Tag Archives: thermonuclear burst

Bent out of shape

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When neutron stars warp their surroundings

Over the past decade it has become increasingly clear that thermonuclear explosions on the surface of neutron stars can have a significant impact on their surrounding accretion disks and on the outflows that they produce. This provides us with something that is very rare and truly unique in astrophysics: a highly repeatable and (somewhat) controllable experiment. By carefully choosing our subjects and measurement equipment, we can use thermonuclear bursts to study fundamental structures and processes that remain challenging to unravel after decades of study. Had it previously already been shown how thermonuclear bursts can enlighten the dynamic physical properties of accretion disks, hot electron coronae, radio jets and disk winds, we can now add another intriguing phenomena that can be studied in this way: accretion disk warps.

Thermonuclear X-ray bursts happen when material piles up on the surface of an accreting neutron star and suddenly ignites, burning away an entire surface layer in an instant, much like the head of a matchstick at stroke. These explosions generate a flash of X-ray emission that last seconds to to hours and are easily detectable with many X-ray telescopes because they are so whooping bright. We can thus easily pinpoint when these explosions are happening and can also readily measure the amount of radiation and energy that is released during these events. The explosions also act like a sudden floodlight, illuminating the surrounding gas and allowing us to watch how the accretion disk reacts to extreme heat and radiation. Usually, the X-ray bursts follow a predictable pattern of a sharp rise in brightness followed by a smooth, graceful fade. Every now and then, however, some of the longer and more energetic bursts instead show rapid, chaotic flickering as the bright emission from the explosion gradually fades.

For years, the cause of the flickering fluctuations in long bursts, where the X-ray emission jumps up and down by as much as 70%, has remained unknown. But now we have found evidence that these flickers are likely the result of the accretion disk literally getting bent out of shape due to the impact of the explosion. By performing a systematic and analysis of a sample of long bursts caught by the Swift satellite, we found that the direct light that we normally see from the neutron star’s surface vanishes from view when erratic flickering occurs. We concluded that when a burst is long and energetic enough, the sheer pressure of the radiation pushes against the disk, triggering an instability that warps it into a three-dimensional, wavy structure.

The most exciting part of this discovery is what it tells us about the physics of the disk itself. This warping instability is incredibly sensitive to the viscosity of the disk, essentially the cosmic glue or friction that determines how rapidly the gas flows toward the neutron star. Only disks with high viscosity are likely to get bent out of shape in this way. By studying long energetic thermonuclear bursts that flicker, we thus aren’t just watching a light show but we are actually measuring the sticky properties of ultra-hot gas in some of the Galaxy’s most extreme environments.

Understanding the viscosity of accretion disk is important for big picture questions about the universe. However, it is very difficult to know the value of this parameter from theoretical principles or to determine it from astrophysical observations. We now know that catching and studying long bursts with erratic fluctuations with (next-generation) sensitive X-ray satellites allows us to watch accretion disks ripple and bend in real-time, providing a new view of the hidden mechanics that drive the growth of neutron stars and black holes.

Ballantyne & Degenaar 2026, appearing in ApJ, arXiv:2608.07758: Do Accretion Disks Get Bent Out of Shape? Investigating the Origin of Rapid Fluctuations in the Tails of Long Thermonuclear X-ray Bursts

Paper link: SciX

Example of a long, energetic thermonuclear X-ray burst that shows erratic flickering; in this case this happens between approximately 500 and 1100 s (about 8-18 min) after the peak of the explosion. We know understand that this flickering is caused by the accretion disk being warped and bent out of shape during this time, causing large variations in the light that we see from the disk and the neutron star in its center.

New kid on the block

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The center of our Milky Way is like the cosmic Times Square; a crowded, chaotic neighborhood where stars are closely packed and a supermassive black hole, Sgr A*, is the center of attention. For over 20 years, the Swift satellite has been our eyes on the ground, taking daily X-ray snapshots to catch the sudden activation of otherwise quiet inhabitants in this busy region. In February 2024, suddenly a shy new face appeared in the crowd: a faint X-ray source popped up and now carries the name Swift J174610−290018.

