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

Paper link: 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).

Gone with the wind: Discovery of a persistent UV outflow from a neutron star

X-ray binaries consist of a neutron star or a black hole that are accompanied by another star (e.g. one like our Sun, a red giant, or a white dwarf). Neutron stars and black holes are not friendly neighbors, however, and will relentlessly rip gap from their companion and swallow it. This cannibalistic process is called accretion. At the same time, some of the gas inswirling gas is propelled back into space through dense winds or highly collimated jets. It is of prime interest to understand how much gas is swallowed versus blown away, as this determines how fast a neutron star or a black hole can grow, but also how long it will take for them to close in on their companion star and eventually collide with them to generate a burst of gravitational waves.

The most common signatures of outflowing material from astronomical objects are associated with “warm” gas. Despite this, only winds of “hot” or “cold” gas have been observed in X-ray binaries… until now! In this new study, we observed the recent accretion eruption of the X-ray binary known as Swift J1858 with a menagerie of ground-based and space-based observatories, including NASA’s Hubble Space Telescope (HST), the European Space Agency’s XMM-Newton satellite (XMM), the European Southern Observatory Organisation’s Very Large Telescope (VLT) located in Chile and the Spanish Gran Telescopio Canarias (GTC) located at La Palma (Canary Islands).

The results of our campaign, which was a joint effort of a team of researchers from 11 countries and was published in the journal Nature, showed persistent signatures of a warm wind at ultraviolet wavelengths occurring at the same time as signatures of a cold wind at optical wavelengths and hints of a hot wind at X-ray wavelengths. This is the first time that winds from an X-ray binary have been seen across different bands of the electromagnetic spectrum. This new discovery that provides key information about the messy eating patterns of these cosmic cookie monsters.

Designing the an ambitious observing campaign, built around the best telescopes on Earth and in space, was a huge challenge. This is mainly because it requires coordinating different observatories located at different parts of the Earth and space to look at your target all at the same time. So, it is incredibly exciting that all this work has paid off and allowed us to make a key discovery that would not have been possible otherwise.

Some press coverage: Independent

Paper link: ADS

Artist’s impression of a wind blown from the inner part of the accretion disk around a neutron star devouring gas from a companion. Image credit: Gabriel Pérez (IAC).

A new type of cosmic explosion discovered: Meet the micronova!

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

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

Volatile flickering of the Milky Way’s supermassive black hole

At the heart of our Milky Way galaxy lurks a supermassive black hole. It is called Sagittarius A*, or Sgr A* for short, and contains as much mass as 4 million Suns together. Our Milky Way is not unique in this respect: it is thought that every galaxy in our Universe harbors a supermassive black hole in its center. As they are notorious for, black holes suck up material from their surroundings, but they also blast large amounts of gas and energy into space. By doing so, the monstrous black holes in the centers of galaxies play an important role in how galaxies evolve over cosmic timescales and also how large-scale structures formed in our Universe. Studying the table manners of supermassive black holes is therefore a very active area of research.

The material that is irrevocably dragged towards a black hole lights up and provides us with a measure of how (much) the black hole is eating. Our very own supermassive black hole does not appear to have a large appetite as we see relatively little light coming from its surroundings compared to other supermassive black holes. However, roughly once a day we see a flare of (X-ray) emission coming from the position of Sgr A*. Despite that this flickering has long been known, we do not know yet what is causing it. Possibly, these are instances that the supermassive black hole is taking a gollup of material for instance a comet or some other small astrophysical object. Another possibility is that magnetic fields that must be treading the material swirling around Sgr A* are playing up.

Over the past two decades there have been many different efforts to understand the flickering behavior of Sgr A*, for instance by studying the energy of flares, to map out the distribution of flare properties, or attempting to constrain how often they occur. A challenge for the latter is that the flares are relatively sporadic. It therefore requires a lot of staring with telescopes to collect a sufficiently large number of flares to be able to draw firm conclusions on their recurrence rate. Most telescopes do not allow for such studies, either because it is technically not possible to take long stares at a single point in the sky, because their detectors do not have sufficient angular resolution to separate Sgr A* from the many nearby stars, or because the competition for telescope time is so high that long stares can usually not be granted.

