Astronomers find X-ray pulsar in record-fast orbit

By Brooks Hays  |  May 11, 2018 at 10:09 AM
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May 11 (UPI) -- Scientists have found a pair of stars orbiting one another at record speed.

The duo orbit each other once every 38 minutes, the shortest-known orbital period for its specific class of pulsar binary systems.

The unusual pair, named J17062, was discovered while interrogating the first batch of data recorded by the Neutron star Interior Composition Explorer mission, or NICER mission.

According to researchers, the system is made up of a rapidly spinning, superdense neutron star called an accreting millisecond X-ray pulsar and a hydrogen-poor white dwarf.

"It's not possible for a hydrogen-rich star, like our sun, to be the pulsar's companion," Tod Strohmayer, an astrophysicist at NASA's Goddard Space Flight Center, said in a news release. "You can't fit a star like that into an orbit so small."

Neutron stars are the extremely hot, superdense stellar cores sometimes left over after supernovae. The stars are so hot, they emanate X-rays. Their violent spin and its accretion from its companion causes intense beams of X-rays to pulse from their poles.

NICER data showed J17062 pulses 163 times per second, which means the pulsar is spinning at a speed of 9,800 revolutions per minute. Though J17062 boasts a record orbit, its spin rate is average. Some pulsar spin some 700 revolutions per second, several orders of magnitude greater than J17062 spin rate.

The hotspots that produce the neutron star's polar pulses are created by the accretion of material from its companion, the hydrogen-poor white dwarf. Stolen material is pulled into an accretion disk, much like a black hole. Because the pulsar has an intense magnetic field, the material in the accretion disk are pulled into the stellar core unevenly, creating hotspots.

Over time, the pulsar will accrue more material than it can handle. The excess mass will eventually trigger a thermonuclear reaction, a massive release of energy in the form of X-rays. NICER can detect such releases, but has yet to measure an outburst from J17062.

While the pulsar's donor star is rather puny, it still has a small effect on the orbital path of J17062. That the slight perturbation can be measured is a testament to the sensitivity of NICER's instruments.

"The distance between us and the pulsar is not constant," Strohmayer said. "It's varying by this orbital motion. When the pulsar is closer, the X-ray emission takes a little less time to reach us than when it's further away. This time delay is small, only about 8 milliseconds for J17062's orbit, but it's well within the capabilities of a sensitive pulsar machine like NICER."

Strohmayer and his colleagues detailed their investigation of J17062 in a new paper, published this week in the Astrophysical Journal.

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