 3 signal detection layers is 3.6 cm thick at 1.0 atmosphere. Methane is used as a quench gas. The front window and a window separating the propane and the xenon/methane chambers are both aluminized mylar of thickness 25 um. The propane layer may also be used as a signal layer in the energy range 1-3 keV.

The PCA is effective over the range 2-60 keV with 18% energy resolution at 6 keV and 255-channel pulse-height discrimination. The gain of the counter is monitored continuously with an americium radioactive source for which detection of the alpha particle identifies the calibration X-rays. The 1o FOV (FWHM) of the tubular (hexagonal) collimators yields a source confusion limit at ~0.1 mCrab.

Fig. 9 (K): PCA

The Crab nebula will yield 8700 c s-1 (2-10 keV) and 1200 c s-1 (10-30 keV) in the PCA. The backgrounds in these 2 bands are respectively 20 and 24 c s-1, corresponding to 2 and 20 mCrab respectively. With these backgrounds, an AGN source of intensity 1.3 mCrab (2 - 10 keV) and energy index 0.7 will be detected at >2[[sigma]] in only 1 s at 2- 10 keV and at 3[[sigma]] in 10 s at 10-30 keV. Monitored anticoincidence rates will provide a measure of the background to at least 10% of its value.

The PCA electronics provide digital pulse-height data to the flight experiment data system (EDS) for binning and on board analyses. The PCA is being provided by NASA's Goddard Space Flight Center.

Experiment Data System (EDS) for the PCA and ASM

The EDS serves to pre-analyze and compress the PCA X-ray data prior to its transmission to the ground via the relatively limited telemetry bandwidth. The EDS bins and analyzes data according to flexible criteria that can be defined for each observation. Its multiple Event Analyzers can process the data from a given source simultaneously in a variety of modes. It also controls the ASM rotation and processes ASM data.

The EDS is a microprocessor-driven data system used for the on-board processing of the PCA and ASM data (Fig. 10). The system will process count rates from the PCA up to O(>,~) 500,000 c s-1 (Sco X-1 yields ~160,000 c s-1) and will be able to time photon arrivals to ~1 us. The PCA data stream can be binned and telemetered in 6 different modes simultaneously by 6 independent Event Analyzers (EA) which operate in parallel, each analyzing the total PCA data stream. For example, a pulsar fold, a high-resolution spectrum, a low-resolution spectrum, and an autocorrelation function with 1 us bins could all be carried out simultaneously.

Fig. 10 (KK): EDS

Each EA includes a Digital Signal Processor (DSP) chip that will rapidly bin the individual events according to highly flexible criteria (e.g., non uniform pulse height bin widths and arbitrarily chosen timing bin widths) which may be specified for each observation. The DSP is used in conjunction with a table-lookup scheme to provide the required speed of classification for each event. (The tables hold the binning criteria for a given observation.) An additional microprocessor in each EA serves as the EA manager.

Each EA can also perform searches to capture X-ray bursts at high time resolution, pulsar folding, autocorrelation functions which yield Fourier power spectra to 5 x 105 Hz, cross correlation functions, and encoding of a single-bit high-speed mode for optimum transmission of the data stream with maximum possible time resolution. In addition, at low count rates, it will provide the familiar mode (Event Mode) wherein all bits describing each X-ray event are transmitted. The telemetry rate assigned to each EA may be changed from observation to observation to accommodate the scientific requirements.

Two of the 6 PCA EA's are intended to be reserved for two standard PCA modes with timing and spectral parameters that will remain unchanged throughout the mission to provide a uniform mission data bank. Currently, the two modes are a time series mode ([[Delta]]t = 0.1 s, 6 energy channels) and a spectral mode ([[Delta]]t = 16 s, 128 channels). Both would be compatible with the HEXTE standard data modes. If necessary, though, these modes can be reprogrammed in flight. Two other EA's will process the ASM data and control its rotation. These EA's will be similar in hardware structure to the 6 assigned to the PCA. The EA's will create data packets for transfer to the spacecraft memory from which they will be transmitted via the telemetry stream to the ground at a time average rate of 21 kb/s or at 256 kb/s for ~30 minutes a day. The EDS is being provided by the Massachusetts Institute of Technology.

High-Energy X-ray Timing Experiment (HEXTE)

The HEXTE features a large area and low background with a 1deg. field of view coaligned with the PCA field of view. Eight "phoswich" detectors are arranged in 2 clusters, each of which rocks on and off the source. This and automatic gain control for each of the 8 detectors together yield a well determined background which permits the spectral measurement of a faint source (1/1000 of the Crab nebula) at 100 keV in about 1 day.

The HEXTE experiment consists of two rocking clusters of NaI/CsI phoswich detectors that cover the energy range 20 - 200 keV (Fig. 11). The detectors are improved versions of the HEAO-1 A4 LED detectors which attained the lowest in-orbit background for large-area scintillators to date. Each detector consists of a 3-mm thick NaI primary detector coupled to a 38-mm thick CsI anticoincidence crystal that also serves as a light guide to the photomultiplier tube. Each detector has 200 cm2 net effective area. Each cluster contains 4 detectors; the total net area of the entire system is 1600 cm2. The field of view is 1o FWHM and is coaligned with the PCA when on source.

Fig. 11 (L): HEXTE

Each cluster will be rotated ("rocked") on or off the source every ~15 s to provide alternate source and background measurements. Each cluster will sample background positions on two opposing sides of the source, and the two clusters will rock in mutually perpendicular directions by either +/-1.5deg. or +/-3.0deg.. Thus 4 background positions will be monitored. The rocking will be phased so that the source is continuously viewed by at least one of the clusters. A 5-sided plastic scintillator 'box', viewed with photomultiplier tubes, serves as an anticoincidence shield for background reduction. Automatic gain control on each detector reduces systematic uncertainties from gain variations.

