Weinberg using NSF grant for simulation project to learn more about neutron stars

A theorist in astrophysics at The University of Texas at Arlington is leading a project to study the explosive phenomena of X-ray bursts in order to better understand neutron stars.

Nevin Weinberg, associate professor of physics, will head the study, titled “Spectral and Radiation Hydrodynamic Models of Photospheric Radius Expansion X-ray Bursts,” which is being funded by a three-year grant from the National Science Foundation’s Division of Astronomical Sciences. He will be joined by students from UTA and by physicists from the University of Virginia.

A neutron star is formed when a massive star explodes in a supernova, and the remnants of the star condense and collapse in upon itself, caused by extremely powerful gravitational. This material is packed in so tightly that protons and electrons combine to make neutrons, thus giving these objects the name neutron stars.

If the neutron star is in a binary system with another star, the neutron star can draw hydrogen-rich or helium-rich material from the other star. This material is deposited on the surface of the neutron star and a thin surface layer of helium accumulates. Once it reaches critical mass, it ignites in a thermonuclear explosion, heating the entire surface of the neutron star to tens of millions of degrees Kelvin and releasing a sudden burst of X-rays.

Around 20 percent of these X-ray bursts are photospheric radius expansion (PRE) bursts, in which the luminosity of the burst is so high that radiation forces drive an optically thick wind that lifts the photosphere (the outer shell from which light is radiated) off the neutron star’s surface.

“A neutron star is the most dense, compact known stellar object, apart from a black hole,” Weinberg said. “It has a mass that’s 1½ to 2 times the mass of our Sun, but a radius of only 10 kilometers, so it’s extremely compact. If you took a teaspoon of this material, it would weigh more than a billion tons. The densities are so high that we don’t really have a good understanding of how matter behaves at such high densities in the core of a neutron star.”

The study will utilize state-of-the-art numerical simulation tools to provide the most sophisticated simulation of PRE X-ray bursts to date. The researchers will compare the results of their simulations directly with observations from telescopes.

X-ray bursts can help constrain the structure of neutron stars which in turn can constrain properties of the strong force in an extreme environment that cannot be reproduced in labs on Earth. The strong force -- one of the four known forces in the universe, along with gravity, electromagnetism, and the weak force -- binds the nuclei of atoms together and is responsible for processes of particle creation in high-energy collisions. Neutron stars are unique environments in which all four fundamental forces of nature are simultaneously important.

“Neutron stars are interesting objects, not just for astrophysics but for fundamental physics,” Weinberg said. “At these very high densities – densities higher than the nucleus of an atom – we don’t actually know how matter behaves. So, if we can learn about that through neutron stars, we can make progress on these important questions related to quantum chromodynamics and the properties of the strong force that have been around for a long time.”

X-ray bursts have been detected by telescopes since the 1970s, but much remains unknown about them.

“This is where our project comes in. We’re going to try to improve the models of X-ray bursts,” Weinberg said. “We’re going to be doing the first general relativistic hydrodynamic simulations of X-ray bursts and their winds. The goal is to understand them better and to have a better agreement between the models and the observations, which will allow us to make more precise statements about the underlying neutron star that’s supporting the burst.”

The teams from UTA and the University of Virginia will study the physics of PRE X-ray bursts using a combination of sophisticated computer models to simulate X-ray burstsin one-dimensional symmetry and two-dimensional axisymmetry. The anticipated results will allow astronomers to better understand neutron stars, the burning processes during X-ray bursts, and the composition of the wind ejected during a burst.

“There have definitely been advances in the study of neutron stars in the past few decades,” Weinberg said. “A big one just recently is the use of more powerful X-ray telescopes. There’s a telescope called NICER, and it’s especially good at looking at these X-ray bursts and detecting spectral lines and features from these bursts. One of the motivations for our project is some of the anomalous things that NICER has seen – indications of spectral lines that we think are heavy elements that were synthesized during the birth itself and ejected into this wind. We’re trying to find out if we can see that in our models and simulations as well.”

NASA’s NICER (Neutron star Interior Composition Explorer) telescope was installed on the International Space Station in 2017 and provides high-precision measurements of neutron stars by using X-ray timing and spectroscopy. One of NICER’s goals is to answer the longstanding question of how big neutron stars are.

The project will provide three years of funding for a graduate student in Weinberg’s research group and will allow him to mentor undergraduates from UTA’s Louis Stokes Alliances for Minority Participation (LSAMP) Summer Research Academy, which provides research opportunities for students from groups historically underrepresented in STEM education.

Weinberg first began studying X-ray bursts as an undergraduate student at the University of Chicago and is excited for the opportunity to return to the topic now.

“Neutron stars in general are really neat because you have to use all different types of physics to understand them,” he said. “There’s hydrodynamics; there’s quantum mechanics, since they’re such dense objects that quantum mechanical effects become important; there’s interesting statistical physics. So, there’s all different types of physics that come into play and are fun to think about in the context of these objects. Then there are these fantastic observations that X-ray telescopes are making, so there’s a nice interplay between theory and observation.”

Alex Weiss, professor and chair of the UTA Department of Physics, said Weinberg’s project could yield exciting new insights into the properties of neutron stars.

“The computer simulations of X-ray bursts that will be used in this study, together with observations from X-ray telescopes, could provide answers to some of the questions we have about neutron stars,” Weiss said. “This project being led by Dr. Weinberg is a great example of the kind of innovative and collaborative work being done in the Department of Physics.”

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