Observations by the Chandra X-ray observatory have helped scientists to find the youngest X-ray binary star ever discovered. X-ray binary stars are some of the brightest objects in the sky, when viewed in the X-ray spectrum, and are formed when one member of a binary star system dies in a violent supernova explosion. At only 4500 years old, Circinus X-1 is one of the youngest astronomical objects of any kind known to humanity.
X-ray binaries start life as one of the most common star configurations: a binary star system, composed of two or more stars orbiting each other. For the binary system to evolve into an X-ray binary, two conditions have to be met: The stars must orbit each other very closely, and one of them must be extremely massive – more than eight times larger than our own Sun. The giant star will burn its fuel extremely fast so that it only lasts a few hundred million years (compared to the ten billion expected for stars like the Sun, and trillions of years for red dwarfs) before beginning an accelerating sequence of burning through heavier and heavier fuels, each the ash from the previous burn, and eventually detonating in a catastrophic supernova explosion. All that remains of the star after this is a neutron star, or if it was truly massive, a black hole. Because it is so close to its partner star, it is then able to begin scooping material from the stellar surface. Tidal forces and internal friction sculpt this material into a super-heated accretion disk as it spirals down to the surface of the neutron star, much of which manages to escape at the last moment by joining the powerful jets shooting out from the neutron star’s poles at temperatures of millions of Kelvins. Gas this hot doesn’t shine red, or white, or even ultra-violet, but instead glows luminously in X-rays, and it is these emissions that Chandra detects.
Astronomers know of many such X-ray binaries, although most are hundreds of thousands, if not millions, of years old. They are closely studied because so much of our knowledge of neutron stars is theoretical, and watching them interact with their partner stars provides valuable data to test those theories and reveal their workings. But the age of these systems puts a limit on how much scientists can learn, since they do not get any data from when the neutron star was first formed. Circinus X1 stands out because the shell of ejected material from its originating supernova explosion is very small at only 76 light years across. According to Sebastian Heinz and his team at the University of Wisconsin-Madison (UW), this sets a maximum age for the neutron star, and therefore the entire X-ray binary system, of 4600 years. “It’s critical that we see what these X-ray binaries are doing at all stages of their lives,” said Paul Sell, also of UW and co-author of a paper describing the new discovery. “Circinus X-1 is showing us what happens in a cosmic blink of an eye after one of these objects is born.”
“X-ray binaries provide us with opportunities to study matter under extreme conditions that would be impossible to recreate in a laboratory,” Heinz said. “For the first time, we can study a newly minted neutron star in an X-ray binary system.”
Previous observations of Circinus X-1 had failed to detect the supernova remnant, as its light was overwhelmed by the brightness of the neutron star itself. But by pointing Chandra at the system when the neutron star was briefly in a dim phase, the scientists were able to see X-rays emitted by the shockwave of the remnant ploughing through the interstellar medium in surrounding space. The results were described in a paper published in the Dec. 4 issue of The Astrophysical Journal.