Soon after it stuck its head above the crowd, it became clear that Swift J174610−290018 was a member of the X-ray binary class of objects. These are binary star systems wherein a neutron star or a black hole devours its companion star through a process that we call accretion. However, while most X-ray binaries draw attention by shining very brightly in X-rays, this new kid on the block appeared much more shy, radiating only faint X-ray emission. This sub-set of X-ray binaries are called Very Faint X-ray Transients (VFXTs) and they appear to take only light bites instead of full meals, which makes them much harder to spot and to study in detail. Swift J174610−290018 showed up for about 50 days in 2024, only to vanish and reappear for a brief, five-day rendez-vous in April 2025,

Figuring out the exact nature of Swift J174610−290018, for instance if it contains a black hole or a neutron star, required some serious detective work in the X-ray data archives of NASA and ESA. Digging through data from the XMM-Newton satellite, we found a single, bright X-ray flare coming from the position of Swift J174610−290018 that had occurred in 2004 and hence had been hiding in the archives for two decades. This X-ray flare had all the hallmarks of a thermonuclear X-ray burst: a sudden thermonuclear explosion on the surface of an accreting neutron star. Such an event is the ultimate ID card showing that Swift J174610−290018 most host a neutron star, because black holes don’t have a solid surface to catch falling gas and hence cannot trigger these kinds of explosions.

Very-faint X-ray binaries are among the Galactic center’s most elusive inhabitants but may well represent the largest population of X-ray binaries in the Milky Way. For this reason finding, identifying and characterizing them, as we did for Swift J174610−290018, remains an important endeavor to understand the make-up of our Galaxy and to understand how pairs of binary stars live and end their lives. The discovery of this new X-ray source is another win for long-term monitoring programs which have proven very effective in catching these faint transients and allowing to trace their history across decades of data. Swift J174610−290018 also proves that even in a region we’ve watched daily for 20 years, there are still new neighbors waiting to be introduced.

Stel, Ponti, Degenaar, Sidoli, Mereghetti, Mori, Bao, Illiano, Mondal, Reynolds, Jin, Lian, Mandel, Scaringi, Zhang, Sanger-Johnson, Wijnands, Miller, Kennea, Zhu 2026, Astronomy & Astrophysics 705, 135: The very faint X-ray transient Swift J174610-290018 at the Galactic center

Paper link: SciX

Neighborhood map showing newly the discovered very-faint X-ray transient Swift J174610–290018 nested in the crowded hearth of our Milky Way’s center, spatially close to our supermassive black hole Sgr A*. Shown is the accumulated X-ray image from the EPIC-pn camera onboard the XMM-Newton satellite taken in the spring of 2024. Swift J174610–290018 is located 6.7 arcmin east of Sgr A*. The neutron star X-ray binary AX J1745–2901 is often active and can also be clearly seen in this image, being the brightest X-ray source in the field of view. The inset displays the position uncertainties of the newly identified 2024 transient (green circle) and the positions from previously cataloged X-ray sources reported.

A cosmic speed camera

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Jets are collimated streams of gas and energy that are produced by a variety of astrophysical objects and phenomena. Jets are, for instance, produced by young forming stars, by neutron stars and black holes in X-ray binaries, and by supermassive black holes that lurk in the centers of galaxies. Moreover, jets are seen during explosive, cataclysmic phenomena such as supernova explosions, gamma-ray bursts and bursts of gravitational waves produced by the mergers of compact stars. Despite that jets are so omnipresent in the universe, it is not understood yet how jets are launched in different circumstances and how fast they travel through space.

In an attempt to break new grounds in our understanding of jets, we designed an out-of-the-box experiment to test if thermonuclear explosions that regularly occur on the surface of neutron stars could cause measurable variations in their jets. Our thought was that the radiative power of such explosions would blow away the launch region of jets, causing them to temporarily break down and rebuild thereafter. Observing that would provide unique new insight into how neutron stars produce jets. Since it was not possible to predict how large any effect would be, hence if we would be able to detect anything at all, it was not feasible to obtain observing time for this experiment through regular routes of proposing our idea to a time-allocation committee. Therefore, we used a special opportunity provided by the Dutch national research council (NWO) to apply for a small grant to fund high risk research (the NWO XS grant). With this grant we bought 80 hours of observing time on the Australian Telescope Compact Array (ATCA) radio telescope to perform our crazy jet experiment.

