The Rossi X-ray Timing Explorer (RXTE) was launched on December 30, 1995 from NASA's Kennedy Space Center. The mission is managed and controlled by NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. RXTE features unprecedented time resolution in combination with moderate spectral resolution to explore the variability of X-ray sources. Time scales from microseconds to months are covered in an instantaneous spectral range from 2 to 250 keV. Originally designed for a required lifetime of two years with a goal of five, RXTE has passed that goal and is still performing well.

The prime scientific objectives of the Rossi X-ray Timing Explorer (RXTE) were the study of astrophysical compact objects: black holes (galactic and extragalactic), many types of neutron stars, and accreting white dwarfs. RXTE was successful in achieving its original observing objectives of large area and high time resolution observations with broadband (2–200 keV) spectra, scheduled flexibly enough to enable observations of targets of opportunity on any timescale greater than a few hours.

These capabilities enabled qualitatively new discoveries about dynamical timescale phenomena related to neutron stars and black holes, phenomena which probe basic physics in the most extreme environments of gravity, density, and magnetic fields. RXTE has extended its lifetime by applying the proportional counter area selectively and maintains schedule flexibility by making use of the distribution of targets around the sky.


Rossi was launched usinf a Delta II rocket at 8:48 EST Saturday December 30, 1995. Launch Date: 1995-12-30 at 13:48:00 UTC
Launch Vehicle: Delta II 7920
Launch Site: Cape Canaveral, United States


It was put into a low-earth circular orbit at an altitude of 580 km, corresponding to an orbital period of about 90 minutes, with an inclination of 23 degrees, and is still going strong, making unique contributions to our understanding of these extreme objects.


Rossi is still a very useful instrument even long after its initial mission has been completed. The characteristics that have made, and are still making it a major scientific research instrument are that:
1. XTE can explore fast and ultra-fast X-ray variability in relatively bright sources, XTE should be unparalleled. The effective area will give about 15,000 counts/s from the Crab nebula in the energy range 2-200 keV. Events are tagged with microsecond accuracy. Data modes can be chosen to optimize the selection of data transmitted and achieve high rates of information retrieval.
2. XTE can perform simultaneous information across the 2-200 keV band, XTE offers co-aligned 2-60 keV and 20-200 keV detectors which all have a 1 degree peak to zero response. The energy resolution is 18% at 6 keV, 10% at 20 keV, and 17% at 60 keV.

3. XTE can look at AGN of various types (Seyferts, BL Lacs, Quasars), at energies above 10 keV, XTE offers detectors with background sufficiently low to detect 0.2 milliCrab sources.
4. XTE has the capability to produce X-ray data simultaneous or quasi-simultaneous with radio, optical, UV, other X-ray, or gamma-ray observations, XTE can point anywhere outside of 30 degrees to the sun and can be scheduled to support other observations within a few days of the observation (although planning ahead will, in general, be necessary).
5. XTE can observe an unanticipated “target of opportunity”, the relatively flexible XTE observing program will accommodate scientifically compelling Targets of Opportunity (TOOs).


The three instruments on XTE are the Proportional Counter Array (PCA), co-pointed with detectors on the High Energy X-Ray Timing Experiment (HEXTE), and the All-Sky Monitor (ASM). The mission carries two pointed instruments, the Proportional Counter Array (PCA) developed by GSFC to cover the lower part of the energy range, and the High Energy X-ray Timing Experiment (HEXTE) developed by UCSD covering the upper energy range. HEXTE has half its area co-pointed with the PCA, with the other half accumulating background from nearby positions. These instruments are equipped with collimators yielding a FWHM of one degree. In addition, RXTE carries an All-Sky Monitor (ASM) from MIT that scans about 80% of the sky every orbit, allowing monitoring at time scales of 90 minutes or longer. Data from PCA and ASM are processed on board by the Experiment Data System (EDS), also built by MIT

Proportional Counter Array (PCA)

The Large Area X-ray Proportional Counter (LAPC) provides for the study of temporal/spectral effects in the X-ray emission from galactic and extragalactic sources. The PCA is an array of five proportional counters with a total collecting area of 6500 square cm.

