X-ray rings reveal neutron star’s distance

Dust cloud reflections plus trigonometry puts Circinus X-1 at 31,000 light-years away

blast of X-rays from a neutron star

BULL’S-EYE  A blast of X-rays from a neutron star (center) illuminate dust clouds between the star and Earth (colored rings), as seen in this composite image. Red, green and blue correspond to low-, medium- and high-energy X-rays.

S. Heinz et al/U. of Wisconsin–Madison/CXC/NASA (X-ray); DSS (optical)

A cosmic bull’s-eye is helping astronomers pinpoint the distance to a neutron star. Concentric rings of X-rays bouncing off interstellar dust indicate that the stellar remnant is about 31,000 light-years away in the constellation Circinus, researchers report in the June 20 Astrophysical Journal.

The X-rays, seen blasting out of the neutron star in 2013, are reflections off dust clouds that sit between Earth and the star, designated Circinus X-1. Each of the four rings comes from a different cloud, Sebastian Heinz, an astrophysicist at the University of Wisconsin–Madison, and colleagues report. The rings are the largest and brightest known, thanks to an exceptionally powerful flare from the star and copious amounts of dust in that part of the galaxy.

Heinz’s team used the Chandra and XMM-Newton satellites to image the rings. The researchers then mined old radio telescope data to figure out the distances to the dust clouds. By combining the ring diameters with the cloud distances — and with the help of some trigonometry — Heinz and colleagues triangulated the distance to the neutron star.

The distance — which had previously eluded astronomers — reveals fundamental properties of Circinus X-1, such as how much energy it pumps out. That, in turn, helps researchers understand the physics of neutron stars and the supernovas that give birth to them.

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