To complement the purchased radio observations that can detect a jet, we obtained time on the INTEGRAL satellite from the European Space Agency (ESA) to detect thermonuclear bursts. Setting up these strictly simultaneous observations was quite a challenge, but we managed to do two runs (each lasting 3 days) on two different bursting neutron stars: 4U 1728-34 and 4U 1636-536. During both runs we detected many bursts and…. we did see a clear jet response!! But it was completely opposite of what we expected: instead of seeing the jets fade (from breaking down) in response to the explosions, we observed a marked brightening of the jet. The fact that the jets persist despite of the bursts provides key information on these outflows are launched and crucial constraints for computer simulations that model the launch of jets.

Another exciting implication from our successful experiment is that the timescale of the response of the jet allowed us to measure, for the first time, the speed of the jet from a neutron star. We found that it is blasted into space at a dazzling speed of 300 million kilometers per hour (or traveling about 90 thousand kilometers in just a single second!). While neutron star jets thus have an enormous velocity, it a factor of 2-3 slower than the velocity measurements that we have for a handful of black holes. This suggest that the properties of the jet-launching object (e.g. their mass, their rotation rate or their magnetic field strength) must play a role in how jets are launched and powered. The important breakthrough of this discovery has opened up a completely new window to understand how jets are connected to the individual properties of a system, which provides us with fundamental insight into the launching of jets on all physical scales.

Never before were we able to anticipate and directly watch how a certain amount of gas got channeled into a jet and accelerated into space. Only the explosions on the surface of the neutron star could give us the clean and isolated view of this process to perform these measurements. Because of the high scientific impact, our results will be published in the journal Nature. Moreover, with the successful demonstration of the experiment it will no longer be difficult to obtain observing time through regular routes to take the same measurements for (many) other neutron stars. Lastly, the spectacular results of our crazy jet experiment demonstrate how valuable it is for science that there are opportunities to support high-risk research. Such projects may, by their very nature, often fail, but it is exciting and fun to try and can turn out to be very high gain.

Russell, Degenaar, van den Eijnden, Maccarone, Tetarenko, Sanchez-Fernandez, Miller-Jones, Kuulkers, Del Santo 2024, Nature 627, 763: Thermonuclear explosions on neutron stars reveal the speed of their jets

Paper link: Nature, ADS

Press releases: ESA, NOVA

Animation: mp4 (source ESA)

Main result of the crazy jet experiment. The top panels a-c show the X-ray light curves obtained with INTEGRAL for 3 consecutive days. The bright spikes of X-ray emission are 9 thermonuclear X-ray bursts. The bottom panels d-f show the simultaneous radio light curves obtained with the ATCA telescope at two different radio frequencies (5.5 GHz in red, 9 GHz in blue). The vertical grey lines indicate the times of the thermonuclear bursts. It is clear that shortly after each burst the radio emission is brightening as a result from extra material being pumped into the jet during a burst.

Discovery of a hyperburst

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A once-in-a-lifetime explosion inside a neutron star!

One of the many spectacular phenomena displayed by neutron stars are thermonuclear bursts. These events are observed from neutron stars that live their lives with a companion star which they steal gas from. Neutron stars can remove gas from their companion star at a startling rate of about 40 trillion kilograms per second. This gas accumulates on the surface of the neutron star, where the temperature and pressure increase as more and more matter piles up. Within hours, so much material builds up that nuclear reactions start occurring in the accumulated gas layer. This leads to the explosive ignition of the layer, causing it to burn away within seconds, somewhat like the head of a matchstick at stroke. The energy that is released in the explosion causes a bright flash of X-ray emission called a thermonuclear X-ray burst.

The most common type of thermonuclear bursts are the result of the ignition of a thin layer of hydrogen and/or helium, which are visible as flashes that last about 1 minute. Despite their short duration, an enormous amount of energy is produced in each of such bursts: about 1032 J (1039 erg). This is roughly as much as 1015 , which is one thousand trillion, nuclear bombs! Still, even more energetic bursts have been observed. The so-called superbursts are 1000x more energetic than common bursts, last for several hours and are the result of the ignition of a thick layer of carbon that was build up below the surface of the neutron star.

