In late 2013 a neutron star at the center of a strange supernova gave off a massive burst of X-rays that bounced off clouds of dust, creating "echoes" that astronomers can now use as a measuring stick.
Circinus X-1 is a "freak" of the Milky Way; it consists of a nebula and a neutron star still intertwined in orbit with its companion star in what is called an X-ray binary system, the University of Wisconsin-Madison reported. The system emits X-rays as material from the companion star moves into the much denser neutron star, and is heated to scorching temperatures. New observations have allowed researchers to determine the odd object is 30,700 light-years away from Earth.
"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," said University of Wisconsin-Madison astronomy Professor Sebastian Heinz. "Then it turned dark again."
The X-rays from the binary star system were monitored by a detector aboard the International Space Station and the Chandra and XMM-Newton telescopes, the observations revealed four bright rings of X-rays around the neutron star. These rings are believed to be "light echoes" from Circinus X-1's X-ray burs, and each one indicates a dense cloud of dust between the supernova remnant and Earth. Each grain of dust can deflect the X-ray away from its original trajectory, creating a triangular path. The findings could allow scientists to calculate the distance to Circinus X-1, a measurement that was previously unobtainable because the supernova is hidden in dust.
"We can use the geometry of the rings and the time delay to do X-ray tomography," Heinz said. "Because the X-rays have traveled on a triangular path rather than a straight path, they take longer to get to us than the ones that were not scattered."
The team combined these measurements with observations of the dust clouds made by Australia's Mopra radio telescope, allowing them to determine which clouds were responsible for each light ring.
"Using this identification, we can determine the distance to the source accurately for the first time. Distance measurements in astronomy are difficult, especially to sources like Circinus X-1, which are hidden in the plane of the galaxy behind a thick layer of dust -- which makes it basically impossible to observe them with optical telescopes," Heinz said. "In this case, we used the dust that otherwise gets in the way to pioneer a new method of estimating distances to X-ray sources."
The findings were published in a recent edition of The Astrophysical Journal.
