This is the fastest orbit ever found in a binary star system that contains an X-ray pulsar.
The first readings from NASA’s Neutron star Interior Composition Explorer (NICER) mission are in and they’re already blowing astronomers’ minds. For the first time ever, NICER detected a binary star system in which an X-ray pulsar is orbited by a white dwarf in only 38 minutes.
This has now become the fastest-known orbital period for this type of binary star system, NASA explains in a news release. The system in question is called IGR J17062–6143 (J17062 for short) and is located about 16,300 light-years away from our planet, Sci News reports.
Pulsars are superdense neutron stars — objects with extremely dense matter that are close to collapsing into black holes — which spin at incredible speeds and appear to pulse at Earth. The pulsar in J17062 falls into the category of accreting millisecond X-ray pulsars (AMXP), which emit light that passes into the X-ray portion of the magnetic spectrum and have a rotational period that doesn’t exceed 10 milliseconds.
J17062’s pulsar spins about 9,800 times per minute and shoots out X-rays with a frequency of 163 times per second. The star system was first discovered in 2006 and has been studied before with NASA’s Rossi X-ray Timing Explorer, but only NICER was able to capture the record-fast orbit of J17062’s stars.
“Neutron stars turn out to be truly unique nuclear physics laboratories, from a terrestrial standpoint,” said NICER lead scientist Zaven Arzoumanian, an astrophysicist at 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,” he pointed out.
NICER uncovered that the superdense pulsar is orbited by a “lightweight” white dwarf, no bigger than 1.5 percent the mass of our sun, that gallops around it completing a full orbit in 38 minutes flat. By comparison, the X-ray pulsar is a lot heavier and weighs around 1.4 solar masses.
Being substantially more massive, the pulsar has a strong gravity pull, that steals away material from the white dwarf and draws it into a surrounding accretion disk (pictured above). This is why the pulsar is defined as “accreting.”
NICER’s discoveries about the J17062 binary star system are detailed in study just published in The Astrophysical Journal Letters.
Apart from the stellar “thievery,” which has been known to occur before, another striking thing about the J17062 binary system is that it’s ultracompact. The two stars sit very closely to one another, at a distance of only about 186,000 miles (300,000 km). This is less than the lunar distance, which means that the pulsar and its white dwarf companion would fit quite comfortably between the Earth and the moon.
This very short distance between the two stars is why astronomers deduced that the pulsar’s companion must be a hydrogen-poor white dwarf in the first place.
“It’s not possible for a hydrogen-rich star, like our sun, to be the pulsar’s companion,” said lead study author Tod Strohmayer, an astrophysicist at Goddard.
“You can’t fit a star like that into an orbit so small,” he added.
The big difference in mass between J17062’s two stars has led astronomers to believe that they orbit a point located around 1,900 miles (3,000 km) from the X-ray pulsar. Their orbital motion also impacts the distance between the pulsar and our planet, which Strohmayer notes that it’s “not constant” but actually “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,” he explained.
“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,” said Strohmayer.
The new data about the J17062 star system comes from more than seven hours of observations performed last August by NICER over a period of five and a half days. Coupled with additional observations made in October and November, the data was recently analyzed by Strohmayer’s team, which came across these remarkable findings.
When NICER started observing J17062 in August, the mission had just been launched for nearly two months. Installed on the International Space Station in June 2017, NICER is tasked with taking high-precision measurements of neutron stars.
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