Astronomers using data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton telescope have discovered a bright X-ray light echo in the form of four well-defined rings around the neutron star at the center of an X-ray binary system called Circinus X-1. The new Chandra data have also provided a rare opportunity to determine the distance to this bizarre system.
A set of four rings around Circinus X-1 can be seen in this composite image where X-rays from Chandra are red, green, and blue corresponding to low, medium, and high-energy X-rays respectively, which have been combined with a view in visible light from the Digitized Sky Survey. The sharp edges are caused by the large size of the X-ray rings compared to the relatively small field-of-view of the Chandra detectors, providing only partial coverage. Image credit: NASA / CXC / University of Wisconsin-Madison / S. Heinz et al / DSS.
Circinus X-1 is an old, low-mass X-ray binary in the plane of our Milky Way Galaxy containing a neutron star, the dense remnant of a massive star pulverized in a supernova explosion.
The neutron star is in orbit with an evolved companion star, and is shrouded by thick clouds of interstellar gas and dust. The system is also the source of a surprisingly powerful jet of high-energy particles.
“In 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. Then it turned dark again,” said team member Prof Sebastian Heinz of the University of Wisconsin-Madison.
Prof Heinz and co-authors mounted a series of follow-up observations with Chandra and XMM-Newton to discover a set of four rings that appear as circles around Circinus X-1’s neutron star.
They then determined that these rings are light echoes from the 2013 burst. The echoes are similar to sound echoes that we may experience here on Earth. Instead of sound waves bouncing off a canyon wall, the echoes around Circinus X-1 are produced when a burst of X-rays from the star system ricochet off of clouds of dust between Circinus X-1 and our planet.
“Each of the four rings indicates a dense cloud of dust between us and the supernova remnant,” Prof Heinz said.
This artist’s illustration shows in detail how the ringed structure seen by Chandra is produced. Each ring is caused by X-rays from the Circinus X-1 flare bouncing off of different dust clouds. If the cloud is closer to us, the ring appears to be larger. The result, as seen by Chandra, is a set of concentric rings with different apparent sizes depending on the distance of the intervening cloud from us. The physical sizes of the rings, using the labels given the illustration, are 41 light years (ring a), 49 light years (ring b), 55 light years (ring c), and 52 light years (ring d). Image credit: Sebastian Heinz / University of Wisconsin-Madison.
“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,” he added.
The light echo shows that Circinus X-1 is located about 30,700 light years from Earth.
The observation settles a large difference amongst previous results, one similar to this work and one indicating a much smaller distance of about 13,000 light years.
Such a difference in distance estimate to the system would have implications for other properties that have been observed before in Circinus X-1.
For example, if it is over twice as far away as some have previously thought, then this means its light output is much greater.
Prof Heinz’s team also determined that the speed of the jet of high-energy particles produced by Circinus X-1 is at least 99.9 percent of the speed of light. This extreme velocity is usually associated with jets produced by a black hole.
“Circinus X-1 acts in some ways like a neutron star and in some like a black hole. It’s extremely unusual to find an object that has such a blend of these properties,” said co-author Dr Catherine Braiding of the University of New South Wales, Australia.
Circinus X-1 is thought to have originally become an X-ray source about 2,500 years ago, as seen from Earth. This makes Circinus X-1 the youngest so-called X-ray binary known.
These results have been published on June 23 in the Astrophysical Journal (arXiv.org preprint).
_____
S. Heinz et al. 2015. Lord of the Rings: A Kinematic Distance to Circinus X-1 from a Giant X-Ray Light Echo. ApJ 806, 265; doi: 10.1088/0004-637X/806/2/265
