abstract

IceCube discovered a flux of cosmic neutrinos originating in extragalactic sources with an energy density close to that in gamma rays and cosmic rays. A multimessenger campaign triggered by the coincident observation of a gamma-ray flare and a 290-TeV IceCube neutrino pinpointed the cosmic-ray accelerator TXS 0506+056. Subsequently, the IceCube archival data revealed a 3-month burst of 13 cosmic neutrinos in 2014–2015 that dominates the neutrino flux of the source over the 9.5 years of observations. The original identification of the source as a blazar was puzzling because it requires a major accretion event onto the rotating supermassive black hole to accommodate the neutrino burst. Subsequent high-resolution radio images of the source with the VLBA brought to light a merger of two galaxies, revealed by the interaction of two jets entangled in the source. Recently, the blazar PKS 1502+106 was found in the direction of a 300-TeV neutrino alert, IC-190730. OVRA radio observations at 15 GHz indicate that the neutrino also coincides with the highest flux density of a flare that started five years ago. This matches the similar long-term outburst seen from TXS 0506+056 and may indicate merger activity. Also, the dominant hotspot in the 10-year IceCube neutrino sky map, NGC 1068 (Messier 77), is a Seyfert galaxy undergoing a major accretion event onto the black hole. A few-percent fraction of such special sources, now labeled gamma-ray blazars, is sufficient to accommodate the diffuse cosmic neutrino flux observed by IceCube. While rapid progress seems likely, the observations also convincingly make the case for the construction of more and larger neutrino telescopes with better angular resolution.

abstract

The first part of this article, based on lectures given at the Cracow School of Theoretical Physics held in Zakopane in June 2019, is devoted to a brief exposition of the physics associated with the development of extensive air showers. The latter parts deal with methods of detection of extensive air showers in a general way, and include a description of the latest measurements of the energy spectrum, the arrival direction distribution and the mass composition of cosmic rays of energy above \(10^{18}\) eV made, primarily, with the Pierre Auger Observatory. There is a brief discussion of future projects.

abstract

The past few years have been essential for multimessenger astrophysics, with the first detection of gravitational waves from the merging of two neutron stars and the recent announcement of a high-energy neutrino event detected by IceCube coincident in direction and time with a gamma-ray flare from a blazar detected by the Fermi gamma-ray satellite. Gravitational Wave and Neutrino sources and their electromagnetic counterparts, together with new developments in transient astronomy, are an active field where the nature of many phenomena is still unknown or debated. Furthermore, the generation of new sensitive, wide-field instrumentation across the entire electromagnetic and astroparticle spectrum (SKA, CTA, KM3NeT, ELT, Athena) is set to radically change the way we perceive the Universe. In the next decade, space and ground-based detectors will jointly explore the Universe through all its messengers.

abstract

The gamma-ray astronomy will reach its acme with the next generation instruments: the Cherenkov Telescope Array (CTA) and the Large High-Altitude Array Shower Observatory (LHAASO). CTA is an array of Imaging Air Cherenkov Telescopes (IACT), which detect the Cherenkov light produced by the charged particles present in the shower produced by cosmic rays when impinging on the atmosphere. LHAASO can also detect the Cherenkov and fluorescence light produced by the shower, but mostly aims at detecting directly the charged particles in the shower. Though quite different and complementary, their characteristics are deeply tied to the physics of the air shower, whose knowledge is then fundamental for understanding their design. In this paper, I will go first through the interaction between radiation and matter needed for the next step of discussing the physics of air showers. Once the physics base is set, I will go through some detection technique, illustrate the IACT approach and the EAS array, and compare them.

abstract

A number of properties of dense matter can be understood semiquantitatively in terms of simple physical arguments. We begin with the outer parts of neutron stars, and consider the density at which pressure ionization occurs, the density at which electrons become relativistic, the density at which neutrons drip out of nuclei, and the size of the equilibrium nucleus in dense matter. Subsequently, we treat the so-called “pasta” phases expected to occur at densities just below the density at which the transition from the crust to the liquid core of a neutron star occurs. We then consider aspects of superfluidity in dense matter. Estimates of pairing gaps in homogeneous nuclear matter are given, and the effect of the dense medium on the interaction between nucleons is described. Finally, we turn to superfluidity in the crust of neutron stars and especially the neutron superfluid density, an important quantity in the theory of sudden speedups of the rotation rate of some pulsars.

abstract

This article is a pedagogical introduction to relativistic quantum mechanics of the free Majorana particle. This relatively simple theory differs from the well-known quantum mechanics of the Dirac particle in several important aspects. First, we present its three equivalent formulations. Next, a so-called axial momentum observable is introduced, and the general solution of the Dirac equation is discussed in terms of eigenfunctions of that operator. We also present pertinent irreducible representations of the Poincaré group. Finally, we show that in the case of massless Majorana particle, the quantum mechanics can be reformulated as a spinorial gauge theory.