High Energy Astrophysics at the University of Turku

The high-energy astrophysics group works on accreting black holes and neutron stars. We develop atmosphere models for rapidly rotating neutron stars in low-mass X-ray binaries with the aim to determine the equation of state (EoS) of cold dense matter of neutron stars and to understand the physics of accretion in these objects. We also construct atmosphere models for rotation-powered millisecond pulsars that are used by the NICER team to constrain the EoS. We work on hydrodynamical models of boundary/spreading layers on weakly magnetized neutron stars to understand the nature of kHz quasi-periodic oscillations. We also study X-ray pulsars and ultra-luminous X-ray pulsars, both observationally and theoretically. We construct models for the accretion column and for the accretion disc around magnetized neutron stars as well as monitor the sources to study the transition to the propeller regime to measure the neutron stars magnetic field.

Another direction of research is accreting black holes of all scales. We model broadband spectra and timing properties of accreting black holes both in the optical/infrared and the X-rays. The group uses both ground-based ESO VLT and the Nordic Optical Telescope (NOT) as well as space telescopes (XMM-Newton, Chandra, RXTE, INTEGRAL, Fermi, Swift, NuSTAR, SRG). We are also involved in preparation for the new X-ray missions being science group members of the NASA’s Imaging X-ray Polarimeter Explorer (IXPE) and of the Chinese enhanced X-ray Timing and Polarimetry (eXTP) missions. In particular, we develop models for X-ray polarization from X-ray pulsars, accounting for relativistic motion of the emission region in the case of millisecond pulsars.

We are also doing optical polarimetric studies with the in-house built high-precision Double-Image Polarimeters -2 (DIPol-2) and -Ultra Fast (DIPol-UF) with the aim to determine the accreting black hole emission mechanisms and to determine orbital orientation in space. Combining these data with the known jet orientation we will be able to measure the misalignment angle between the black hole spin and the orbital angular momentum, which is important for understanding black hole formation mechanisms.