The HEXTE flight data system will provide the following modes: binned, event encoded, pulsar fold, burst trigger, and an optimum high-speed code as well as a standard output mode. The telemetry rate for HEXTE will be ~5 kb/s. The Crab nebula will yield 170 c s-1 (15-30 keV) and 130 c s-1 (>30 keV) in 1 cluster (1/2 HEXTE). The background in these energy bands will be ~6 and ~29 c s-1 respectively. At 100 keV, the background is about 100 mCrab (1 x 10-4 cts cm-2 s-1 keV-1). In the 90-110 keV band, the limiting sensitivity for detailed spectral analysis is expected to be about 1 mCrab (1 x 10-6 cts cm-2 s-1 keV-1) or 1% of the instrument background. This sensitivity can be reached at 3 [[sigma]] in 105 s. The instrument is being provided by the University of California at San Diego.

All Sky Monitor (ASM)

The ASM is the watchdog that alerts XTE to flares and changes of state in X-ray sources. It consists of three rotating Scanning Shadow Cameras (SSC) that can scan about 80% of the sky in 1.5 hours. The cameras provide measurements of intensities of about 75 known celestial sources in a day and can measure the position of a previously unknown source with a precision of about 3'.

The All-Sky Monitor consists of 3 Scanning Shadow Cameras (SSC) on one rotating boom with a total net effective area of 90 cm2 (180 cm2 without masks) (Fig. 12). Each SSC is a one-dimensional 'Dicke camera' consisting of a 1-dimensional mask and a 1-dimensional position-sensitive proportional counter. The gross field of view of a single SSC is 6o x 90o FWHM, and the angular resolution in the narrow (imaging) direction is 0.2o. A ~5[[sigma]] detection provides a single line of position of 3' x 90o. Two of the units view perpendicular to the rotation axis in nearly the same direction except that the two detectors are each rotated by +/-12deg. about the view direction so that they serve as 'crossed slat collimators'. The crossed fields provide a positional error region of 0.2o x 1o for a weak source and 3' x 15' for a ~5[[sigma]] detection. A spacecraft maneuver could reduce this to 3' x 3'. With high-statistics detections, precisions <1' should be attainable. The third SSC unit views along the axis of rotation. It serves in part as a 'rotation modulation collimator' and surveys one of the 2 poles not scanned by the other two cameras.

Fig. 12 (M): ASM

Each SSC detector is a sealed proportional counter filled to 1.2 atm with xenon-CO2, and has a sensitive depth of 13 mm. It has 8 position-sensitive anodes, a 50-um beryllium window, a sensitive area of 60 cm2 of which only 1/2 can view a given celestial position through the mask at a given time, anticoincidence chambers on the sides and rear, and sensitivity to 2-10 keV X-rays with three energy channels. The one-dimensional design of the SSC's greatly minimizes the required telemetry rate compared to two-dimensional systems. The data are telemetered in a spatial-image mode and in a time-series mode.

A motorized drive will rotate the three SSC's from field to field in 6deg. steps. At each resting position, a ~100-s exposure of the X-ray sky will be made; a complete rotation is thus completed in ~100 min. Since the 'crossed-field detectors' are stepped by only the 6deg. FWHM angle, each source is viewed twice. In this manner, each source gives rise to the entire mask pattern in the accumulated data, thus minimizing aliasing and side bands in the deconvolved results. During each rotation, ~80% of the sky will be surveyed to a depth of ~20 mCrab (about 50 sources). Frequent spacecraft maneuvers will make it likely that 100% of the sky is surveyed each day. In one day, the limiting sensitivity becomes O(<,~)10 mCrab (~75 sources). The drive can be commanded to stop for an extended observation of a given source, e.g. to obtain a precise position of a nova. The drive has a total rotation angle of ~500deg.; it can be stepped or moved rapidly in either direction.

The intensities and other basic results derived from the data will immediately be made available in the XTE Science Operations Center and to the community in general via computer links. The results will make possible rapid acquisition by the PCA/HEXTE of sources when they undergo a change of state, e.g. when a transient appears or when a low-mass binary system moves to the horizontal branch of the color-color plot.


The XTE will be carried into a low-earth orbit on a Delta II rocket scheduled for 1996 launch. A new spacecraft design has been developed to make possible flexible operations through rapid pointing, high data rates, and nearly continuous receipt of data at the Science Operations Center via a Multiple Access channel of TDRSS.

The XTE instruments and the service hardware (reaction wheels, star trackers, transmitters, etc.) are all integrated onto a common spacecraft structure (Fig. 13). The spacecraft components and their locations are chosen to optimize the scientific performance of XTE. The XTE is highly maneuverable (>6o/min), and the PCA/HEXTE field of view can be pointed to any position on the sky on any day of the year provided the angle to the sun is >30o. The pointing accuracy is < 0.1 deg with knowledge of aspect from star trackers and gyroscopes to O(<,~)1'. Rotatable solar panels make possible anti-sun pointing so that coordinated observations with optical telescopes can be carried on throughout the ground-based night.

Fig. 13 (N): S/C

The command and data links will be through the NASA TDRSS communication satellites. Both the multiple access and single access channels of TDRSS will be used to provide nearly continuous telemetry of data (Fig. 14). At least one command link per orbit will be available. The time-averaged data rate will be about 32 kb/s of which 26 kb/s will be available for the scientific instruments. In addition, 256 kbs for about 30 min. a day will be available.

