Multimessenger astrophysics

Realtime Multi-messenger and multi-energy studies of Blazars with the IceCube Neutrino Observatory
E. Bernardini, M. Mallamaci

IceCube is the largest operating instrument dedicated to the study of astrophysical neutrinos with energy above few hundred GeV. It has recently provided the first strong evidence for the existence of astrophysical neutrinos in the TeV-PeV energy range. The astrophysical objects producing these neutrinos are to date unknown. Many candidate source types exist, with Blazars among the most prominent. In these AGNs, the central supermassive black hole converts gravitational energy of accreting matter and/or the rotational energy of the black hole into powerful relativistic jets, within which particles can be accelerated to high energies.
As traditional time-integrated searches for point-like sources of neutrinos have not been able to isolate individual astrophysical sites, variable and transient sources shic into focus. Such sources may also exhibit variable behaviour in other observations, such as gamma-rays. Using IceCube’s capabilities to observe the
entire sky around-the-clock, it is possible to continuously search for neutrino flares, and alert the community with the lowest possible latency in the case of a detection. An infrastructure to generate alerts was recently implemented to properly notify the community whenever significant single neutrino events are recorded, or significant neutrino multiplets develop on time-scales from days to several weeks. The system also allows to quickly respond to interesting external astrophysical triggers, such as the observation of gravitational waves. The IceCube realtime plaeorm has proven it’s power with the coincident observation of the high energy event IceCube-170922A and a flaring state in gamma-ray of the the Blazar TXS 0506+056. This thesis project aims at further developing the IceCube real-time infrastructure, with particular attention to its extension in energy down to the lowest range accessible by the experiment (below 100 GeV) and the shortest time-scales (down to minutes). A class of models predict connected high-energy and low-energy  neutrino signatures opening the possibility to synergies with other low-energy neutrino detectors, like Superkamiokande.
The candidate will have the opportunity to deepen his/her competencies in the fields of neutrino astronomy and multi-messenger astrophysics, explore the most advanced computing algorithms and socware, necessary for both event selection purposes (machine learning techniques) and time-evolution analysis (maximum likelihood methods), join a lively international scientific environment.

A self-consistent lepto-hadronic modelling for multi-messenger studies of the origin of very-high-energy gamma rays and neutrinos in Blazars
E. Bernardini

The spectral energy distribution (SED) of Blazars span a wide energy range from radio up to very-high- energy gamma-rays and typically exhibit a two-hump structure. While the origin of the first hump is unanimously attributed to leptonic processes, the origin of the higher energy hump is debatable and can be attributed to both hadronic as well as leptonic emission scenarios. Hence to understand the emission mechanisms giving rise to especially the high-energy and very-high-energy gamma-rays in the blazar spectrum, is crucial to investigate its link with cosmic rays.
Theoretical modelling of the broadband SED of Blazars provides a wealth of information regarding the underlying radiative processes, source energetics and implications for neutrino emission. This involves a detailed and accurate simulation of all the relevant leptonic and hadronic emission scenarios in Blazars, taking into account the contribution from all nearby radiation fields and obtaining reasonable constraints on the source parameters. Investigating a hadronic or lepto-hadronic origin of the gamma-ray emission is especially important for characterizing the blazar's potential as a cosmic-ray accelerator. Some of the common hadronic processes that can give rise to the high-energy electromagnetic component of Blazars are direct synchrotron emission by protons or photo-meson interactions of protons with the ambient target photon fields and the subsequent secondary pair cascades. By modelling the electromagnetic observations, a level of the plausible neutrino emission can then be predicted, which thus helps to infer the Blazar's potential for cosmic-ray acceleration.
The main aim of this thesis is to investigate Blazars and their link to the most powerful particle accelerators of our Universe. Such studies will be performed in light of the recent multi-messenger observations and phenomenological interpretations. Special emphasis will be put on developing a self-consistent lepto- hadronic modelling framework for understanding the origin of the very-high-energy gamma rays, under different assumptions of the source geometry within the current observational and theoretical limits. Implications will be drawn for sub-classes of Blazars that can provide the most optimal environment for generating detectable neutrino emission, the unambiguous tracer of hadronic particle interactions.

Binary neutron star mergers, short GRBs and kilonovae
R. Ciolfi

The first multimessenger observation of a binary neutron star (BNS) merger in 2017 marked a major milestone in the investigation of these extreme astrophysical events. Among numerous results, it confirmed that they can be power short gamma-ray bursts (SGRBs) and produce a large amount of very heavy elements via r-process nucleosynthesis.
On the other hand, many open questions still remain on the dynamical evolution that followed the merger and thus on the physical origin of the observed firework of electromagnetic signals.
In this PhD project, the student will study such a system via stat-of-the-art magnetohydrodynamics simulations in general relativity. The main step foward will consist in a first systematic investigation of the merger and post-merger evolution including simultaneously the effects of strong magnetic fields and neutrino radiation. The impact on the baryon-polluted environment surrounding the merger remnant will be crucial to better understand the conditions for SGRB jet formation, while a detailed study of the different matter ejection mechanisms will improve our current interpretation of the radioactively-powered transients (named "kilonovae") associated with heavy element nucleosynthesis.

Cosmological propagation of gamma rays and fundamental laws of physics
A. De Angelis

Observations of high-energy gamma rays are performed nowadays: from space, by the Large Area Telescope onboard the Fermi satellite and by the AGILE and DAMPE satellite, and from Earth, by the Imaging Air Cherenkov Telescopes H.E.S.S., MAGIC and VERITAS, and by the Extensive Air Shower detectors HAWC and LHAASO. These instruments have discovered different populations of gamma-ray emitters and studied in detail the non-thermal processes producing this high-energy radiation. Their scientific objectives include also questions related to fundamental physics. Using gamma-ray instruments we study the origin of cosmic rays, and the cosmic electron-positron spectrum at high energies; moreover, we can search for signatures of dark matter. By observing the gamma-ray emission from sources at cosmological distances, we measure the intensity and evolution of the extragalactic background radiation, measure the energy of the cosmic vacuum searching for axion-like particles in a domain complementary to laboratory experiments, and perform tests of Lorentz invariance. This is why the field is in rapid development, and many new experiments involving experimental particle physicists are in construction and in project. A new high-energy gamma ray detector is going to be built in the Southern hemisphere (site to be decided between Argentina, Chile and Peru), DWGO. We look for PhD students interested in the study of phenomenology, and in the development of SWGO.

Multimessenger astrophysics: propagation of neutrinos and gamma rays near the sources and through their cosmic voyages
A. De Angelis

Multimessenger astronomy, using cosmic hadrons, X and gamma rays, hadrons, neutrinos and gravitational waves, is the astronomy for the new century. Multimessenger relations connecting neutrino, gamma ray, and cosmic ray observations depend on the generation and propagation of different particles, which are presently not well known. We plan to develop simulations on these different messengers, studying in particular the propagation of neutrinos and gamma rays near the sources and through their cosmic voyages to the Earth.