Rossi X-ray Timing Explorer (RXTE)

The Rossi X-ray Timing Explorer, named after MIT astronomer Bruno Rossi, probed the physics of cosmic X-ray sources by making sensitive measurements of their variability over time scales ranging from milliseconds to years. How these sources behave over time is a source of important information about processes and structures in white dwarf stars, X-ray binaries, neutron stars and black holes. RXTE launched on Dec. 30, 1995, atop a Delta II rocket into low-Earth orbit (600 km altitude and 23-degree inclination). RXTE could maneuver quickly to point its instruments at a source, which allowed it to study short-lived or new sources as they were discovered. With instruments sensitive to a wide range of X-ray energies (from 2,000 to 250,000 electron volts), RXTE operated for 16 years before being decommissioned in 2012; the satellite re-entered Earth’s atmosphere on April 30, 2018. The astronomical community has recognized the importance of RXTE research with five major awards. These include four Bruno Rossi Prizes (1999, 2003, 2006 and 2009) from the High Energy Astrophysics Division of the American Astronomical Society and the 2004 NWO Spinoza Prize, the highest Dutch science award, from the Netherlands Organization for Scientific Research.

Additional RXTE highlights

Rossi X-ray Timing Explorer Learning Center

Content Contact:

Multimedia stories including RXTE results

  • NASA's RXTE Satellite Catches the Beat of a Midsize Black Hole
    Astronomers from the University of Maryland, College Park (UMCP) and NASA's Goddard Space Flight Center have uncovered rhythmic pulsations from a rare breed of black hole in archival data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite. The signals provide compelling evidence that the object, known as M82 X-1, is one of only a few midsize black holes known. Dying stars form modest black holes measuring up to around 25 times the mass of our sun. At the opposite extreme, most large galaxies contain a supermassive black hole with a mass tens of thousands of times greater. Just as drivers traveling a highway packed with compact cars and monster trucks might start looking for sedans, astronomers are searching for a middle range of the black hole population and wondering why they see so few. M82 X-1 is the brightest X-ray source in Messier 82, a galaxy located about 12 million light-years away in the constellation Ursa Major. While astronomers have suspected the object of being a midsize, or intermediate-mass, black hole for at least a decade, estimates have varied from 20 to 1,000 solar masses, preventing a definitive classification. Working with Mushotzky and Strohmayer, UMCP graduate student Dheeraj Pasham sifted through about 800 RXTE observations of M82 in a search for specific types of brightness changes that would help pin down the mass of the X-ray source. As gas streams toward the black hole it piles up into a disk around it. Friction within the disk heats the gas to millions of degrees, which is hot enough to emit X-rays. Cyclical intensity variations in these X-rays reflect processes occurring within the disk. Scientists think the most rapid changes occur near the inner edge of the disk on the brink of the black hole's event horizon, the point beyond which nothing, not even light, can escape. With such close proximity to the black hole, the effects of Einstein's general relativity come into play, resulting in X-ray variations that repeat at nearly regular intervals. Astronomers call these signals quasi-periodic oscillations, or QPOs, and have shown that for black holes produced by stars, their frequencies scale up or down depending on the size of the black hole. When astronomers study X-ray fluctuations from many stellar-mass black holes, they see both slow and fast QPOs, but the fast ones often come in pairs with a specific 3:2 rhythmic relationship. For every three flashes from one member of the QPO pair, its partner flashes twice. The combined presence of slow QPOs and a faster pair in a 3:2 rhythm effectively sets a standard scale that gives scientists a powerful tool for establishing the masses of stellar black holes. A decade ago, Strohmayer and Mushotzky showed the presence of slow QPO signals from M82 X-1. In order to apply the tried-and-true relationship used for stellar-mass black holes, the researchers needed to identify a pair of steady fluctuations exhibiting the same 3:2 beat in RXTE observations. By analyzing six years of data, they located X-ray variations that reliably repeated about 3.3 and 5.1 times each second, just the 3:2 relationship they needed. This allowed them to calculate that M82 X-1 weighs about 400 solar masses — the most accurate determination to date for this object and one that clearly places it in the category of intermediate-mass black holes. Read the paper at Read the press release at
  • NASA's RXTE Helps Pinpoint Launch of 'Bullets' in a Black Hole's Jet
    Using observations from NASA's Rossi X-ray Timing Explorer (RXTE) satellite and the National Science Foundation's (NSF) Very Long Baseline Array (VLBA) radio telescope, an international team of astronomers has identified the moment when a black hole in our galaxy launched superfast knots of gas into space.

