Author Archives: degenaar

Unknown's avatar

About degenaar

Astrophysicist (associate professor) at the University of Amsterdam

Bent out of shape

Posted on by

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.

The clue of the missing jet

Posted on by

The cosmic neighbor that kept us in the dark

Some X-ray binaries scream for attention, others just quietly hum along. 4U 1556–60 is one belonging to the latter category, known for over 50 years and thought to be a run-of-the-mill X-ray binary system living near the center of our Galaxy, like so many. But as it turns out, we were looking at it all wrong.

A surprising shortcut was delivered by the Gaia satellite, which measures the distances to billions of stars with incredible precision. Distilling the optical counterpart of 4U 1556–60 from the many stars Gaia has detected, we found that Instead of being roughly 25,000 light-years from Earth, this X-ray binary must actually be no less than ten times closer! At a distance of only 700 pc, it is one of the closest X-ray binaries ever discovered and this proximity drastically alters everything we thought we knew about it. To put it in perspective, it’s like thinking a faint light in the distance that you’ve seen forever is a massive lighthouse miles away, only to realize it’s a tiny candle right across the street.

Because it is so much closer, we now know it is 100 times fainter than previously believed. Its persistent low luminosity is quite unusual for an X-ray binary: Usually, these systems either flare up brightly or remain dark in a state of quiescence, but 4U 1556–60 just quietly simmers at a very low level. To figure out if the system’s heart is a black hole or a neutron star, we used the Australia Telescope Compact Array (ATCA) to look for a radio jet. The result was absolutely nothing: one of the lowest radio limits ever recorded for an X-ray binary.

While black holes are notoriously noisy in radio waves, neutron stars produce much weaker jets, even when their X-ray emission shines similarly bright. The deep radio silence is the strongest evidence yet that 4U 1556-60 hosts a neutron star. Despite that no thermonuclear bursts, smoking-gun evidence for the presence of a neutron star, have ever been detected from the source this is likely because it is eating its companion so slowly that it takes months or even years to build up enough fuel for a single pop. The very low X-ray luminosity and low fueling rate likely implies that the neutron star in 4U 1556-60 is orbiting very closely to its companion that is just a tiny, stripped-down remnant of a star. While faint in X-rays, such ultra-compact X-ray binaries might actually be screaming in gravitational wave emission. Due to its close proximity, 4U 1556-60 is therefore a prime target for future gravitational wave detectors like LISA.

Pattie, Maccarone, Russell, Bachetti, Degenaar, Kupfer 2026, A&A 707, 193: 4U 1556-60 as a very faint neutron star X-ray binary at 700 pc with an undetected radio jet

Paper link: NASA ADS

Radio and X-ray brightness of a large number of black hole and neutron star X-ray binaries, with our close neighbor 4U 1556-60 indicated as the red star, showing its comparatively faint X-ray emission and missing radio jet.

New kid on the block

Posted on by

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.

Rewriting a compact binary’s life story

Posted on by

Compact binaries may merge quicker than we thought due to disk winds.

In the world of X-ray binaries, disk size matters. These binary star systems are cosmic pairings where the dead(ly) remains of a massive star, a neutron star or a black hole, feeds on its neighboring star by pulling in gas that swirls into a vast, glowing whirlpool known as an accretion disk. Generally, the larger the orbital path between a cosmic cannibal and its unfortunate companion, the larger this accretion disk will be, the brighter it will shine and the more gas it will blow away in a disk wind. At least we thought so.

For long, it has been established that only X-ray binaries with fairly large accretion disks had the right conditions to blow disk winds into space. Compact binaries were thought to be too tiny to manage it. When we discovered footprints of an outflowing wind in the compact neutron star X-ray binary UW CrB this therefore came as an unexpected surprise. However, as excitingly puzzling as this discovery was, we had too little information to determine if this was an sporadic breeze or a persistent storm, nor if the outflow we picked up was actually a wind blown off the accretion disk rather than some gas stream that happened to pass through our line of sight.

To get to the bottom of UW CrB’s windy secret, we launched a monster campaign wherein we pointed several major space-based and ground-based observing facilities at this neutron star located a few tens of thousands of light years away from us. By catching as much of its emitted light as we possibly could, cutting through the electromagnetic spectrum from radio waves to optical and infrared light to UV and X-ray radiation, we hoped to find more features of a wind and to map thet overall behavior of the X-ray binary. Within a window of 6 days in July 2023, the Hubble Space Telescope (UV), XMM-Newton and Swift satellites (both X-ray) observed from space, while the Very Large Telescope (VLT, optical/infrared) was watching from Chile and Gran Telescopio Canaria (Grantecan, optical) from La Palma. In a later campaign, performed in February 2024, we also monitored UW CrB with the Arcminute Microkelvin Imager (AMI, radio) telescope located near Cambridge UK, and the Swift X-ray satellite from space, to see if the neutron star launches a jet.