Recently we found evidence for the existence of yet another type of explosion from neutron stars that even diminishes the superbursts. This new kid in town, the hyperburst, is thought to be the result of the explosive burning of a deep layer of oxygen or neon, that has been steadily building up in the neutron star over hundreds or thousands of years of stealing gas from its companion. Hyperbursts would be the most energetic neutron star thermonuclear explosion known. releasing as much energy in about three minutes as the Sun releases in 800 years!

What does a hyperbursts look like? The tricky thing with hyperbursts is that they would not be directly visible as X-ray flashes, like the superbursts and common burst are detected. This is because the layer that ignites lies so deep inside the neutron star that all the energy that is released in the explosion gets absorbed by the layers above it and hence won’t escape through the stellar surface into our view.

So how did we discover a hyperburst then? Even if a hyperburst may not be directly visible, the enormous amount of energy it releases should leave its imprint on the neutron star, in particular on its outer layers that absorb the explosion. This is what we think we observed in the neutron star MAXI 0556–332. This source spend about a year feasting on its companion star in 2010-2011 and when we measured its temperature afterwards it turned out to be incredibly hot. We had taken similar measurements of 10 other neutron stars before, but MAXI 0556-332 was much more hot than any of them. Its unusually high temperature puzzled astrophysicists for many years, as it was difficult to understand why this neutron star was so different then the rest.

Over the past decade we observed MAXI 0556-332 to gradually cool down as the neutron star is radiating heat from its surface. From its cooling trajectory we can infer how much heat must have been injected into it when it was swallowing gas and also at what depth that energy must have been generated. We then realized that at this depth we to find layers of oxygen and neon and when we calculated how much energy the ignition of such a layer would generate, that turned out to match the energy that was needed to heat MAXI 0556-332 to its observed high temperature. So we indirectly observed what we coined a hyperburst.

When will we observe the next hyperburst? Igniting any type of explosion requires very high pressure and temperatures. We estimate that the temperature and pressure needed to ignite a deep layer of oxygen or neon may only be reached once in 1,000 years in any neutron star. Hyperbursts should thus be exceedingly rare and we might just never see one again. If this is indeed the case is what we hope to answer next as we launch new investigations to understand precisely under what circumstances hyperbursts can occur.

Some press coverage: New Scientist and Sci Physics

Page et al. 2022, the Astrophysical Journal 933, 216: A “Hyperburst” in the MAXI J0556-332 Neutron Star: Evidence for a New Type of Thermonuclear Explosion

Paper link: ADS

Artist’s impression of a neutron star lying in the center of a gaseous accretion disk. During quiescent episodes the disk is cold and residing far away from the neutron star, preventing it from swallowing gas from it. During such quiescent episodes, sensitive X-ray satellites can detect the thermal glow of the hot neutron star, which allows to measure its temperature.

Meet the micronova!

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A new type of cosmic explosion discovered.


Staring up at the night sky, it may appear that the endless universe is calm and serene. However, the vastness of space harbors many extreme objects like black holes, neutron stars and white dwarfs. Despite being dead stars, their extreme properties causes them to produce all kinds of violent explosions like gravitational wave mergers, supernovae, gamma-ray bursts, X-ray bursts, fast radio bursts and what not. As of 2022, we can add a new type of explosion to this menagerie: the micronova.

Micronova are thermonuclear explosions that occur on the surface of a white dwarf. In just a matter of hours, an amount of matter equivalent to 3.5 billion Great Pyramids of Giza is burned into flames. Despite being so magnificent, micronovae are just tiny explosions on astrophysical context. In particular, micronovae are about a million times smaller than the common nova explosions that have been known for many decades. Nova explosions occur when a white dwarf slurps gas from a nearby companion; when accumulating on the surface of the cannibal, this gas gets so enormously hot that hydrogen atoms explosively fuse together to form helium. The energy released in this process makes the entire surface of the white dwarf light up and shine bright for several weeks. This makes it easy to spot and study novae. Micronovae, however, are both shorter and less intense, which explains why they went undiscovered for so long.

