Astronomers Find Youngest-Ever X-Ray Binary
December 5, 2013

Chandra Helps Astronomers Locate Youngest-Ever X-Ray Binary

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Using data obtained from NASA's Chandra X-ray Observatory and the Australia Telescope Compact Array, researchers have identified the remains of a supernova that exploded just 2,500 years ago, making it the youngest astronomical object of its kind ever discovered.

The object is an X-ray binary located between 24,000 and 30,000 light years from Earth in the constellation Circinus, and as such it has been dubbed Circinus X-1. An X-ray binary, NASA explains, is a star system comprised of two parts: a compact stellar remnant (such as a neutron star) and a companion star (a normal star like our sun).

As the two objects orbit each other, the stellar remnant attracts gas from the companion star, heating it to millions of degrees and producing intense X-ray radiation. As a result, this type of star system is one of the brightest sources of X-rays in the sky, the US space agency reported.

The authors of the new study found that Circinus X-1 is less than 4,600 years old, which makes it the youngest X-ray binary system ever identified. The discovery will allow scientists to gain new, previously unavailable insight into how neutron stars and supernovas form, as well as the effect of the explosion on a nearby companion star.

According to University of Wisconsin-Madison astronomy professor Dr. Sebastian Heinz, the majority of X-ray binaries are so old that they no longer have a supernova remnant, making Circinus X-1 a rare find. “It basically creates the historical record of that explosion,” he said. “They don't last very long, so they are quite accurate clocks for finding things that are recent.”

While researchers have located hundreds of X-ray binaries throughout the Milky Way and in other galaxies, most of them are millions of years old and provide information only about the later stages of the evolutionary process. Circinus X-1 will allow astronomers to “study matter under extreme conditions that would be impossible to recreate in a laboratory,” Heinz said. “For the first time, we can study a newly minted neutron star in an X-ray binary system.”

In order to determine this X-ray binary’s age, Heinz and his colleagues needed to examine the material surrounding the orbiting pair of stars. Typically, the extreme brightness of the neutron star makes such observations too difficult. However, the investigators were able to recently observe the neutron star in a faint state, so that it was dim enough for them to detect the X-rays that resulted from the supernova shock wave.

Circinus X-1’s  relative youth helps account for its unusual swings in brightness, as well as the unusual orbit of its two stars, Penn State University astronomy and astrophysics professor Niel Brandt said. Those phenomena had puzzled Brandt and his colleagues for many years.

“For a system this young that recently has gone through a supernova event, the orbit is likely to be eccentric, and the neutron-star's spin axis, the companion-star's spin axis, and the binary pair's orbital axis are likely to be quite misaligned,” he explained. “Such misalignment will induce changes over time, which can help to explain the peculiarly strong long-term changes in the X-ray light that we see coming from Circinus X-1.”

A paper describing these results is available online and appears in the Dec. 4 issue of The Astrophysical Journal.