The Neil Gehrels Swift observatory, shortly called Swift, is a satellited that was launched by NASA in 2005 and was specifically designed to be able to rapidly point to different positions in the sky. Because of this rapid slewing capability, it is no trouble for Swift to very frequently visit a certain position in the sky, hence effectively build up a long stare at that position. Taking advantage of this exceptional capability, Swift has been taking brief (about 20-min long) snapshots of the center of our Milky Way galaxy with its onboard X-ray telescope nearly every day since 2006. Having accumulated over 2000 X-ray pictures of Sgr A* by now, these Swift data provide a truly unique opportunity to study its flickering behavior over a 15 year baseline.

In the summer of 2019, Alexis Andes, an undergraduate student in El Salvador at the time, came to Amsterdam as part of the ASPIRE program. This program provides research opportunities for students that cannot easily study astrophysics in their home country. Alexis’ research project was to perform a statistical analysis of Swift’s X-ray data of Sgr A* to understand if its flickering behavior is constant over time. Excitingly, this turned out not to be the case! Our study revealed that there are sequences of years in which our supermassive black hole is frequently flickering but also stretches of years in which it is much more quiet. It was not known before that Sgr A* changes its behavior in this way.

Our results do not provide a conclusive answer on the cause of the flickering just jet. However, we noted that years of enhanced flickering seemed to occur after orbiting stars closely passed Sgr A*, such as the object G2 did a few years ago. Possibly, this can stir up the gas swirling around the supermassive black hole, causing it to be more restless for some period of time. While this is highly speculative at present, we can test this idea with continued Swift monitoring of Sgr A*: we predict that the flickering will become more quiet again within 1-2 years as the gas flow settles again after G2’s passage. So stay tuned!

Press release (English): NOVA

Andes, van den Eijnden, Degenaar, Evans, Chatterjee, Reynolds, Miller, Kennea, Wijnands, Markoff, Altamirano, Heinke, Bahramian, Ponti, Haggard 2022, MNRAS 510, 285: A Swift study of long-term changes in the X-ray flaring properties of Sagittarius A*
Paper link: ADS


Three-color accumulated Swift X-ray Telescope Image of the Galactic center (2006-2014).

To be or not to be superfluid

Neutron stars have masses up to about 2.5 times the mass of our Sun, but their radii are nearly 50 thousand times smaller. This means that the matter that makes up neutron stars must be squeezed incredibly close together. In fact, neutron stars present the densest observable form of matter in the Universe. While in black holes matter is even more tightly packed, it is all hidden behind an event horizon and hence beyond our reach. For neutron stars, however, we can observe radiation from their surface and use that to infer what they look like on the inside. Neutron stars allow us to study how matter behaves when it is crushed to enormously high densities, conditions that we cannot replicate and study in laboratories on Earth.

Neutron stars are thought to have a 1-km thick solid crust, below which matter occurs in liquid form. It is generally thought that this liquid must be superfluid, meaning that it swirls around freely without experiencing any friction. On Earth, such fascinating, odd behavior is observed when liquid helium is cooled down to extremely low temperatures of about -270 degrees Celsius. There are, however, many uncertainties about how a superfluid works at the millions of degrees Celsius temperatures inside neutron stars. As a result, it is far from understood what the superfluid properties of neutron stars are exactly. This is very important to establish, as it likely has a profound effect on how various neutron star properties, such as their magnetic field strength, spin and interior temperature, evolve over their lifetime. Moreover, several observable phenomena of neutron stars, such as occasional distortions in their rotation called glitches or sudden cracking of their crust analogous to Earth quakes, likely depend on the detailed properties of their interior superfluid.

