A very radio-bright neutron star

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

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.

What do neutron stars look like inside?

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.

Zooming in on an intriguing neutron star

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.

The accretion flow around a black hole

Black holes are infamous for their relentless gravitational pull through which they drain matter and energy from their surroundings. However, with their enormous power, these tantalizing objects also blast matter back into space via ultra-fast collimated jets and dense winds. Understanding the exact connection between how black holes accrete from – and supply feedback to – their environment is one of the outstanding challenges of modern astrophysics.

X-ray binaries are excellent laboratories to study the eating habits of black holes. In these binary star systems a black hole orbits a Sun-like star close enough to pull off and accrete the outer layers of its unfortunate companion. This accretion process liberates enormous amounts of energy that is emitted across the electromagnetic spectrum. Studying the accretion flow in X-ray binaries thus warrants a multi-wavelength approach.

We recently performed such a study for the newly discovered X-ray binary Swift J1910.2-0546.  In 2012 May the Swift satellite suddenly discovered a new, bright X-ray point source in the sky and very soon it became clear that the X-ray emission was powered by accretion onto a black hole. Using the X-ray and UV telescopes onboard Swift, we continued to monitor this new X-ray binary for about three months. To complement these observations, the source was also closely followed at optical and infrared wavelengths (B, V, R, I, J, H, and K filters) using the 1.3-m SMARTS telescope located at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. Finally, a high-resolution X-ray spectroscopic observation was obtained with the Chandra satellite.

This monitoring campaign allowed us to map out the accretion morphology around the black hole in Swift J1910.2-0546. Firstly, X-ray spectroscopy revealed two peculiarities: although disk winds appear to be ubiquitous in black hole X-ray binaries when they are at their brightest, our Chandra observations did not reveal any emission or absorption features that are the imprints of an accretion disk wind. Since such winds are thought to be concentrated in the equatorial plane, this may imply that we are viewing the binary at relatively low inclination. Moreover, even during the brightest stages of its outburst tracked by Swift, the temperature of the accretion disk did not reach above 0.5 keV (about 6 million degrees Kelvin), whereas most black hole disks are much hotter with temperatures above 1 keV. This could plausibly be a geometrical effect, again suggesting that the inclination angle of the binary is relatively low.

Comparing the overall light curves of the outburst in different wavebands revealed two other striking features. A sharp and prominent flux dip appeared in the X-rays almost one week later than at UV, optical and infrared wavelengths. The detailed properties of this flux dip appear to point to a global change in accretion flow geometry, possibly related to the formation of a collimated jet or the condensation of the inner part of the accretion disk. In addition, when the activity of the black hole started to cease, we found that the X-rays steadily decreased whereas the UV emission suddenly was rising again. The observed strong anti-correlation between the X-ray/UV flux also indicates a global change in accretion flow.

Degenaar, Maitra, Cackett et al. 2014, ApJ 784, 122: Multi-wavelength Coverage of State Transitions in the New Black Hole X-Ray Binary Swift J1910.2-0546

Paper link: ADS

Artist impression of the accretion flow around a black hole.  Credit: NASA/Dana Berry, SkyWorks Digital

Artist impression of the accretion flow around a black hole.
Credit: NASA/Dana Berry, SkyWorks Digital