NASA’s Neutron Star Interior Composition Explorer Finds Pulsar in Ultracompact Orbit
Astronomers using NASA’s Neutron star Interior Composition Explorer (NICER) have found evidence that an accreting millisecond X-ray pulsar called IGR J17062–6143 (J17062 for short) resides in an ultracompact binary system. In this system, J17062 and its companion white dwarf star orbit the common center of mass in only 38 minutes.
J17062, also classified as SWIFT J1706.6−6146, is about 16,300 light-years from Earth.
The white dwarf star in the system is a ‘lightweight,’ only around 1.5% of our Sun’s mass. The pulsar is much heavier, around 1.4 solar masses.
The data from NICER show that J17062’s stars revolve around each other in a circular orbit, which is common for accreting millisecond X-ray pulsars.
The stars are only about 186,000 miles (300,000 km) apart, less than the distance between Earth and the Moon.
“The distance between us and the pulsar is not constant. It’s varying by this orbital motion,” said team leader Dr. Tod Strohmayer, an astrophysicist at NASA’s Goddard Space Flight Center.
“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.”
[youtube https://www.youtube.com/watch?v=B_JcarjiuDQ?rel=0]
Based on the pair’s breakneck orbital period and separation, Dr. Strohmayer and co-authors think the second star is a hydrogen-poor white dwarf.
“It’s not possible for a hydrogen-rich star, like our Sun, to be the pulsar’s companion. You can’t fit a star like that into an orbit so small,” Dr. Strohmayer said.
A previous 20-minute observation by NASA’s Rossi X-ray Timing Explorer (RXTE) in 2008 was only able to set a lower limit for J17062’s orbital period.
NICER, which was installed aboard the International Space Station in June 2017, has been able to observe the system for much longer periods of time. In August, the instrument focused on J17062 for more than seven hours over 5.3 days.
The 2008 RXTE observation of J17062 found X-ray pulses recurring 163 times a second. These pulses mark the locations of hot spots around the pulsar’s magnetic poles, so they allow astronomers to determine how fast it’s spinning. J17062’s pulsar is rotating at about 9,800 revolutions per minute.
“Hot spots form when a neutron star’s intense gravitational field pulls material away from a stellar companion — in J17062, from the white dwarf star — where it collects into an accretion disk,” the astronomers explained.
“Matter in the disk spirals down, eventually making its way onto the surface. Neutron stars have strong magnetic fields, so the material lands on the surface of the star unevenly, traveling along the magnetic field to the magnetic poles where it creates hot spots.”
“The constant barrage of in-falling gas causes accreting pulsars to spin more rapidly. As they spin, the hot spots come in and out of the view of X-ray instruments like NICER, which record the fluctuations. Some pulsars rotate over 700 times per second.”
The team’s results appear this week in the Astrophysical Journal Letters.
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T.E. Strohmayer et al. 2018. NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062-6143. ApJL 858, L13; doi: 10.3847/2041-8213/aabf44