# About me

I am a researcher working on fields like relativistic astrophysics and computational physics focusing mainly on compact objects like neutron stars and black holes. For this I use tools such as pen & paper and supercomputers.

Currently I am a Nordita Fellow in Nordic Institute for Theoretical Physics (Nordita, Stockholm, Sweden). I am also an avid open-source science & software advocate so you can find most of my research and codes freely available.

### Research

I have a wide range of interdisciplinary research interests. Mostly I, however, like to focus on computational astrophysics.

**High-energy astrophysics**: accretion & accretion disks; neutron stars, pulsars, X-ray bursters; black holes**Plasma physics & (magnetized) fluid dynamics**: collisionless plasma dynamics; turbulence

**Dense plasma**: equation of state of cold ultra-dense matter; universal relations**General relativity**: ray tracing**Statistics**: (Markov Chain) Monte Carlo methods; Bayesian analysis

**Computer sciences**: high performance computing; parallellization paradigms; neural networks**Mathematics**: topology; knot theory

## Recent publications & blog posts

## Introduction to Julia

My teaching material for an introductory course on the new high-level, high-performance programming language called Julia is freely available here.

## Bayesian M-R constraints for accreting millisecond pulsars

Together with Tuomo Salmi and Juri Poutanen we studied the possibility of applying Bayesian inference to accreting millisecond pulsar X-ray light curves.

## Arcmancer: general purpose ray tracer

In the project led by Pauli Pihajoki, we took ray tracing and numerical differential geometry to the next level. Images of black holes, disks, neutron stars? Can do!

## Constraining the radius of neutron stars with atmosphere model fits

We used new nested hierarchical Bayesian modeling framework to constrain radius of the neutron star in 4U 1702-429 to be R=12.4km. The presented method turns out to be one of the most accurate ones there is!

## Radiation from rapidly rotating neutron stars

Bender is finally here! Together with Pauli Pihajoki, we developed a ray tracing platform for rapidly rotating oblate neutron stars. And yes, it's open source!

## Helium composition in 4U 1820-303

In the work led by Valery Suleimanov, we applied the direct cooling tail method to constrain the atmosphere composition of a neutron star in 1820. Our results show that helium is strongly favored.

## Variable spreading layers of neutron stars

In the work led by Jari Kajava, we studied the interface between the neutron star and the accretion disk. This so-called spreading layer seemed not as constant as people have previously assumed!

## Dynamic powerlaw method for X-ray bursts

How do X-ray bursts really cool? Is it a single-index powerlaw or maybe an exponential? We investigated this together with our student Jere Kuuttila.

## Direct cooling tail method

Here we derived some corrections to our existing cooling tail method. This new method is then applied to a previously measured neutron star, identifying about 500 meter calibration error.

## Scientific writing tips

A collection of scientific writing tips for Master students working with their thesis.

## Traces of heavy metals in neutron star atmospheres

Some peculiar X-ray bursts are found to have atmospheres that are made almost completely of heavy metals. A radiatively driven wind might then ejects some of the burning ashes into the interstellar space.

## UNIX introduction

3 hour introductory lecture material into the world of UNIX.

## Equation of state of dense matter from X-ray bursts

Equation of state of the cold dense matter has remained mystery for decades. In this recent work, we set new constraints to the EoS parameters by using X-ray bursts that show signs of passive cooling.

## Heavy metal neutron star atmospheres

What happens to the observed spectrum of neutrons stars when the atmosphere is full of heavy metals from the nuclear burning? On this article, published in A&A we test the effects by computing detailed atmosphere models.

## Exploration of numerical hydrodynamics

Contains some doodlings I did in the world of hydrodynamics. At some point I became interested in how do fluids actually move so I build my own hydro code to find out.

## Influence of accretion geometry on the spectral evolution during X-ray bursts

In this work we show how the accretion modifies the cooling of X-ray bursts on top of neutron stars. This will then lead to (at least) two different classes of bursts, that differ considerably.