In late 2013, when the neutron star at the heart of one of our galaxy's oddest supernovae gave off a massive burst of X-rays, the resulting echoes -- created when the X-rays bounced off clouds of dust in interstellar space -- yielded a surprising new measuring stick for astronomers. Circinus X-1 is a freak of the Milky Way. Located in the plane of the galaxy, Circinus X-1 is the glowing husk of a binary star system that exploded a mere 2,500 years ago. The system consists of a nebula and a neutron star, the incredibly dense collapsed core of the exploded star, still in the orbital embrace of its companion star.

The system is called an X-ray binary because it emits X-rays as material from the companion star spirals onto the much denser neutron star and is heated to very high temperatures.

"In late 2013, the neutron star underwent an enormous outburst for about two months, during which it became one of the brightest sources in the X-ray sky," explains University of Wisconsin-Madison astronomy Professor Sebastian Heinz. "Then it turned dark again."

The flicker of X-rays from the odd binary system was monitored by a detector aboard the International Space Station. Heinz and his colleagues quickly mounted a series of follow-up observations with the space-based Chandra and XMM-Newton telescopes to discover four bright rings of X-rays, like ripples in a cosmic pond, all around the neutron star at the heart of Circinus X-1. 

Their observations were reported June 23 in The Astrophysical Journal.