Spider binary systems contain a pulsar — the superdense, rapidly rotating remains of a star that exploded as a supernova — that slowly erodes its companion.
An orbiting star begins to eclipse its partner, a rapidly rotating, superdense stellar remnant called a pulsar, in this illustration. Image credit: NASA / Sonoma State University / Aurore Simonnet.
Spider systems develop because one star in a binary evolves more swiftly than its partner. When the more massive star goes supernova, it leaves behind a pulsar.
This stellar remnant emits beams of multiwavelength light, including gamma rays, that sweep in and out of our view, creating pulses so regular they rival the precision of atomic clocks.
Early on, a spider pulsar ‘feeds’ off its companion by siphoning away a stream of gas.
As the system evolves, the feeding stops as the pulsar begins to spin more rapidly, generating particle outflows and radiation that superheat the companion’s facing side and erode it.
Astronomers divide spider systems into two types named after spider species whose females sometimes eat their smaller mates: (i) black widow systems contain companions with less than 5% of the Sun’s mass; (ii) redback systems host bigger companions, both in size and mass, weighing between 10% and 50% of the Sun.
“One of the most important goals for studying spiders is to try to measure the masses of the pulsars,” said Dr. Colin Clark, an astrophysicist at the Max Planck Institute for Gravitational Physics.
“Before NASA’s Fermi Gamma-ray Space Telescope, we only knew of a handful of pulsars that emitted gamma rays,” added Fermi project scientist Dr. Elizabeth Hays, an astronomer at NASA’s Goddard Space Flight Center.
“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.”
The astronomers can calculate the masses of spider systems by measuring their orbital motions.
Visible light observations can measure how quickly the companion is traveling, while radio measurements reveal the pulsar’s speed. However, these rely on motion towards and away from us.
For a nearly face-on system, such changes are slight and potentially confusing.
The same signals also could be produced by a smaller, slower-orbiting system that’s seen from the side.
Knowing the system’s tilt relative to our line of sight is vital for measuring mass.
The tilt’s angle is normally measured using visible light, but these measurements come with some potential complications.
As the companion orbits the pulsar, its superheated side comes in and out of view, creating a fluctuation in visible light that depends on the tilt.
However, researchers are still learning about the superheating 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, unaffected by debris, unless blocked by the companion.
If gamma rays disappear from the data set of a spider system, scientists can infer that the companion eclipsed the pulsar.
From there, they can calculate the system’s tilt into our sight line, the stars’ velocities, and the pulsar’s mass.
PSR B1957+20 was the first-known black widow, discovered in 1988.
Earlier models for this system, built from visible light observations, determined that it was tipped about 65 degrees into our line of sight and the pulsar’s mass was 2.4 times the Sun’s.
That would make PSR B1957+20 the heaviest-known pulsar, straddling the theoretical mass limit between pulsar and black hole.
By looking at the Fermi data, the study authors found 15 missing gamma-ray photons.
The timing of the gamma-ray pulses from these objects is so dependable that 15 missing photons over a decade is significant enough that they could determine the system is eclipsing.
They then calculated that the binary is inclined 84 degrees and the pulsar weighs only 1.8 times as much as the Sun.
“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 Dr. Matthew Kerr, a physicist at the U.S. Naval Research Laboratory.
“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 findings were published today in the journal Nature Astronomy.
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C.J. Clark et al. Neutron star mass estimates from gamma-ray eclipses in spider millisecond pulsar binaries. Nat Astron, published online January 26, 2023; doi: 10.1038/s41550-022-01874-x
