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

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.

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

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.