The High Energy X-ray Timing Experiment (HEXTE)

The High-Energy X-ray Timing Experiment (HEXTE) is a scintillator array for the study of temporal and temporal/spectral effects of the hard X-ray (20 to 200 keV) emission from galactic and extragalactic sources.

The HEXTE consists of two clusters each containing four `phoswich scintillation detectors. Each cluster can `rock' (beamswitch) along mutually orthogonal directions to provide background measurements 1.5 or 3.0 degrees away from the source every 16 to 128 s. Automatic gain control is provided by using a 241Am radioactive source mounted in each detector's field of view.

All-Sky Monitor (ASM)

The All-Sky Monitor (ASM) provides all-sky X-ray coverage, to a sensitivity of a few percent of the Crab Nebula intensity in one day, in order to provide both flare alarms and long-term intensity records of celestial X-ray sources.

The ASM consists of three wide-angle shadow cameras equipped with proportional counters with a total collecting area of 90 square cm.

Experiment Data System (EDS)

The EDS consists of eight Event Analyzers (EA), of which six are dedicated to the PCA and two to the ASM. Each EA contains an Intel 80286 processor and associated memory. The EAs can be programmed independently to process incoming events from the instruments in many modes.


Proportional Counter Array (PCA)

The instrumental properties are:
Energy range: 2 - 60 keV
Energy resolution: < 18% at 6 keV
Time resolution: 1 microsec
Spatial resolution: collimator with 1 degree FWHM
Detectors: 5 proportional counters
Collecting area: 6500 square cm
Layers: 1 Propane veto; 3 Xenon, each split into two; 1 Xenon veto layer
Sensitivity: 0.1 mCrab
Background: 2 mCrab
Events detected by the PCA will be processed on board by the EDS before insertion into the telemetry stream. The PCA was built by the EUD (formerly 'LHEA') at GSFC.

The High Energy X-ray Timing Experiment (HEXTE)

The HEXTE's basic properties are:
• Energy range: 15 - 250 keV
• Energy resolution: 15% at 60 keV
• Time sampling: 8 microsecond
• Field of view: 1 degree FWHM
• Detectors: 2 clusters of 4 NaI/CsI scintillation counters
• Collecting area: 2 times 800 cm^2
• Sensitivity: 1 Crab = 360 count/s per HEXTE cluster
• Background: 50 count/s per HEXTE cluster
Events detected by HEXTE will be processed on board by its own data system before insertion into the telemetry stream at an average data rate of 5 kbit/s. Data products include event mode, binned spectra and light curves, and a burst-triggered event buffer. The HEXTE was designed and built by the Center for Astrophysics & Space Sciences (CASS) at the University of California, San Diego.

All-Sky Monitor (ASM)

The instrumental properties are:
• Energy range: 2 - 10 keV
• Time resolution: 80% of the sky every 90 minutes
• Spatial resolution: 3' x 15'
• Number of shadow cameras: 3, each with 6 x 90 degrees FOV
• Collecting area: 90 square cm
• Detector: Xenon proportional counter, position-sensitive
• Sensitivity: 30 mCrab
Events detected by the ASM will be processed on board by the EDS before insertion into the telemetry stream.

Experiment Data System (EDS)

EDS can process events in any of the following modes:
• Transparent mode, using 1, 2, or 3 EAs
• Event mode, using 1 or 2 EAs
• Binned mode (time and/or energy)
• Burst catcher mode
• Fourier transform mode
• Pulsar fold mode
• Autocorrelation mode


Rossi's mission has the primary objective to study the temporal and broad-band spectral phenomena associated with stellar and galactic systems containing compact objects in the energy range 2--200 keV and in time scales from microseconds to years.

Some pulsars spin faster than a thousand times a second. And a neutron star produces a gravitational pull so powerful that a marshmallow striking the star's surface would hit with the force of a thousand hydrogen bombs.