    Racing outward at about one-quarter the speed of light, these "bullets" of ionized gas are thought to arise from a region located just outside the black hole's event horizon, the point beyond which nothing can escape.

    The research centered on the mid-2009 outburst of a binary system known as H1743-322, located about 28,000 light-years away toward the constellation Scorpius. Discovered by NASA's HEAO-1 satellite in 1977, the system is composed of a normal star and a black hole of modest but unknown masses.

    Their orbit around each other is measured in days, which puts them so close together that the black hole pulls a continuous stream of matter from its stellar companion. The flowing gas forms a flattened accretion disk millions of miles across, several times wider than our sun, centered on the black hole. As matter swirls inward, it is compressed and heated to tens of millions of degrees, so hot that it emits X-rays.

    Some of the infalling matter becomes re-directed out of the accretion disk as dual, oppositely directed jets. Most of the time, the jets consist of a steady flow of particles. Occasionally, though, they morph into more powerful outflows that hurl massive gas blobs at significant fractions of the speed of light.

  • RXTE Detects 'Heartbeat' Of Smallest Black Hole Candidate
    Data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite has identified a candidate for the smallest-known black hole. The evidence comes from a specific type of X-ray pattern — nicknamed a "heartbeat" because of its resemblance to an electrocardiogram — that until now has been recorded in only one other black hole system.

    Named IGR J17091-3624 after the astronomical coordinates of its sky position, the binary system pairs a normal star with a black hole that may weigh less than three times the sun's mass, near the theoretical boundary where black-hole status first becomes possible. Flare-ups occur when gas from the normal star streams toward the black hole and forms a disk around it. Friction within the disk heats the gas to millions of degrees, which is hot enough to radiate X-rays.

    The record-holder for ubiquitous X-ray variability is another black hole binary named GRS 1915+105. This system is unique in displaying more than a dozen highly structured patterns — typically lasting between seconds and hours — that scientists distinguish by Greek-letter names. Seven of these patterns are now seen in IGR J17091, including the so-called rho-class oscillations that astronomers describe them as the "heartbeat" of black hole systems.

    It's thought that strong magnetic fields near the black hole's event horizon eject some of the gas into dual, oppositely directed jets that blast outward at nearly the speed of light. The peak of its heartbeat emission corresponds to the emergence of the jet. Changes in the X-ray spectrum observed by RXTE during each beat in GRS 1915 reveal that the innermost region of the disk emits enough radiation to push back the gas, creating a strong outward wind that staunches the inward flow, briefly starving the black hole and shutting down the jet. This corresponds to the faintest emission. Eventually the inner disk gets so bright and so hot that it essentially disintegrates and plunges toward the black hole, re-establishing the jet and beginning the cycle anew.

    In GRS 1915+105, which at 14 solar masses is by for the more massive of the two, this cycle can take as little as 40 seconds. In IGR J17091, the emission can be 20 times fainter than GRS 1915, and the heartbeat cycle can occur up to eight times faster.

    Download the animations here.

  • RXTE Sees Eclipses from Fast X-ray Pulsar
    Astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) have found the first fast X-ray pulsar to be eclipsed by its companion star. Further studies of this unique stellar system will shed light on some of the most compressed matter in the universe and test a key prediction of Einstein's relativity theory.

    Known as Swift J1749.4-2807 — J1749 for short — the system erupted with an X-ray outburst on April 10. During the event, RXTE observed three eclipses, detected X-ray pulses that identified the neutron star as a pulsar, and even recorded pulse variations that indicated the neutron star's orbital motion. More information here.

