accretion discs around black holes, magnetic instabilities, jet formation.
binary compact objects (neutron stars, black holes), deformed compact stars.
The study of compact objects, like (binary) black holes, (binary) neutron stars, their mergers and subsequent formation of a single compact object surrounded by an accretion disk are very important for contemporary astrophysics (and physics) for several reasons. Binary neutron-star systems are particularly interesting, because they are the best candidate source for explaining a class of gamma-ray bursts, which are in general the most energetic explosions in the Universe. Gamma-ray bursts are observed daily by satellites and telescopes, but their origin and mechanism is not known in detail. Binary neutron-star and/or black-hole systems are also powerful sources of gravitational radiation. Gravitational waves are a fundamental prediction of General Relativity but have been detected for the first time only in 2015 (it was a binary black-hole merger; the first binary neutron-star merger was instead observed in 2017). Since then, tens of binary black-hole mergers and a few binary neutron-star mergers and black-hole—neutron-star mergers have been observed. The measurement of gravitational waves has opened a completely new observational window on the Universe, through which we are getting to know things that are completely or partially inaccessible to electromagnetic observations, like most of black-hole dynamics, the internal structure of compact stars, the equation of state of ultra-high density matter, and so on, in addition to indications on the internal engine of gamma-ray bursts, and on the viability of gravitational and cosmological theories. Gravitational-wave measurements are performed by interferometers in the US (LIGO) and Italy (Virgo), while the Japanese detector, KAGRA, has fully joined them since observation runs in 2022. In order to physically interpret the measurements, accurate knowledge of gravitational waveforms is of crucial importance and hence numerical simulations of the sources are necessary. Given the high nonlinearity of the Einstein equations for General Relativity, numerical solutions are in fact the only means to study the internal structure and the most violent dynamics of such objects in detail. Luca Baiotti is currently part of the OUTAP group (Osaka University Theoretical AstroPhysics group) in the Department of Earth and Space Science. Prospective students, in particular, are invited to look at the web pages of the OUTAP group.
Rest-mass density and magnetic-field lines of a black hole surrounded by a disc, a system produced by the merger of binary neutron stars.
Gravitational waveforms of binary neutron-star coalescence, merger, and post-merger.