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.
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.
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.
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.
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.
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.
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.
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.
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 Crab): 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 Crab) 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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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):
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
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)
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)
Precession periods of accretion disks
Variability in active galactic nuclei, esp. BL Lac objects
Pulsar rotation-rate changes
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
The large area, low background, and wide energy response of the XTE detectors facilitate these observations.
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.
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.
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.
Jean Swank, PI
Richard Rothschild, PI
ASM and EDS (MIT)
Hale Bradt, PI
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)
Project & Engineering Management (GSFC)
Dale Schulz (XTE Program Manager)
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
PARAMETERS OF POINTED INSTRUMENTS
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
ALL-SKY MONITOR ('ASM'; MIT)
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
SPACECRAFT and OPERATIONS
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
USER (GUEST) PROGRAM
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