The X-ray Legacy of RXTE

The RXTE satellite, which reentered Earth's atmosphere over the tropics on April 30th, leaves behind a legacy of discoveries and data.

RXTE in orbit

With a mass of 3½ tons, NASA's Rossi X-ray Timing Explorer circles Earth in an orbit 375 miles (600 km) high. It ceased operation in early 2012 after 16 years of observations.
NASA / GSFC

On April 30th the Rossi X-ray Timing Explorer (RXTE), a satellite launched on December 30, 1995, finally came back home to Earth. RXTE’s projected lifetime was five years, but it operated for 16 years and was decommissioned in 2012.

“RXTE was unique in several ways,” explains NASA’s Tod Strohmayer, who was part of the RXTE team. “It was a ‘big telescope,’ meaning it had a very large collecting area, so as to obtain as many X-rays from targets as possible.” The satellite’s primary targets were neutron stars and black holes in binary systems with normal stars.

Black holes are invisible on their own, but when they’re in a mutual orbit with a star, the black hole’s gravity can pull matter off the star and into an accretion disk, which lights up with X-rays.

The X-ray emissions depend on how much gas the black hole is feeding on, becoming brighter and dimmer on surprisingly short timescales. RXTE measured these fluctuations, which told scientists how hot the accretion disk is and how fast the material is orbiting around the black hole.

Black holes are simple creatures, their nature determined entirely by two things: their spin and their mass. Mass is easily determined by watching the orbit of the companion star, but spin is another matter. If a black hole is spinning, it’ll drag spacetime — and the matter in it — as it spins, an effect called frame-dragging. This causes the inner disk to wobble in a way predicted by Einstein’s theory of general relativity. In addition, matter can come closer to the event horizon of a spinning black hole before falling in entirely. Relativity predicts this relation as well. RXTE observations enabled astronomers to use relativity’s predictions to witness frame-dragging and measure the spin of stellar-mass black holes.

RXTE also observed neutron stars, allowing astronomers to measure their magnetic fields by their X-ray activity. In the process, RXTE helped astronomers discover magnetars, a rare type of neutron star with 1,000 times stronger magnetic fields.

The Legacy Continues

Bursting Pulsar before transition (artist's concept)

A spinning neutron steals matter from its companion star in this artist's concept. But this flow won't last forever. The so-called Bursting Pulsar may be one object in a transitional phase, where the flow of matter, and the X-rays it produces, are sputtering out.
University of Southampton

Despite the satellite’s demise, RXTE data is still fueling new discoveries. Researchers at the University of South Hampton recently used archived RXTE observations to better understand the so-called Bursting Pulsar (GRO J1744-28), a unique type of transitional pulsar.

Regular transitional pulsars — spinning neutron stars — operate much like binary black holes do, stripping matter off their companion star. As the matter approaches the neutron star’s surface, it heats up and emits X-rays. But this flow of matter can’t last forever, and as it tapers off, X-ray emission sputters out. The Bursting Pulsar, however, spins about 100 times slower than other transitional pulsars; meanwhile its magnetic field is 100 times stronger than its siblings. The magnetic field may be what causes the neutron star to “hiccup,” swallowing the stolen matter in bursts.

The addition of the Bursting Pulsar to the class of transitional pulsars enables astronomers to test their understanding of these rare systems. And RXTE is still doing its part — six years after the mission ended.

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