Research groups

Center of Excellence in Neutron-Star Physics involves five different research groups studying neutron stars from all perspectives.

Nuclear Astrophysics - Anu Kankainen

The research group led by PI Anu Kankainen focuses on experimental studies of exotic nuclei and their properties. With novel production and ion trapping techniques, we can determine atomic masses of nuclides extremely precisely and determine their nuclear binding energies. So far, we have determined atomic masses for more than 400 nuclides, including more than 70 long-lived excited nuclear states known as isomers. This makes our team as one of the world-leading research groups in such mass measurements. We also explore decay properties of nuclei, e.g., half-lives, that also influence the studied astrophysical processes. Nuclear masses and half-lives serve as key inputs for nucleosynthesis calculations in type I x-ray bursts and neutron-star mergers.

Not all nuclei involved in the astrophysical rapid neutron capture process (r process) are experimentally reachable. Thus, it is crucial to benchmark nuclear models predicting the experimentally inaccessible nuclei and their properties. We actively collaborate with nuclear theory groups, such as the one led by PI Kortelainen.

For the rapid proton capture (rp process) taking place, e.g. in type I x-ray bursts, the involved nuclei will become experimentally accessible within the coming decade. As such, we have a great opportunity to model the process with improved nuclear data.

Theoretical Nuclear Physics - Markus Kortelainen

The research group led by PI Markus Kortelainen focuses on development and application of advanced theoretical models for nuclear structure. At the core of our work lies nuclear density functional theory and energy density functional-based approaches, which serve as the principal framework for our investigations.

Our central research goal is to achieve a universal and predictive description of the properties of finite nuclei and nuclear matter. To achieve this, we employ a variety of nuclear structure models that provide input for astrophysical processes, such as nucleosynthesis and neutron star physics, as well as for tests of fundamental symmetries in physics. We actively collaborate with experimental groups, offering theoretical insights that help interpret newly measured data.

Plasma Astrophysics - Joonas Nättilä

Plasma astrophysics is a new, emerging research field aiming to understand the dynamics of astrophysical plasmas - hot ionized gases - from first principles. The Computational Plasma Astrophysics research group led by PI Joonas Nättilä at the University of Helsinki uses theoretical and computational methods to study the most extreme plasma environments around neutron stars and black holes. Their group focus on understanding the radiative plasma physics of magnetar flares, neutron-star mergers, accretion flows around black holes, and enigmatic fast radio bursts.

High-performance computing solutions and open-source simulation tools are essential to the group’s research. To perform the numerical studies, the group maintains their own “computational laboratory” with a dedicated in-house, >5000-core supercomputer HILE. The cluster is used to develop the open-source plasma simulation framework runko.

High-Energy Astrophysics - Juri Poutanen

The high-energy astrophysics group at the University of Turku, led by PI Juri Poutanen, works on a broad range of topics related to accreting neutron stars. We have been at the forefront of developing atmosphere models for X-ray bursting neutron stars in low-mass X-ray binaries, which are used to constrain the equation of state (EoS) of cold, dense neutron-star matter. We also proposed the method of using pulse profiles of X-ray millisecond pulsars to determine neutron-star parameters—a technique now employed by the NICER team for rotation-powered millisecond pulsars.

Our group has produced state-of-the-art radiation–hydrodynamical models of boundary and spreading layers on weakly magnetized neutron stars, aiming to reveal the physical origin of kHz quasi-periodic oscillations. We are deeply engaged in both observational and theoretical studies of X-ray pulsars and ultra-luminous X-ray pulsars, including the development of models for accretion columns and for accretion discs around magnetized neutron stars.

Among our most recent efforts, we analyze X-ray polarization data from a wide variety of compact objects observed with NASA’s Imaging X-ray Polarimeter Explorer (IXPE). We lead the IXPE Topical Working Group on Accreting Neutron Stars, as well as the Strong Magnetism Working Group for the upcoming Chinese enhanced X-ray Timing and Polarimetry (eXTP) mission. In addition to observational work, we have developed pioneering models of X-ray polarization in X-ray pulsars that incorporate the relativistic motion of the emission regions in millisecond pulsars.

Theoretical Particle Physics - Aleksi Vuorinen

The research group of PI Aleksi Vuorinen concentrates on the first-principles description of dense quark matter, expected to be found in the inner cores of massive neutron stars and created in their binary mergers. In this work, we apply the machinery of perturbative thermal quantum field theory, with which we have derived state-of-the-art equations of state (EoSs) for dense quark matter both at vanishing and nonzero temperatures [1]. This work also requires the active development of novel computational tools for high-order thermal-field-theory computations, including the recent introduction of the so-called thermal Loop Tree Duality framework [2] in a line of work led by Risto Paatelainen.

We also apply these methods to phenomenological studies of the neutron-star-matter EoS, where we have actively developed model-agnostic, data-driven approaches to equation-of-state inference, incorporating information from theoretical calculations within nuclear and quark matter as well as astrophysical observations. Notably, we were the first group to apply the tidal-deformability constraints from the GW170817 merger to EoS inference [3], and the first to provide a quantitative and model-independent case for the presence of quark-matter cores in massive neutron stars [4,5].