Witnessing Light Echoes from a Neutron Star

X-ray echoes from binary star system Circinus X-1 are helping astronomers measure its distance from Earth.

Imagine ripples spreading out from a drop of water falling on a tepid lake. The concentric circles that radiate out from the center all have a common center, a geometry common in wave interactions, such as sound waves radiating from a speaker or light echoing in space.

Two composite images of Circinus X-1 and background stars. The low, medium, and high-energy X-rays captured by the Chandra X-Ray Observatory are shown in red, green, and blue, respectively. The background stars in this composite image come from visible-light Digitized Sky Survey images. The right image highlights the four X-ray rings. NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al.

Two composite images of Circinus X-1 and background stars. The low, medium, and high-energy X-rays captured by the Chandra X-Ray Observatory are shown in red, green, and blue, respectively. The background stars in this composite image come from visible-light Digitized Sky Survey images. The right image highlights the four X-ray rings.
NASA/CXC/Univ. of Wisconsin-Madison/S.Heinz et al.

Light echoes occur when a flash of radiation from an astronomical source collides with intervening matter, giving astronomers a new perspective on celestial happenings. In the case of the X-ray-emitting binary system named Circinus X-1, light echoes provide an unexpected opportunity to measure its distance directly, putting an end to years of debate. On June 20th Sebastian Heinz (University of Wisconsin-Madison) and colleagues reported on X-ray light echoes around this system in the Astrophysical Journal.

In late 2013 the neutron star at the center of Circinus X-1 flared, creating four concentric rings that astronomers spotted a few months later. The brief flare bounced off intervening dust clouds to form the concentric circles, which look like they circle the neutron star — but it turns out this is an optical illusion

How did the Rings Form?

When the neutron source flared, it emitted a brief flash of X-rays in every direction. Some of these X-rays traveled straight to Earth, but some of them scattered off intervening dust. The scattered X-rays take longer to arrive, and the lag in their arrival time gives a precise geometric measurement of the distance to the source.

Diagram showing the geometry of the light echo effect. Credit: Sebastian Heinz, et al.

Diagram showing the geometry of the light echo effect.
Credit: Sebastian Heinz, et al.

The figure on the right outlines light echoes’ optical illusion. An X-ray travels outward from the neutron star at an angle “α,” but bounces off a screen of dust between the neutron star and the observer. Because of its detour, the observer sees the photon arrive at an angle “θ,” as if it came not from the neutron star but from above it. In a given moment, the observer will see all the X-rays scattering at a certain angle. Just like a protractor, all of these scattered photons come in at the same angle and form an illusory ring of light. If astronomers watched for long enough, they would see this circle ripple outward, as X-rays come in from ever larger angles.

In the case of Circinus X-1, there’s not just one but four dust clouds between the neutron star and us. So rather than watch a single ring ripple outward over time, the astronomers spotted four concentric rings.

The team collected the X-ray data via the Chandra X-Ray Observatory. They also studied carbon monoxide maps of the clouds themselves, compiled by the Mopra radio telescope in Australia, which told them how far away the clouds were from Earth. Together, these data sets allowed them to calculate Circinus X-1’s distance from Earth using simple geometry: 30,700 light-years, more than twice a previously published distance.

Knowing an object’s distance from Earth can tell us a lot about it, such as its intrinsic brightness. Just as a lamp right next to you seems to shine brighter than one across the street, knowing the distance to Circinus X-1 puts its apparent brightness into perspective. Now astronomers can begin to understand how the neutron star’s flares fit into its total energy output.

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Anne McGovern

About Anne McGovern

Anne is the 2015 summer Editorial Intern at Sky & Telescope and a graduate student in Science and Medical Writing at Johns Hopkins University. She is sustained by science and literature, and loves to travel the world.