The mysterious micronovae were spotted in data collected with NASA’s Transiting Exoplanet Survey Satellite (TESS), which takes highly frequent snapshots of large part of the sky. While designed to find new planets around other stars, its high-cadence optical monitoring can reveal many other interesting phenomena to the keen observer. It is in this way that a bright flash of optical light lasting for a few hours was found from a known white dwarf binary. Searching further revealed similar signals in two other systems. Their origin remained a puzzle until subsequent follow-up observations with ESO’s Very Large Telescope revealed a common denominator for the three binaries showing the hours-long optical flashes: all contained white dwarfs with magnetic fields strong enough to channel the siphoned gas onto the magnetic poles rather than it splashing over the entire surface. Putting things together and working out the energetics of the optical flashes revealed that these can be thermonuclear explosions that are confined to the magnetic poles, opposed to the well-known nova explosions that happen over the entire white dwarf surface. The smaller area involved in the explosion explains both the shorter duration and the lower energy output.

The discovery of micronovae shows for the first time that detonations apparently can happen in confined regions, which was not known before. The challenge is now to find more micronovae and to study them in detail to learn more about thermonuclear explosions on stellar cannibals.

Press release: ESO

Scaringi et al. 2022, Nature 604, 447: Localized thermonuclear bursts from accreting magnetic white dwarfs

Paper link: ADS

Artist’s impression of a localized explosion on the surface of a white dwarf that is swallowing gas from a companion star. Image credit: Mark Garlick

Crazy jet experiment

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Neutron stars and black holes are notorious for their strong gravity that allows them pull gas from their surrounds. However, apart from swallowing material, these stellar cannibals also spit large amounts of it back into space via so-called jets.

Jets are streams of gas and energy that are being blown into space by an astronomical body that is accreting. X-ray binaries are a prime example of accreting systems that produce jets, but these outflows are seen in a wide variety of astronomical systems, including young forming stars, white dwarfs and supermassive black holes that lurk in the centers of galaxies. Jets play a fundamental role throughout the universe, including the birth and death of stars, the growth and evolution of galaxies, and the formation of large-scale structures (the cosmic web).

X-ray binary jets have an enormous impact on a variety of processes. Firstly, jets remove mass from an X-ray binary. This strongly affects how the accreting object and its companion star revolve around each other, moving closer on a timescale of billions of years until they eventually collide and produce a burst of gravitational waves. Secondly, jets slam into the interstellar medium; the gas that fills the space between stars in galaxies and in which new stars are born. As jets plough through the interstellar medium, the gas is stirred up, heated, and magnetized. This affects the birth rate of new stars and how the galaxy evolves over time. Despite their omnipresence and undisputed importance, however, it remains a mystery how and where jets are launched.

Owing to a Klein-XS grant from NWO, a funding scheme recently installed to support high risk/high gain research, we are going to conduct a very exciting experiment that can potentially shed new light on how jets form. Considering that change is a very powerful diagnostic in astrophysics, my co-workers and I are going to test whether jets may be temporarily destroyed and rebuild in response to thermonuclear X-ray bursts.

Thermonuclear X-ray bursts are brief flashes of X-ray emission that result from runaway nuclear fusion reactions in the gas that accumulates on the surface of an accreting neutron star. These explosions have a devastating power of 1032 Joule (equal to 1015 nuclear bombs!), last a few seconds and repeat every few hours. Recent calculations suggests that thermonuclear bursts can blow away the region where a jet is launched. This could cause the jet to weaken or disappear during a thermonuclear X-ray burst, and rebuild once the explosion has passed. If we can truly detect an response of the radio jet to a thermonuclear bursts, this can prove to be a completely new and powerful way to watch in real time how jets are formed.

Jets emit their energy mainly at radio wavelengths, and are best studied at frequencies of about 8 GHz with sensitive radio telescopes. With the awarded NWO grant, we will buy observing time on the Australia Telescope Compact Array (ATCA)  telescope to study the radio jets of a few neutron stars that regularly fire off thermonuclear X-ray bursts. We will perform simultaneously X-ray observations with the Integral satellite to know at what times the thermonuclear bursts are occurring. If we see any change in the radio emission at those times, this implies that the explosions can indeed affect the radio jet. Stay tuned!

