For more than two decades, astronomers have been systematically tracking the mysterious sources of high-energy gamma rays.
One, however, went undetected – 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 was found.
Now astronomers have solved the mystery and discovered a second star by examining gamma-ray data from 2008 to 2018. Together, the two stars make up one of the strangest binary systems we've ever seen.
“The binary star system and the neutron star at its heart, now known as PSR J1653-0158, have set new records,” said astronomer Lars Nieder of the Albert Einstein Institute of Hanover in Germany.
“We have discovered the galactic dance of a super heavyweight with a lightweight: a neutron star, with a mass just over twice the mass of our Sun, is extremely heavy. Its companion has about six times the density, but only 1 percent of our Sun's mass.
“This 'strange pair' rotates every 75 minutes, faster than all known comparable binaries.”
Since 2009, it has been thought that the gamma rays detected by the system should be produced by a gamma pulsar. Then, in 2014, X-ray and optical observations of a gamma-ray source discovered a variable star with a period of 75 minutes.
It was a tiny companion star, and astronomers believed that the 75-minute period corresponds to the orbital period, and the second star is the source of gamma rays.
The second star was considered a pulsar. It is a type of fast spinning neutron star that emits radiation from its poles as it rotates. These beams are a bit like a beacon, blinking (or pulsing) past the observer at the speed of the star's rotation.
However, the pulsar rotates very quickly, more than 500 times per second. Millisecond pulsars do spin extremely fast; that's what makes up the 'millisecond' part of their name. But PSR J1653-0158 has one of the highest rotational speeds ever observed in pulsars.
In addition, the star has a very weak magnetic field.
The satellite is also quite strange, as it has an incredibly low mass. The team believes it is a helium white dwarf that has been absorbed by the pulsar. Such a system is known as the black widow binary system.
“The remnant of the dwarf star orbits the pulsar 1.3 times the Earth-Moon distance in just 75 minutes at over 700 kilometers per second,” Nieder said.
'This unusual duo could have arisen from an extremely close binary system in which matter initially flowed from a companion star to a neutron star, increasing its mass and causing it to spin faster and faster, while simultaneously weakening its magnetic field.'
Visualization of the system (bottom) compared to the Earth and the Moon (top). (Knispel / Clark / Max Planck Institute for Gravitational Physics / NASA).
This hypothesis is confirmed by studies of a group of radio waves. If the pulsar emits them, we cannot detect them; this could be because the system is surrounded by a dense cloud of material from a dwarf star. Gamma radiation could have penetrated this cloud, but not radio waves.
In any case, PSR J1653-0158 is just the second millisecond pulsar found that does not emit any detectable radio waves.
“In binary systems like the one we have now discovered, pulsars are called 'black widows' because, like the spiders of the same name, they eat their partners, so to speak,” Clarke said.
“The pulsar vaporizes its satellite with radiation, filling the stellar system with plasma impenetrable to radio waves.”
The study is published in The Astrophysical Journal Letters.
