Abu Dhabi researchers observe 11,000-year-old space event

Scientists witness a star drawing in space material and expelling X-rays thousands of times brighter than our sun

An illustration of an accreting pulsar, drawing material in from a nearby star and emitting X-rays. Courtesy: Nasa

Extraordinary details of an event that happened in space 11,000 years ago have been observed fully for the first time by Abu Dhabi researchers.

They have detailed, in a new study, how an object called a neutron star pulled in material from another star and then emitted a burst of X-rays thousands of times brighter than our sun.

This process, named accretion, has happened many times across the universe, but never before has the full sequence of events been observed by astrophysicists.

“We’ve been very excited because this is the very first time it’s been possible to observe this entire process with a proper X-ray telescope and a ground-based optical telescope,” said Dr Cristina Baglio, a postdoctoral researcher at the Centre for Astro, Particle and Planetary Physics at NYU Abu Dhabi and one of the study’s authors.

“We decided to start monitoring this in mid-July (2019) because we expected the outburst to come. We were able to advertise it to the entire scientific community, who started to observe this system.”

The researchers analysed what is known as an accreting neutron star system. Neutron stars are what is left behind when a star much bigger than our sun has used up most of its energy, exploded in a supernova and then collapsed in on itself.

The one observed by the NYUAD researchers is a rotating neutron star, or pulsar, just a few tens of kilometres across and spinning 400 times per second.

Likely to be several billion years old, the neutron star is orbited by another star, from which it draws in material by gravity into a structure around itself called an accretion disk. While this material is drawn in continually, it only actually reaches the neutron star every few years, when an outburst happens.

Using automated systems to pick up changes in activity in space, the researchers spotted the first signs of light emitted by the outburst, followed by the explosive burst of X-ray emissions, then the end of the outburst.

Land-based optical telescopes (which detect visible light) and telescopes in space – including one on the International Space Station – that observe X-rays showed that it took 12 days for the material to swirl inward and collide with the pulsar, several times longer than astrophysicists expected.

Until now, researchers had not picked up such an event early enough to follow it all the way through from start to finish.

The findings were presented this month at an online meeting of the American Astronomical Society and are due to be published in the Monthly Notices of the Royal Astronomical Society. Other authors include Dr Daniel Bramich of NYU Abu Dhabi and Adelle Goodwin from Monash University in Australia, the lead researcher.

“I’ve been observing these objects for about 15 years. In the last few years we set up an automated system that searches for new outbursts,” said Dr David Russell, an assistant professor of physics at NYU Abu Dhabi and another of the paper’s authors.

“In the past we detected many outbursts but were not able to collect all of the X-ray data soon enough. I was very excited with the results.”

The system is about 10,000 light years away – one light year is about 5.88 trillion miles – meaning that the observed event actually took place around 10,000 years ago. Despite the enormous distances involved, the system is actually within our own galaxy, the Milky Way.

The pulsar will continue to pull in material from the orbiting star every few years, leading to more X-ray outbursts, something likely to continue for the next 100,000 years or so until just a small star is left orbiting the pulsar.

“Einstein's theory of General Relativity led to the prediction of pulsars. By studying pulsars, we can test the theory of General Relativity in the most extreme gravitational fields – conditions we cannot replicate in a lab on Earth,” said Dr Russell.

“Testing General Relativity is important, because not only does it describe the nature of space-time, it is also needed for GPS satellites to help us navigate on Earth, and your mobile phone's maps and positioning functions would not work without it.”

The work by NYU Abu Dhabi has come at a time when the UAE is strengthening its interest in space, especially with the planned launch next month of its Hope mission to Mars.

Dr Russell said amateur astronomers with telescopes containing mirrors 30cm to 40cm in diameter may have been able to observe the burst of light emitted by the accreting neutron star system.

“The object was only visible from the southern hemisphere as a bright dot, but it’s in a very crowded part of the sky. We’re peering through the plane of the galaxy. There are a lot of other stars nearby,” he said.

“If you didn’t know what you were looking for, you wouldn’t see it.”