An article published in the journal “Nature Astronomy” reports the detection of gamma-ray eclipses in binary systems formed by a normal star or a brown dwarf that have a pulsar as a companion that is defined as a spider pulsar because they steal gas from the companion behaving like a black widow. A team of researchers used data collected over a decade by NASA’s Fermi Space Telescope to find cases in which the star passes in front of the pulsar. Seven binaries have been identified with a spider pulsar being eclipsed by its companion. This identification made it possible to measure the pulsar’s mass, a result useful to conduct tests on the theory of relativity and understand the behavior of matter in extreme conditions.

A neutron star can steal gas from a companion, whether it’s a normal star or a brown dwarf. Even a red dwarf is much larger than a pulsar because a neutron star is extremely compact, which is why its gravitational force is enormous. In this case, the gas-stealing pulsar is jargon for a redback, named after a species of Australian black widow. When the pulsar’s companion is much smaller, such as a brown dwarf, the pulsar is simply called a black widow.

The Fermi Space Telescope accumulated years of observations, including of pulsars. This made it possible to map the gamma-ray emissions that come from millisecond pulsars and therefore from the binary systems formed by a pulsar of that type and a companion. Together with observations at optical frequencies of the companions’ motion, that allowed reconstructing the systems’ orbits and consequently also the masses of the two objects that form each system.

The inclination of a binary system with respect to the Earth is an important value for correctly measuring the masses. The analysis of the observations conducted with Fermi allowed identifying the cases of an eclipse in which the companion passed in front of the spider pulsar from the Earth’s point of view. These steps allow to measure the system’s inclination and therefore to obtain precise measurements of its components’ masses.

The pulsar PSR B1957+20, or simply B1957, the first black widow detected, had a surprise in store. Estimates of its mass led to it being very massive, about 2.4 times the Sun, but this study indicates that, with the system inclined 84°, its mass is around 1.8 times the Sun’s.

Eclipses from these pulsars can also be used to observe relativistic effects because a neutron star’s gravity can act as a gravitational microlens that temporarily increases the companion’s brightness. Estimating that effect may be another way to measure the pulsar’s mass, but it will take extremely sensitive instruments to do that.

Precise measurements of the masses of known pulsars are important for discovering massive ones near the neutron star-black hole boundary. Spider pulsars steal gas from their companions and that could push them over that edge. It’s yet another reason to seek and study them.