Astronomers date youngest X-ray binary
A blast from the past has allowed an age for the youngest X-ray binary to be estimated.
This composite image shows the energies streaming toward Earth from this object — X-rays in blue and the radio emission in purple. These energy detections have been overlaid in this image on an optical field of view from the Digitized Sky Survey.
At less than 4,600 years old, an X-ray binary – dubbed Circinus X-1 – has been revealed as the youngest to date after a team of astronomers using data from NASA’s Chandra X-ray Observatory detected the system’s neutron star ripping into its stellar companion and belching out a torrent of X-rays.
The record breaking Circinus X-1, which rests in the constellation Circinus some 24,000 to 30,000 light years away, contains a neutron star – a stellar remnant that results from the gravitational collapse, or supernova explosion, of a massive star – in a dance with a smaller star companion. As the pair orbit one another, with the neutron star pulling material from its partner, gas is heated to millions of degrees. Intense radiation is then produced, making these star systems some of the brightest X-ray sources in the sky.
And it was these rays that allowed the team of astronomers, led by Sebastian Heinz, an Associate Professor of Astrophysics and Astronomy at University of Wisconsin-Madison, to lock down an age for the pairing.
“The age is derived from its size and the velocity with which the supernova remnant expands,” Heinz tells All About Space. “It is straight forward to measure the size from the image. If we had a very accurate measurement of the velocity, we could do the mathematical equivalent of Google Maps (essentially dividing the distance by velocity, with a bit more sophistication, because the velocity decreases as the remnant grows) to derive a very accurate measurement of the time it took for the supernova remnant to reach its current size (such as its age).”
However, measuring the velocity directly is a bit of problem and, as Heinz explains, they have had to rely on the temperature of the shocked gas observed in the glow of X-rays.
“And [even] that measurement is not very accurate,” he warns. “In fact, we can really only say that the temperature of the gas in the shock wave must be larger than 3.5 million degrees Celsius (otherwise we would not see X-rays from the remnant at the observed level).”
From this sizzling temperature, Heinz and his team suggest that the supernova is expanding by at least 500 kilometres per second. “But that could be higher,” he says. “The way that the X-rays are measured does not allow us to confidently claim a narrow range in velocity.”
Despite being unable to lock down a precise measurement for Circinus X-1, Heinz and his team are happy with their result – whichever way they look at it, the X-ray binary is still, by far, the youngest known and a fairly young supernova remnant to boot.
Artist’s concept of the life of X-ray binary systems and the young and turbulent history of Circinus X-1, which formed in a supernova explosion less than 4,600 years ago.
However, Heinz does let All About Space in on another estimate of Circinus X-1. “We can provide a current best estimate of approximately 2500 years,” he says. “But as explained, that estimate is fairly uncertain and could be much younger still. It would require more observations to narrow the range in temperature, velocity and age.”
Circinus X-1 has puzzled astronomers for years, but realising that the X-ray binary is incredibly young allows Heinz and his team to have some kind of an explanation for its wild swings in brightness along with the highly unusual orbit of its two stars which sees the orbital time decrease by several minutes each year.
“For a system this young that recently has gone through a supernova event, the orbit is likely to be eccentric and the neutron-star’s spin axis, the companion-star’s spin axis, and the binary pair’s orbital axis are likely to be quite misaligned,” says Niel Brandt of Penn State University who contributed to the interpretation of the data on the X-ray binary. “Such misalignment will induce changes over time, which can help to explain the peculiarly strong long-term changes in the X-ray light that we see coming from Circinus X-1.”
New radio observations from the Australia Telescope Compact Array were also critical in probing the X-ray binary, providing a clearer view of the entire supernova remnant.
“Most previous observations were much blurrier and did not show the sharp edge that can be seen in the image. It also did not show how the remnant is brighter at the edges,” adds Heinz. “Those are the exact features one would expect of a supernova remnant, where the radio light comes from relativistic electrons just behind the blast wave that plows into the interstellar gas.”
Heinz and his team also found out several important new conclusions about the binary system. Firstly that the neutron star has a much lower magnetic field than what’s expected for its young age, challenging how these stars get their magnetic fields or how that field evolves after birth. Secondly a few details about the jets that emanate from Circinus X-1 became clearer.
“The X-ray binary is known as a microquasar – it produces powerful narrowly focused bundles of hot, magnetized gas that shoot away from the neutron star at very high velocity,” explains Heinz. Astronomers call those bundles “jets” and it had originally been assumed that they were responsible for creating the bright radio emission from Circinus X-1. “We now know that the radio light comes from the shock wave of the supernova remnant,” Heinz adds. “That really changes the picture we have about how X-ray binary jets behave on scales of light years, because before, Circinus X-1 was somewhat of a poster child for X-ray binaries with bright jets (we now have to rethink how the jet of Circinus X-1 should be viewed).”
In addition to the supernova remnant, the jet of the neutron star has revealed itself to be of further interest. “We showed that the jet must have been active for about 1000 years,” says Heinz. “That means that the neutron star has been active from basically the very beginning of its life, sucking matter away from its companion and funneling some of it into the jets we still observe today.”
Images courtesy of NASA/CXC/University of Wisconsin-Madison/S. Heinz et al, DSS & CSIRO/ATNF/ATCA (top) and University of Wisconsin-Madison (bottom)