Astronomers using NASA’s Chandra X-ray Observatory have reported the discovery of the youngest X-ray binary ever found in our Milky Way Galaxy.
This composite image shows the X-ray binary Circinus X-1. Image credit: X-ray – NASA / CXC / University of Wisconsin-Madison / S.Heinz et al; optical – DSS; radio – CSIRO / ATNF / ATCA.
The object, named Circinus X-1, is located 24,000 to 30,000 light years away in the constellation Circinus. It is a glowing nebula created when the star exploded and, inside of it, the collapsed core of the exploded star, a neutron star, still clinging to its former companion star. It is the only known example of such a system in our galaxy.
Circinus X-1 is about 2,500 years old, which makes the system the youngest known X-ray binary.
“In terms of a human lifetime, it would be roughly akin to a newborn just a few days old,” said Dr Paul Sell from the University of Wisconsin-Madison, who is the second author of a paper published in the Astrophysical Journal (arXiv.org).
When a once-massive star explodes as a supernova, it creates either a black hole or a neutron star, a condensed, rapidly-spinning cinder with extraordinary gravitational pull. Most neutron stars, which pack the mass of about 1.4 suns into a ball just 20 or so kilometers in diameter, exist in isolation.
“In the Milky Way, there are only about a hundred other neutron stars we are aware of that have held onto their companion stars,” said lead author Dr Sebastian Heinz, also from the University of Wisconsin-Madison.
It is much rarer still to observe the hallmark of a supernova explosion, the glowing cloud of ionized gas created by the shock wave of the blast as it crashes into the interstellar gas around the stars.
“The shock wave from a supernova can only be seen for about 100,000 years or so before it dissipates and merges with the rest of the gas and dust around it in space. This is only a small fraction of the lifetime of an X-ray binary, making the chance to find one in this early phase of its life very small,” Dr Sell said.
Astronomers have found that the neutron star in Circinus X-1 is less than 4,600 years old, making it much younger than any other X-ray binary known in the Milky Way. Image credit: University of Wisconsin-Madison / S.Heinz et al.
“The fact that we have this remnant along with the neutron star and its companion means we can test all kinds of things,” Dr Heinz added.
“Our observations solve a number of puzzles both about this object and the way that neutron stars evolve after they are born. For example, the unusual elliptical orbit on which these two stars swing around each other is exactly what you would expect for a very young X-ray binary.
However, the observations also pose new questions: previous X-ray observations revealed that the Circinus X-1’s neutron star has a relatively small magnetic field.
“General theory holds neutron stars are born with a large magnetic field. This newly minted neutron star has a field much smaller than expected,” Dr Heinz said.
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Heinz S et al. 2013. The Youngest Known X-Ray Binary: Circinus X-1 and Its Natal Supernova Remnant. ApJ 779, 171; doi: 10.1088/0004-637X/779/2/171
