Universiteit Leiden

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Discovering new physics in extremely bright neutron stars

Astronomer Alexander Mushtukov, currently working at the University of Amsterdam, received a Veni grant of 250,000 euro’s which he will execute in Leiden. Supervised by Simon Portegies Zwart, Mushtukov will use advanced simulations to understand the unknown physics in extremely bright neutron stars.

Leftovers from supernova explosions

Neutron stars are extremely compact remnants of supernova explosions. Their masses are similar to the mass of the Sun, while the radii are a mere 10 kilometers – the radius of the sun is approximetaly 700,000 km). Very recently, it has been discovered that neutron stars can be extremely bright. These radiant neutron stars are one of the driving forces behind so-called Ultraluminous X-ray sources (ULXs) – which can also be black holes. And these ULXs are the subject of interest of Mushtukov. But what makes them so interesting?

Strongest magnets around

Neutron stars are the strongest magnets in our Universe. Their magnetic fields are up to a million times stronger than the strongest magnetic field that scientist can produce in labs here on Earth. With such extremely powerful magnetic fields, standard rules in chemistry and physics do not apply anymore. So, how do particles behave under these extreme conditions? And how is it possible that these neutron stars are so extremely bright? Mushtukov will try to answer these questions by comparing observational data with advanced simulations.  

Not a black hole

Theoretically, neutron stars cannot be brighter than a certain value. But four years ago, astronomers discovered that some ULXs – which were thought to be black holes – actually were neutron stars. ‘These extremely bright neutron stars have a companion, forming a binary system’, says Mushtukov. ‘In these cases, neutron stars can grab and absorb material from their companion star. Often this process results in the emission of X-rays, which can be detected by X-ray space observatories orbiting our planet. I have a hypothesis that explains how these neutron stars can be so extremely bright, and the strong magnetic field is a key ingredient in my model. I will model this idea on the computer, and then compare our model with real observations of neutrons stars.


Mushtukov wrote his proposal together with Simon Portegies Zwart, who is an international expert in this field. ‘The calculations I will use in this research are too complicated for normal computers, so I need a supercomputer. And Simon Portegies Zwart can help me with this. Here, at Leiden University, Simon leads a strong team focusing on advanced computational astrophysics. That is the expertise I need for this research.’

Understanding the Universe

‘We expect to construct the most detailed model of highly magnetized neutron stars at extreme conditions’, says Mushtukov. ‘With this model, we hope to clarify the nature and origin of the brightest neutron stars in the Universe. Our models and solutions can be ground-breaking for the understanding of the physics of radiation and matter interaction under conditions of extreme magnetic fields and temperatures. With a successful model, we will be able to distinguish accreting neutron stars from black holes. This will also help in determining the population of accreting neutron stars among ULXs, which is relevant for understanding the evolution of massive stars. In addition, our research will enable us to probe a challenging and longstanding problem of fundamental physics: the correctness of Quantum Electrodynamics in extremely high magnetic fields, where the theory has not been verified so far.’

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