Neutron stars are some of the most extreme objects in the Universe. They are incredibly dense, extremely hot and have very strong gravity. Although they are only about the size of a city, they can contain more mass than the Sun.
These strange objects can also produce powerful explosions called X-ray bursts. Scientists have now found that some of these bursts may be doing something unexpected: they may be bending and disturbing the disk of gas surrounding the neutron star.
A new study by Ballantyne and Degenaar examined five unusually long X-ray bursts that showed sudden changes in brightness. The researchers found evidence that these strange fluctuations may be connected to changes in the shape of the surrounding accretion disk.
The discovery could help scientists understand one of the biggest mysteries in astrophysics: how material moves through the disks around neutron stars and other compact objects.
What Is a Neutron Star?
A neutron star is the extremely dense leftover core of a massive star that has exploded in a supernova.
Imagine taking more mass than our Sun and squeezing it into a sphere only around 20 kilometres wide. That is roughly what a neutron star is like.
Because so much matter is packed into such a small space, its gravity is incredibly powerful.
Many neutron stars have a companion star nearby. The neutron star can pull gas away from its companion. This gas does not usually fall directly onto the neutron star. Instead, it forms a rapidly rotating disk around it.
This is called an accretion disk.
As the gas moves inward, friction and other processes make it extremely hot. The material can then produce powerful X-rays.
When a Neutron Star Explodes
Sometimes, gas collected on the surface of a neutron star becomes unstable.
A runaway nuclear reaction suddenly begins, producing a huge amount of energy. This creates what astronomers call a thermonuclear X-ray burst.
The burst can become extremely bright in a very short time.
Most X-ray bursts last only around 10 to 20 seconds. They rise quickly to a peak and then slowly become weaker as the neutron star's surface cools.
But some bursts are much longer.
Certain events can last for hundreds of seconds, while the most powerful ones, called superbursts, can continue for thousands of seconds.
During these explosions, a neutron star can release an enormous amount of energy—around 10³⁹ to 10⁴¹ ergs.
That radiation does not only affect the neutron star.
It also hits the material surrounding it.
The Burst Can Disturb the Surrounding Disk
The powerful radiation from an X-ray burst can dramatically change the accretion disk.
It can heat the gas to extremely high temperatures and change the atoms in the disk through a process called ionization.
The radiation can also affect the hot region above the disk, known as the corona.
It may even change the position and structure of the inner edge of the disk.
But scientists think something even more interesting may happen.
The intense radiation may cause the accretion disk to bend or warp.
Normally, we imagine an accretion disk as a flat structure rotating around the neutron star.
But if radiation pushes different parts of the disk in different ways, the disk may become tilted, twisted or warped.
This is known as a radiation-driven warp.
The Strange Brightness Fluctuations
The researchers were interested in a small group of unusual X-ray bursts.
These bursts do not simply fade smoothly.
During their fading stage, their brightness suddenly goes up and down.
The changes can be very large—sometimes around 70 percent.
The fluctuations can happen on timescales of about one second to one minute. They may continue for tens or even hundreds of seconds before suddenly disappearing.
This behavior has puzzled scientists for years.
The systems showing these fluctuations are not known to be eclipsing systems or typical "dippers." That makes it unlikely that another object is simply blocking the X-rays from our view.
The timing of the fluctuations provides an important clue.
Scientists believe that the changes are probably caused by an interaction between the X-ray burst and the surrounding accretion disk.
Scientists Looked at Five Unusual Bursts
Ballantyne and Degenaar studied five long X-ray bursts that showed these rapid brightness fluctuations.
Instead of looking only at the overall brightness, they examined the X-ray spectrum of each burst.
They studied the spectra before the fluctuations started, while the fluctuations were happening and after they stopped.
This allowed them to see whether the material surrounding the neutron star was changing.
The results were fascinating.
All five bursts showed evidence of ionized reflection from the accretion disk during at least one stage.
Reflection occurs when X-rays from the neutron star hit the surrounding disk and are reflected back into space.