  • RXTE Views X-ray Pulsar Occulted by the Moon
    On October 13, 2010, the Rossi X-ray Timing Explorer (RXTE), a satellite in low-Earth orbit, observed the x-ray pulsar IGR J17480-2446 as it passed behind the Moon. This was an unusual opportunity to calculate the precise position of the pulsar by using the times at which it disappeared and reappeared at the edge of the Moon's disk. As shown in this animation, ingress (the moment when the pulsar disappeared) occurred on the Moon's eastern limb just above the equator. Egress, 8 minutes 32 seconds later, was near the south pole on the western limb. The timing of ingress and egress depended delicately on the shape of the terrain. In other words, it mattered whether the pulsar passed behind a mountain or a valley. So the calculation relied on the detailed topography measured by both JAXA's Kaguya and NASA's Lunar Reconnaissance Orbiter (LRO). The animation faithfully reproduces the angle of the Sun, the position of RXTE, the position and orientation of the Moon as seen from the satellite, the Moon's topography, and the starry background. RXTE's position was derived from the Goddard Flight Dynamics Facility ephemeris for day 6129 of the satellite's orbit, while the Sun and Moon positions came from JPL's DE421 solar system ephemeris. All of the positions and the viewing direction were transformed into Moon body-fixed coordinates, so that in the animation software, the Moon remained stationary at the origin, while the camera moved and pointed appropriately. The Moon, the stars, the pulsar, and the clock were all rendered separately and layered together. (A note about the name of the pulsar: The IGR prefix refers to INTEGRAL, the satellite that discovered it. The numbers are its position in right ascension (hours, minutes) and declination (degrees, minutes). So the pulsar is at J2000 right ascension 17h 48m, declination -24°46'.)
  • The Cloudy Cores of Active Galaxies
    At the hearts of most big galaxies, including our own Milky Way, there lurks a supermassive black hole weighing millions to billions of times the sun's mass. As gas falls toward a supermassive black hole, it gathers into a so-called accretion disk and becomes compressed and heated, ultimately emitting X-rays. The centers of some galaxies produce unusually powerful emission that exceeds the sun's energy output by billions of times. These are active galactic nuclei, or AGN.

    Using data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite, an international team has uncovered a dozen instances where X-ray signals from active galaxies dimmed as a result of a cloud of gas moving across our line of sight. The new study triples the number of cloud events previously identified in the 16-year archive.

    The study is the first statistical survey of the environments around supermassive black holes and is the longest-running AGN-monitoring study yet performed in X-rays. Scientists determined various properties of the occulting clouds, which vary in size and shape but average 4 billion miles (6.5 billion km) across – greater than Pluto's distance from the sun — and twice the mass of Earth. They orbit a few light-weeks to a few light-years from the black hole.

  • RXTE Data Link Pulsar Pulses with a QPO
    A quasi-periodic oscillation, or QPO, is a flicker of X-ray light from an astronomical object that hovers around certain frequencies. The X-rays are thought to be emitted near the inner edge of an accretion disk where gas falls onto a compact object such as a white dwarf, neutron star (also known as a pulsar) or black hole. For pulsars like SAX J1808.43658 (SAX J1808 for short), gas channeled onto the neutron star’s magnetic poles creates hot spots that are a strong source of X-rays. The object rotates 401 times a second, and as its hot spots wheel into view from Earth, spacecraft like NASA’s Rossi X-ray Timing Explorer (RXTE) detect strong pulses. RXTE also detects a QPO flickering between 300 and 700 times a second. For the first time, RXTE observations have shown that the pulses and the QPO have a direct relationship, providing insight into the inner structure of the accretion disk. The pulses from the hot spots are twice as bright when the QPO frequency matches or is faster than the pulsar’s spin, and its brightness dims by the same amount when the QPO fluctuates more slowly than the pulsar’s rotation. RXTE observed these changes during outbursts in 2002, 2005 and 2008. This result strongly suggests that the QPO is a region of especially hot gas at the inner edge of the accretion disk and that its fluctuations trace its orbital motion. When the QPO orbits more slowly than the neutron star’s spin, the flow of matter onto the pulsar becomes inhibited by the pulsar’s magnetic field. During an outburst, the inner edge of the disk is forced closer to the pulsar, resulting in a faster-moving QPO and compression of the magnetic field. When the QPO matches or bests the pulsar’s 401 hertz spin, the flow of matter onto the neutron star is enhanced, with more gas reaching the magnetic poles, which produce brighter pulses. During these episodes, matter may also flow directly onto the pulsar's equatorial regions (lateral accretion).
  • RXTE Photos
    Technicians work on RXTE in 1995.

