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 Latest Papers:

• Upstream neutrino production and delayed jet emission in the blazar GB6 J1542+6129

We investigate the physical origin and location of high-energy neutrino emission in active galactic nuclei (AGN) using the blazar GB6 J1542+6129 as a case study, testing whether neutrinos are produced in compact regions near the black hole or in parsec-scale jets. This question is central to understanding the conditions under which hadronic processes become efficient in AGN environments. We perform a multimessenger analysis combining ~17 years of Fermi-LAT gamma-ray data, including a 5% adaptively binned light curve and Bayesian block decomposition, with ~14 years of VLBI/MOJAVE observations to derive the Doppler factor evolution of the radio core. These are compared to the temporal properties of a suspected IceCube neutrino flare with a duration of 147 days. We find that the suspected neutrino flare precedes both a gamma-ray flare and a pronounced increase in the VLBI core Doppler factor by ~1 year. This delay is consistent with the propagation time of a disturbance from the central engine to the 15GHz radio core. The duration of the post-flare gamma-ray activity is similar to that of the neutrino flare. The broadband gamma-ray spectral energy distributions remain consistent in shape across the full, flare, and post-flare intervals, indicating stable particle acceleration conditions. The temporal ordering favors neutrino production upstream of the VLBI core. GB6 J1542+6129 provides evidence for spatially separated neutrino and gamma-ray/radio emission regions in AGN. The observations are consistent with a disturbance-driven, multi-zone scenario in which neutrinos are produced in a compact, photon-rich region near the central engine, while the same disturbance later enhances Doppler-boosted leptonic emission at the parsec-scale VLBI core. These results demonstrate the power of multimessenger observations in constraining the origin of astrophysical neutrinos. 

• Solar events and solar wind conditions associated with intense geomagnetic storms

Intense magnetic storms pose a systemic threat to the electric power grid. In this study we examined the solar/interplanetary causes of such storms, their peak theoretical and observed intensities, and their occurrence frequency. Using coronal mass ejection (CME) and solar wind data, we selected the 18 intense magnetic storms from 1996 to 2021 with disturbance storm time (Dst) index of less than – 200 nT and analyzed solar events and solar wind conditions associated with them. Approximately 83% of the CMEs associated with the storms were full halo type and more than 83% of the flares associated with the storms were located within 30 degrees in longitude of solar central meridian. The integrated dawn-to-dusk electric field in the solar wind (Ey) showed a good correlation with |min. Dst| of the storms and the peak Ey (Eyp) and the peak southward interplanetary magnetic field showed next good correlations with |min. Dst|. We obtained the Eyp of 236 mV/m for |min. Dst| of 2500 nT of the expected upper limit of Earth’s magnetosphere using the empirical equations from the correlations between |min. Dst| and solar wind parameters and showed that this value of Ey is possible according to the past observations. The Eyp of 54 mV/m for the 13 March 1989 storm and that of 165/79 mV/m for the Carrington storm (|min. Dst|= 1760/850 nT) were also obtained. The analysis using the complimentary cumulative distribution function suggested the probabilities of Ey of 100, 200, 250, and 340 mV/m over the next 100 years to be 0.563, 0.110, 0.060 and 0.026, respectively.