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X-ray echoes map the distance to a neutron star

Interstellar echoes from a flaring neutron star have allowed astronomers to solve a mystery -- pinpointing the star's location.

NASA/CXC/U. Wisconsin/S. Heinz

This is not a crazy space rainbow. It doesn't lead to a pot of gold. This is, in fact, shimmering light echoes caused by X-rays, and it leads to the location of a neutron star.

They come from a specific neutron star called Circinus X-1, a binary star system on the galactic plane. The system consists of a neutron star -- a collapsed giant star post-supernova on its way to becoming a black hole -- in orbit around another supermassive star, which isn't so unusual. What is unusual is that the neutron star exhibits X-ray jets normally only found in black holes.

It is these highly unusual jets that have allowed astronomers using NASA's Chandra X-ray Observatory to pinpoint the star's exact location. The paper is available online via arXiv.

"It's really hard to get accurate distance measurements in astronomy and we only have a handful of methods," said study leader Sebastian Heinz of the University of Wisconsin in Madison. "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."

In late 2013, the neutron star flared with a burst of X-rays. These X-rays produced light echoes. Much like traditional sound echoes, light echoes occur when light is reflected from a source, and this reflection is seen some time after the original burst.

The X-ray burst of Circinus X-1 reflected off clouds of dust, creating a ring of echoes. Each dust cloud produced its own reflection and by comparing that data with images of dust clouds previously taken by the Mopra Telescope in Australia, scientists were able to pinpoint its location.

Using radio data to determine the distance to the different clouds, combined with the X-ray echo data, researchers were able to pinpoint Circinus X-1's location to about 30,700 light-years from Earth.

"We like to call this system the 'Lord of the Rings,' but this one has nothing to do with Sauron," said co-author Michael Burton of the University of New South Wales in Sydney, Australia. "The beautiful match between the Chandra X-ray rings and the Mopra radio images of the different clouds is really a first in astronomy."

The finding has allowed the team to settle the dispute about Circinus X-1's location from conflicting previous studies, but the results also mean several interesting things about the binary system. For example, Circinus X-1 is much brighter in X-rays and other types of light than previously thought -- in fact, its brightness is more in keeping with a black hole than a neutron star.

The aforementioned X-ray jets have also been confirmed to be very black-hole like. The researchers determined that the high-energy particle jets are travelling at at least 99.9 percent of the speed of light -- speeds usually only seen in black holes.

"Circinus X-1 acts in some ways like a neutron star and in some like a black hole," said co-author Catherine Braiding of the University of New South Wales. "It's extremely unusual to find an object that has such a blend of these properties."