X-ray echoes used as a cosmic yardstick

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Science  17 Jul 2015:
Vol. 349, Issue 6245, pp. 280-281
DOI: 10.1126/science.349.6245.280-b

Determining accurate distances to astronomical x-ray sources is notoriously difficult, but Heinz et al. have developed a new technique for doing so. They exploited “light echoes” from the object Cir X-1, in which x-rays emitted during a large flare bounced off foreground dust and appeared months later as delayed rings of emission around the source. By comparing the x-ray rings to radio data, they were able to identify the foreground clouds of dust and thereby measure the distance to Cir X-1 with an accuracy of 10%. Previous estimates had varied by almost a factor of 3.

Astrophys. J. 806, 265 (2015).

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