Needless to say, analyzing this impressive multi-wavelength, multi-facility data set was a massive amount of work and it required some serious detective work to figure out all clues that were hidden in the data. But the eventual result was more than worth it: we found several more indications, from all wavelength regimes we studied, that UW CrB is consistently blowing a disk wind into space. Having established that X-ray binaries with small disks are capable of doing this makes us rethink binary aging. If even compact binaries like UW CrB can launch persistent winds, it means they are losing much more mass and angular momentum than previously assumed. This then implies the two stars may move faster towards each other than current binary evolution models would predict, which would in turn impact predictions for the gravitational wave emission associated with compact binary star systems.

After this exciting effort to figure out the behavior of UW CrB, the queste is no on to find disk winds in other similar compact binary systems and refine our understanding of their life cycle.

Fijma, Degenaar, Castro Segura, Maccarone, Knigge, Armas Padilla, Mata Sánchez, Muñoz-Darias, Hernández Santisteban, Rhodes, Bright, van den Eijnden, Green 2025, MNRAS 544, 4702: A multiwavelength view of the outflowing short-period X-ray binary UW CrB

Paper link: SciX

Overview of our multi-wavelength, multi-observatory monster campaign on UW CrB aiming to unravel the properties and nature of its outflows. The top panel shows the first two observing blocks of 2023 July during which we had X-ray, UV, optical and infrared coverage. The bottom panel shows the third observing block that was focused on radio monitoring, with supporting X-ray observations and a re-run of part of the HST UV observations.

A clumpy secret

Posted on by

Everybody experiences a dip sometimes, but some X-ray binaries have a lot of dips. In the world of X-ray binaries, 4U 1624-490 is quite a legend. Known as Big Dipper, this binary system, consisting of a neutron star snacking on a nearby companion, is famous for the dramatic, periodic drops in its X-ray light. For long, these “dips” are thought to happen because a massive bulge of gas on the edge of the accretion disk swings into our line of sight, temporarily blocking the view. The properties of this bulge, such as its structure, size and temperature, remain largely unknown, however, and so is the role that these may play in setting the dynamics of the flow of accretion towards the neutron star.

A new, deep look with the XMM-Newton satellite has revealed that the bulge isn’t a solid wall of gas, but more like a cosmic sandstorm. Slicing the X-ray data into different segments revealed that the material hitting the disk is surprisingly complex. Instead of a uniform cloud, the bulge is a multi-phase mixture of both cold gas and highly ionized hot gas. The most striking discovery, however, is just how clumpy the system is. The dipping phenomena isn’t observed as a smooth fade-out and fade-in, but rather as a chaotic series of flickers caused by individual clumps of dense gas passing through our line of sight. By calculating the size of the bulge compared to the shortest flickers in the light curve, we estimated that this turbulent region contains at least 7,000 individual clumps.

Even during episodes that 4U 1624-490 is not dipping, we found evidence of a highly ionized atmosphere surrounding the entire disk. It seems that it is much messier than previously thought, with the impact of the gas stream potentially creating a splash so turbulent that it generates a swarm of thousands of mini-clouds that we detect as an atmosphere. This study shows that dipping light curves are powerful tracers that let us map out the hidden, chaotic structures of accretion disks in unprecedented detail.


Caruso, Costantini, Degenaar, Diaz Trigo 2026, Astronomy & Astrophysics 705, 176: An XMM-Newton long look at the accretion disk plasma in the dipping neutron star LMXB 4U 1624-490

Paper link: NASA ADS

X-ray light curve, recorded by the XMM-Newton satellire, of 4U 1624-490 showing different phases of obscuration (dipping).

How to launch a jet

Posted on by

Whenever a gigantic explosion occurs in the cosmos, or an astrophysical object guzzles up matter from its surroundings, so-called jets are shot out: collimated streams of plasma that hurdle through space at speeds of hundreds of thousands to billions of kilometers per hour. As jets carry enormous energy and travel very large distances, they may significantly impact their cosmic environment, for instance enriching it with exotic chemical elements or compressing gas clouds to the extent that these start to contract to form new stars. Moreover, a jet might carry away significant amounts of energy, mass and rotation, from the object that launches it hence changing its properties and evolution.

