Scientists have discovered the first gamma-ray eclipses from a special type of binary star system using data from NASA’s Fermi 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 – that is slowly destroying its companion.
An international team of scientists studied more than a decade of Fermi observations to find the seven spiders that undergo these eclipses, which occur when a low-mass companion star passes in front of a pulsar from our point of view. The data allowed them to calculate how the systems tilt relative to our line of sight 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 work. “Pulsars are basically balls of the densest matter we can measure. The maximum mass they can reach limits the physics of these extreme conditions, which cannot be replicated on Earth.”
The research paper was published on January 26 in Astronomy of nature.
Spider systems evolve because one star in a binary system evolves faster than its partner. When a more massive star goes supernova, it leaves behind a pulsar. This stellar remnant emits beams of multi-wavelength light, including gamma rays, that pass in and out of our field of vision, creating pulses so regular that they can rival atomic clocks in accuracy.
At the very beginning, the spider pulsar “feeds” on its companion, sucking out the gas flow. As the system evolves, the power stops as the pulsar begins to spin faster, creating an outflow of particles and radiation that overheats the side of the satellite and eats away at it.
Scientists divide spider systems into two types, named after species of spiders whose females sometimes eat their smaller partners. Black widows have satellites with less than 5% of the mass of the Sun. Redback systems host larger satellites, 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 Hayes, 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 and collected a long, nearly continuous dataset that enables the community to engage in groundbreaking science.”
Researchers can calculate the masses of spider systems by measuring their orbital motions. Visible light observations can measure how fast the satellite is moving, while radio measurements show the speed of the pulsar. However, they depend on movement towards us and from us. For the near-eye system, such changes are minor and potentially confusing. A smaller system with a slower orbit that can be seen from the side may also be giving off the same signals. Knowing the tilt of the system relative to the field of view is essential for mass measurement.
Tilt angle is usually measured using visible light, but these measurements involve some potential complications. As the satellite orbits the pulsar, its superheated side moves in and out of view, creating tilt-dependent oscillations in visible light. However, astronomers are still studying the reheating process, and models with different heating patterns sometimes predict different pulsar masses.
Gamma rays, however, are only generated by the pulsar and have so much energy that they travel in a straight line without being hit by debris unless blocked by a satellite. If the gamma rays disappear from the spider system data set, scientists can conclude that the satellite has eclipsed the pulsar. From there, they can calculate the angle of inclination of the system to our line of sight, the velocity 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 of this system, based on visible-light observations, determined that it was tilted about 65 degrees in our field of vision and that the pulsar had a mass 2.4 times that of the Sun. This would make B1957 the heaviest pulsar known, exceeding the theoretical mass limit between a pulsar and a black hole.
Looking at the Fermi data, Clarke and his team found 15 missing gamma photons. The timing of gamma-ray bursts from these objects is so reliable that 15 missing photons per decade is significant enough for the team to determine that the system is dimming. They then calculated that the binary system is tilted by 84 degrees and the pulsar weighs only 1.8 times that of the Sun.
“There is a search for massive 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 US Naval Research Laboratory in Washington. . “They went through a very extreme mass transfer process from the companion star to the pulsar. Once we fine-tune these models, we will know for sure whether these spider systems are more massive than the rest of the pulsar population. .”
The Fermi Gamma-ray Space Telescope is a partnership in astrophysics and particle physics managed by Goddard. Fermi was developed in collaboration with the US Department of Energy, with important input from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.
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