Tag Archives: radio

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.

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.

A very controversial black hole

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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

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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.

A very radio-bright neutron star

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Black holes and neutron stars are notorious for swallowing gas from their surroundings. However, these extreme objects also spit large amounts of matter and energy back into space via collimated streams of gas that are called jets. These jets emit radio emission that can be detected with large radio telescopes such as the Australian Telescope Compact Array (ATCA) that is located in Australia. Black holes seem better at producing jets, since their radio emission is on average a factor ~10 brighter than that emitted by neutron stars.

In 2018 August, the X-ray telescopes orbiting the Earth detected a new X-ray source in the Sky that was named IGR J17591-2342 (after its position in the Sky and after the telescope that discovered it, ESA’s INTEGRAL mission). Within a few days after this discovery, we observed this new X-ray source with ATCA, to investigate if it was producing a jet. We detected such bright radio emission that we suggested that this object was likely a black hole. However, a pulsed X-ray signal was detected from IGR J17591-2342; such a signal requires an object with a solid surface and therefore rules out that this source contains a black hole. The detection of X-ray pulsations instead showed that IGR J17591-2342 contains a neutron star, spinning at a dazzling rate of 527 rotations per second, that is swallowing gas from a nearby companion star.

The distance to the new X-ray source IGR J17591-2342 is unknown, but its X-ray emission is strongly absorbed by interstellar gas, which would suggest that the source is relatively distant. For distances larger than 3 kpc, the radio brightness of IGR J17591-2342 is very similar to that of black holes and much brighter than that of neutron stars. It is not yet understood why this neutron star is able to produce such a bright radio jet.

Russell, Degenaar, Wijnands, van den Eijnden, Gusinskaia, Hessels, Miller-Jones 2018, ApJ Letters 869, L16: The Radio-bright Accreting Millisecond X-Ray Pulsar IGR J17591-2342

Paper link: ADS

Lx_Lr_IGRJ1759_russel2018

Radio and X-ray luminosities of a large collection of black holes (black circles) and different classes of neutron stars (grey circles, pink squares and cyan triangles). The location of IGR J17591-2342 in this diagram depends on its unknown distance and is indicated by the different coloured symbols. Unless the source is very nearby (less than 3 kpc), it is unusually radio bright for a neutron star.

A new class of jet sources

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Accretion is a fundamental physical process that plays an important role at all spatial scales encountered in the universe. Whenever accretion occurs, it appears to be inevitable that jets are produced; collimated beams of matter and energy that are spit into space by the astrophysical object that is accreting. For decades, strongly magnetized neutron stars stood out as the only objects that accreted and did not seem to produce jets. This led to the paradigm that their strong magnetic fields prevent the formation of jets. Earlier this year, we made a ground-breaking discovery that disproves this.

Despite decades of jet studies of X-ray binaries, strikingly, no radio emission was ever detected from accreting neutron stars that have strong magnetic fields. For decades, it was therefore assumed that these objects do not produce radio emission because they are incapable of producing jets. Originally set out to provide more stringent upper limits on the radio emission, we exploited the upgraded sensitivity of the Very Large Array (VLA) radio telescope to perform deep radio observations of two strongly magnetic neutron stars, Her X-1 and GX 1+4. Somewhat surprisingly, we detected radio from both objects for this first time. Though very exciting, we were not able to prove that these  detections pointed to the presence of a jet, since other emission processes could produce the observed radio emission. Nevertheless, this motivated us to dig deeper into the question if strongly magnetic neutron stars could produce jets after all.

In late 2017, we were fortunate to run into an ideal test case. A previously unknown X-ray binary suddenly exhibited an accretion outburst, making it shine very bright in X-rays. When it was discovered that the accreting object in this newly discovered source, dubbed Swift J0243.6+6124, was a strongly magnetic neutron star, we requested observations with the VLA to search for radio emission from a jet. And this is exactly what we found.

Our observations of Swift J0243.6+6124 unambiguously proved that we were watching an evolving radio jet. Firstly, we clearly observed a coupling between the radio emission and the X-ray emission, as is seen in black holes and weakly magnetic neutron stars. Secondly, by performing the radio emission in multiple frequency bands, we were able to measure the radio spectral index and evolution therein, which too followed exactly the same behavior as seen for other X-ray binaries. Our observations thus disproved the long-lasting paradigm that strongly magnetic neutron stars cannot produce jets, which has far-reaching consequences.