Despite their prominent role in shaping our universe, how jets are actually produced has puzzled astronomers for over a century, ever since the first recording of an astrophysical jet in 1918. The answer to this seemingly basic question is, however, essential to fully understand the wide impact of jets. This is because the launch mechanism determines the physical properties of the jet, such as its power, speed, and composition. For neutron stars, pressing questions are whether the star’s magnetic field is involved in launching jets and to what extent their jet production mechanism resembles that of black holes.

A rather unique neutron star to study the role of the stellar magnetic field on jet production is one with the stage name The Rapid Burster (formally called MXB 1730-335). It is thought for this neutron star there is a tug-of-war between its magnetic field, pushing gas outwards, and its accretion disk through which gas flows from its companion star flows towards it. During so-called Type-II X-ray bursts, flashes of bright X-ray emission that last seconds to minutes, the magnetic field is thought to be temporarily pushed inwards, allowing a sudden strong increase in the gas supply to the neutron star. Seeing if, and how, the radio jet responds to these Type-II bursts thus provides an excellent setting to study the role of the stellar magnetic field in launching jets.

As with the thermonuclear burst / jet experiment, it was again an exciting challenge to design and execute the observing campaign to study the jet of the Rapid Burster. This is because this neutron star is dormant most of its time and only occasionally gobbles up gas from its companion star. Luckily, the Rapid Burster is a rare case where its meal times are rather regular, allowing to predict when a new episode of activity is about to occur. Making use of this, we devised a strategy that involved 4 different observatories. First, we monitored our target for signs of increased X-ray activity through the MAXI satellite, which is continuously scanning the sky in X-rays. When it detected the onset of a new accretion outburst, we started to monitor the source with the Swift satellite for accurate flux measurements and chart its X-ray bursts. As soon as Swift showed us that the Rapid Burster had become bright enough and had started showing type-II bursts, we initiated pre-arranged observations carried out simultaneously with the Very Large Array (radio) and Integral (X-rays).

During our observing campaign, the Rapid Burster showed both short, rapidly recurring Type-II bursts, as well as a much longer one that was followed by a burst-free episode. Interestingly, we witnessed that the jet of the neutron star was solidly on when displaying the short bursts, but appeared to switch off after the longer Type-II burst. This could point towards a crucial role for the stellar magnetic field in launching jets, at least for this particular neutron star. Having conducted this successful pilot experiment, we can confirm this hypothesis by conducting a more extensive campaign to catch more longer Type-II bursts and study the associated jet response. Comparing these results a more systematic radio study of other neutron stars that do not display Type-II bursts will further allow to understand the role of the magnetic field.

van den Eijnden, Robins, Sharma, Sánchez-Fernández, Russell, Degenaar, Miller-Jones, Maccarone 2024, MNRAS 533, 756: The variable radio jet of the accreting neutron star the Rapid Burster

Paper link: ADS

Results of the simultaneous VLA radio (top) and Integral X-ray (bottom) observations of the Rapid Burster in 2020. The jet seemed to on during episodes where short successive Type-II X-ray bursts were occurring (epochs I, III and IV; top label), but switched off after a stronger/longer Type-II X-ray burst (epoch II).

A cosmic speed camera

Posted on by

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.

An unexpected companion

Posted on by

The more gas neutron stars or black holes take in, the brighter they shine in X-rays. Many are glutenous, swallowing as much as 1018 (a million trillion!) of gas per second, which makes them the brightest X-ray sources in the sky. However, more and more neutron stars and black holes are discovered to emit only dim X-ray light, implying that these are not taking in a lot of gas from their companion star. It is not clear, however, why they don’t. The two leading theories are either that these neutron stars and black holes have very small companions and are hence just not very well fed, or that they have a normal gas supply but somehow spit much of this back into space. Charting the demographics of these dim X-ray binaries important for several areas of astrophysics, including the study of gravitational waves, supernova physics and binary evolution.

Many of the neutron stars and black holes that we have found in our Galaxy are not solitary but are instead orbiting through space with another star. Owing to their relentless gravitational pull, neutron stars and black holes are able to nibble gas from their companion. This makes them light up in X-rays and therefore these star pairs are called X-ray binaries. Studying X-ray binaries is important for a variety of reasons, including understanding how binaries with neutron stars and black holes are formed and evolve, how supernova explosions work, how black holes grow, and how matter behaves when subject to extreme conditions that cannot be mimicked in laboratory experiments on Earth (e.g. extreme magnetic fields, severe radiation, ultra-high densities, super-strong gravity).

