My astrophysics research mostly focuses on

  • Growth of
    Black Holes

    In Astrophysics, I research growing black holes, their demographics and their immediate environments. Before matter falls into a black hole, it swirls around it and heats up, creating radiation. This makes distant galaxies shine bright as Active Galactic Nuclei (AGN). I observe this radiation because it tells us how much the black hole is currently growing.

    My research focuses on the time when this growth occured most (z=0.5-3). For my PhD project I reconstructed the total growth of black holes over cosmic time using a large sample of distant AGN (2000, CDFS, COSMOS, AEGIS, XMM-XXL). This also requires a good understanding of the observations (astro-statistics) and the obscuration of AGN.

  • The Obscurer around
    Active Galactic Nuclei

    In most AGN, much of the radiation is swallowed by thick columns of gas and dust near the black hole. In my research I try to understand these gas clouds, in particular their location, extent/covering and relation to the black hole. During my PhD I investigated different obscurer geometries. I could also place the best constraints to date on the intrinsic covering fraction of the obscurer (77% Compton-thin, 38% Compton-thick obscured). The large-scale gas in galaxies can also obscure AGN and gamma-ray bursts — in two recent papers I constrained how important this effect is. To understand the nuclear obscurer is crucial to correctly infer the intrinsic emission and therefore the black hole growth. Also, the mechanisms making these clouds is currently unknown.

  • Astrostatistics &

    I have published in Statistics, where I focus on nested sampling Monte Carlo algorithms and their performance. Population demographics (hierarchical Bayesian models) interests me. I enjoy helping others with statistics problems in diverse applications.

    I write a lot of software for various purposes (>100 github repos), many of them are also used by others: I am the author of the Bayesian inference packages PyMultiNest and UltraNest. Bayesian X-ray Astronomy (BXA) package for X-ray spectral fitting, and the lead developer of the popular state-of-the-art NWAY multi-catalog matching code. I think about new algorithms and solutions daily.