Simulations Show X-Rays from Neutron Stars Blasting Surrounding Plasma

Results open a new way to study the physics of accretion disks

January 7, 2020 | Atlanta, GA

Editor's Note: This story is adapted from a press release published January 6 by the College of Charleston.

X-ray bursts are highly energetic releases of radiation from the surfaces of neutron stars. They are triggered by the explosive burning of material on the star’s surface. Because the radiation escapes almost instantly, anything around the neutron star will be blasted.

Surrounding many neutron stars is an accretion disk, a swirl of plasma caught in the star’s gravitational field. X-ray instruments, such as the NICER mission on board the International Space Station, have given astronomers tools to study in detail these X-ray bursts from neutron stars and their effects on the accretion disk.

Neutron stars are so important to astrophysicists because they represent the densest state of matter in our universe. Understanding how matter behaves under these conditions is important in unlocking the mysteries of subatomic physics and extreme gravity. However, understanding neutron stars requires understanding the total radiation we receive from them. That includes contributions from the neutron star itself and from the surrounding disk.

This is where new research by Chris Fragile and his student Aidan Blankenship at the College of Charleston comes into play. They performed multiple computer simulations of the interactions of X-ray bursts with accretion disks.

The simulations uncovered many remarkable results. The most notable is a significant disruption of the inner parts of the accretion disk. Several effects revealed in the simulations appear to match evidence of disrupted disks seen by X-ray telescopes over the past 15 years.

Results were published on Jan. 6, 2020, in Nature Astronomy.

“I was really excited to see these results,” says David Ballantyne, an associate professor in the Georgia Tech School of Physics and a collaborator on the work. “I've been studying these systems for over a decade, trying to understand what the data are telling us about how these disks respond to bursts. The details revealed by these simulations opens a whole new way of studying the physics of accretion disks.”

Seeing the disk being disrupted by the burst and then bouncing back as the burst fades provides a way to study the internal processes causing accretion to work. “I like to say we're giving the disk a kick and watching what happens,” Ballantyne says. “Seeing how quickly a disk responds to such a strong impulse allows us to peer into its insides. It’s similar to how scientists use earthquakes to learn about the interior of Earth.”

Ballantyne hopes to use these results and make predictions for future observations. Eventually, the team hopes to be able to reverse engineer what is happening in real neutron star systems and accretion disks.

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