Scientists have detected the first gamma-ray eclipses from a private binary star system using data from NASA’s Fermi Space Gamma-ray Telescope. Each of these so-called spider systems contains a pulsar, the super-dense, fast-spinning remnants of a star that exploded in a supernova, slowly destroying its companion. An international team of scientists scoured ten years of Fermi observations to find seven spiders that experienced these eclipses, which occur when a low-mass co-star from our side passes in front of a pulsar. The data allowed the systems to calculate how they leaned relative to our field of view and other information.

“One of the most important goals of studying spiders is to try to measure the masses of pulsars,” said Colin Clark, an astrophysicist at the Max Planck Institute for Gravitational Physics in Hannover, Germany, who led the study. “Pulsars are basically balls of the densest matter we can measure. The maximum mass they can reach limits physics in these extreme environments that cannot be replicated on Earth.”

Spider systems evolve because a star in a binary system evolves faster than its mate. When a larger star goes supernova, it leaves a pulsar behind. This stellar remnant emits multi-wavelength beams of light, including gamma rays, that enter and exit our field of view in pulses so regular that it rivals the precision of an atomic clock.

Initially, the spider pulsar “feeds” its mate by absorbing the gas stream. As the system evolves, as the pulsar begins to spin faster, power is cut off, creating a stream of particles and radiation that overheats and destroys the rear of its companion.

Scientists divide spider systems into two species, named after the species of spiders whose females sometimes eat their smaller mates. Black widows contain moons with a mass less than 5% of the mass of the Sun. Redback systems contain larger moons ranging in both size and mass from 10% to 50% of the Sun.

“Before Fermi, we only knew of a few pulsars that emit gamma rays,” said Elizabeth Hayes, a Fermi project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “After more than a decade of observations, the mission has identified more than 300 observations and has collected a long, nearly continuous dataset that enables the community to do groundbreaking science.”

By measuring their orbital motion, researchers can calculate the masses of spider systems. Visible light observations can determine how fast a satellite is moving, while radio measurements show the speed of a pulsar. However, they depend on us towards and action from us. For an almost direct system, such changes are trivial and can be confusing. The same signals can be produced by a smaller system with a slower trajectory when viewed from the side. To measure mass, it is very important to know the tilt of the system relative to our line of sight.

The tilt angle is usually measured using visible light, but these measurements come with some potential complications. As the companion orbits the pulsar, its overheated side enters and exits the field of view, creating tilt-dependent oscillations in visible light. However, astronomers are still studying the overheating process, and models with different heating patterns sometimes predict different pulsar masses.

However, gamma rays are only produced by the pulsar and have so much energy that they travel in a straight line unaffected by debris unless blocked by a satellite. If gamma rays disappear from the spider system dataset, scientists may conclude that the satellite has eclipsed the pulsar. From there, they can calculate the inclination of the system to our line of sight, the velocities of the stars, and the pulsar’s mass.

PSR B1957+20, or B1957 for short, was the first known black widow discovered in 1988. Previous models of this system, based on visible light observations, showed that it was inclined about 65 degrees in the direction of our line of sight and pulsar. the mass was 2.4 times greater than the mass of the Sun. This makes B1957 the heaviest known pulsar that exceeds the theoretical mass limit between a pulsar and a black hole.

Clark and his team looked at the Fermi data and found 15 missing gamma-ray photons. The timing of gamma-ray bursts from these objects is so reliable that the 15 missing photons over the course of ten years are significant enough for the team to determine that the system is dimming. They then calculated that the binary system tilted 84 degrees and the pulsar was only 1.8 times the weight of the Sun.

“There’s a search for large pulsars, and these spider systems are considered one of the best ways to find them,” said Matthew Kerr, co-author of the new paper and a research physicist at the U.S. Naval Research Laboratory in Washington. “They went through a very extreme mass transfer process from the companion star to the pulsar. When we actually tune these models, we’re going to make sure these spider systems are larger than the rest of the pulsar population.”