This discovery opens up a completely new regime to study astrophysical jets. In particular is can shed new light on the open question how these outflows are launched. This is because strongly magnetic neutrons stars have a completely different accretion geometry than black holes and weakly magnetic neutron stars, because their strong magnetic field pushes the accretion disk out to hundreds of kilometers. Any jet launching model must thus be able to explain that material is accelerated into a jet from such large distances. Moreover, several models prescribe that the power of a jet should scale with the rotation rate of the accreting object. This has been very difficult to test with black holes, because their spin rates cannot be unambiguously measured, or with weakly magnetic neutron stars, because these exhibit only a very narrow range in spin rates. Neutron stars with strong magnetic fields, however, are observed with a very wide range of accurately measured rotation rates, from sub-seconds to thousands of seconds. This finally allows to test the predicted correlation between that the radio brightness and the spin rate.

Because of the important scientific impact for jet studies, our results are published in the October issue of Nature (2018). Following up on our discovery, we have already started to perform a large,  systematic radio survey of accreting strongly magnetized neutron stars. The important next steps are to test if, and how, these jets are coupled to the properties of the accretion flow, and if we can detect any dependence of the jet properties on the spin of the neutron star. Stay tuned.

van den Eijnden, Degenaar, Russell, Wijnands, Miller-Jones, Sivakoff, Hernández Santisteban 2018, Nature 562, 233: An evolving jet from a strongly magnetized accreting X-ray pulsar

Paper link: ADS
Selection of press items: NOVA and NRAO
Explanatory movie (English): youtube

lc_swj0432_short

Discovery of a radio jet launched by the strongly magnetic neutron star in Swift J0243.6+6124. Shown is the X-ray light curve from Swift/BAT in black together with our radio observations from VLA in red. After an initial non-detection in the radio, we detected the jet emission during the peak of the outburst and watched it fade in tandem with the decrease in X-rays.

Facts and myths about neutron star jets

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Jets are collimated outflows of matter and energy produced by accreting astrophyical objects. Such jets are found on many different scales in the universe, ranging from young stars to supermassive black holes in the centers of galaxies. Black holes and neutron stars that accrete gas from a companion star in an X-ray binary are prominent jet producers too. In these systems, the collimated jets are most prominently detected at radio wavelengths.

Starting in the 1970s, the radio jets of X-ray binaries have been studied in great detail. One key characteristic is that there is a very strong correlation between the radio brightness and the X-ray luminosity, which suggests a strong coupling between the inflow of matter (traced by the X-rays) and the outflow (traced by the radio emission). Early studies suggested, however, that the coupling between the X-rays and the radio emission, parametrized by the coupling index beta, is different for black holes and neutron stars. A plausible explanation for this difference could be that neutron stars have a solid surface; whereas gas that reaches a black hole can be carried across the event horizon without emitting any radiation, all energy contained by the gas will be converted into X-ray radiation when it hits the surface of a neutron star. This could translate into a different X-ray/radio correlation.

Collecting the largest sample of radio/X-ray points of X-ray binaries to date, we set out to perform a rigorous statistical analysis to investigate if the jets of neutrons stars are  fundamentally different from those of black holes. Our analysis contained a total of 35 individual black holes, and 41 neutron stars and let to several important conclusions. Several common conjectures about neutron star jets were disproved by our analysis, while others were strengthened, leading to the following facts and myths:

Facts:

Our rigorous analysis reinforces previous conjectures that the radio emission of neutron stars is fainter, by a factor ~20, than that of black holes accreting at similar X-ray luminosity. Correcting for different factors that might influence the comparison (e.g. their difference in mass, different bolometric correction factors and the extra X-ray emission of neutron stars coming from their surface) does not lift this difference. Therefore, we are left to conclude that, in general, neutron stars produce less bright radio emission than black holes accreting at similar rates.

Myths:

1) For decades, the number of neutron stars observed in the radio band was much more modest than that of black holes, partly driven by the fact (see above) that neutron stars were considerably fainter in the radio band, hence more difficult to observe. However, exploiting the current generation of upgraded radio facilities, much more neutron stars have been observed in the radio band. In fact, our study included 41 different neutron stars, compared to 35 different black holes. Neutron stars are thus no longer underrepresented in radio studies.