An operations scenario patterned after the International Ultraviolet Explorer (IUE) is being planned wherein the XTE Science Operations Center has the prime responsibility for routine operations including pointing. Also, the scientific operations are being decoupled from engineering and servicing constraints. For example, since the two pointable antennas can transmit to any point on the sky, XTE can maintain communication with a TDRSS even if XTE is maneuvered to a previously unscheduled celestial target. No schedule changes need be negotiated with TDRSS, and on-board memory allocation is not subject to changing TDRSS schedules. These features should present an observer with the degree of flexibility and control similar to that experienced at a major ground-based observatory.

Fig. 14 (O): Telemetry links

XTE will not carry expendables that would artificially limit the active lifetime in orbit. It will be in a low-earth orbit of altitude ~600 km and ~23deg. inclination. The XTE mission will be placed in orbit on a Delta II rocket and should remain operational for 2 - 5 years.

Community of XTE Users

User Program

The XTE will be available for 100% of its usable time to the international community of observers. A large community of theoretical astrophysicists will create and apply their models of the compact systems studied with XTE .

The XTE observing program will be devoted 100% to guest observers, or Users, after a 30-d checkout period. The community will propose for observations with the PCA and HEXTE instruments. The ASM observing plan is dictated for the most part by the requirements of the continuous monitoring. The basic ASM results (e.g., intensities, hardness ratios, routine FFT's) will be placed in the public domain in near real time by the PI team for the use of proposal writers, XTE observers, multifrequency observers, and paper writers. Proposals to use ASM data for specialized analyses or limited observations should also be possible. Proposal evaluation will be carried out by a NASA-appointed peer group. PI-institution observers will compete in this process to obtain observing time.

A User committee will help set proposal guidelines. For example, it is anticipated that proposals will be accepted for contingency observations, e.g. for a transient with particular characteristics, that a proper balance will be maintained between individual-source studies and extended class studies, and that the handling of multifrequency proposals will consider the schedules of other observatories. Specialized analyses of the ASM data may also be an appropriate proposal.

A User may carry out and/or monitor the observation at the Science Operations Center (SOC, at GSFC), at UCSD or MIT, or at the user's home institution through a remote terminal. The focus of operations and observer support will reside at the SOC, but some support will be available at UCSD and MIT. Standard analysis programs and computer facilities will be available at the SOC for current observers. Standard programs will be made available to Users for use at their home institutions. Data will be provided promptly to Users in standard formats.

Real-time operations at the SOC will include examination of the data from the instruments and the sending of commands to adjust instrument parameters and possibly times for maneuvers to new targets, e.g., if a source is found to be "off". The ASM data will be examined for transients and changes of state of sources. This could dictate a change in the observing program. The management of the data system and the flexible maneuvering of XTE will help minimize the disruption to the preplanned schedule.

The international community of observers will have access to XTE through the peer review process. A User committee will be formed to advise NASA and the Science Working Group on guidelines for proposals, observing, operations, and data rights. Currently, the plan is that the observer would have proprietary data rights for one year.

Multifrequency Observers

XTE will be haighly suited to multifrequency studies that involve many additional radio, optical, UV, and gamma-ray observers.

The importance of multi-frequency observations is recognized by the XTE team. The state of knowledge of accreting binary systems (cataclysmic variables and neutron-star binaries) is now sufficiently developed so that concurrent optical, ultraviolet, and X-ray observations may be required for detailed studies such as building geometric models. Similarly, the current attempts to construct unified models of AGN require concurrent observations from the radio to gamma rays.

The timing and spectral studies that will be made with XTE are thus often best served by complementary observations at [[gamma]]-ray, UV, optical, IR, or radio frequencies. It is a specific goal of the XTE mission to facilitate multifrequency observing plans when scientific justification is provided. Accordingly, the XTE spacecraft will be able to point the instruments anywhere beyond a 30deg. cone centered on the sun on any day of the year. Spacecraft systems are being designed to simplify operations. Close coordination with potential observers and non-XTE telescope assignment committees is planned.

XTE Science

Global Scientific Objectives

The prime targets of study with XTE are systems containing compact objects (galactic and extragalactic). These systems are characterized by very high gravitational fields, plasmas of extremely high temperatures (~107 K), and often ultra-high magnetic fields (to ~1012 G).

The timing and spectral data obtained by XTE probe numerous fundamental physical processes (see Fig. 1). The galactic objects are mostly binary systems with an accreting neutron star, white dwarf, or possibly black-hole, i.e. systems near the end points of their evolution. The X-rays provide direct information about: (1) the conditions very close to the compact object, e.g. geometries, temperatures, magnetic fields, (2) the evolution of the binary systems (e.g. relation of low-mass systems to the millisecond radio pulsars), and (3) the nature of the compact object itself (e.g. neutron-star masses and internal structure of neutron stars).

The extragalactic objects include emission-line AGN (Seyferts and quasars) as well as BL-Lac type objects. Here again, the X-rays provide direct information about the regions closest to the putative ~108 MO(O,.) solar mass black hole. The X-rays play a substantial role in the overall energetics and in the conditions of the optical and radio emitting regions. Measurements of the high-energy spectra of AGN to >100 keV are critically needed to bound the power-law model and the AGNs' bolometric luminosities. Such data will also answer specific questions about the origin of the diffuse X-ray background.

The objects accessible to XTE are the thousand or more brightest X-ray sources in the sky. These are the closest or most luminous objects of the several classes (compared to the much fainter deep-sky objects detected with Einstein or AXAF). The relative brightness of these objects (e.g. QSO's at V ~ 15 - 16) makes them most desirable for detailed studies at other frequencies as well as in X-rays.

