This Irish scientist is helping us get to the core of what neutron stars are

15 Feb 2023

Dr Mark Kennedy of University College Cork. Image: UCC

Based in UCC, Dr Mark Kennedy is part of a global team that has stumbled upon a stellar discovery that answers, and raises, questions about neutron stars.

Dr Mark Kennedy’s research over the years has entailed the entire life cycle of stellar evolution from white dwarfs and neutron stars to black holes, a “perfect trifecta” as he calls it.

His latest contribution in the field of astrophysics is being part of an international team that has deepened our understanding of one of the most exotic objects in the known universe: neutron stars.

Working with data from NASA’s Fermi Gamma-ray Space Telescope, the team led by Dr Colin Clark of the Max Planck Institute for Gravitational Physics in Hannover discovered the first ‘gamma-ray eclipses’ from a special type of binary star system known as a spider system.

“So neutron stars are the dense core remnants of a star that’s finished its lifecycle. It’s burned through all of its fuel and undergone a supernova explosion, which has produced and left behind the neutron star,” Kennedy explained to in a recent interview.

Kennedy is a scientist and lecturer based in University College Cork (UCC). He has previously conducted postdoctoral research in neutron star binaries as a Newton International Fellow at the University of Manchester.

While there are a lot of interesting things about neutron stars, Kennedy noted the two most interesting things about them. The first is the fact that we don’t know what their ‘equation of state’ is, i.e., how the matter responds to changes in temperature and pressure.

“Knowing the equation of state is really important because that tells us how massive a neutron star can be before it gets too massive and collapses in on itself,” he said.

And that raises the second interesting question: when a neutron star gets too big, can it become a black hole? And what does that type of event look like? These are just some of the questions that the latest discovery will help astrophysicists come closer to answering.

The Black Widow Pulsar

The latest discovery involves the study of spider systems, which are basically binary star systems that contain a rapidly rotating neutron star slowly evaporating a nearby companion star.

“This window, which involves looking for a dip in the number of detectable gamma-rays emitted by the neutron star as a companion star passes between the neutron star and us, allows us to measure the mass of the neutron star independently of previous techniques,” Kennedy explained.

Perhaps the most important result of this method is that it has led to a revision of the mass of the neutron star in the binary star system named PSR B1957+20, also known as the Black Widow Pulsar because of its tendency to “cannibalise” its companion star.

“Previously thought to contain a neutron star with a record-breaking mass equal to 2.4 times that of the sun, we now know because of the gamma-ray data of B1957 that the actual mass is 1.8 times that of the sun,” Kennedy said.

“This reopens the question behind the maximum mass of neutron stars, and suggests significant work is required in the near future to explain why previous neutron star masses may have been so far off the mark.”

‘Invisible Monsters’ project

Originally from Tramore, Co Waterford, Kennedy did his undergraduate studies in astrophysics at UCC, graduating in 2013. He was then awarded a Naughton Fellowship to do a PhD between UCC and the University of Notre Dame, completing it in 2017 before moving to Manchester.

Kennedy returned to Ireland in 2021 after being awarded a Government of Ireland postdoctoral fellowship. At UCC, he works – among other projects – on the Invisible Monsters project, that is on a mission to find ‘hidden’ black holes using machine learning and all-sky surveys.

“Our galaxy evolution models suggest that there should be about maybe a million stellar mass black holes, which are black holes that have a mass of somewhere between one and 20 times the mass of the sun,” he said, adding that there are only nine known black holes in the Milky Way.

“So the key question that I’m trying to ask is, where are all the other black holes hiding?”

This is the project that Kennedy says most excites him, hoping one day that the ‘invisible monsters’ will become visible. At the moment, he has some “candidate systems” that might have black holes in them based on data collected so far.

“I’m working on writing proposals to get follow-up data to try and confirm whether there are black holes in it or not. But yeah, I think that’s probably the most exciting project – the one I get excited about when I sit down to type away at work.”

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Vish Gain is a journalist with Silicon Republic