Since we cannot perform experiments on Earth that teach us how superfluids inside neutron stars may work, we need to reverse the problem. One promising way to do this is to track how a neutron star cools down after it has been heated up by gobbling up gas from a companion star. We call this cannibalistic action accretion and often this happens in spurts that we call  outbursts. The superfluid properties of its interior should influence both the extend to which the neutron star can be heated during accretion outbursts, and how its temperature then subsequently decreases when accretion stops and hence there is no more heat generation. One such neutron star that we previously established to be heated and subsequently cooling down is called HETE J1900.1-2455. In a previous study we tracked its temperature decrease and our modeling suggested that the neutron star would cool further. After having waited a few years, we took a new observation of the source with the Chandra X-ray satellite to verify this.

Chandra pointed towards our neutron star for over half a day and surprisingly saw almost nothing! HETE J1900.1-2455 turned out to have cooled so much that it was hardly generating any detectable X-ray emission, even for the very sensitive X-ray detector of Chandra. In the 16 hours that Chandra started at it, a mere 5 X-ray light particles were capture from the neutron star. In order words, only every 3 hours was the Chandra detector hit by an X-ray light particle from HETE J1900.1-2455. Perhaps somewhat counter-intuitive, the fact that we detected so little light from the source actually provides us with very interesting constraints on its interior properties. Performing a rigorous and conservative analysis (based on those 5 light particles!) we inferred the likely temperature of the neutron star and performed highly advanced simulations that take into account the physical properties of neutron stars, including their interior superfluid.

Our analysis revealed that the low temperature of HETE J1900.1-2455 must imply that it is able to cool its interior extremely rapidly. Two exciting possibilities that would allow for this is either that the neutron star is very massive or that a large fraction of its core is not superfluid. The latter option would contrast the general belief and would have profound implications for our understanding of many observable properties of neutron stars. The former option would be a very important finding because the most massive neutron stars put the strongest constraints on the behavior of ultra-dense matter.  Although both options are equally exciting, our current observations do not allow us to distinguish between the two. However, because of the significant science impact, Chandra will again point towards HETE J1900.1-2455 to allow us to obtain a new temperature measurement. This time, it will not stare at the neutron star for 16 hours, but for 6 full days!! If its temperature remained stable, we will then collect 50 X-ray light particles and be able to obtain a much more reliable temperature estimate. If it cooled further, however, we might not see any light at all. Ironically, the latter case would be the most exciting outcome. Part of the data has recently been taken and the remaining observations will come in soon. I can wait to find out what we will (not) see in those new Chandra observations!

Degenaar, Page, van den Eijnden, Beznogov, Renolds, Wijnands 2021, MNRAS 508, 882: Constraining the properties of dense neutron star cores: the case of the low-mass X-ray binary HETE J1900.1-2455

Paper link: ADS

Observations (green points) and simulations (colored curves) of the temperature evolution of the neutron star in HETE J1900 during and after its 11-year long accretion outburst. Whereas we derive an ultra-low temperature of around 35 eV from our 2018 observation, the simulations predict that the neutron star may cool even further to an all-time low of about 15 eV.

The mystery of unbreakable radio jets

Black holes and neutron stars are notorious for sucking material from their surroundings towards themselves, a process that we call accretion. However, both types of objects also blast material back into space, for instance via highly collimated streams of gas and energy that we call jets. The material that is hurdling towards black holes and neutron stars, the accretion flow, is hot and emits heat radiation at X-ray wavelengths. The jets, on the other hand, emit radiation at radio wavelengths.