Many of these objects have only recently become available through identifications and classifications of HEAO-1 sky-survey sources. The identification of ROSAT sky survey and IPC slew survey sources is providing many additional objects. XTE can stimulate detailed multifrequency studies as it brings to bear the unique capabilities of large aperture and high-energy response.

Unexplored Measurement Phase Space

XTE is designed to make possible or facilitate studies of important domains of science not easily accessible to previous missions, specifically sub-millisecond timing, the study of early phases of X-ray novae, continuous sensitive spectra over the entire 2 - 200 keV band, and long term variability studies of faint sources.

Specific objectives for XTE studies follow from the domains of largely unexplored measurement phase space that XTE provides. This phase space also has potential for unanticipated discoveries. The domains are (with examples of their utility):

Sub-millisecond timing regime explored

High telemetry rates (32 kbs continuous and daily 256 kbs), high-rate processing capability to ~5 x 105 ct/s (e.g. ~3 x Sco X-1), and timing to ~1 us allow exploration of this temporal regime. The natural time scale for matter near a 10-km neutron star is ~ 0.1 ms (size / free fall speed ~ dynamical time scale).

Search for X-ray millisecond pulsars - precursors of radio ms pulsars

Compton scattering in QPO's (soft vs hard delays)

Matter in last orbits around black holes

Rapid response to temporal phenomena

The ASM, rapid maneuvering capability (180o in 30 min), and efficient operations make possible early detection and rapid acquisition of temporal phenomena with the PCA/HEXTE within a few hours of detection. Alarms from other observatories (e.g. Gamma Ray Observatory) can also alert XTE.

Episodic accretion in high-mass X-ray binaries ('hard X-ray transients' = Be-star binary)

Sudden accretion flow in low-mass X-ray binaries ('soft X-ray transients')

Change of state of Cyg X-1

Turn-on of Sagittarius gamma-ray repeating burster (GRO alarm)

Spectra up to 200 keV with high sensitivity.

The exposures obtained by HEXTE will be greater than HEAO-1 by factors 100 - 1000. HEXTE, with proven instrumentation, will be highly sensitive through its large aperture, low background, and well-controlled systematic uncertainties.

Turnover of AGN spectra which typically rise (in [[nu]]F[[nu]]) with energy in 2-20 keV region

Magnetic fields of neutron stars through the study of cyclotron features

Temperatures of the hard components of magnetic cataclysmic variables

Hard tails in X-ray spectra of low-mass X-ray binaries (black holes?)

Intracluster magnetic fields through hard-tail observations

(inverse Compton of radio synchrotron electrons on 3deg. K radiation)

Long-term and very sensitive monitoring of fluxes and pulsar phases.

The flexible maneuvering capability of XTE permits frequent repeated observations with the PCA. (The ASM will perform routinely such measurements for the brighter sources.)

Precession periods of accretion disks

Variability in active galactic nuclei, esp. BL Lac objects

Pulsar rotation-rate changes

Timing Studies with XTE

The pervasive variability of the X-ray sky provides a rich set of X-ray phenomena for XTE to study. X-ray pulsars, novae, low-mass binaries, precessing systems, and X-ray bursts all have their distinctive temporal signatures.

Pulsing from spinning neutron stars and white dwarfs

X-ray pulsars (see Fig. 6) are due to the accretion of gas onto a spinning magnetized neutron star. Heated regions at the magnetic poles rotate into and out of view as the neutron star rotates, and the radiation will be beamed into preferred directions. Thus an observer on earth sees a periodic X-ray flux which can be analyzed to obtain period and pulse-shape values and the variations of these quantities. Such studies can reveal the conditions inside the neutron star as well as in its environs.

The infalling accreting matter applies a torque to the star, causing it to spin faster or slower. Fluctuations in the spin period may be accentuated because the shell of the neutron star may not be rigidly coupled to the core. The degree of this coupling can be obtained from XTE data. XTE can also continue to track the spin variation of pulsars due to the accretion torques to determine the evolutionary behavior of these systems (Fig. 15). Pulse shapes as a function of X-ray energy pertain directly to the beaming of X-rays in the strong magnetic fields and hot plasmas of the polar regions of the neutron star.

Fig. 15 (P): Vela X-1 spin history

The moderately strong magnetic fields of the white dwarfs in some cataclysmic variable systems (e.g., DQ Her objects) focus the accreting material onto a small region of the pole of the white dwarf, with the result that hard X-rays are emitted. The spinning of the white dwarf then gives rise to periodic modulation of the X-ray flux. Periodic modulation in hard X-rays is therefore the definitive means of classifying DQ Her objects.

The flexible pointing, rapid maneuverability, and sensitivity of XTE makes possible efficient studies of many pulsars. The Pulsar Folding Modes of the on-board data systems permit efficient pulse-phase spectroscopy.

X-ray novae

Some binary X-ray sources are only occasionally active. Most of the time, the matter from the normal star in the binary system does not flow onto the neutron star and hence it emits no X-rays. But occasionally the accretion can turn on very strongly for a week or more, and the source appears in the sky as a brilliant X-ray source, known either as an X-ray nova (Fig. 16) or a hard transient (Fig. 17). The nova phenomenon is due to the sudden (and not understood) onset of accretion from a low-mass star onto a neutron star or black hole. The hard transient phenomenon is due to outbursts of plasma from a massive Be star. The plasma reaches the orbit of the neutron star, or the neutron star (possibly in a highly eccentric orbit) enters a region of plasma, and accretion commences. Temporal and spectral studies are a diagnostic of the geometry and constituents of these systems. For example, study of the early stages of novae could lead to an explanation of the sudden onset of accretion. The hard transients can yield the spatial and temporal character of the plasmas ejected by the Be stars.