Astronomers study changes that happen from microseconds to months in cosmic objects to learn about how gravity works near black holes, how pulsars in binary systems are affected by mass transferring from one star to the other, and how the giant engines in distant galaxies are powered.

For RXTE, the trick to observing these kinds of objects is all in the timing -- an ability to observe changes in X-ray brightness that occur in a mere thousandths of a second, or over several years. Learn more about how this one-of-a-kind satellite has reshaped our understanding of what goes on in the most violent and bizarre regions of the Universe.

Mission Objectives

General Objectives

The objectives of RXTE are to investigate:
• periodic, transient, and burst phenomena in the X-ray emission from a wide variety of objects,
• the characteristics of X-ray binaries, including the masses of the stars, their orbital properties, and the exchange of matter between them,
• the inner structure of neutron stars, and properties of their magnetic fields,
• the behavior of matter just before it falls into a black hole,
• effects of general relativity which can be seen only near a black hole,
• properties and effects of super-massive black holes in the centers of active galaxies,
• and the mechanisms which cause the emission of X-rays in all these objects.

For the Future

Proposed future observations emphasize opportunity to discover and study additional millisecond pulsars, pursue the high frequency quasi-periodic oscillations in black hole transients, and connect high frequency phenomena with longer-term characteristics. RXTE will continue to strongly support, for both galactic and extragalactic targets, combining RXTE observations with other wavelengths (from IR to TeV) or with other capabilities, such as high spectral resolution.

Mission Summary

ROSSI has been extremely successful by any objective measure. It has opened up new windows on high energy processes and the objects that create them. It has greatly increased our understanding of Black Holes and Pulsars. It has helped to answer many questions and so has contributed to the increasing accuracy in our theoretical models of high energy phenomena. It has long outlasted its original life expectancy and mission objectives and there is no reason not to believe it will continue to do so into the foreseeable future. Certainly the data collected to date will keep theoreticians busy for literally decades to come.


The XTE orbit is at an altitude of 580 km with an inclination of about 23 degrees and a period of about 100 minutes. For many targets there may be 15-30 minute gaps in coverage due to earth occultation and the satellite's passage of the South Atlantic Anomaly regions of high charged particle density. The gaps will be minimized by scheduling when longer, uninterrupted data trains are important.

Orbital Parameters
Periapsis409.0 km
Apoapsis 409.0 km
Period 92.5999984741211 minutes
Inclination 28.5°
Eccentricity 0.0 km

Comming Soon


The spacecraft was designed and built by the Engineering Directorate at NASA Goddard Space Flight Center. The Rossi X-ray Timing Explorer (RXTE) is a satellite that observes the fast-moving, high-energy worlds of black holes, neutron stars, X-ray pulsars and bursts of X-rays that light up the sky and then disappear forever.

The scientific instruments consists of two pointed instruments, the Proportional Counter Array (PCA) and the High-Energy X-ray Timing Experiment (HEXTE), and the All Sky Monitor (ASM), which scans over 70% of the sky each orbit.All of the XTE observing time will be available to the international scientific community through a peer review of submitted proposals. XTE uses a new spacecraft design that allows flexible operations through rapid pointing, high data rates, and nearly continuous receipt of data at the Science Operations Center at Goddard Space Flight Center via a Multiple Access link to the Tracking and Data Relay Satellite System (TDRSS). XTE is highly maneuverable with a slew rate of greater than 6 degrees per minute.

The PCA/HEXTE can be pointed anywhere in the sky to an accuracy of less than 0.1 degree, with an aspect knowledge of around 1 arc-minute. Rotatable solar panels enable anti-sunward pointing to coordinate with ground-based night-time observations. Two pointable high gain antennas maintain nearly continuous communication with the TDRSS. This, together with 1 GB (approximately four orbits) of on-board solid-state data storage, give added flexibility in scheduling observations.

The Rossi Spacecraft
Rossi Spacecraft Schematic

                         HEXTE Characteristics
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Comming Soon