Circinus X-1

Circinus X-1

A luminous galactic X-ray source, showing recurrent flaring at a 16.6 day period. Spectral characteristics and variability on timescales down to milliseconds, similar to Cygnus X-1, initially suggested Cir X-1 as a black hole candidate. Discovery by EXOSAT of X-ray bursts have now shown that Cir X-1 is more likely to be a neutron star in an eccentric 16.6 day orbit with an early-type nondegenerate companion star. More recent observations with Asca suggest the rapid X-ray variability may result from dense gas clouds passing across in front of the X-ray emission region.
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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.
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.
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.
Now astronomers know that Circinus X-1, one of the Milky Way's most bizarre objects, is 30,700 light-years from Earth.
The team discovered the age of this record-breaking pair, named Circinus X-1, by using data from NASA's Chandra X-ray Observatory, which revealed faint remnants of the supernova explosion that created the neutron star.
Niel Brandt, Distinguished Professor of Astronomy and Astrophysics, said that he was perplexed by the unusually strong evolution of the orbit of Circinus X-1 since my graduate-school days, asserting that the discovery now of this system's youth provides a satisfying explanation for why its orbit evolves so strongly - as the system likely still is settling down after its violent birth.
The research team, which was led by Sebastian Heinz at the University of Wisconsin-Madison, determined that Circinus X-1 is less than 4,600 years old.
Similarly, a team led by Roberto Mignani (European Southern Observatory) used the Hubble Space Telescope to set an upper limit of 5 milliarcseconds per year on the proper motion of Circinus X-1, an accreting neutron star in a binary system.