One ring to rule them all and in the darkness... tell us the exact position of a neutron star in the Milky Way. This epic image gave scientists the exciting opportunity to pinpoint its location in our galaxy, with better accuracy than ever before.
The neutron star, called Circinus X-1, has produced some spectacular rings of X-rays – the largest and brightest ever observed, in fact. “We like to call this system the ‘Lord of the Rings,’ but this one has nothing to do with Sauron,” said Michael Burton of the University of New South Wales (UNSW), who coauthored the new study.
The source of these rings is neutron X-ray bursts. These occur when a type of stellar remnant called a neutron star and a low-mass main sequence (sun-like) star are in orbit around each other. While doing its dangerous, cosmic dance, the main sequence star can shuffle too close to the neutron star. The neutron star has a powerful gravitational attraction so strong that it sucks hydrogen from the outer layers of the star and stores it in the spinning disk of material encircling it, known as an accretion disk.
The stolen hydrogen swirls around the accretion disk and makes its way toward the surface of the neutron star. The intense heat and pressure of the neutron star immediately fuse the hydrogen into helium when it touches the surface. Over time, a thin layer of helium builds up on the surface of the neutron star. Once the amount of helium reaches critical mass, it ignites explosively, and X-rays pour out.
This doesn't explain why Circinus X-1 is surrounded by rings, though. The reason for this is found in between the neutron star and Earth in the form of dust clouds. Innocent clouds of dust intercept the X-rays spewing from the neutron star, scattering the X-rays and changing the angle at which they arrive at Earth. This creates a light echo, in the form of a ring. Circinus X-1 has four echo rings that were each formed by a different dust cloud.
By comparing this data with previous observations of the neutron star, the scientists could accurately predict the distance of Circinus X-1 from Earth. “It’s really hard to get accurate distance measurements in astronomy and we only have a handful of methods,” said Sebastian Heinz, University of Wisconsin. “But just as bats use sonar to triangulate their location, we can use the X-rays from Circinus X-1 to figure out exactly where it is.”
The X-ray echoes confirm that Circinus X-1 is located 30,700 light years from Earth. This new figure has interesting implications for the neutron star: it is brighter than originally thought. In fact, it is too bright to be stable, a feature more commonly associated with black holes than neutron stars. The neutron star has another black-hole-like feature: the velocity of the particles being produced in and around the neutron star are close to the speed of light.
“Circinus X-1 acts in some ways like a neutron star and in some like a black hole,” said coauthor Catherine Braiding, UNSW. “It’s extremely unusual to find an object that has such a blend of these properties.”
This neutron star is thought to have become an X-ray source only 2,500 years ago, making it the youngest known X-ray binary system.
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