NASA's Fermi detects gamma-ray eclipses from 'spider' star systems

An international team of scientists discovered the first gamma-ray eclipses from a "spider" star system using NASA's Fermi Gamma-Ray Space Telescope, a blog post from the U.S. space agency reveals.

A spider star system is a particular type of binary star system that contains one regular star and one pulsar, the rapidly rotating, superdense remnants of a dead star.

The new observations add to the growing scientific literature on pulsars, which are made up of the densest material ever observed.

Studying "spider" star systems

The team of scientists was sifting through a decade of Fermi data when they discovered the seven spider systems where these eclipses took place. As NASA points out in its blog post, these eclipses "occur when the low-mass companion star passes in front of the pulsar from our point of view."

The new data allowed the scientists to calculate the system's tilt relative to our line of sight and other important information that could alter our understanding of pulsars. "One of the most important goals for studying spiders is to try to measure the masses of the pulsars," explained Colin Clark, an astrophysicist at the Max Planck Institute for Gravitational Physics in Hannover, Germany, and study lead.

"Pulsars are basically balls of the densest matter we can measure," Clark continued. "The maximum mass they can reach constrains the physics within these extreme environments, which can't be replicated on Earth."

Spider systems develop when a star in a binary — a two-star system — evolves faster than its partner. When the more massive star goes supernova, it turns into a pulsar and emits beams of multiwavelength light, including gamma rays. The pulses emitted from pulsars occur so regularly that they rival the precision of atomic clocks.

In the Fermi data they were looking at, Clark and his team found a measly 15 missing gamma-ray photons. The timing of gamma-ray pulses is so constant and dependable from pulsars, though, that this was enough to determine the system they were observing is eclipsing. This allowed them to calculate that the binary is inclined by 84 degrees and that the pulsar weighs 1.8 as much as our Sun.

NASA's Fermi telescope "allows the community to do trailblazing science"

NASA highlights the fact that its Fermi Gamma-Ray Space Telescope has altered our understanding of pulsars since it was launched to orbit on June 11, 2008.

"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."

When the gamma rays were obscured, the scientists could use this to infer that the companion eclipsed the pulsar. This allowed them to use the gamma rays emitted by the spider systems to calculate the system’s tilt into our sight line, the stars’ velocities, and the pulsar’s mass.

"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 scientists published their findings in a paper published in the journal Nature Astronomy.

Abstract:

Reliable neutron star mass measurements are key to determining the equation of state of cold nuclear matter, but such measurements are rare. Black widows and redbacks are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. Although inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly understood variability. Using data from the Fermi Large Area Telescope, we have searched for gamma-ray eclipses from 49 spider systems, discovering significant eclipses in 7 systems, including the prototypical black widow PSR B1957+20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. For PSR B1957+20, the eclipse implies a much lighter pulsar (1.81 ± 0.07 solar masses) than inferred from optical light curve modelling.