Väitös (tähtitiede): MSc Pouya Kouch
MSc Pouya Kouch esittää väitöskirjansa ”Multi-messenger emission of relativistic jets launched by supermassive black holes” julkisesti tarkastettavaksi Turun yliopistossa perjantaina 4.9.2026 klo 13.00 (Turun yliopisto, Quantum, Auditorio, Turku).
Vastaväittäjänä toimii vanhempi tutkija Luigi Foschini (Kansallinen astrofysiikan instituutti, Breran tähtitieteellinen observatorio, Italia) ja kustoksena professori Seppo Mattila (Turun yliopisto). Tilaisuus on englanninkielinen. Väitöksen alana on tähtitiede.
Tiivistelmä väitöstutkimuksesta:
Neutrinos, like electrons, are elementary particles. However, unlike electrons, they do not interact through the electromagnetic force and are weakly affected by gravity due to their extremely low mass. The weak force is the only way through which they interact with matter, albeit exceedingly rarely. As such, most neutrinos pass through normal objects, even the Earth, without a trace — hence earning the nickname "ghost particles".
Although infrequent, neutrino-matter interactions are used to indirectly detect neutrinos. In 2013, the IceCube Neutrino Observatory detected neutrinos coming from cosmic sources at the highest energies (TeV—PeV). At these energies, neutrinos can only be created in processes involving protons of similar energies. As such, these neutrinos and ultra-high-energy cosmic rays — protons of EeV energies coming to Earth from unknown cosmic sources — must be produced in the same sources. As protons are charged, they are deflected on their way to Earth. However, neutrinos travel unimpeded from their source and, thus, enable solving the six-decade-long mystery of the most extreme proton accelerators in the universe. Unfortunately, neutrino astronomy faces inherent challenges that make it difficult to accurately localize the arrival direction of the neutrinos on the sky. Hence, the origin of cosmic neutrinos has also remained unknown for the past decade. This doctoral thesis aimed to shed light on their origin.
Black holes are compact cosmic objects, from whose gravitational pull even light cannot escape. Supermassive black holes, which are billions of times more massive than our Sun, are often found in the center of massive galaxies. Their immense gravitational potential can generate exceedingly high thermal energy as matter accretes onto them, leading to an active galactic nucleus (AGN). Some AGN are observed to launch powerful jets, with the ejected plasma approaching the speed of light. Although much about how these jets are launched and accelerated is not yet understood, they are known to accelerate particles — electrons and possibly protons — to extreme energies. Thus, they could be sources of ultra-high-energy cosmic rays and high-energy neutrinos. When the jet is viewed at a small angle, the relativistic approach of the plasma toward the observer greatly enhances the brightness of its electromagnetic and any neutrino emission. Therefore, AGN jets that happen to point toward Earth — also known as blazars — are some of the brightest objects in the universe and prime source candidates for high-energy neutrinos.
Although the electromagnetic output of AGN jets and, by extension, blazars is known to vary stochastically, the most prominent jet outbursts are often linked to features such as magnetized plasma shocks traversing down the jet. They can accelerate particles which then radiate away their kinetic energy through non-thermal emission processes. Theoretically, neutrino emission is expected to peak during these acceleration events. An observational hint for this came in 2017 when the blazar TXS0506+056 was experiencing a prominent outburst and a high-energy neutrino was observed from its direction. This thesis conducted four original studies on the blazar–neutrino connection by focusing on neutrinos that arrive from the direction of blazars when the blazars are in outburst. This included the most extensive blazar–neutrino correlation search to date, which concluded fewer than 8% of IceCube neutrinos came from major blazar flares. Thus, the mystery continues and the search should be expanded to other potential source candidates and energy ranges.
The thesis also conducted two other original studies utilizing multiwavelength polarization observations of two blazars. These showed that shock acceleration is likely the most dominant mechanism leading to multiwavelength outbursts. It was also shown that hadronic emissions are subdominant, inline with the lack of a strong blazar–neutrino connection.