    Credit: NASA’s Goddard Space Flight Center

  • NASA Missions Take an Unparalleled Look into Superstar Eta Carinae
    Eta Carinae is a binary system containing the most luminous and massive star within 10,000 light-years. A long-term study led by astronomers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, combined data from NASA satellites, ground-based observing campaigns and theoretical modeling to produce the most comprehensive picture of Eta Carinae to date. New findings include Hubble Space Telescope images that show decade-old shells of ionized gas racing away from the largest star at a million miles an hour, and new 3-D models that reveal never-before-seen features of the stars' interactions. Located about 7,500 light-years away in the southern constellation of Carina, Eta Carinae comprises two massive stars whose eccentric orbits bring them unusually close every 5.5 years. Both produce powerful gaseous outflows called stellar winds, which enshroud the stars and stymy efforts to directly measure their properties. Astronomers have established that the brighter, cooler primary star has about 90 times the mass of the sun and outshines it by 5 million times. While the properties of its smaller, hotter companion are more contested, Goddard's Ted Gull and his colleagues think the star has about 30 solar masses and emits a million times the sun's light. At closest approach, or periastron, the stars are 140 million miles (225 million kilometers) apart, or about the average distance between Mars and the sun. Astronomers observe dramatic changes in the system during the months before and after periastron. These include X-ray flares, followed by a sudden decline and eventual recovery of X-ray emission; the disappearance and re-emergence of structures near the stars detected at specific wavelengths of visible light; and even a play of light and shadow as the smaller star swings around the primary. During the past 11 years, spanning three periastron passages, the Goddard group has developed a model based on routine observations of the stars using ground-based telescopes and multiple NASA satellites. According to this model, the interaction of the two stellar winds accounts for many of the periodic changes observed in the system. The winds from each star have markedly different properties: thick and slow for the primary, lean and fast for the hotter companion. The primary's wind blows at nearly 1 million mph and is especially dense, carrying away the equivalent mass of our sun every thousand years. By contrast, the companion's wind carries off about 100 times less material than the primary's, but it races outward as much as six times faster. The images and videos on this page include periastron observations from NASA's Rossi X-ray Timing Explorer, the X-Ray Telescope aboard NASA's Swift, the Hubble Space Telescope's STIS instrument, and computer simulations. See the captions for details.
  • A Flickering X-ray Candle
    The Crab Nebula, created by a supernova seen nearly a thousand years ago, is one of the sky's most famous "star wrecks." For decades, most astronomers have regarded it as the steadiest beacon at X-ray energies, but data from orbiting observatories show unexpected variations, showing astronomers their hard X-ray "standard candle" isn't as steady as they once thought. From 1999 to 2008, the Crab brightened and faded by as much as 3.5 percent a year, and since 2008, it has faded by 7 percent. The Gamma-ray Burst Monitor on NASA's Fermi satellite first detected the decline, and Fermi's Large Area Telescope also spotted two gamma-ray flares at even higher energies. Scientists think the X-rays reveal processes deep within the nebula, in a region powered by a rapidly spinning neutron star — the core of the star that blew up. But figuring out exactly where the Crab's X-rays are changing over the long term will require a new generation of X-ray telescopes.
  • Fermi finds the first extragalactic gamma-ray pulsar
    Researchers using NASA's Fermi Gamma-ray Space Telescope have discovered the first gamma-ray pulsar in a galaxy other than our own. The object sets a new record for the most luminous gamma-ray pulsar known. The pulsar lies in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a small galaxy that orbits our Milky Way and is located 163,000 light-years away. The Tarantula Nebula is the largest, most active and most complex star-formation region in our galactic neighborhood. It was identified as a bright source of gamma rays, the highest-energy form of light, early in the Fermi mission. Astronomers initially attributed this glow to collisions of subatomic particles accelerated in the shock waves produced by supernova . However, the discovery of gamma-ray pulses from a previously known pulsar named PSR J0540-6919 shows that it is responsible for roughly half of the gamma-ray brightness previously thought to come from the nebula. Gamma-ray pulses from J0540-6919 have 20 times the intensity of the previous record-holder, the pulsar in the famous Crab Nebula. Yet they have roughly similar levels of radio, optical and X-ray emission. Accounting for these differences will guide astronomers to a better understanding of the extreme physics at work in young pulsars.