It is natural to assume that there is some kind of connection between how (much) material is flowing in and how (much) is pushed out in a radio jet. Indeed, a strong correlation between the X-ray and radio brightness is observed for both black holes and neutron stars, which points towards a strong connection. For black holes, it has also been observed that when material is flowing in extremely rapidly, it is no longer possible to push out a continuous jet. Rather, when matter is pushing in at high speed, it is spewed out in spurts while the continuous steady jet seen at low accretion rates disappears. The latter is observed as a sudden strong reduction in the radio brightness once the X-ray luminosity, hence the rate of matter inflow, climbs up to very high levels. Surprisingly, some neutron stars do not show a strong reduction of their radio brightness when we see them move up to high accretion rates. It therefore appears that, somehow, these neutron stars are able to sustain their continuous, steady radio jets. It is a long-standing puzzle why this is the case.

In a recent study, we investigated the coupled radio and X-ray behavior of the accreting neutron star 4U 1820-30. This is one of those few neutron stars that was thought to sustain its continuous radio jets because its radio brightness never becomes very low. What we found, however, is that the brightness at different radio frequencies does vary by a lot causing the radio energy spectrum to change strongly. In particular, we found that between X-ray “low and high modes” that differ a factor of about 10 in X-ray brightness, 4U 1820-30 is switching between sending out a steady continuous jet and ballistic ejections, represented by the two different radio spectra. Contrary to what was thought, the neutron star is thus not sustaining its steady jets, but behaving in the same way as black holes. These findings motivate similar studies of other neutron stars as well as a more detailed study of 4U 1820-30 itself to resolve the changes on shorter timescales (days or even hours) than we have done now (weeks).

Russell, Degenaar, van den Eijnden, Del Santo, Segreto, Altamirano, Beri, Diaz Trigo, Miller Jones 2020, MNRAS 508, L6: The evolving radio jet from the neutron star X-ray binary 4U 1820-30

Paper link: ADS

Radio observations of 4U1820 during its X-ray high and low modes. The figure shows that there is a clear difference in the radio spectrum between low modes (for instance the green, red and yellow data points+curves) and high modes (for instance the blue, purple and pink data points+curves). These large changes are highly surprising since the change in X-ray luminosity between the two modes is very small (a factor of 2 or so) and also the X-ray spectrum remains largely the same.

Calling all telescopes for duty

In late 2018, the Neil Gehrels Swift observatory (Swift), discovered a new bright source lighting up the X-ray sky. It was called Swift J1858.6-0814, or shortly Swift J1858, and soon realized to be an X-ray binary: a system of two stars orbiting around each other where one of the two is a black hole or a neutron star and the other a regular star. These objects shine bright in X-rays (and at other wavelengths) when the black hole or neutron star is able to pull gas from its companion towards itself. Often this happens only sporadically during episodes that we call outbursts.

About two hundred X-ray binaries are currently known in our Galaxy and many of these have been extensively studied since the dawn of X-ray astronomy in the late 1960s. Swift J1858 immediately stood out, however, by displaying extreme behavior in which the X-ray emission changed by orders of magnitude on short (hours) time scales. Only a handful of other X-ray binaries had ever been observed to display similarly volatile behavior as Swift J1858. Perhaps the most prominent one of those is the infamous black hole V404 Cygni. Based on this analogy, Swift J1858 was therefore expected to habor a black hole too.

The extreme behavior of Swift J1858 drew a lot of attention in the X-ray binary community and motivated a massive multi-wavelength campaign involving many ground-based and space-based observatories. The fleet of facilities pointing to Swift J1858 involved, for instance, ESA’s XMM-Newton satellite (X-rays), NASA’s NICER mission located on the International Space Station (X-rays), the Hubble Space Telescope (UV), the Very Large Telescope in Chile (UV/optical/infrared), the 10-m Grantecan telescope on La Palma (optical/infrared), the Very Large Array in New Mexico USA (radio) and the Atacama Telescope Compact Array in Australia (radio). All these efforts allowed for an unprecedented characterization of the binary and its extreme variability.