We tried to solve the riddle of dim X-ray emission for a neutron star called 1RXH J173523.7-354013. To do so, we turned to optical and near-infrared telescopes. In particular, we took a near-infrared spectrum using the Very Large Telescope (VLT) aiming to determine the type of companion star and used data from the Visible and Infrared Survey Telescope for Astronomy (VISTA) in an attempt to determine how long it takes for the two stars to orbit around each other (i.e. how wide the binary is). Expecting to find a very small (white dwarf) companion star and a very small orbital period (<1 hour), our studies instead revealed the complete opposite: a red giant star and an orbital period of about 8 days!

Since red giants can donate large amounts of gas to a neutron star or a black hole, it remains a puzzle why 1RXH J173523.7-354013 is such a dim X-ray source. We speculate that gas pulled off from the companion is accumulating in a reservoir near the companion and, as it becomes hotter and denser filling up, will at some point cross a critical threshold that allows all stored gas to suddenly stampede towards the neutron star. If our hypothesis is right, 1RXH J173523.7-354013 should one day become whoppingly bright in X-rays. Let’s see if this comes true! In mean time, we continue our quest to determine what companion stars dim X-ray binaries have, which may lead us to stumble across more of them having big companions and wide binary orbits like 1RXH J173523.7-354013. Stay tuned.

Shaw, Degenaar, Maccarone, Heinke, Wijnands, van den Eijnden 2024, MNRAS 527, 7603: The nature of very-faint X-ray binaries: near-infrared spectroscopy of 1RXH J173523.7-354013 reveals a giant companion

Paper link: ADS

Near-infrared (NIR) spectrum obtained with the SINFONI instrument on the Very Large Telescope (VLT) in Chile. Numerous lines can be seen that correspond to neutral atoms (e.g., Na I, Ca I and Mg I), as well as molecular bandheads (CO). All these are classic features seen in the NIR spectra of giant stars (of spectral type K or M), which came as a huge surprise! This is because objects like 1RXH J173523.7-354013 are generally thought to harbor very small donor stars (e.g. white dwarfs) instead.

A windy surprise

Posted on by

Binary star systems that contain a neutron star are important for probing fundamental theories of physics and for studying a large variety of astrophysical processes. For instance, the most energetic explosive phenomena seen in the cosmos, such as supernovae, kilonovae, gamma-ray bursts, gravitational wave mergers and fast radio bursts, often involve neutron stars in binary systems. Furthermore, they serve as an important testbed for Einstein’s General Relativity Theory, and binaries containing neutron stars are also excellent laboratories to study the behavior of cold ultra-dense matter. Finally, studying populations of binaries with neutron stars further allow us to several key processes of stellar evolution.

A particularly important phase in the life and evolution of neutron stars in binary systems is when the neutron star accretes mass from its companion star. This is when the system manifests itself as an X-ray binary. However, neutron stars do not only swallow gas, they also blow matter and energy back into space via outflows. These can be observed as highly collimated streams that are called jets and thought to be shot out with velocities of tens to hundreds of thousands of kilometers per second, or dense winds that have a larger opening angle and travel at lower speeds of a few hundreds to thousands of kilometers per second.

As in any astrophysical system where accretion takes place, outflows are ubiquitous among neutron star X-ray binaries. However, two key aspects of jets and winds are not understood yet: how these outflows are actually launched and how much mass can be lost from the binary in this way. Determining the mass loss is important, for instance, for understanding how long it will take for the neutron star to close in on its companion star and eventually collide with it, generating a burst of gravitational waves. The amount of mass contained in a wind is closely related to the mechanism that drive the wind.

Studies of X-ray binaries containing black holes have shown that disk winds are likely driven by thermal processes: X-rays produced in the inner parts of the accretion disk heat the outerparts of the disk, causing these to puff up. If the disk is large enough, the gas may at some point in the disk puff up to such an extent that it’s able to escape the gravitational pull of the black hole and flow away as a disk wind. Based on theoretical knowledge, it is expected that black hole X-ray binaries should have orbital periods of more than 8 hours to be able to have large enough disks to launch thermal winds. So far, this was consistent with observations, since disk winds have almost exclusively been detected in X-ray binaries with orbital periods exceeding 8 hours.

