Welcome to the Astro - Space Journal Club!

In November 2024, the High-Energy Astrophysics Group and the Space Physics Group launched a Journal Club for the Section of Astrophysics, Astronomy, and Mechanics in the Department of Physics at the National and Kapodistrian University of Athens.

From undergraduate students to experienced researchers, the Journal Club (JC) is open to everyone who shares an interest in exploring diverse topics within astrophysics.

Each session features discussions on up to three selected papers, offering insights into different research areas of astrophysics.

Check out our upcoming JC sessions, and we’d love to have you join the conversation!

Abstracts of the Upcoming Papers:

• Grand minima and maxima of solar activity: new observational constraints

Aims.Using a reconstruction of sunspot numbers stretching over multiple millennia, we analyze the statistics of the occurrence of grand minima and maxima and set new observational constraints on long-term solar and stellar dynamo models.

Methods.We present an updated reconstruction of sunspot number over multiple millennia, from 14C data by means of a physics-based model, using an updated model of the evolution of the solar open magnetic flux. A list of grand minima and maxima of solar activity is presented for the Holocene (since 9500 BC) and the statistics of both the length of individual events as well as the waiting time between them are analyzed.

Results.The occurrence of grand minima/maxima is driven not by long-term cyclic variability, but by a stochastic/chaotic process. The waiting time distribution of the occurrence of grand minima/maxima deviates from an exponential distribution, implying that these events tend to cluster together with long event-free periods between the clusters. Two different types of grand minima are observed: short (30–90 years) minima of Maunder type and long (>110 years) minima of Spörer type, implying that a deterministic behaviour of the dynamo during a grand minimum defines its length. The duration of grand maxima follows an exponential distribution, suggesting that the duration of a grand maximum is determined by a random process.

Conclusions.These results set new observational constraints upon the long-term behaviour of the solar dynamo.

• Ultra-luminous X-ray pulsars as sources of TeV neutrinos

We explored the expected properties of the neutrino emission from accreting neutron stars in X-ray binaries using numerical simulations. The simulations are based on a model in which neutrinos are produced by the decay of charged pions and kaons, generated in inelastic collisions between protons accelerated up to TeV energies in the magnetosphere of a magnetized (B ∼ 10^12 G) neutron star and protons of the accretion disc. Our results show that this process can produce strong neutrino emission up to a few tens of TeV when the X-ray luminosity is above ∼1039 erg s^(−1), as in ultra-luminous X-ray (ULX) pulsars. We show that neutrinos from a transient Galactic ULX pulsar with Lx ≈ 5 × 10^39 erg s^(−1) can be detected with kilometre-scale detectors such as IceCube if the source is within about 3–4 kpc. We also derived an upper limit on the neutrino flux from the Galactic ULX pulsar Swift J0243.6+6124 using IceCube data, a result that has not been previously reported. Our findings establish a new benchmark for future astrophysical neutrino observations, critical for interpreting data from current and upcoming instruments with significantly improved sensitivity.