About me
I am an astrophysicist focusing on topics like cosmic plasmas, high-energy astrophysical phenomena, and computational physics. I’ve also done some research on computer sciences, statistics, and machine learning. For my research I use tools such as pen & paper and supercomputers.
Currently, I have a joint position as a Postdoctoral Research Scientist in Columbia University and as a Flatiron Research Fellow in Center for Computational Astrophysics, Flatiron Institute (New York, USA). Previously, I was a Nordita Fellow in Nordic Institute for Theoretical Physics (Stockholm, Sweden). I am also an avid open-source science & software advocate — you can find my research and codes freely available from GitHub.
Research
I have a wide range of interdisciplinary research interests. These reflect some of my latest publications:
- High-energy astrophysics: accretion (accretion disks); compact objects (neutron stars, black holes)
- Plasma physics: collisionless plasma dynamics; turbulence; particle acceleration
- Nuclear physics: equation of state of cold ultra-dense neutron matter
- General relativity: ray tracing
- Statistics: Bayesian inference; Monte Carlo methods
- Computer sciences: high performance computing; parallellization paradigms; machine learning; Julia language
- Mathematics: cellular automata models; topology
Notes on physics
- Relativistic plasma physics
- MHD turbulence
- Digital filtering
- Introduction to neutron stars (my PhD thesis)
Most of these research notes are done in connection to some publication. If you want to cite these, let me know and I’ll point you to a right paper.
Teaching
- CSC High-performance Summer School
- latest materials from 2019
- Introduction to Julia (CSC)
- Introduction to UNIX (Univ. Turku)
- Scientific writing tips
Undergraduate/PhD collaborators
- Tuomo Salmi (host for Nordita PhD visit; MSc and BSc thesis co-supervisor)
- Maarja Kruuse (host for Nordita PhD visit)
- John Hope (MSc thesis supervisor)
- Jere Kuuttila (MSc thesis co-supervisor)
- Eemeli Annala (PhD collaboration)
Senior collaborators
- Axel Brandenburg (Nordita)
- Andrei Beloborodov (Columbia Univ.)
- James Cho (Flatiron)
- Aleksi Kurkela (CERN)
- Cole Miller (Univ. Maryland)
- Sasha Philippov (Flatiron)
- Juri Poutanen (Univ. Turku)
- Lorenzo Sironi (Columbia Univ.)
- Andrew Steiner (Univ. Tennessee & ORNL)
- Valery Suleimanov (Univ. Tubingen)
- Aleksi Vuorinen (Univ. Helsinki)
- Alexandra Veledina (Nordita & Univ. Turku)
Recent publications & blog posts
Alfven Wave Turbulence
PRL on how Alfven-wave turbulence can heat astrophysical plasmas around black holes and neutron stars.
Relativistic Boris spectrally deferred correction algorithm
In a work led by Kris Smedt we studied the so-called Boris-SDC algorithm for updating particle velocities in kinetic simulations.
Computer vision & turbulence simulations
Together with Maarja Bussov we constructed a new machine learning computer vision algorithm to help us segment turbulence simulations.
Coherent radio emission from magnetized shocks
In our PRL we studied the production of radio waves from magnetized collisionless shocks as a viable FRB emission mechanism.
Kinetic plasma model for orphan gamma-ray flares
Together with Emanuele Sobacchi and Lorenzo Sironi we discuss a new astrophysical model for orphan gamma-ray flares based on reconnection-mediated turbulence
Multimessenger constraints for neutron star matter
In this work, we did a detailed study on how much do we actually know about neutron star equation of state given all the most recent multimessenger constraints.
Combining EM and GW observations to measure neutron star radii
In a Phys. Rev. Letter led by M. Al-Mamun and A. Steiner we combined state-of-the-art EM and GW observations to constrain the size of the neutron radius.
Radiative turbulent flares
We performed new radiative kinetic simulations of reconnection-mediated turbulent flares - these flares are a new way to energize plasmas around neutron stars and black holes.
Bending of polarized light rays around neutron stars
Together with V. Loktev and T. Salmi we extended our previous work on light bending around neutron stars to account for polarization of the light.
Milli-second pulsar atmospheres
In a work led by T. Salmi we computed new return-current-heated neutron star atmosphere models for millisecond pulsars.
Spreading layers around neutron stars
When an accretion disk around a neutron star touches the star, it can form a so-called spreading layer. In a work led by P. Abolmasov, we designed a new 2D spherical spectral code to simulate the dynamics of these layers. Code is, obviously, calle...
Runko: toolbox for plasma simulations
Runko simulation framework is finally available for everybody. Check it out at GitHub!
Pulsar-wind heated accretion disks
In a work led by Alexandra Veledina we presented a new model for the transitional millisecond pulsars in terms of wind-heated disks.
Exploring helical dynamos with machine learning
Together with Farrukh Nauman we explored helically forced 3D magnetohydrodynamic turbulence simulations with ensemble machine learning techniques.
Quark-matter cores in neutron stars
Our new EOS analysis published in Nature Physics indicates that neutrons stars actually have two distinct phases of matter inside: normal hadronic and a sizable quark matter core!
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.
Effect of accretion on the NS mass and radius measurements: 4U 1608-52
On this article, published in MNRAS, we show how the accretion can affect the NS mass and radius measurements that are done using the X-ray bursts.