Residing in the plane of the Milky Way, where it cannot be observed by
optical telescopes because of obscuring clouds of interstellar dust,
Circinus X-1 is the glowing husk of a binary star system that exploded
in a supernova event just 2,500 years ago. It consists of a very dense
neutron star locked in the orbital embrace of a companion star. Hi-res Image
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.
Sebastian Heinz
“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.
The rings are light echoes from Circinus X-1’s X-ray burst. Each of
the four rings, says Heinz, indicates a dense cloud of dust between us
and the supernova remnant. When X-rays encounter grains of dust in
interstellar space they can be deflected, and if the dust clouds are
dense they can scatter a noticeable fraction of the X-rays away from
their original trajectory, putting them on a triangular path.
That phenomenon, Heinz and his colleagues recognized, could give
astronomers an opportunity to use the geometry of the rings and a time
delay between deflected and undeflected X-rays to calculate the distance
to Circinus X-1, a measurement previously unobtainable because the
supernova is hidden in the dust that permeates the plane of our galaxy
We can use the geometry of the rings and the time delay to do X-ray
tomography,” Heinz explains. “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.”
Combining those measurements with observations of the dust clouds by
Australia’s Mopra radio telescope, Heinz and his colleagues were able to
determine which dust clouds were responsible for each of the four light
echoes.
“Using this identification, we can determine the distance to the
source accurately for the first time,” according to the UW-Madison
astronomer. “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.
“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,” Heinz
says.
Now astronomers know that Circinus X-1, one of the Milky Way’s most bizarre objects, is 30,700 light-years from Earth.