Fig. 16 (R): Nova GS 2000+25
Fig. 17 (Q): Be-star binary cartoon

During the rise and fall of the X-ray intensity, the falling gas can exert a torque on the neutron star so that its spin rate changes. If the neutron star is also a pulsar, the rate of spinup can be correlated to the X-ray luminosity which is a measure of the amount of accreting matter flowing onto the neutron star (Fig. 18). The X-ray pulse shape also changes as the accretion rate varies. Such data illuminate the interplay among the neutron star, the infalling material, and the emerging X-rays.

Fig. 18 ( S): 2030 Spin rate change

There is strong evidence that at least one of the nova systems (A0620-00) contains a black hole. XTE can discover and identify X-ray novae which can then be studied in visible light for further evidence of additional black holes.

The All-Sky Monitor provides the alarm that the nova has occurred leading to sensitive studies with the PCA/HEXTE. The ASM also provides a position sufficiently accurate to lead to an optical identification. Raster scans with the PCA will also provide accurate positions.

Low-mass binaries, quasi-periodic oscillators, supernova 1987A

A class of X-ray sources characterized by non-pulsing and no eclipses are called "low-mass X-ray binaries" because it is believed that the normal star (companion to the neutron star) is less massive than the neutron star. A common characteristic of these sources is a quasi-periodic oscillation (QPO) of the X-ray flux (Fig. 19). The rapid pulsing at 5 or more times a second is not strictly periodic, and it changes character as a function of source accretion rate (Figs. 20). Some aperiodic pulsing may indicate an interaction between the magnetic field of the neutron star and the matter circulating near the magnetosphere.

Fig. 19 (T): Rapid burster QPO
Fig. 20 (U): Sco X-1 QPO: freq and intensity
Fig. 21 (V): Cyg X2 Z plot

The complex, yet repeatable, character of the QPO phenomenon (Fig. 21) makes it an excellent prospect for the revelation of important facets of the accretion processes in low-mass X-ray binaries. This phenomenon might be due to an undetected neutron star that spins nearly a thousand times a second. XTE can search for the coherent pulsing from such a pulsar.

The famous supernova SN1987A that exploded in February 1987 may well have an underlying neutron star spinning this rapidly, with a period of less than 10 ms. Even if it is now emitting pulsed radiation, debris from the supernova outburst is probably obscuring the radiation. As the debris begins to thin out, gamma and X-rays would most easily penetrate it. XTE could well be the first observatory to detect this pulsar. Pulse-timing studies would then address fundamental issues, specifically the evolution of the progenitor, the magnetic moment of the pulsar, and the internal structure of the neutron star.

The large area of XTE collects the large numbers of X-rays required to study the rapidly varying QPO X-ray signals and to search for millisecond pulsars. The experiment data systems have the capability to handle the high throughput (~5 x 105 X-rays per second).

Precessing accretion disks and long-term light curves

The gaseous matter accreting onto a neutron star often forms a spiraling accretion disk, not unlike Saturn's rings. Under the influence of the gravity of the two stars in the system, the disk may slowly precess (like a spinning gyroscope). It can thereby block the X-rays coming from the central neutron star on a periodic basis. The resultant modulation informs us of the precession, and the irregularities provide details about the form of the accretion disk. At present, three such systems are known, Her X-1, SS433, and LMC X-4, and others are suspected (e.g. LMC X-3). The long-term light curves thus reveal precessing as well as orbital motions.

Long-term light curves may be obtained from the All-Sky Monitor for the brighter ~75 X-ray sources. The daily or weekly monitoring of some dozen individual faint sources can be monitored with the PCA/HEXTE in a series of brief measurements, made possible by the rapid maneuvering capability of the spacecraft.

X-ray bursters

As material accretes onto a neutron star, it builds up on the surface until it is so deep that the pressure at the bottom becomes very high. When it is great enough, the atomic nuclei of the gas can combine or burn with an explosive release of energy. (This is similar to the reaction that powers the sun and hydrogen bombs.) The outburst is seen on Earth as an X-ray burst, a flash of X-rays in the sky that lasts from seconds to minutes (Fig. 22). Another type of burst (Type II) is believed to be due to instabilities that develop as the accreting gases penetrate the magnetic field of the neutron star. The temporal and spectral evolution of the X-ray emission is a diagnostic of these dramatic energetic processes.

Fig. 22 (W): X-ray Burst: Discovery ANS

The study of X-ray bursts with high temporal and spectral resolution requires the large sensitive area of XTE. The burst events can be captured in the data systems at high resolution with burst catcher modes for later transmission at a low rate. This mode can work autonomously in a single Event Analyzer while other modes are used for other studies of the same source. A burst detected in the PCA can trigger the HEXTE burst catcher and vice versa.

Rapid aperiodic flickering and black holes

The emission from regions close to the compact object may vary on particularly short time scales. Rapid aperiodic flickering has been observed in white-dwarf cataclysmic variables, neutron-star systems (e.g. Cir X-1), and black-hole candidates (e.g. Cyg X-1; Fig. 23). The nature of this variability, if studied with sufficient sensitivity, can potentially distinguish between a neutron star and a black hole. Quasi-periodic variability due to matter in the innermost stable orbits of a black hole might be detectable.

Fig. 23 (Y): Cyg X-1 ms flaring

The large area of the PCA in multiple independent detectors and the throughput of the data systems make possible studies of these rapidly varying signals. The EDS can carry out fast signal analyses, e.g. time difference histograms and autocorrelation functions to times as short as 1 us.

