How can we weigh lone pulsars?
Wynn Ho has come up with a new way to check the mass of neutron stars without cosmic companions
OINTERVIEWED BY PAUL SUTHERLAND
ne of the most exotic objects in the Universe is the pulsar. It spins at an extremely rapid rate like a supercharged cosmic lighthouse, firing beams of light at precise intervals. We know today that a pulsar is a type of highly magnetised neutron star formed by the collapse of the heart of a supernova. The material inside is so tightly packed that it is about 100 trillion times more dense than water.
Studying pulsars is important because their extreme conditions allow us to test our understanding of fundamental, particle and nuclear physics. The results are useful across a lot of different areas. To understand it, one of the main things you need to know about a pulsar is its mass, and the usual way to work this out for an object in space is by using Newton’s laws to observe how its gravitational pull affects an object orbiting it.
Going solo
However, whereas most stars in the Universe appear to be in double or multiple systems where you can easily apply Newton’s laws, only around a tenth of known pulsars have companions. That’s because the explosion produced when a star goes supernova usually destroys the companion star it previously had, or unbinds the system. This means about 90 per cent of pulsars are solitary objects that can’t be weighed the conventional way.
My team decided to approach the problem of weighing a lone pulsar from a different angle: by examining the physical properties of the object itself, using principles of nuclear physics rather than gravity, to determine the mass. We did this by observing glitches in the pulsar that occasionally disrupt its spin, making it speed up. We realised that by discovering exactly what was happening during the glitches, we could measure the pulsar’s mass without having a companion star nearby. Dr Wynn Ho is based at the University of Southampton, where his primary research interests are in theoretical and high-energy astrophysics, including getting under the crust of compact stars like pulsars.
So what is happening in such a pulsar? As an analogy, imagine you have a cup of water on a table and the cup is spinning. Friction between the cup and the table will cause the cup’s spin to slow until it comes to a stop. But the water inside continues to spin fast as the cup slows down. Because a pulsar is emitting radiation, it’s also losing energy, like the cup, and slowing down. But the incredibly dense interior of the pulsar (material that we call a superfluid) continues to spin rapidly, even as the rest of the star slows down. Once in a while there’ll be some interaction between the superfluid and the crust of the pulsar. The superfluid transfers some of its rotational energy to the crust, causing the glitch that makes it briefly spin up. You need a detailed understanding of superfluidity to use this method of determining a pulsar’s mass. The size and frequency of the glitches depend on the amount of superfluid in the pulsar and how the superfluid behaves. By combining observational information with an understanding of the nuclear physics involved, you can ‘weigh’ the pulsar.
We tested our new technique by applying it first to a number of solitary pulsars. Our results are at a similar level of accuracy to those typically obtained from pulsars in binary systems, using gravity. So we’re very pleased with it. It shows the technique has the potential to revolutionise the way we make this kind of calculation. In the future, we hope to check our results by measuring the mass of the same pulsar using both methods.
Observing pulsars is one of the key science goals of the Square Kilometre Array (SKA) – a major radio telescope being constructed in South Africa and Australia. Our technique promises to underpin that research. And as new telescopes such as the SKA detect many more pulsars and monitor them for glitches, then we can refine our technique and use it to understand them a lot better.