News items: NWO and NOVA

rxsj1804_xmm_bursts

An X-ray observation of an accreting neutron star that shows highly repetitive thermonuclear X-ray bursts. The 11 distinct spikes of X-ray emission each represent a single thermonuclear X-ray burst. These powerful explosions can likely repeatedly destruct or weaken the radio jet.

Astrophysical pollution

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The outflows, i.e. the jets and disk winds, that are produced by accreting black holes and neutron stars can potentially have a significant impact on the environment of X-ray binaries. The space in between stars and binary star systems is not empty: it’s filled with tenuous gas and dust that is referred to as the interstellar medium (ISM). Jets and disk winds can slam into this ISM, thereby stirring it and heating it. Moreover, extremely powerful thermonuclear X-ray bursts may eject material into the surroundings of the neutron star and create the same effect. These interactions can may have far-reaching consequences, perhaps influencing the formation of stars and thereby influence the evolution of the entire galaxy. However, it is not yet established if the majority of X-ray binaries truly impact their surroundings; this likely depends on the power of the outflows and the density of the ISM.

Whether or not an X-ray binary interacts with its environment may be determined by looking for shocks in the surrounding ISM. Such shocks produce ionized radiation that are characterized by strong emission lines, e.g. one produced by hydrogen gas at a wavelength of 650 nm (H-alpha emission). Some telescopes are equipped with filters that allow you to look at such a specific wavelength; by taking images with a H-alpha filter, shocked regions around X-ray binaries may be revealed. In addition, a camera with a very wide field of view is required, because the shocked regions may be lying quite far away from the X-ray binaries (and hence would be missed when looking with a camera that has a narrow field of view).

Determined to find out if X-ray binaries generally create shocks in their surroundings, we set up a very large campaign to take H-alpha images of many tens of X-ray binaries. For this purpose we are using the Wide Field Camera (WFC) mounted on the Isaac Newton Telescope that is located on La Palma, Spain. To be able to also access X-ray binaries that are located in the Southern hemisphere,  we are also using the Las Cumbres Observatory, which consists of network of telescopes located across the globe, and the Very Large Telescope located in Chile. To pull off this massive observing campaign, my group and I are joining forces with researchers from the University of Sounthampton in the United Kingdom, St Andrews University in Schotland, the Instituto de Astrofísica de Canarias in Spain, and New York University Abu Dhabi in the United Arab Emirates. Stay tuned for the results!

INT_bigger

The Isaac Newton Telescope on La Palma. Photo credit: see here

What do neutron stars look like inside?

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Everything around us is constructed of atoms, which themselves consist of electrons and nucleons (i.e. protons and neutrons). This familiar structure of matter is, however, disrupted when matter is compressed to very high densities that reach beyond the density of an atom, called the nuclear density. It is one of the prime pursuits of modern physics to understand what happens to matter beyond this point. It is not possible to generate supra-nuclear densities in terrestrial laboratories on Earth. However, neutron stars are extreme objects in which matter is compressed to enormously high densities. These stellar bodies therefore serve as exciting, natural laboratories to further our understanding of the fundamental behavior of matter.

Neutron stars are the remnants of once massive stars that ended their life in a supernova explosion. A defining property of neutron stars is that these objects are very compact; while being roughly a factor of 1.5 more massive than our Sun, their radius is almost 100.000 times smaller. Due to their extreme compactness, neutron stars are the densest, directly observable stellar objects in our universe. These fascinating objects come in a wide range of manifestations, e.g. as single stars or as part of a binary, and can be detected at different wavelengths.

Unfortunately it is not possible to travel to a neutron star to conduct experiments of how their interiors look like. However, the macroscopic properties of neutron stars, such as their mass, radius and rotation rate, provide indirect yet powerful information about their interiors. The electromagnetic radiation coming directly from the surface of neutron stars, or from matter that revolves around them, can be used to measure these macroscopic properties. These observational constraints can then be used to infer for instance how high their central density is, what kind of particles are present, and what the superfluid properties of their interior are.