The reflected X-rays carry information about the temperature, chemical state and shape of the disk.
The Disk Appeared to Change
In several of the bursts, the researchers found that the properties of the reflecting region changed dramatically when the brightness fluctuations started or stopped.
For example, the level of ionization in the disk changed.
The apparent viewing angle of the reflecting region also changed in some cases.
Even more importantly, the amount of direct X-ray emission coming from the neutron star's surface changed significantly.
At certain times, the direct emission from the neutron star appeared to be hidden, while the reflected X-rays from the disk became much more important.
This strongly suggests that the geometry of the system was changing.
A changing or warped disk could explain why.
Scientists Compared Them With Five Normal Bursts
To make the result stronger, the researchers also studied five similar long X-ray bursts that did not show rapid fluctuations.
This was an important control group.
Four of these five bursts were best explained by a simple blackbody spectrum, with almost no evidence of strong reflection from the accretion disk.
Only one burst, from XTE J1810-189, showed reflection-dominated emission similar to the unusual group.
This difference gives scientists an important clue.
The bursts with rapid fluctuations appear to have surrounding disks that behave differently from most of the bursts without fluctuations.
Longer Bursts May Be More Powerful Enough to Warp the Disk
The researchers found another important difference.
The bursts with rapid fluctuations were generally longer and more energetic than the bursts without fluctuations.
This fits the idea of radiation-driven warping.
A powerful burst sends an enormous amount of radiation into the surrounding disk.
If the burst lasts long enough, the radiation may disturb the disk strongly enough to make it warp.
As the shape of the disk changes, different parts of it may block, reflect or redirect X-rays.
From Earth, we would see this as sudden changes in brightness.
In simple terms, the strange X-ray flickering may be the visible sign of an accretion disk changing its shape.
Could This Reveal the Disk's Viscosity?
One of the most exciting parts of this research is the connection to something called viscosity.
Viscosity describes how material inside the disk interacts and how efficiently matter and angular momentum move through it.
Scientists know that viscosity is extremely important for accretion disks, but measuring it directly is difficult.
Theoretical models suggest that radiation-driven warping is easier to produce when the disk has a relatively high viscosity.
Therefore, if the rapid X-ray fluctuations are really caused by radiation-driven warping, scientists may be able to use them as a tool for estimating the viscosity of these extreme disks.
That would give researchers a new way to study how matter behaves under some of the most extreme conditions in the Universe.
Why Do Some Bursts Show No Reflection?
The study also offers a possible explanation for the bursts without strong reflection.
These systems may have lower accretion rates, meaning less material is flowing through their disks.
Their disks could have lower surface densities.
When the powerful X-ray burst heats the disk, the gas may expand and become more spread out.
The disk could then become Compton-thin, making it much less effective at reflecting X-rays.
So, a lack of reflection does not necessarily mean that the disk is unaffected.
It may simply mean that the disk has responded differently to the burst.
The Future of This Research
The study shows that X-ray bursts can act like natural experiments.
Every time a neutron star produces a powerful burst, scientists get an opportunity to watch how extreme radiation interacts with the surrounding accretion disk.
Future X-ray observatories with much higher sensitivity and faster observations could provide an even clearer picture.
Missions such as STROBE-X, eXTP and NewAthena could potentially track these changes in much greater detail.
Scientists may eventually be able to watch an accretion disk heat up, become ionized, change its shape and return to normal.
The strange flickering of neutron stars may therefore be more than just a mysterious signal.
It could be a cosmic window into the hidden physics of accretion disks—revealing how matter moves, how disks respond to extreme radiation and how viscosity controls one of the most important processes in high-energy astrophysics.
Reference: D.R. Ballantyne, N. Degenaar, "Do Accretion Disks Get Bent Out of Shape? Investigating the Origin of Rapid Fluctuations in the Tails of Long Thermonuclear X-ray Bursts", Arxiv, 2026. https://arxiv.org/abs/2608.07758
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