Active galactic nuclei

Aperiodic variability of the X-ray intensity is a common characteristic of many active galactic nuclei (Fig. 24). Power density spectra show a -1 slope down to a period of 1000 s indicating no characteristic time scale. XTE observations could test this behavior down to a period of ~60 s. Dramatic changes in flux provide evidence in a couple of cases that the X-ray emission must be beamed (Fig. 25). Additional examples studied by XTE will further elucidate the conditions close to the massive black holes of active galactic nuclei.

Fig. 24 (Z): MCG-6-30-15
Fig. 25 (CC): Quasar Flare: PKS 0558-504

The maneuverability of XTE and the large effective areas of the detectors make possible the measurement of very-low-frequency variations with brief (5-10 min) observations of 10 or 12 AGN once per day.

Spectral Studies with XTE

X-ray spectra from the various types of systems reveal the detailed emission processes and the locations of the emitting regions, e.g. blackbody radiation from neutron-star polar caps or cyclotron lines from intense magnetic fields of neutron stars. The empirical power-law character of AGN, well established in the 2-10 keV range, will be investigated in depth with the extensive bandwidth of XTE.

Compact stellar objects and black holes

The accretion of gas onto the surface of a compact object is a complicated process, the nature of which depends greatly upon the particular characteristics of the system. The depth of the potential well and the strength of the magnetic field of the compact object can vary greatly from one type of system to another. Some cataclysmic variables are so magnetic (AM Her type objects) that the infalling matter travels along field lines and never forms an accretion disk. Accretion disks have hotter inner regions and cooler outer regions and sometimes a corona, and/or a "hot spot" where the accretion stream collides with the outer boundary of the disk. X-ray emission can arise from the surface of the compact object where the matter impinges, from an accretion shock above the surface, or from the inner accretion disk itself. A black hole, of course, has no "surface"; no surface component would be expected.

Each of these characteristics contribute to the net X-ray spectrum, often in well-defined ways. For example, the "temperature" of the line emission from iron atoms distinguishes between emission from a cold plasma illuminated by high-energy photons (line at 6.4 keV) or from a hot plasma which self-excites the iron emission (line at 6.7 keV). The radiation from the polar region of the surface of a neutron star can appear as a black body spectrum while the radiation from an accretion shock will have the spectral form of optically-thin thermal bremsstrahlung. The high magnetic fields of neutron stars constrain the motions of the electrons in the infalling matter so that they emit characteristic cyclotron lines which provide direct information about the magnetic fields and the emitting plasmas (Fig. 26). The 511 keV radiation from the annihilation of electrons and positrons in compact stellar sources may have been detected as a line-like feature at the backscatter energy of 170 keV. This too can be studied with XTE, in conjunction with the Gamma-Ray Observatory.

Fig. 26 (AA): 1538-52 cyclotron

The spectra of many (stellar) black-hole candidates have a pronounced hard component or high-energy tail. The extent (in energy) and cause of this spectral component is presently unknown. XTE will provide more frequent, high-quality measures of this phenomenon leading to valuable constraints on the emission physics and a better understanding of its utility as a black-hole indicator.

The spectrum of an object can change with time: during the pulse cycle of a pulsar or during the rise and fall of an X-ray burst. A combined temporal/spectral attack on the emission processes is an especially powerful diagnostic. For example, the variation of the X-ray spectrum during a burst allows one to watch the lifting of the exploding gases above the neutron-star surface and their subsequent cooling. Also the phase lags between high and low energy X-rays from quasi-periodic oscillators (QPO) reveal possible Comptonization of the photons by high energy electrons in the system. Pulsars may be studied with pulse-phase spectroscopy such as that shown in Fig. 26.

The large dynamic range (2 - 200 keV) of XTE spectra serves to better distinguish emission components.

Active galactic nuclei

The overall shape of the spectra from active galactic nuclei has proved useful in distinguishing different types of AGN. Seyfert 1 and QSO spectra typically exhibit a logarithmic slope of -0.7 for the power spectrum whereas BL Lac object slopes are typically -1 or steeper. Such distinctions are vital to the construction of unified models of AGN; it is possible that, for the BL Lac objects, the earth is directly in the beam of the jet (Fig. 4). A steep component at low X-ray energies in some objects has been attributed to black-body emission from an accretion disk around the massive black hole. Also, complex absorption and emission due to iron in these objects has indicated the existence of cold matter near the central continuum source.

XTE will study not only the classical subclasses of AGN (broad and narrow-line Seyferts, quasars, BL Lac objects), but also subgroups such as emission-line blazars, AGN with extensive Fe II emission, and AGN with strong or weak blue bumps.

The diffuse X-ray background is believed to arise at least in part from the combined emission of AGN. Above ~30 keV, the power-law spectra of the AGN appears to be at odds with the steeper thermal spectrum of the background (Fig. 27). The measurement of the high energy spectra of AGN therefore is one of the high-priority objectives of XTE.

Fig. 27 (BB): CXB Spectrum

The large area, low background, and wide energy response of the XTE detectors facilitate these observations.

Multifrequency Studies

The understanding of high-energy processes requires a broad range of data extending over frequencies from radio to gamma ray. For sources that vary in intensity, concurrent observations are important. XTE's spacecraft and operations are being designed to facilitate this type of science.