We recently reviewed how different types of electromagnetic observations can be employed to learn more about the interior of neutron stars. This included commonly used techniques of combining radio pulsar timing with optical spectroscopic studies to measure neutron star masses, as well as various techniques to measure neutron star radii from X-ray data. In addition, we touched upon various techniques that have not yielded strong constraints to date, but have great potential to be further developed in the future and can be particularly interesting when combined with other methods. Finally, we provided an outlook of the potential for neutron star research of the future generation of ground-based  observatories such as the Square Kilometer Array and the new class of 30-m telescopes, as well as new and upcoming X-ray facilities such as NICER, eXTP, Athena and X-ray polarimetry missions.

Degenaar & Suleimanov 2018, book chapter in The Physics and Astrophysics of Neutron Stars, Springer Astrophysics and Space Science Library: Testing the equation of state of neutron stars with electromagnetic observations

Paper link: ADS

SKA

Artist impression of the Square Kilometer Array (SKA), which is currently in the design phase. This radio facility is expected to be transformable in many areas of science, including measuring the masses (and spins) of neutron stars.

Zooming in on an intriguing neutron star

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Neutron stars and black holes are the collapsed remnants of once massive stars that ended their life in a supernova explosion. A defining property of neutron stars and black holes is that their mass is compressed into a very small volume and therefore these stellar corpses are also referred to as compact objects.

One direct consequence of their compactness is that neutron stars and black holes exert immense gravity. When they are part of a binary star system, this allows them to pull off gas from their companion star and swallow this material to their own benefit (e.g. to increase their own mass and spin). This process of mass transfer is called accretion and plays an important role throughout the universe. Understanding exactly how neutron stars and black holes eat, and how much they spit back into space, is therefore a very active area of research.

Accretion onto compact objects leads to the liberation of enormous amounts of gravitational energy, which is carried into space as electromagnetic radiation. Most of the energy is released in the inner part of the gaseous disk that forms around the neutron star or black hole. The temperatures in this part of the disk are billions of degrees Celsius, which implies that the radiation is visible at X-ray wavelengths. For this reason, accreting neutron stars and black holes are called X-ray binaries.

Despite that X-ray binaries radiate most prominently in the X-rays, the cooler parts of their accretion disks emit at ultra-violet (UV), optical and infrared wavelengths, while the material that is blown back into space is typically detected in the radio band. Furthermore, their companion star also emits optical, infrared, and sometimes UV, radiation. Although the accretion in X-ray binaries is typically studied with X-ray telescopes, forming a complete picture of all components involved in the accretion process requires studying X-ray binaries at all wavelengths, from X-ray and UV to optical and infrared, all the way to radio. Such multi-wavelength studies are highly challenging, however, because every different wavelength requires another observatory and the data acquisition, reduction and analysis techniques are widely different.

In an effort to elucidate the puzzling nature of the intriguing neutron star X-ray binary IGR J17062-6143, we carried out an ambitious multi-wavelength observing campaign. We used three satellites (NuSTAR, XMM-Newton, and the Neil Gehrel’s Swift observatory), as well as two large ground-based telescopes (Gemini South and Magellan) to understand i) if the neutron star in this X-ray binary is stopping the accretion flow with its magnetic field, ii) if part of the accreted gas is blown away in a wind, and iii) if the accretion disk has a size similar to other X-ray binaries or is comparatively small.

Jakob utilized a total of four different X-ray analysis techniques (broad-band X-ray spectral fitting, reflection spectroscopy, high-resolution X-ray spectroscopy and coherent X-ray timing) to zoom in on the properties of the hot inner part of the accretion flow, near the neutron star. Among his main findings are that the inner part of the gas disk does not extend close to the neutron star as is usually the case in X-ray binaries, but is truncated well away from it (question i above). Secondly, he found evidence for an outflowing wind (question ii above), which may be related to the fact that the inner disk is vacated (e.g. the magnetic field of the neutron star may be pushing the gas away and expelling it into a wind). Finally, he found evidence for unusually high abundances of oxygen in the gas surrounding the neutron star. This could indicate that the companion star is very old, which ties in with our other multi-wavelength data analyzed by Juan.

