For more than two decades, astronomers have been systematically tracking mysterious sources of high-energy gamma rays to their sources.
One, however, remained stubborn – the brightest unidentified source of gamma rays in the Milky Way. It seemed to come from a binary system 2,740 light-years away, but only one of the stars could be found.
Now astronomers have solved the mystery and fixed the other star by searching in gamma-ray data obtained between 2008 and 2018. Together, the two stars form one of the strangest binary systems we have ever seen.
“The binary star system and the neutron star in its heart, now known as PSR J1653-0158, set new records,” says astronomer Lars Nieder of the Albert Einstein Institute Hannover in Germany.
“We have discovered the galactic dance of a super-heavyweight with a flying weight: at a little more than twice as much as our sun, the neutron star is extremely heavy. Its companions have about six times the lead density, but only about 1 percent of the mass of our sun.
“This” odd pair “circuits every 75 minutes, faster than any known comparable binary.”
Since at least 2009, it has been thought that the gamma radiation detected from the system must be produced by a gamma ray pulse. Since 2014, X-ray and optical observations of the source of the gamma rays showed a variable star with a 75-minute period.
This was the small companion star, and astronomers believed that the 75-minute period was compatible with an orbital period in which the other star was the source of the gamma rays.
“But all the searches for the neutron star in it have so far been in vain,” said astronomer Colin Clark of the Jodrell Bank Center for Astrophysics at the University of Manchester in the United Kingdom.
The other star was considered a pulsar. It is a type of rapidly rotating neutron star that radiates from its poles as it spins. These rays are a bit like a lighthouse, flashing (or pulsing) past the observer as the star rotates. Radio pulsars are more common, but gamma radiation pulses are also known.
To confirm the presence of the other star, you must find the pulsations in time with its rotation. So the team went hard. They shattered a decade’s worth of gamma-ray data collected by the Large Area Telescope (LAT) aboard NASA’s Fermi Gamma-ray Space Telescope, using computing power donated by tens of thousands of members of the Einstein @ Home civil science program.
In just two weeks, they found their pulses.
That’s a little weird. The pulsar rotates extremely fast, more than 500 times per second. Millisecond pulsars rotate extremely fast; that’s what’s the millisecond ‘of their name. But the PSR J1653-0158 has one of the fastest rotational speeds ever seen in pulsars.
In addition, the star has an extremely weak magnetic field. It is within the lower three that have ever been discovered for the strength of magnetic pulsar magnet.
The companion is also quite strange, as it has an extremely low mass. The team believes it is a helium-white dwarf cannibalized by the pulsar and leaves a remnant. This type of system is known as a “black widow” binary.
“The rest of a dwarf star orbits the pulsar at just 1.3 times the Earth’s distance in just 75 minutes at a speed of more than 700 kilometers per second (435 miles per second),” Nieder said.
“This unusual duo may have originated in an extremely close binary system, where matter originally flowed from the accompanying star to the neutron star, increasing its mass and causing it to rotate faster and faster while attenuating the magnetic field.”
Above: Visualization of the system (bottom) compared to the earth and the moon (top).
This hypothesis is supported by the team’s search for radio waves. If the pulsar emits someone, we can not detect them. this may be because the system is surrounded by a dense cloud of material from the cannibalized dwarf star. Gamma radiation can penetrate this cloud, but not radio waves.
However, the PSR J1653-0158 is just the second millisecond pulsar that emits no detectable radio waves.
“In binary systems such as the one we have just discovered, pulsars are known as ‘black widows’ because those spiders of the same name eat their partners, so to speak,” Clark said.
“The pulsar evaporates its companion with its radiation and a particle wind and fills the star system with plasma that is impermeable to radio waves.”
That we think the system is so strange may be due to the limitations of technology. With tools like Einstein @ Home, which mainly uses free computing time to provide supercomputer features, we may be heading for a new era of pulse detection.
“In the catalog of gamma-ray sources found by the Fermi satellite, there are dozens more that I would bet on having binary pulsars in them,” said astronomer Bruce Allen of the Max Planck Institute for Gravitational Physics in Hanover and director and founder of Einstein @Hem.
“But so far no one has been able to detect the characteristic pulsation of their gamma rays. With Einstein @ Home, we hope to be able to do just that – who knows what other surprises await us.”
The research has been published in The Astrophysical Journal Letters.