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RXTE mission graphic

Rossi X-ray Timing Explorer

Phase: Past

Launch Date: December 30, 1995

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Program(s):Astrophysics Explorers, Explorers

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Over the course of its 16 year mission, NASA's Rossi X-ray Timing Explorer (RXTE) revolutionized our view of the dynamic X-ray sky. With a powerful and unique combination of large collecting area, broad-band spectral coverage, high time resolution and highly flexible observing, RXTE's capabilities led to many breakthroughs in our physical understanding of the extreme environments of accreting compact objects, including neutron stars and Galactic and extragalactic black holes.

How fast and how energetic are these objects? Some pulsars spin faster than a kitchen blender, 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, how the giant engines in distant galaxies are powered, and how physics operates under the most extreme conditions in the universe.

RXTE used three separate but complementary detectors to observe the X-ray sky. The All Sky Monitor (ASM), built at the Massachusetts Institute of Technology (MIT), provided views of the whole X-ray sky about once per spacecraft orbit. Searching for new X-ray sources or changes in known ones, the ASM watched for new observing opportunities for RXTE's larger area detectors. The Proportional Counter Array (PCA), a set of five nearly-identical proportional counter detectors built at NASA's Goddard Space Flight Center, was RXTE's "workhorse" telescope, providing large area, low background and high time resolution for detailed studies of X-ray sources in the 2 - 60 keV energy band. The Higher Energy X-ray Timing Experiment (HEXTE), built at the University of California San Diego (UCSD), extended RXTE's fast timing capabilities to higher energies, providing sensitive spectroscopy from 20 - 200 keV.

RXTE was launched into low-Earth orbit on December 30, 1995. After launch it was renamed in honor of Bruno Rossi (and got the "R" added to its acronym), a famous Italian-American physicist instrumental in pioneering the detectors that enabled the first cosmic X-ray observations. After an extraordinarily productive 16 years of scientific discovery the RXTE observatory was decommissioned in January 2012.

Science Highlights

  • RXTE discovered millisecond X-ray oscillations from accreting neutron stars and black holes. These are the fastest oscillations known from cosmic sources and provide the closest view of black holes and neutron stars currently achievable. These oscillations come from matter which is within just a few miles of the surface of the neutron star or the black hole event horizon.

  • RXTE made the first accurate measurements of the magnetic fields of the most strongly magnetized neutron stars known. These neutron stars are known as "magnetars," and their magnetic fields can be 100 trillion times stronger than the Earth's magnetic field. RXTE also discovered new magnetars, and greatly expanded our understanding of this population of neutron stars.

  • RXTE revealed amazing new properties of neutron stars that produce powerful X-ray bursts caused by nuclear fusion of matter accumulating on the star. RXTE discovered that the spin rates of neutron stars could be seen during X-ray bursts, as the surface is heated unevenly by the nuclear fusion. Some of the neutron stars seen in this way are spinning faster than 600 times per second.

  • RXTE solved the long-standing puzzle of the origin of the fastest spinning radio pulsars. It was thought that these neutron stars were "recycled" from slow spin rates to fast by accreting matter in a binary system. The matter accreting onto the neutron star in this way can spin up the star. RXTE found the first such accreting millisecond X-ray pulsar being caught in the act of being recycled. Indeed, RXTE has discovered all of the known accreting millisecond X-ray pulsars.

  • RXTE made the first measurements of the characteristic variability timescales of supermassive black holes in active galactic nuclei (AGN). These black holes can be a billion times more massive than the Sun. By measuring these timescales for a number of different black holes RXTE found that the timescale increases in proportion to the mass of the black hole.

  • RXTE observations enabled the measurement of the spin rates of black holes. Matter orbiting a spinning black hole can approach more closely to it than a non-spinning hole. RXTE X-ray spectral measurements can determine the size of the accretion disk and how closely it approaches the black hole.

  • RXTE discovered fast oscillations during very powerful giant outbursts or flares from two magnetars. These oscillations appear to be associated with vibrations of the solid crust of neutron stars, and can be used to measure the thickness of the crust. The discovery of these X-ray oscillations were totally unexpected.

  • RXTE found the fastest oscillation yet seen from a black hole, at 450 cycles per second. This is so fast that in order for the oscillation to be caused by orbital motion near the black hole, the black hole must be spinning, otherwise the orbital frequency could not be high enough to account for the observed oscillation frequency.

  • RXTE made the first detailed measurements of a new kind of X-ray burst called a "superburst" that is 1,000 times more powerful than previously known bursts. These are caused by a much thicker and deeper layer of nuclear fuel that explodes on the neutron star. RXTE was able to see the superburst light reflected off of the material orbiting in an accretion disk very close to the surface of the neutron star. RXTE also saw for the first time the spin rate of a neutron star during a superburst.

Additional information on RXTE scientific discoveries can be found here,


Research based on RXTE observations and discoveries has been recognized with a number of significant awards, including four Rossi Prizes. The Rossi Prize is the highest award for "a significant contribution to High Energy Astrophysics," given by the High Energy Astrophysics Division (HEAD) of the American Astronomical Society (AAS). The four Rossi Prizes associated with RXTE or RXTE-related work are:

i) 1999 Jean Swank and Hale Bradt
"The 1999 Rossi Prize of the High Energy Astrophysics Division of the American Astronomical Society is awarded to Drs. Jean Swank and Hale Bradt for their key roles in the development of the Rossi X-Ray Timing Explorer, and for the resulting important discoveries related to high time resolution observations of compact astrophysical objects."

ii) 2003 Robert Duncan, Christopher Thompson, & Chryssa Kouveliotou
"The 2003 Rossi Prize of the High Energy Astrophysics Division of the American Astronomical Society is awarded to Robert Duncan and Christopher Thompson for their prediction, and to Chryssa Kouveliotou for her observational confirmation, of the existence of magnetars: neutron stars with extraordinarily strong magnetic fields."

iii) 2006 Tod Strohmayer, Deepto Chakrabarty, and Rudy Wijnands
"The 2006 Rossi Prize of the High Energy Astrophysics Division of the American Astronomical Society is awarded to Tod Strohmayer, Deepto Chakrabarty, and Rudy Wijnands for their pioneering research which revealed millisecond spin periods and established the powerful diagnostic tool of kilohertz intensity oscillations in accreting neutron star binary systems."

iv) 2009 Charles D. Bailyn, Jeffrey E. McClintock and Ronald A. Remillard
"The 2009 Rossi Prize of the High Energy Astrophysics Division of the American Astronomical Society is awarded to Charles D. Bailyn, Jeffrey E. McClintock and Ronald A. Remillard for their measurement of the masses of Galactic black holes."

Last updated: April 24, 2015