Category Archives: jets

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.

Calling all telescopes for duty

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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

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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.

Astrophysical pollution

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

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

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

INT_bigger

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

Don’t overfeed the neutron star

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The laws of physics dictate that there is a maximum amount of food that neutron stars and black holes can digest. Once you reach the so-called Eddington limit, the radiation that is produced by the consumption of gas becomes so strong that it blows away the in-falling material. Theoretically, it is therefore predicted that if you overfeed a neutron star or a black hole, strong outflows are produced: in the regime of super-Eddington accretion, we expect both jets and winds to be created. Jets are usually detected at radio wavelengths, whereas winds often reveal themselves as narrow absorption lines in high-resolution X-ray spectra.

There are a number of neutron stars and black holes identified that are likely accreting at very high rates. Most of these are located in other galaxies, and referred to as ultra-luminous X-ray sources (ULXs), because the high rate of food consumption makes them very bright X-ray emitters. For several of these ULXs, signatures of disk winds have been detected. A few other ULXs have radio bubbles around them that suggest that these objects are producing strong jets. However, to date there is no ULX known that is known to produce both winds and jets at the same time. It therefore remains to be established if super-Eddington accretion indeed causes both types of outflows.

Swift J0243.6+6124 is an accreting neutron star that is located in our Milky Way galaxy and was discovered in late 2017 when it suddenly started to feed of its companion star. Following its discovery, the object kept brightening until after a few weeks it reached super-Eddington accretion rates. We previously detected a jet from this neutron star using the Very Large Array (VLA) radio telescope. Following this detection, and known that the source was in the super-Eddington regime, we also requested high-resolution X-ray observations with the Chandra telescope with the aim to search for the presence of a disk wind.

Detecting a disk wind in Swift J0243.6+6124 was not an easy task because it was so overwhelmingly bright that it was causing issues for all X-ray satellites: just as the NS cannot eat fast enough, our X-ray detectors couldn’t process the light received from the source fast enough. Luckily, Chandra could be operated in a very special setting that allowed us to look at the source anyway. Excitingly, the spectra that we obtained with Chandra contained a number of narrow absorption lines that can arise from a disk wind. The properties of these absorption lines suggest that the wind is blown away from the neutron star at a dazzling speed of 20% of the speed of light: a speed of about 200 million kilometers per hour! Similar wind speeds have been measured for ULXs in other galaxies.

Our Chandra and VLA observations thus revealed that indeed jets and winds are produced at the same time in the super-Eddington accretion regime, just like theory predicts.

van den Eijnden, Degenaar, Schulz et al. 2019, MNRAS 487, 4355: Chandra reveals a possible ultrafast outflow in the super-Eddington Be/X-ray binary Swift J0243.6+6124 

Paper link: ADS

Spacecraft labled new 2019

Schematic overview of the Chandra X-ray satellite, with which we performed this research. Image credit: NASA

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.

Zooming in on an intriguing neutron star

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

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

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

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

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

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

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

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

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

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

Paper link: ADS

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

Paper link: ADS

global_sed_flux

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

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.