Many classes of X-ray sources emit copious radiation over a large range of photon frequencies. The multifrequency spectrum (from radio to X-ray or gamma ray) of a source can consist of a variety of emission components that arise from distinct physical processes. For example, in X-ray binaries the temperature within an accretion disk varies enormously from the hot inner region near the compact object to the cooler edge near the source of the accreting material. X-rays emerge from the inner regions, while the outer disk radiates UV and optical photons. In some active galactic nuclei, ultraviolet and optical emission in excess of an underlying power-law continuum may indicate the presence of a massive accretion disk, while radio and infrared emission have been associated with much larger structures such as jets and a circum-nuclear molecular toroid. The high-energy spectrum of an AGN will be enhanced by combining XTE and gamma-ray results, the latter from the Compton Observatory (GRO).

The variability of the fluxes often makes concurrent observations very important. Cataclysmic variable systems have been convincingly modeled with concurrent X-ray and optical light curves and spectra, leading to highly detailed descriptions of the accretion flow. For example, mysterious X-ray/optical correlations on time scales of a few minutes has been observed in the AM Her object BY Cam (Fig. 28). X-ray bursts are also known to have slightly delayed optical flashes which locate disk material illuminated by the X-ray burst. In AGN, comparison of variability in the X-ray, ultraviolet, optical, and 1 um-infrared bands have helped to decouple the various emission components that underlie these spectral regions. Simultaneous optical monitoring of the dramatic X-ray flare from a quasar (Fig. 25 above) would have been immeasurably valuable.

Fig. 28 (X): H053+608 flickering

Studies of New Prototype Objects

The survey missions (HEAO-1, Einstein Slew Survey, ROSAT) are continually infusing the samples of X-ray bright sources with new objects that have unusual characteristics. The pool of potential XTE targets is continually being enriched.

Specific compelling objectives for XTE also follow from the study of new bright (>0.3 uJy at 5 keV) objects through optical identifications of cataloged X-ray sources, i.e. the HEAO-1 'LASS' catalog, the Einstein slew survey, and the current ROSAT survey. The optical identifications of these sources yield systems that extend substantially the canonical parameters of known classes. Recent examples that merited followup X-ray observations include (1) a QSO with unusually steep X-ray spectrum and with very strong optical Fe II emission (highly pertinent to the formation of broad-line clouds and to massive accretion disks), (2) a possible slightly asynchronous AM Her type object, (3) a BL Lac object with the highest known optical polarization (17%) among X-ray selected objects, and (4) a cataclysmic variable with an orbital period within the "period gap".

The numbers of known, bright, optically identified (or X-ray classified) objects with hard (>2 keV) emission in each of the major X-ray classes (e.g. low-mass neutron-star binaries, QSO's, Sy 1, AM Her type, etc.) is currently not high, from ~10 to ~100. Thus newly identified or classified objects may well have unique properties and hence be especially worthy of detailed study. Some objects are likely to become prototypes of new subclasses. The population characteristics of the ~1000 brightest X-ray sources in the sky are not yet fully known, and XTE will help complete this task with new objects discovered in current surveys.

Targets of Opportunity

The All-Sky Monitor on the XTE will provide early warning of changes of state of X-ray sources. This will make possible detailed studies with the HEXTE/PCA of variable-state sources such as QPO 'Z' sources, Be-star binaries in outburst, and X-ray novae in their early stages.

The pervasive variability of the X-ray sky and the unpredictability of this behavior in many instances require a flexible approach to the observing program. Events that may require a maneuver of the PCA/HEXTE fields of view to a previously unscheduled target are the beginning stages of a transient outburst from a Be-star binary, the precursor outburst of an X-ray nova, the entry of Cyg X-1 into the "high" spectral state, or the move of a low-mass binary to the horizontal branch of the two-color diagram. The ASM, or frequent sampling of galactic and extragalactic sources by the PCA/HEXTE, can alert the SOC to the event. It is anticipated that deviations from the preplanned observing schedule will be relatively infrequent (2-4 times per month). With this capability, the XTE can efficiently obtain data from unusual events in order to address specific scientific questions.

The decision to go to a target of opportunity will be the observatory director's responsibility. It is anticipated that many such observations will be carried out on the basis of previously selected contingency proposals from the community. The proposer would immediately be contacted so he/she could direct the observation. For truly unanticipated events, the data rights could belong to either the current observing team or to the community through a rapid after-the-fact proposal selection. The User committee must help set this policy.

The Current XTE Team

The scientific definition and planning for the mission and the development of the instruments is being carried out currently by scientists at 8 institutions. These are organized into 3 instrument teams, a Science Working Group (SWG), and a Science Operations Center (SOC) team. At a later time, closer to the launch date, a User Committee will be formed. The current (1991) active science members and senior management and engineering personnel are:


Jean Swank, PI

Keith Jahoda

Frank Marshall

Weiping Zhang


Richard Rothschild, PI

Duane Gruber

Paul Hink

Michael Pelling


Hale Bradt, PI

Alan Levine

Edward Morgan

Ronald Remillard

J. Garrett Jernigan (UCB)


Instrument Teams (see above)

Alan Bunner (NASA)

John Deeter (UWA)

John Grunsfeld (CIT)

Paul Hertz (NRL)

Louis Kaluzienski (NASA)

Shri Kulkarni (CIT)

Dan Schwartz (SAO)


Jean Swank (Project scientist)

Instrument Teams (see above)

Nick White

Leon Herreid

Nand Lal

Arnold Rots

Project & Engineering Management (GSFC)

Dale Schulz (XTE Program Manager)

William Davis

Richard Day

Gabriel Epstein

John Robinson

Instrument Management Lois Workman (PCA)

David Bundas (PCA)

Edwin Stephan (HEXTE)

Fred Duttweiler (HEXTE)

William Mayer (ASM, EDS)

Robert Goeke (ASM, EDS)



H. Bradt, J. Swank, R. Rothschild, A. Levine, R. Remillard, and colleagues.


C. Wilson (MIT Graphic Arts)


E. Moyer & O. Sheinman (GSFC)

R. Cutlip & G. Bacon (Advanced Technology & Research Corp.)


Dundee Litho Inc.

