Scientists have recorded the first gamma-ray eclipses from strange star systems “spiders”

Astronomers have recorded the first gamma-ray eclipses from the spider star system, in which a superdense, rapidly spinning neutron star called a pulsar feeds on a stellar companion. These never-before-seen gamma-ray eclipses are caused by a low-mass pulsar companion star moving in front of it and blocking high-energy photons for a very short time.

An international team of scientists has discovered seven spider systems surviving such gamma-ray eclipses by studying more than 10 years of data from NASA’s Fermi Gamma-ray Space Telescope. In one case, the finds helped scientists figure out how the spider system is tilted relative to Earth and determine the mass of the pulsars in such systems. In the future, the study could help scientists determine what mass marks the dividing line between neutron stars and black holes.

“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 Institute for Gravitational Physics. Max Planck in Germany and head of the research team. (will open in a new tab).

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How web systems are born

Like all neutron stars and black holes, pulsars form when massive stars run out of fusion fuel and run out of external energy to keep them from gravitational collapse. When the core of such a star collapses and the outer material is blown away by the supernova, the core’s spin is greatly increased, just like a skater clasping his hands together to speed up the spin.

The collapse of the core results in the formation of a neutron star, a body with the mass of the Sun or more, compressed to a diameter of about 12 miles (17 kilometers), about the width of a city here on Earth – so dense that an ordinary teaspoon of it would weigh 4 billion tons. , which is equivalent to 600 Great Pyramids of Giza stacked on a spoon.

If the star is massive enough, the internal force of gravity overwhelms this material, which is 95% neutrons, and causes a complete collapse, causing the birth of a black hole. However, where the dividing line is is not clear.

“Pulsars are basically balls of the densest matter that we can measure,” Clarke said. “The maximum mass they can achieve limits physics under these extreme conditions, which cannot be replicated on Earth.”

Pulsars are also considered the extreme remnants of stars because they emit intense radiation. Because these beams are not aligned with their axis of rotation, they sweep through space and their radiation appears as pulses at regular intervals as they turn to face the Earth, almost like a cosmic beacon.

Scientists believe that gossamer systems form when one star in a binary system evolves faster than its partner, forming a pulsar with beams of light, including gamma rays, that come and go from our field of vision on Earth.

At the beginning of its existence, the pulsar “feeds” on the material of its binary companion, carrying this material away with a gas flow that has angular momentum. The accretion of this gas onto the pulsar adds angular momentum to the stellar remnant and speeds up its rotation, or causes it to “spin up”.

As the pulsar spins faster, it stops feeding and starts spewing high-energy particles and radiation at its stellar companion, overheating and destroying the pulsar-facing side of the star.

These spider systems fall into two categories, with corresponding arachnid-inspired nicknames; The black widow system contains a pulsar and a stellar companion with a mass less than 5% of the Sun’s, while the red back system combines a pulsar with a larger stellar companion with a mass between 10% and 50% of the Sun’s mass. (In both species, female spiders sometimes eat their partners.)

Gamma rays interfere with observations of the spider system

Astronomers have been able to glean vast amounts of information about spider systems from the light they emit. For example, visible light can show how fast a satellite is moving, and measurements of radio waves can show how fast a pulsar is spinning.

But these observations are based on motions towards and away from the Earth, and are therefore affected by the angle at which these systems are oriented with respect to the Earth. For systems we see face to face, changes in this motion are subtle and can produce signals that are confusing, like signals from a smaller system with a slower orbit seen from the side. This difficulty means that knowing the slope of a system is vital to understanding that system and its mass.

Astronomers can use visible light observations to estimate the tilt of a system, but these measurements can be tricky. For example, if the superheated side of a companion star moves into and out of the field of view, this can cause fluctuations in the system’s visible light signature. In addition, astronomers are only just beginning to understand the overheating of stars, so models built on different heating regimes can give different results.

In the spider system, gamma rays are only generated by the pulsar, not the companion star, and are of such energy that they are not affected by dust and debris in the system and can only be blocked by the companion star, so if the ray’s gamma signal goes out, astronomers can be sure that the pulsar has been eclipsed by a companion star. This is a clear sign that astronomers are viewing the system from the side, allowing scientists to confirm the companion star’s speed and pulsar’s mass.

But these gamma-ray eclipses have eluded astronomers, hence the new study.

Spider in the spotlight

One of the spiders the team studied turned out to be particularly productive.

PSR B1957+20 was the first black widow to be discovered, identified in 1998. But in more than a decade of Fermi data, Clark and his team found 15 missing gamma-ray photons that make up particles of light. Fifteen photons might not sound like a lot, but it’s an important finding because of how precise the timing of pulsars is.

Initially, scientists calculated the tilt of PSR B1957+20 to be 65 degrees compared to our line of sight. This measurement, made using visible light, led to an estimate of the mass of the pulsar at 2.4 times the mass of the Sun. The calculation made PSR B1957+20 the heaviest known pulsar and reached the theoretical mass limit that separates a neutron star from a black hole.

With the new data, Clarke and his team calculated that PSR B1957+20 is actually tilted by 84 degrees, reducing the pulsar’s mass to 1.8 solar masses – a measurement much more in line with the theory of neutron star formation.

“The challenge is to find massive pulsars, and these spider systems are considered one of the best ways to find them,” says Matthew Kerr, project co-author and research physicist at the US Naval Research Laboratory in Washington. DS,” the same statement said. “They have undergone a very extreme process of mass transfer from the companion star to the pulsar. Once we actually tune these models, we’ll know for sure if these spider systems are more massive than the rest of the pulsar population. .”

The new work not only marks a step forward in our understanding of spider systems and pulsars in general, but also illustrates the impact of the Fermi Gamma-ray Space Telescope on high-energy astronomy.

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

The team’s study was published Thursday (January 26) in the journal Nature Astronomy. (will open in a new tab). (will open in a new tab)

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