Juan did an amazing job at combining the information from various different wavelengths to understand the size of the accretion disk in IGR J17062-6143 in (question iii above). Fitting the multi-wavelength spectral-energy distribution to accretion models, he found that the gas disk in this X-ray binary must be exceedingly small compared to other systems. In particular, he found that the companion star must be orbiting the neutron star in less than an hour, which implies that the companion must be a very old, small star. Such old stars have lost all their hydrogen and as a result the accretion that they feed has a more exotic chemical composition, which can explain the abundance of oxygen found in Jakob’s X-ray analysis.

X-ray binaries with very small orbits and old companions are called ultra-compact X-ray binaries. Only about a dozen of such systems are known, but their is high desire to find more of them. For instance, characterizing ultra-compact binaries is very important for understanding how binary stars evolve. Furthermore, these systems are expected to emit gravitational waves that should be detectable with future gravitational wave detectors such as LISA.

Our efforts demonstrated the power of multi-wavelength studies to gain a deeper understanding of accretion processes and to find rare, exotic X-ray binaries.

van den Eijnden, Degenaar, Pinto et al. 2018, MNRAS 475, 2027: The very faint X-ray binary IGR J17062-6143: a truncated disc, no pulsations, and a possible outflow

Paper link: ADS

Hernández Santisteban, Cuneo, Degenaar et al. 2019, MNRAS in press: Multi-wavelength characterisation of the accreting millisecond X-ray pulsar and ultra-compact binary IGR J17062-6143

Paper link: ADS

global_sed_flux

Accumulation of multi-wavelength data for the neutron star X-ray binary IGR J17062-6143. Shown is the spectral-energy distribution that was obtained with three different satellites and two ground-based telescopes. This image is adopted from Juan’s paper.

The devastating impact of X-ray bursts

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When plasma falls onto a neutron star it undergoes thermo-nuclear reactions that can cause an extremely energetic explosion called an X-ray burst. Such explosions are extremely common: tens of thousands of X-ray bursts have been recorded to date with different X-ray detectors and on some neutron stars the explosions repeat every few hours.

X-ray bursts occur on neutron stars that are surrounded by a gaseous disk in which material that is pulled off the companion star spirals at increasing speed until it finally plunges into the neutron star. Apart from this accretion disk, a neutron star is also surrounded by a hot plasma, called a corona. The formation and properties of accretion disks are much better understood than that of the corona.

It has long been appreciated that the properties of the accretion flow (i.e. the accretion disk and the corona) affect the observable properties of X-ray bursts such as their peak brightness, duration, recurrence rate and variability properties. However, in recent years evidence for the reverse interaction have been accumulating too: the devastating power of X-rays bursts can destruct the accretion disk and corona that surround the neutron star. Shortly after the surge of energy from the X-ray burst is over, the disk and corona should return to their original status.

Change is always a very powerful diagnostic in astronomy. The destruction and re-formation of accretion disks and coronae in response to an X-ray burst can therefore reveal intriguing new insight in the properties of accretion flows. Given that X-ray bursts are very common, they can thus serve as a powerful, repeating probe to study the poorly known properties of coronae (such as their geometry) and how an accretion disk responds to a sudden shower of intense radiation.

We recently reviewed all the observational evidence for X-ray bursts interacting with the accretion flow. Based on our current understanding of these interactions, we looked ahead and studied how new and concept X-ray missions such as ASTROSAT (launched in 2015), NICER and HXMT (both launched in 2017), eXTP and STROBE-X (mission concepts currently under study) can further this research field. We also proposed various multi-wavelength strategies can be leveraged to learn more about accretion flows using X-ray bursts.

Degenaar, Ballantyne, Belloni et al. 2018, Space Science Reviews 214, 15: Accretion Disks and Coronae in the X-Ray Flashlight

Paper link: ADS

corona

Schematic overview of three different possible geometries for the corona in an X-ray binary. The neutron star is indicated as the red ball, the accretion disk as the brown surface, and the corona as the grey structure.