For more than two decades, astronomers have been systematically tracing mystery 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 be coming 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 pinned down that second star by searching gamma-ray data obtained between 2008 and 2018. Together, the two stars constitute one of the weirdest binary systems we've ever seen.
"The binary star system and the neutron star at its heart, now known as PSR J1653-0158, set new records," said astronomer Lars Nieder of the Albert Einstein Institute Hannover in Germany.
"We have discovered the galactic dance of a super heavyweight with a flyweight: At slightly more than twice the mass of our Sun, the neutron star is extraordinarily heavy. Its companion has about six times the density of lead, but only about 1 percent the mass of our Sun.
"This 'odd couple' orbits every 75 minutes, more quickly than all known comparable binaries."
It's been thought since at least 2009 that the gamma radiation detected from the system must be produced by a gamma-ray pulsar. Then, in 2014, X-ray and optical observations of the source of the gamma rays turned up a variable star with a 75-minute period.
This was the tiny companion star, and astronomers thought that the 75-minute period was consistent with an orbital period with the second star being the source of the gamma rays.
"But all searches for the neutron star in it have so far been in vain," said astronomer Colin Clark of the Jodrell Bank Centre for Astrophysics at the University of Manchester in the UK.
The second star was thought to be a pulsar. That's a type of rapidly rotating neutron star that beams radiation from its poles as it spins. Those beams are a little like a lighthouse, flashing (or pulsing) past the observer at the rate of the star's rotation. Radio pulsars are more usual, but gamma-ray pulsars are known too.
In order to confirm the presence of the second star, you'd need to find the pulsations in time with its rotation. So the team went hard. They crunched a decade's worth of gamma-ray data collected by the Large Area Telescope (LAT) on board NASA's Fermi Gamma-ray Space Telescope, using computing power donated by tens of thousands of members of the citizen science program Einstein@Home.
In just two weeks, they found their pulsar.
It's a bit of an oddball. The pulsar is rotating extremely fast, more than 500 times a second. Millisecond pulsars do rotate extremely fast; that's what accounts for the "millisecond" part of their name. But PSR J1653-0158 has one of the fastest rotation rates ever seen in pulsars.
In addition, the star has an extremely weak magnetic field. It's within the bottom three ever detected for pulsar magnetic field strength.
The companion is also quite strange, since it has incredibly low mass. The team believes it's a helium white dwarf that's been cannibalised by the pulsar, leaving behind a remnant. This kind of system is known as a 'black widow' binary.
"The remnant of a dwarf star orbits the pulsar at just 1.3 times the Earth-Moon distance in only 75 minutes at a speed of more than 700 kilometres per second (435 miles per second)," Nieder said.
"This unusual duo might have originated from an extremely close binary system, in which matter originally flowed from the companion star onto the neutron star, increasing its mass and causing it to rotate faster and faster while simultaneously dampening its magnetic field."
(Knispel/Clark/Max Planck Institute for Gravitational Physics/NASA)
Above: Visualisation of the system (bottom) compared to Earth and the Moon (top).
This hypothesis is supported by the team's search for radio waves. If the pulsar is emitting any, we can't detect them; this could be because the system is surrounded by a dense cloud of material from the cannibalised dwarf star. Gamma radiation could penetrate this cloud, but not radio waves.
Either way, PSR J1653-0158 is only the second millisecond pulsar found that's not emitting any detectable radio waves.
"In binary systems like the one we have now discovered, pulsars are known as 'black widows' because, like spiders of the same name, they eat their partners, so to speak," Clark said.
"The pulsar vaporises its companion with its radiation and a particle wind, filling the star system with plasma that is impenetrable to radio waves."
That we find the system so peculiar may be because of the limitations of technology. With tools like Einstein@Home, which essentially uses idle computing time to provide supercomputing capabilities, we may be at the cusp of a new era of pulsar discovery.
"In the catalogue of gamma-ray sources found by the Fermi satellite, there are dozens more that I would bet have binary pulsars in them," said astronomer Bruce Allen of the Max Planck Institute for Gravitational Physics in Hannover and Director and founder of Einstein@Home.
"But so far no one has been able to detect the characteristic pulsation of their gamma rays. With Einstein@Home, we hope to do just that - who knows what other surprises await us."
The research has been published in The Astrophysical Journal Letters.