Using information from NASA’s Fermi Gamma-ray Space Telescope, researchers have found the first gamma-ray eclipses from a particular kind of binary star system. The superdense, quickly revolving remnants of a star that erupted in a supernova and progressively eroded its companion are known as pulsars, and they are present in each of these so-called spider systems.
Seven spiders that experience these eclipses, which happen when the low-mass companion star crosses in front of the pulsar from our perspective, were discovered by an international team of researchers who combed through more than ten years of Fermi observations. They were able to determine other details, such as the systems’ tilt with respect to our line of sight, thanks to the data.
“One of the most important goals for studying spiders is to try to measure the masses of the pulsars,” said Colin Clark, an astrophysicist at the Max Planck Institute for Gravitational Physics in Hannover, Germany, who led the work. “Pulsars are basically balls of the densest matter we can measure. The maximum mass they can reach constrains the physics within these extreme environments, which can’t be replicated on Earth.”
A paper about the study was published on January 26, 2023, in Nature Astronomy.
Because one star in a pair grows more quickly than its partner, spider systems form. A pulsar is left behind by the supernova of a more massive star. This star remnant produces pulses that are so regular that they are comparable to the accuracy of atomic clocks by emitting beams of multiwavelength light, including gamma rays, that sweep in and out of our field of vision.
Early on, a spider pulsar “feeds” off its companion by siphoning away a stream of gas. The feeding ends as the system develops and the pulsar speeds up, producing radiation and particle outflows that superheat and destroy the companion’s facing side.
One of the most important goals for studying spiders is to try to measure the masses of the pulsars. Pulsars are basically balls of the densest matter we can measure. The maximum mass they can reach constrains the physics within these extreme environments, which can’t be replicated on Earth.
Colin Clark
Scientists categorize spider systems into two groups, each named for a species of spider whose females occasionally consume their smaller partners. Companions of black widows have a mass less than 5% that of the sun. Redback systems host bigger companions, both in size and mass, weighing between 10% and 50% of the Sun.
“Before Fermi, we only knew of a handful of pulsars that emitted gamma rays,” said Elizabeth Hays, the Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “After over a decade of observations, the mission has identified over 300 and collected a long, nearly uninterrupted dataset that allows the community to do trailblazing science.”
By observing the orbital motions of spider systems, scientists can determine their masses. The speed of the companion can be determined by visible light studies, whereas the pulsar’s speed can be determined by radio measurements. However, these rely on motion towards and away from us.
For a nearly face-on system, such changes are slight and potentially confusing. The identical signals might potentially be generated by a side-viewing device that is smaller and slower-orbiting. Knowing the system’s tilt relative to our line of sight is vital for measuring mass.
Visible light is typically used to estimate the tilt’s angle, although this method has certain limitations. The superheated side of the companion’s orbit changes our view of it, causing a tilt-dependent variation in visible light. However, as the mechanism of superheating is still poorly understood, different pulsar masses can occasionally be predicted by models with various heating regimes.
Gamma rays, on the other hand, are only produced by the pulsar and have such high energy that, unless the companion blocks them, they travel in a straight line untouched by debris.
Gamma rays disappearing from a spider system’s data set indicates that the companion has overtaken the pulsar, according to researchers. From there, they can determine the system’s tilt toward our line of sight as well as the speeds of the stars and the mass of the pulsar.
PSR B1957+20, or B1957 for short, was the first-known black widow, discovered in 1988. Earlier models for this system, constructed from observations of visible light, found that the pulsar’s mass was 2.4 times that of the Sun and that it was inclined toward our line of sight by around 65 degrees. By crossing the theorized mass threshold between pulsars and black holes, B1957 would become the heaviest pulsar known to science.
By looking at the Fermi data, Clark and his team found 15 missing gamma-ray photons. The gamma-ray pulses from these objects are timed with such precision that the team was able to identify the system was eclipsing from the 15 photons that were missing during a 10-year period. The pair is 84 degrees inclined, and the pulsar weighs only 1.8 times as much as the Sun, according to their calculations.
“There’s a quest to find massive pulsars, and these spider systems are thought to be one of the best ways to find them,” said Matthew Kerr, a co-author on the new paper and research physicist at the U.S. Naval Research Laboratory in Washington.
“They’ve undergone a very extreme process of mass transfer from the companion star to the pulsar. Once we really get these models fine-tuned, we’ll know for sure whether these spider systems are more massive than the rest of the pulsar population.”
The Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership managed by Goddard. Fermi was developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.