Analysing far-UV spectra of a very small neutron star X-ray binary called UW CrB, with the aim to understand the composition of its accretion disk, we surprisingly discovered features of a wind. Since the orbital period of this binary is only 2 hours, it should not be able to launch a thermal wind. Based on this observational discovery, we performed preliminary simulations and actually found that the X-rays emitted from the surface of the neutron star make it possible to drive a wind from smaller accretion disks than would be possible in black hole X-ray binaries (since black holes to have a surface where they can emit X-rays from). The wind in UW CrB does remain mysterious, since it was detected in only a fraction of the data that we analysed. This suggests that winds can potentially switch on and off on a time scale of hours, which was not previously known.

To establish the nature and time-variability of the wind in UW CrB, we have been granted time on several big observing facilities: the space satellites Hubble Space Telescope, XMM-Newton and Swift, as well as the optical/near-infrared Very Large Telescope (VLT, in Chile) and Grantecan (on La Palma). It was a huge challenge to figure out at what exact time all these telescopes could point to UW CrB at exactly the same time, but this ambitious and exciting observing campaign is happening in mid July. Stay tuned for the outcome!

Fijma, Castro-Segura, Degenaar, Knigge, Higginbottom, Hernandez Santisteban, Maccarone 2023, submitted to MNRAS: A transient ultraviolet outflow in the short-period X-ray binary UW CrB

Paper link: ADS

Hubble Space Telescope far-UV lightcurve (left) and a Zoom of the spectrum (right) around the Si-iv emission line (at 1402 Angstrom). The Si line in the right plot shows a P-Cygni profile, which is the hallmark of an outflowing wind. However, this wind feature was seen in only part of the observation, namely in the time interval colored red in the left plot.

A universal accretion instability

Posted on by

Shedding light on an old black hole mystery using… a neutron star!


Neutron stars and black holes are both remnants of massive stars that ended their lives in a supernova explosion. They also both exert very strong gravity and when they are part of a binary star system, this allows them to devour gas from their unfortunate companion star. This gas spirals towards the cannibal forming a disk that is incredibly hot, so hot that it emits X-ray radiation. As these cosmic dinner parties can be spotted as sudden eruptions of X-ray emission, these stellar binaries containing a black hole or a neutron star are called X-ray binaries. However, neutron stars and black holes are greedy and cannot swallow all gas they attract; some of it is flung into space through powerful collimated jets or dense winds.

Despite their similar behavior, there is a distinct difference between the two tribes of cannibals: whereas for neutron stars the attracted gas plunges into their solid surface or anchored magnetic field where it may create observable shocks or explosions, a black hole silently swallows the gas from view beyond its event horizon. However, it has not been established yet how this and other differences between the two types of objects, such as the higher mass and faster spin rate of black holes, affect their eating patterns. Vice versa, comparing how neutron stars and black holes take their meals in can teach us how accretion and the production of outflows fundamentally works.

In 2018, a X-ray binary called Swift J1858.6-0814 was discovered when it suddenly started consuming material from its companion star. Unlike other X-ray binaries, it did so in an incredibly violent way, showing bright sparks, called flares, visible from radio to X-ray wavelengths The origin of this “cosmic fireworks” was unknown, but since it was so extreme, the astronomical community was convinced that this was the work of a black hole. However, over a year after its discovery, Swift J1858.6-0814 suddenly ignited a thermonuclear explosion, which require the presence of a solid surface. This exposed the black hole imposter, revealing that this extreme X-ray binary, in fact, harbored a neutron star.

Because of its extreme behavior, Swift J1858.6-0814 was closely watched, using many different space-based and ground based telescopes, including NASA’s Hubble Space Telescope, ESO’s Very Large Telescope and ESA’s XMM-Newton satellite. For over a year, this suite of observing facilities was used to decipher the complex table matters of the neutron star. This led to the remarkable result that similar patters were found as seen in the notorious black hole X-ray binary GRS 1915+105, which had been standing out for decades because of its extreme behavior. Intense study suggests that the gaseous disk surrounding these compact objects must cyclically empty and fill, causing repeated spectacular ejections of matter into jets (seen at radio waves and infrared wavelengths). The discovery that both black holes and neutron stars experience this instability implies that it is a fundamental (i.e. unavoidable) process that occurs when compact objects are overfed.

Vincentelli et al. 2023, Nature 615, 45: A shared accretion instability for black holes and neutron stars

Paper link: Nature, ADS

Artist’s impression of an X-ray binary containing a black hole (left) and a neutron star (right) swallowing gas from a companion star through an accretion disk. The insets show how the intensity of the emission varies strongly as the inner disk cyclically empties and re-fills. Whereas the timescales are different for the two objects, the underlying mechanism is thought to be the same. Image credit: Gabriel Pérez Díaz (IAC).