24 July 2020 News

A first of its kind white dwarf merger with a dense compact object discovered by students

An artist's depiction of a black hole swallowing a neutron star. Image: Carl Knox/OzGrav ARC Centre of Excellence
An artist's depiction of a black hole swallowing a neutron star. Image: Carl Knox/OzGrav ARC Centre of Excellence

An explosion in space discovered in 2018 by a Queen’s University, Belfast student has turned out to be the first observed merger of its kind, and only the second ever compact merger to be seen via the light it emitted when the cataclysmic event took place.

Back in 2016, the discovery of gravitational waves produced by the merger of two black holes sent ripples of excitement around the globe.

Not only did this scientific breakthrough prove Einstein right, again, but it kick-started a new field of astronomy and had everyone connected to the Laser Interferometer Gravitational-Wave Observatory (LIGO) – the facility that recorded the event – on the hunt for more.

Current reports suggest that binary black hole mergers are being discovered by LIGO and its sister facility, Virgo, at a rate of one discovery per week.

Now though, it is not just cosmic collisions between black hole behemoths that are being observed, but mergers between black holes and neutron stars too.

More recently, a smash up between two neutron stars heralded another new development in the world of gravitational wave discoveries, when a pair were caught colliding 130 million light years away.

Although neutron stars are tiny compared to black holes, they are massive. A tablespoon of material from these small, superdense, collapsed stars weighs more than 900 billion kilograms (one billion tons) — the same approximate weight of Mount Everest.

But what set this revelation apart from the rest was that two seconds after the waves were captured by the interferometers, a gamma-ray burst was also detected.

The discovery, which became known as GW 170817, marked the first cosmic event observed in both gravitational waves and light. And so far, it is the only one.

But that could be about to change.

Just under two years ago on 28th December, 2018, Owen McBrien, a student at Queen’s University, Belfast, spotted an extremely fast evolving astronomical transient.

Transients are typically extreme, short-lived events that last fractions of a second. Some do last weeks or years but nearly always, no matter the length of their occurrence, they are all associated with the total or partial destruction of an astrophysical object.

This particular transient spotted by McBrien was labelled AT2018kzr, and its origin, for the time-being remained a mystery.

Since then, second year PhD student James Gillanders, and his two PhD supervisors, Professor Stephen Smartt and Dr. Stuart Sim, also based at Queen’s, decided to take a closer look at the spectra – the intensity of light emitted over a range of energies – of the transient as it evolved.

Using computer code developed at Queen’s to model the spectra, the trio were able to constrain the abundance of different elements that were produced in the brief flash of light left behind by AT2018kzr.

The team found signatures of oxygen, magnesium, silicon and iron, lots of iron - enough to warrant further investigation.

Their analysis indicated that the iron was a stable isotope (54Fe) and wasn’t produced from the decay of radioactive nickel that was created in the explosion.

Considering the unusually rich abundance of this isotope, coupled with the rapid evolution of the transient and other factors the team examined, they could come up with only one plausible scenario to explain it all; the merger of a white dwarf with either a neutron star or a black hole.

“The explosion is likely to be the result of a merger between a white dwarf (the core of a star after it has evolved and shed all its outer envelope) and either the most dense, or second most dense object in the known Universe (black hole or neutron star, respectively),” says James Gillanders.

“In this case, two stars were orbiting each other and both evolved in different ways. One collapsed into a black hole or neutron star while the other became a white dwarf. This left a ball about the size of the earth, which was ripped apart as it orbited too close to its companion,” he added.

While the battle between the two cosmic heavy-weights didn’t end so favourably for the objects in question, it did end well for Gillanger and colleagues, as this discovery is the first observed merger of its kind, and only the second ever compact merger which has been observed via the electromagnetic spectrum.

And now that the trio know what signal to look out for, more discoveries could follow soon.

“The fast evolution of the transient makes it difficult to follow for an extended period, and so they need to be identified as a potential merger candidate, and observed frequently, soon after discovery,” write the authors in their research paper.

“In the early phases, AT2018kzr was characterised by a hot, blue continuum, with strong Fe [iron] absorption. This unusual combination could be used as a sign that the transient is worth follow-up observations,” Gillanger and colleagues add.

This research has now been published in the Royal Astronomical’s Monthly Notices.

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