I am a computational astrophysicist based in New York. My research is focused on understanding high-energy astrophysical phenomena around neutron stars and black holes.
Starting later this year, I will start as an Associate Research Scientist in Columbia University (2023, New York, USA). Currently, I am a joint Flatiron Research Fellow in Center for Computational Astrophysics, Flatiron Institute & Postdoctoral Reseacher in Columbia University (2020-2023; New York, USA). Before New York, I was a Nordita Fellow in Nordic Institute for Theoretical Physics (2018-2020; Stockholm, Sweden).
In Finnish/Suomeksi:
Olen suomalainen astrofyysikko joka keskittyy suurenergia-astrofysiikan ääri-ilmiöiden tutkimiseen. Tutkin muun muuassa neutronitähtien purkauksia, pulsarien säteilyä, mustien aukkojen kertymäkiekkoja, sekä kosmisia radiopurskeita.
Publications
A list of some of the papers of mine that I like the most (in no particular order):
- Radiative turbulent flares in magnetically-dominated plasmas
- Detailed study on reconnection-mediated turbulent flares and their radiative properties. These flares can be relevant for explaining plasma energization in neutron star magnetospheres and black hole accretion disks.
- Radiation from rapidly rotating oblate neutron stars
- Extensive study on radiation characteristics from rapidly spinning neutron stars. We generalize previous works to include effects from oblate surfaces and quadrupole corrections to the spacetime metric.
- Evidence for quark matter cores inside neutron stars
- We show that when results from modern nuclear physics and perturbative QCD limits are combined together, they imply that the most massive neutron stars can have quark matter cores. Importantly, we show that even if there is no quark matter, the core is still made of exotic material that breaks the conformal limit (speed of sound exceeds square root of c/3).
- Neutron star M-R measurements from atmospheric model fits to X-ray burst cooling tail spectra
- By using a detailed Bayesian fitting procedure we show that it is possible to measure the neutron star radius very accurately from existing X-ray data. From this analysis we obtained a neutron star radius of 12.4 +- 0.4 km that is amazingly consistent with other measurements, and is still one of the most accurate measurements of NS radius.
Research topics
In general, you can categorize my past work to (roughly) these topics:
Astroplasma physics
- Radiative turbulent flares in magnetically-dominated plasmas
- Runko: Modern multi-physics toolbox for simulating plasma
Equation of state of ultra-dense matter
- Combining Electromagnetic and Gravitational-Wave Constraints on Neutron-Star Masses and Radii
- Evidence for quark-matter cores in massive neutron stars
- Bayesian parameter constraints for neutron star masses and radii using X-ray timing observations of accretion-powered millisecond pulsars
- (also neutron star physics)
- Equation of state constraints for the cold dense matter inside neutron stars using the cooling tail method
- (also X-ray bursts)
Neutron star (astro)physics
- Oblate Schwarzschild approximation for polarized radiation from rapidly rotating neutron stars
- Magnetospheric return-current-heated atmospheres of rotation-powered millisecond pulsars
- Pulsar Wind-heated Accretion Disk and the Origin of Modes in Transitional Millisecond Pulsar PSR J1023+0038
- Kilohertz quasi-periodic oscillations from neutron star spreading layers
- Radiation from rapidly rotating oblate neutron stars
- Models of neutron star atmospheres enriched with nuclear burning ashes
X-ray bursts from neutron stars
- Evidence for the Photoionization Absorption Edge in a Photospheric Radius Expansion X-Ray Burst from GRS 1747-312 in Terzan 6
- Basic parameters of the helium-accreting X-ray bursting neutron star in 4U 1820-30
- Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
- Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state
- Flux decay during thermonuclear X-ray bursts analysed with the dynamic power-law index method
- The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii
- Detection of burning ashes from thermonuclear X-ray bursts
- The influence of accretion geometry on the spectral evolution during thermonuclear (type I) X-ray bursts
- The effect of accretion on the measurement of neutron star mass and radius in the low-mass X-ray binary 4U 1608-52
In addition, there are other miscellaneous topics that I’ve dipped my toes in:
Machine learning
General relativity/ray tracing
Collection of my writings & publications
A full list of my publications is here:
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!
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.
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.
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.
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.