In general, my research interests revolve around investigating the role that magnetic fields play in various environments throughout the universe. Magnetic fields are important almost everywhere in astrophysics, mainly because they are so strongly coupled to charged particles (and there are a lot of charged particles floating around out there). They operate on all scales in the universe, from the magnetosphere that shields the Earth from harmful particles in the solar wind, to the incredibly strong fields surrounding the neutron stars associated with pulsars, to the extremely weak but large-scale fields that occupy the space within clusters of galaxies, which are the largest gravitationally bound objects in the universe. Also, to improve our ability to measure the properties of magnetic fields, I'm developing new data and image processing algorithms using the tools of Information Field Theory (IFT), and helping to calibrate the new LOFAR telescope which is already starting to provide an exciting new look at the universe.
Magnetic fields in jets
My PhD research involved observationally characterizing, and attempting to model, the magnetic fields in the jets of the fascinating microquasar SS 433. SS 433 is a system consisting of a star orbiting a black hole (probably, but it could also be another dense stellar remnant like a neutron star). The black hole was once a star itself, but has since gone supernova and collapsed in on itself while shedding its puffy outer layer of gas. The result of this massive explosion can still be seen in the shell of the supernova remnant W50, within which SS 433 is embedded. The black hole is sucking gas from its orbiting companion, but the gas doesn't actually all fall into the black hole. First, the in-falling gas forms a hot accretion disk that glows x-rays, and as the material in the disk approaches the event horizon of the black hole some of it is actually redirected and launched outward in a pair of collimated, highly energetic jets.
These kinds of disk-jet systems are common in the universe, both at stellar scales, as in the case of SS 433, as well as at much larger scales like the jets that are launched from the super-massive black holes at the centers of some galaxies which are millions of times more massive than the Sun. It is thought that magnetic fields within the disk around the black hole get twisted up thanks to the rotation of the gas, and that these twisted magnetic fields redirect the gas in the disk to form jets. The exact mechanism of jet formation is not well understood, nor is the mechanism that keeps the jets confined to a narrow stream as they travel over large distances, but magnetic fields are thought to be crucial for both.
The jets in SS 433 are particularly interesting because they have a corkscrew-like pattern on the sky caused by the precession of the base of the jet, much like a stream of water flowing from a garden hose as you twirl it around in your hand. We know that there are magnetic fields in the jets because we can see their signature in polarized radio emission, and by studying this emission we can hope to learn more about how the fields influence the behavior of these and other jet systems.
I am also involved in projects studying the role of magnetic fields in jets that are emitted from forming stars, rather than stellar remnants. These jets are much less energetic than those in other systems, and it is unclear how they are related to their bigger and faster counterparts, if at all. We hope to learn more about how these jets work, and whether there might be universal properties to all disk-jet systems that scale in a nice way with energy and mass in the system.
At the same time, I am involved in a few projects studying the structure of magnetic fields in the massive outflows from active galactic nuclei. Some theories and simulations predict that the twisted up magnetic fields in the accretion disk should produce helical magnetic fields that are threaded throughout the jets, and that these play a role in keeping the jets focused. If this were the case, we expect to see certain characteristic observational signatures. Some hints of these signatures have been seen already, but results are so far inconclusive. My collaborators and I plan to improve upon previous observations using new polarization imaging techniques that I've developed over the past couple of years with the hope of unraveling the true nature of these powerful jets.
Diffuse magnetic fields in galaxies and clusters of galaxies
Magnetic fields are thought to pervade nearly every part of the universe. Although the magnetic fields in e.g. the interstellar medium of the Milky Way or in the intergalactic medium of galaxy clusters are very weak, they are still a crucial component of these systems and amazingly enough can still be detected by industrious astronomers. These fields play a role in the transport of cosmic rays and other particles throughout space and are excellent tracers of the turbulent properties of the gas in these environments, among other things.
I am interested in understanding the structure of the magnetic field in the Milky Way. This will of course help us better understand our own cosmological backyard and it will also help when studying the rest of the universe in more detail because the Milky Way is always in the foreground of, and thus contaminates, sensitive observations of things like the cosmic microwave background radiation field or the epoch of reionization. To this end, my collaborators and I have predicted observational signatures of magnetic field reversals that we call "Faraday caustics," and we believe that these can be used to help map out the large scale magnetic fields that fill our Galaxy and to study turbulent properties of the interstellar medium. I have also been involved in a project, lead by my colleague Niels Oppermann, to make the most detailed map to date of the Galactic Faraday depth which is a tracer of magnetic fields. As we continue this work, we hope to learn more about the Galactic magnetic fields, and to develop a much more detailed, 3D model of our local environment.
I am also working on projects to study the extremely faint magnetic fields in clusters of galaxies. We know that clusters are magnetized because in some cases we can see the light that is emitted by particles spiraling around the fields and in other cases, even if we can't see this light, we can detect the effects of magnetic fields on light that passes through the cluster medium. Despite the fact that we know they are there, little is known about the properties of these magnetic fields or where they come from. We hope that by looking at polarized radio emission from sources embedded within clusters of galaxies and measuring how the magnetic fields distort the polarized light, we can learn about the strength and structure of the magnetic fields, the turbulent properties of the cluster gas, and the interactions between the embedded sources and the cluster gas.
Improving methods for studying magnetism
Radio telescopes are the best tools for studying magnetic fields. Much of what a radio telescope "sees" is what is known as synchrotron radiation, which is light emitted from highly energetic electrons as they spiral around magnetic fields. Synchrotron radiation has the property that it is often highly polarized, and the orientation of the polarized light is directly related to the orientation of the magnetic field on the sky. So by carefully measuring synchrotron radiation with a radio telescope, we can learn a lot about the strength and structure magnetic fields that were responsible for emitting it. Moreover, as polarized light travels through a region containing magnetic fields, the polarization plane rotates due to an effect called Faraday rotation. By measuring synchrotron radiation AND Faraday rotation together, we can hope to recover the 3D properties of magnetic fields.
I have been working to develop new tools to help study these effects in more detail. These tools will help us make high-fidelity images of the radio sky, in both polarized and unpolarized light, and to connect the information in these images to the physical properties of the source that we're observing. I've developed the technique of Faraday synthesis, which we hope will become the new standard for polarimetric imaging with radio telescopes, and helped to improve the existing technique of RM synthesis. We will continue working to improve our ability to reconstruct complex, diffuse structures on the sky and to connect the structures in the images to the underlying physics. Along the way, we have developed, and will continue to develop, more broadly applicable data processing techniques using statistical inference techniques and information field theory. Using these signal and data processing tools I'm also helping to better calibrate and characterize the properties of the LOFAR telescope, which will be a unique and powerful tool for studying magnetism, among many other things.