X-ray studies suggested that Swift J1858 was very rapidly swallowing gas from its companion, but our radio studies showed that it was also blasting a bright collimated jet into space. Moreover, our X-ray and optical studies showed that it was also blowing material into space via a disk wind. One of the most surprising discoveries was that Swift J1858 turned out to harbor a neutron star rather than a black hole. This was established by the detection of a thermonuclear explosion from the source, a so-called type-I X-ray burst, which cannot be produced by a black hole because they lack a surface. Neutron stars might be tiny, but they can truly be as violent as black holes!

Swift J1858 is now dormant, but our ambitious multi-wavelength campaign has delivered an incredibly rich data set for us to analyze and interpret. A first series of papers reporting on the findings at different wavelengths has already been published, but the analysis is ongoing. In particular, correlating all the data sets obtained at different wavelengths is expected to result in new discoveries that will help us understand how accretion and associated outflows work, and why Swift J1858 showed such extreme behavior. So there is more to come!

Paper links (ADS):

ATCA light curve of Swift J1858 showing that is was also extremely variable in the radio band. This light curve is taken from van den Eijnden et al. 2020

A very controversial black hole

As their name suggests, black holes do not emit any light. However, these dark entities can be spotted through the effect of their strong gravity on their surroundings. For instance, gas that is pulled towards a black hole will light up in X-rays and thereby give away the hiding place of the cosmic cannibal.

Based on what we currently understand of how stars are formed and evolve through their life, it is expected that our Milky Way galaxy should harbor hundreds of thousands, perhaps millions of black holes as the remains of once massive stars. Many of these stellar corpses are expected to be accompanied by another (regular) star. If the black hole and the other star orbit each other close enough, the black hole can steal gas from its unfortunate companion and make the binary discoverable as an X-ray binary. However, only a few tens of X-ray binaries harboring black holes have been found to date (see this catalog called BLACKCAT). We therefore think that there must be many black holes hiding in binaries with orbits that are so wide that the black hole cannot snack off its companion and hence will be discoverable as an X-ray binary. How to find these hidden black holes?

Even if a black hole is not close enough to pull gas from its companion, its gravitational pull will cause the other star to wobble. Stars are too far away to see them wobble against the sky, but we can detect any movement by unraveling their light with spectrometers. The light that we receive from stars contains dark lines, called spectral lines, that will be shifting in wavelength due to the wobble that the black hole is causing. Many millions of stars are currently being surveyed to see if their spectral lines display shifts that can give away the presence of a black hole. Recently one of such surveys, performed with the Chinese LAMOST telescope, uncovered a black hole by these means. The object, called LB-1, was announced to have a mass of no less than 70 times the mass of our Sun. This discovery soon became controversial.

The notorious LB-1 startled the scientific community because within our current understanding of how stars work it is far from trivial to make a black hole that is 70 times as massive as our Sun. In short, the challenge is that such a black hole would likely be the remains of a very massive star, but that such very massive stars are thought to explode in such a violent  supernova that nothing, not even a black hole, is left behind. Within a short time, many alternative ideas have thus been put out to explain how such a massive black hole could theoretically form. At the same time, there are many observational efforts initiated to verify that LB-1 truly contains such a massive black hole.

Observations with the Chandra X-ray satellite showed that if LB-1 contains a black hole, it is not pulling sufficient amounts of gas from its companion star to generate X-ray emission. However, if it would succeed to pull off even the tiniest bit of gas, it could also reveal itself by spitting some of this back into space via jets. The radio emission from the jets that a black hole spits out may be more easily detectable than the X-rays from the gas it is pulling in. Therefore, we’ll be looking at LB-1 with the Very Large Array radio telescope, located in New Mexico. If there is a massive black hole lurking in LB-1 that it secretly stealing some gas from its companion star, it will be caught in the act. Stay tuned!

Shortly after the discovery of LB-1, I gave an interview for Dutch radio to discuss its impact and controversy (for the NPO/radio-1 program “Met het oog op morgen“).

