Electrodynamics of stars and exoplanets

I study coronal and magnetospheric plasma in stars, brown dwarfs and exoplanets, with a long-term focus on modelling the effects of plasma and magnetic fields on the atmospheres and habitability of exoplanets. Radio observations have been a unique tracer of high energy plasma in the Sun and the atmospheres of solar-system planets. With LOFAR's excellent sensitivity and state-of-the-art infrared observation, my collaborators and I are now extending similar observations to extrasolar systems.

I discovered the first compelling evidence of radio emission from the magnetic interaction between a star and its exoplanet. I also recently made the first direct radio discovery of a brown dwarf. These discoveries have the potential to use radio observations as a novel exoplanet discovery technique and to study the magnetic fields of exoplanets for the first time.

Key collaborators: Joe Callingham, Tim Shimwell.

Epoch of Reionization

The 21-cm signal from primordial hydrogen is one of the few probes of the early Universe when the first stars formed. I am a member of the LOFAR Epoch of Reionization key science project that is attempting to measure the sky-averaged brightness of the 21-cm signal as well as the statistical properties of its spatial fluctuations. I am currently interested in detecting the 21-cm signal in absorption against bright background radio sources (quasars).

During and prior to my PhD, I worked on two vexing challenges in the field: (1) the separation of bright astrophysical foreground emission from the faint cosmological signal and (2) the corruption of astrophysical radio-waves as they pass through the Earth's ionosphere. My technique to separate the cosmological signal from foreground emission has been refined and widely adopted by leading groups around the world. My PhD work on ionospheric scintillation noise and its particular application to 21-cm cosmology remain the definitive theoretical treatment of the subject so far. Part of it has been included in the standard graduate textbook on radio-interferometry.

Key collaborators: Leon Koopmans, Ger de Bruyn, Ravi Subrahmanyan.

Fast Radio Bursts (FRB)

FRBs are millisecond duration radio flashes from cosmological distances that are among the most intriguing mysteries in high-energy astrophysics. I maintain a sustained albeit auxiliary interest in FRBs. My work on the brightness distribution of FRBs provided the impetus for a novel low-cost 10-dish FRB localization array at Caltech's Owens Valley Radio Observatory. I wrote the calibration and imaging code for this array. It achieved one of the first FRB localizations. I have also worked on the subtle imprints of the FRB environment on radio wave propagation and have proposed the use of FRBs as probes of the small-scale structure of the circum-galactic media of galaxies.

Key collaborators: Vikram Ravi, Gregg Hallinan, Shri Kulkarni.

Astrophysical plasma

Studying the effects of wave propagation in astrophysical plasma has been a recurring theme in my research. I have worked on extreme radio-wave scintillation caused by dense Galactic plasma clouds that are thus-far of unknown origin. Along with one of the PhD advisers, Ger de Bruyn, I developed a new Faraday imaging technique to image these plasma clouds. More recently, I showed that a dense plasma blob, that could be within the Sun's sphere of gravitational influence, was causing extreme scintillations of an APERTIF-discovered variable source.

Key collaborators: Ger de Bruyn, J-P Macquart, Tom Oosterloo.

Millilensing and dark matter

I stumbled upon dark matter serendipitously, when Tony Readhead and I identified a new peculiar type of variability in a few radio sources. We proposed that the variability is caused by so-called millilensing (gravitational lensing on small angular scales) by mass condensates that would constitute a fraction of dark matter. I have since developed an interest for alternative dark matter models (e.g. ultra-light axions, dissipative dark matter) and am working out their particular millilensing signatures.

Key collaborators: Tony Readhead, Phil Hopkins.