Tag Archives: X-ray

How to launch a jet

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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

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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.

An unexpected companion

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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.

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.

Volatile flickering…

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…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

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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

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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.

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.

Changing emission mechanisms

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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

14U_fits

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.

Puffing up the accretion flow

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X-ray binaries are most easily studied when they are devouring a lot of gas and therefore produce bright radiation at X-ray, UV, optical, infrared and radio wavelengths. During such outburst episodes, the gas that is being stripped off from the companion forms a disk that swirls around the black hole or neutron star. We think that this accretion disk extends very close to the cannibal star, maybe even touching it.

During quiescent episodes, X-ray binaries are consuming much less gas and are therefore orders of magnitude dimmer at all wavelengths. Current theories of accretion prescribe that during quiescence, the accretion disk cannot extend close to the black hole or the neutron star and must lie (tens of) thousands of kilometers away. In between the edge of this disk and the compact star, the gas flow might be very hot and vertically extended. However, because this gas is very tenuous and not producing strong radiation, it is very hard to test this idea with observations.

Accretion theory thus predicts that as an X-ray binary starts to fade from outburst to quiescence, the geometry of the accretion flow is strongly changing. However, it is highly challenging to measure this because 1) it’s not predictable exactly when X-ray binaries transition to quiescence, 2) once they do, the transition happens very rapidly, typically on a timescale of 1-2 weeks, and is therefore easy to miss, and 3) as X-ray binaries decay into quiescence they become increasingly dim and therefore long/sensitive observations are needed in order to study the properties of the accretion stream. Ready to take up this challenge, we recently designed an ambitious observing campaign aimed to reveal the changing accretion flow in an X-ray binary called 4U 1608-52.

One powerful way to measure the location of the inner edge of an accretion disk is X-ray reflection. This produces prominent emission features at certain X-ray energies that can be studied with sensitive X-ray telescope. NuSTAR is one of the telescopes that is optimally suited to study X-ray reflection. In a previous study, we observed the X-ray binary 4U 1608-52 during one of its accretion outbursts. Our NuSTAR observation revealed a beautiful X-ray reflection spectrum that allowed us to determine that the accretion disk was extending very close to the neutron star. Taking advantage of the fact that 1) this X-ray binary goes into outburst once every few years (opposed to some sources for which we have observed only 1 outburst over 5 decades), and 2) it produces a very strong reflection spectrum, we designed an observing campaign to capture the source again with NuSTAR, but then at a factor ~10 lower luminosity. The main aim was to use the reflection spectrum to determine if with this change in brightness, the accretion geometry changes a lot.

When our target 4U 1608-52 was seen to enter a new accretion outburst in 2018, we closely monitored how its brightness evolved by looking at the data obtained with the Japanese MAXI X-ray telescope that is installed on the International Space Station. Once MAXI showed that the X-ray brightness of 4U 1608-52 was decreasing, we performed observations with the much more sensitive Swift and NICER telescopes. This allowed us to continue watching our target once it became too faint to be detected with MAXI. We analyzed each new Swift observation immediately after it was performed and tried to predict how the brightness of our target would decay onward. With a few days lead time, we then triggered our NuSTAR observation, hoping that it would observe our target at exactly the right time.

After an intense 2 weeks of watching 4U 1608-52 closely every day, we succeeded to have NuSTAR point to our target at exactly the right time. Interestingly, our new NuSTAR observation showed that with a factor ~10 change in brightness, the reflection spectrum completely disappeared. We think that this is because the accretion flow is changing from a (flat) disk into a hot (spherical) structure. This constitutes one of the very few observations that supports standard accretion models, so we are very excited about these results.

van den Eijnden, Degenaar, Ludlam, Parikh, Miller, Wijnands, Gendreau, Arzoumanian, Chakrabarty, Bult, 2020, MNRAS 493, 1318: A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608-52

Paper link: ADS

4U1608_combi

NICER and NuSTAR spectra of 4U1608-52. The left panel shows the data obtained during several instances along the 2018 outburst. Data obtained during the 2014 outburst is shown for comparison. A prominent feature can be seen between 5 and 10 keV that is referred to as the iron (Fe-K) line and gives information about the accretion geometry. The right panel shows that this emission feature disappears during the decay of the outburst, which shows that there is a dramatic change in the accretion flow.