640px-Doppler_shiftFigure: Graphic illustration of the method to find hidden black holes. Whereas the black hole itself may be invisible, its gravitational pull causes the normal star to wobble. This wobble is visible as a movement in the spectral lines in the light of the star (the dark lines in the color bar). The same method is used to find planets around stars. Source of gif: Wikicommons.

Crazy jet experiment

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


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.

Changing emission mechanisms

X-ray binaries, in which a neutron star or black hole swallows gas from a companion star, have been known since the dawn of X-ray astronomy in the 1960s. With decades of studies, using many different space-based and ground-based telescopes across the electromagnetic spectrum, we have learned many things about how neutron stars and black holes accrete gas. Nearly all of this knowledge has been assembled for stages during which X-ray binaries are rapidly accreting gas, making them shine bright at all wavelengths and hence can easily be studied. However, most X-ray binaries spend only a fraction of their time being bright and rapidly accreting; the far majority of their time their gas consumption occurs at a very low level. Because X-ray binaries are much dimmer when accreting slowly, it is much more challenging to study them. Our standard  models predict that accretion proceeds very differently at low rates, but we have hardly any observational constraints to test and further develop models of low-level accretion.

There are many different components in an X-ray binary that emit electromagnetic radiation. One of the prime emission components is the accretion disk that, depending on its physical properties like its temperature, radiates at X-ray, UV, optical and near-infrared wavelengths. At low accretion rates, however, theory predicts that this disk evaporates into a more extended hot flow that may emit energy at the same wavelengths as the disk. Apart from the accretion stream, be it a disk or a hot flow, the companion star also emits UV, optical and near-infrared emission  (depending on what type of star it is and whether it is being irradiated by the accretion flow), whereas X-ray binaries also launch jets that can be detected at radio and near-infrared wavelengths, but possibly also in the optical, UV and X-ray bands too. Disentangling the different emission components, and finding which one(s) dominate(s) the total observed emission, can be a powerful way to obtain details about the accretion process. This is not an easy task, however, because for each different wavelength we (generally) need different telescopes and it’s very challenging to coordinate different observatories.

The Swift satellite is a very important observatory to study X-ray binaries. This is for two reasons. Firstly, it is a relatively small satellite that can easily maneuver around, allowing us to take frequent snapshots of sources (which is not possible for bigger satellites). Secondly, Swift carries both an X-ray telescope and a UV/optical telescope, which can observe an astrophysical object at the same time. Combined with its good sensitivity, this makes that Swift is a powerful tool to study how the accretion flow in X-ray binaries changes when it moves (quickly!) from high to low accretion rates. We attempted such a dedicated study for the well-known neutron star X-ray binary Aquila X-1 (aka Aql X-1).

Using Swift data from the NASA archives that covered three different accretion outbursts of Aql X-1, we studied how the X-ray, UV and optical emission changed as the source evolved between high and low accretion rates. We found that the X-ray and UV/optical emission always change together, but that this happens in a different manner when Aql X-1 is bright than when it is fading. This implies that the dominant mechanism producing UV and optical emission changes during the decay of an outburst, as is expected from accretion theory. It might be a hint that an accretion disk is changing into a hot flow, or that the properties of the accretion disk are changing otherwise. Moreover, we found that the UV and optical emission behaves differently during the rise of an outburst than during the decay. This suggests that the accretion flow may have different properties at the start and the end of an outburst. This investigating has exposed some interesting behavior that warrants follow-up by performing similar studies for other X-ray binaries or using additional observatories.

López-Navas, Degenaar, Parikh, Hernández Santisteban, van den Eijnden 2020, MNRAS 493, 940: The connection between the UV/optical and X-ray emission in the neutron star low-mass X-ray binary Aql X-1

Paper link: ADS


The X-ray and UV flux of Aql X-1 over an entire outburst. It can be seen that the emission at the two wavelengths is coupled in a different manner when the source is bright than when it is faint. Moreover, the UV flux is fainter during the decay (yellow/green points) than during the rise (blue/purple points) of the outburst.