2) It is commonly said that neutron stars display a larger scatter in the radio/X-ray plane, i.e. display more chaotic behavior. However, in our study we found that the statistical scatter in the neutron star sample is similar to that in the black hole sample.

3) It is often assumed that neutron stars, in general, show a different (namely steeper) correlation between their radio and X-ray luminosity. However, this conjecture is largely based on a detailed study of one individual neutron star. Considering the sample as a whole, we obtained a coupling index for the neutron stars that was consistent with being the same as that of the black hole sample. It thus appears that neutron stars do not show a different radio/X-ray coupling than black holes.

Apart from comparing the neutron star and black hole samples, we also investigated if  sub-samples among the neutron stars may behave differently. Interestingly, we found that the sub-population of transitional millisecond radio pulsars, statistically behaves differently from the other neutron stars. This suggest that their jet properties are fundamentally different.

Gallo, Degenaar & van den Eijnden 2018, MNRAS Letters 478, L132: Hard state neutron star and black hole X-ray binaries in the radio:X-ray luminosity plane

Paper link: ADS

gallo2018_Lx_Lr

Radio versus X-ray luminosity of about 36 black holes (black filled circles) and 41 neutron stars (red open diamonds). The solid lines and shaded areas represent statistical fits to the correlation between the radio and X-ray luminosity. The resulting coupling index beta is quoted for both populations and is consistent with being the same within the errors.

What do neutron stars look like inside?

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

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

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

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

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

Paper link: ADS

SKA

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

A new regime to study jets?

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Accretion is an important physical process in which an astronomical body gravitationally attracts material from its surroundings. This leads to growth and to the release of gravitational energy. We encounter accretion throughout the universe, on many different scales and in widely varying environments. For instance, stars and planets are formed through accretion, and the accretion behavior of a super-massive black hole determines how its host galaxy evolves over time.

Regardless of the nature of the object that is accreting (e.g. star, black hole), or the environment in which accretion occurs, it seems inevitable that part of the attracted material is spit back into space. This occurs to powerful collimated streams called jets. Despite being ubiquitous, exactly how jets are formed and being powered remains a mystery. To understand this, it is key to study jets in different types of accreting systems. This is one of the prime pursuits of modern astrophysics.

Strikingly, the only accreting systems for which jets had never been detected were neutron stars with strong magnetic fields (over a trillion times – a one with twelve zero’s that is – more powerful than the Earth’s magnetic field). This led to the long-standing paradigm that the presence of very strong magnetic field prevent jets from being formed.

Jets from accreting black holes and neutron stars with low magnetic fields (only a billion times the strength of the Earth’s magnetic field, i.e. a one with only nine zero’s) are most commonly detected at radio wavelengths. Despite various searches, radio emission had never been detected from accreting neutron stars with strong magnetic fields. Luckily, the Very Large Array (VLA) radio facility in New Mexico underwent major technical upgrades in recent years that greatly improved its sensitivity. We therefore decided to revisit if strong magnetic field neutron stars truly do not produce radio jets.

Somewhat to our surprise, Jakob made the startling discovery that the two strong-magnetic field neutron stars that we observed with the VLA, the well-studied sources GX 1+4 and Her X-1, were both detected in the radio band (at 8 GHz). While for GX 1+4 the radio properties allow for a different origin than a jet (e.g. shocks in the magnetosphere of the neutron star), the radio properties of Her X-1 more strongly suggest a jet origin. If confirmed, it would show that high-magnetic field neutron stars can launch jets after all. These findings would have important implications for understanding jet formation in general.

Press release

van den Eijnden, Degenaar, Russell, Miller-Jones, Wijnands, Miller, King, Rupen 2018, MNRAS Letters 473, L141: Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron star?

Paper link Her X-1: ADS

van den Eijnden, Degenaar, Russell, Miller-Jones, Wijnands, Miller, King, Rupen 2018, MNRAS Letters 474, L91: Discovery of radio emission from the symbiotic X-ray binary system GX 1+4

Paper link GX 1+4: ADS

 

jakob_radio_gx14

VLA radio image (9 GHz) of GX 1+4. The cross indicates the position of this high-magnetic neutron star. The color scaling indicates the radio brightness. GX 1+4 is clearly detected.