Binary star system is composed of two stellar remnants in rapid orbit

Rapidly-spinning pulsar is pulling in material from its white dwarf companion.
By Laurel Kornfeld | Dec 13, 2018
A binary star system discovered by NASA's Neutron star Interior Composition Explorer (NICER) is composed of two stellar remnants, a pulsar and a white dwarf, that orbit each other every 38 minutes and are separated by just 186,000 miles, a distance smaller than that between the Earth and Moon.

Designated IGR J17062-6143, the double star system was first observed in 2008 by the Rossi X-ray Timing Explorer (RXTE) over a period of 20 minutes; however, RXTE was unable to determine the binary's orbital speed.

NICER, installed on the International Space Station (ISS) and capable of observing the system for much longer time periods, pinned down its orbital speed and also measured the spin rate of the pulsar.

Pulsars are rapidly spinning neutron stars, which are remnants of massive stars that died in supernova explosions. This particular pulsar is extremely dense and rotates about 9,800 times per minute, making it a millisecond pulsar.

As it spins, the pulsar emits X-rays at varying levels of intensity from its magnetic poles. From these X-rays, scientists can determine how rapidly the pulsar is spinning.

"Neutron stars turn out to be truly unique nuclear physics laboratories, from a terrestrial standpoint," said NICER astrophysicist Zaven Arzoumanian of NASA's Goddard Space Flight Center. "We can't recreate the conditions on neutron stars anywhere within our solar system. One of NICER's key objectives is to study subatomic physics that isn't accessible anywhere else."

As a pulsar sucks in material from a companion through its strong gravitational pull, hot spots form where this material is pulled into an accretion disk. Eventually, the disk of material spirals onto the pulsar's surface, where it lands unevenly. The more material the pulsar accretes, the faster it spins.

When the pulsar spins, these hot spots rotate into and out of NICER's view.

"The distance between us and the pulsar is not constant," noted Tod Strohmayer, also an astrophysicist at Goddard. "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 eight milliseconds for J17062's orbit, but it's well within the capabilities of a sensitive pulsar machine like NICER."

The companion white dwarf is the stellar remnant of a less massive star that scientists believe contained very little hydrogen.

"It's not possible for a hydrogen-rich star, like our Sun, to be the pulsar's companion," Strohmayer said. "You can't fit a star like that into an orbit so small."

While the white dwarf has just 1.5 percent of the Sun's mass, the pulsar is approximately 1.4 solar masses, resulting in the two stars orbiting a point 1,900 miles (3,000 km) from the pulsar.

Findings of the study have been published in The Astrophysical Journal Letters.


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