Plates 1,4 (LMC and Orion nebula): David Malin; taken with the UK Schmidt telescope, (c)Royal Observatory Edinburgh and Anglo-Australian Telescope Board.

Plate 2 (Sizes): C. Jones, C. Stern, W. Forman. (c)Smithsonian Institution Astrophysical Observatory

Plates 3,5 SN1987A and Cen A): David Malin; taken with the 3.9-m Anglo-Australian telescope. (c)Anglo-Australia Telescope Board.

Plate 6 (Cen A jet): Provided by C. Jones, C. Stern, W. Forman; taken with the Einstein X-ray observatory; adapted from Feigelson et al. 1981, Ap. J. 251, 31. (c)Smithsonian Institution Astrophysical Observatory

Figs. 1-6,8-12,14,17 (sketches) H. Bradt, M. Halverson, and students.

Fig. 7: (Sky Map) Wood, et al. 1984. Ap. J. Suppl., 56, 507.

Fig. 13: (Spacecraft): O. Sheinman, F. Leyh (GSFC).

Fig. 15 (Vela X-1 pulse period): Adapted from Nagase et al. 1989. Publ. Astron. Soc. Japan 41, 1.

Fig. 16 (GS2000+25 nova): Adapted from Tsunemi et al. 1989. Ap. J. Lett. 337, L81.

Fig. 18. (2030+375 spin): Adapted from Parmar et al. 1989. Ap. J. 338, 359.

Fig. 19: (Rapid burster QPO): T. Dotani (see Ap. J. 350, 395, 1990.

Fig. 20: (Sco X-1 QPO): Priedhorsky et al. 1986. Ap. J. Lett. 306, L91.

Fig. 21: (Cyg-X2 'Z' plot): Adapted from Hasinger & van der Klis 1989. A&A 225, 79.

Fig. 22: (X-ray burst): Grindlay et al. 1976. Ap.J.Lett. 205, L127.

Fig. 23: (Cyg X1 ms): Rothschild et al. 1974. Ap. J. Lett. 189, L13.

Fig. 24: (MCG-6-30-15): Pounds & McHardy 1988. Phys. of Neutron Stars and Black Holes, ed. Y. Tanaka, p. 285. Tokyo: Universal Acad. Press.

Fig. 25: (PKS 0558-504 flare): Remillard et al. 1991. Nature 350, 589.

Fig. 26: (1538-52 cyclotron): Adapted from Clark et al. 1990. Ap. J. 353, 274.

Fig. 27: (Background spectrum): Adapted from Rothschild et al. 1983. Ap. J. 269, 423.

Fig. 28: (BY Cam flickering): Silber et al 1992. Ap. J. in press.



6250 cm2; Xe Proportional Counters; 2 - 60 keV; 'PCA';

GSFC; HEAO-A2 type; low background.

1600 cm2; NaI/CsI; 20 - 200 keV; 'HEXTE';

UCSD; Rocks on-off source continuously; low background


Net Area 3000 cm2 at 3 keV

6000 cm2 at 10 keV

1200 cm2 at 50 keV (NaI); 800 cm2 at 50 keV (Xe)

1100 cm2 at 100 keV

300 cm2 at 200 keV

Field of View 1o FWHM circular; PCA and HEXTE coaligned

Energy Resolution 18% at 6 keV (Xe)

18% at 60 keV (NaI)

Sensitivity* 0.1 mCrab 2-10 keV (in minutes); limit of source confusion

1 mCrab 90 - 110 keV (3[[sigma]]; 105 s)

Time Resolution 1 us (PCA); 10 us (HEXTE)

Background 2 mCrab 2-10 keV

100 mCrab at 100 keV

Telemetry 18 kb/s (PCA) and 5 kb/s (HEXTE) continuous; 256 kb/s ~30 min/day (PCA)

Flight Data System(MIT) Flexible binning criteria (microprocessor-driven)

for PCA/ASM Pulsar folding and high-time-resolution burst searches

Simultaneous binning with different criteria

Real-time us ms resolution ACF's and CCF's.

High throughput, ~5 x 105 ct s-1

HEXTE Data Modes Binned, Event encoded, pulsar fold, burst trigger, optimum high-speed code


Energy Range 2-10 keV; 3 energy channels

Net Area 90 cm2 net area of 3 detectors (180 cm2 without masks)

Positional Resolution ~3' x 15'

Scan time 90 min; 80% of the sky per orbit

Sensitivity ~20 mCrab in 90 min; O(<,~)10 mCrab in 1 day

Telemetry 3 kb/s

Operations Immediate analyses to search for transient events or changes of state


Maneuverability 6o/min; precise to < 0.1 deg.; aspect to 0.01 deg.

93% of sky accessible (includes anti-sun pointing for coordinated observations)

Response to Transients Target acquisition a few hours after detection


PCA/HEXTE 100% of time competitively assigned. (PI's also compete)

Single object, class studies, or contingency (i.e. transients) proposals allowed

Observing at SOC or at PI institutions (with less support)

Remote observing/monitoring from User's home institution

Multifrequency coordinated observations encouraged

ASM Results placed in public domain (computer access) to aid community in proposal writing, optical observations, etc.

Proposals for specialized analyses possible


* 1 mCrab ~ 1.06 uJy at 5.2 keV