Seminars

"Fresh stellar abundances of Os, Ir, and Pt, and their implications for stellar nucleosynthesis"

Arthur Puls (CAMK, Warsaw)

Heavier elements (atomic number larger than ~30) are produced in stellar nucleosynthesis by neutron-capture, which is known to happen either by the r- (rapid) process and the s- (slow) process. In the context of Galactic Chemical Evolution, the elements in the third r-process peak (Os, Ir, Pt) are among the least studied species of the periodic table. This is due to observational challenges: the lines of these elements featuring in stellar spectra are weak, in crowded wavelengths, and appear in cool (FGK-type) stars, in which those (blue, near-UV) wavelengths have relatively low flux. In this talk, I will present the results of the first study focusing on a homogeneous set of stellar abundances for the third r-process peak, which more than doubled the availability of Os, Ir, Pt stellar abundances from high resolution spectra in the literature. The implications of the results for our understanding of the nature of the r-process, both in terms of physics and astrophysical sites, will also be discussed.

"X-ray variability properties of blazars"

Gopal Bhatta (Institute of Astronomy, University of Zielona Góra)

X-ray emission from blazars provides crucial insights into the underlying AGN processes, as it can arise from synchrotron and inverse-Compton mechanisms in leptonic models, or be part of particle cascades produced in hadronic models. Here we present multi-instrument, multi-epoch investigations of several prominent blazars (OJ 287, PKS 2155–304, 3C 273, and Mrk 421 and samples of other blazars) with a particular focus on their X-ray variability and spectral properties. We perform comprehensive timing and spectral analyses covering intraday to decadal timescales, using long-term observations from Swift, XMM-Newton, NICER, and NuSTAR spanning more than two decades. During the period, the sources consistently display strong variability, with X-ray flux changes reaching factors of ~10–200 during major flares. Timing analysis, including fractional variability, flux and rms distributions, PSD slopes, and rms–flux relations, reveal a combination of normal, lognormal, and bimodal flux states, indicating complex, multiplicative variability processes and transitions between distinct emission states. Spectral analyses using power-law, log-parabola, and broken power-law models demonstrate that log-parabolic or broken spectra best represent the majority of observations, pointing to curvature in the underlying electron energy distributions. Several sources show clear flux-dependent spectral evolution, including “harder-when-brighter’’ behavior and strong correlations between synchrotron peak energy, curvature parameters, and X-ray flux. In OJ 287, broad-band spectral energy distribution (SED) modeling highlights systematic transitions between inverse-Compton–dominated quiescent states and synchrotron-dominated flaring states. Collectively, the observed properties such as Gaussian and non-Gaussian flux distributions, variable PSD slopes, and evolving spectral curvature supports scenarios involving energy-dependent particle acceleration, turbulence, and/or magnetic reconnection within the jet.

"TBA"

Anirudh Nemmani (NCAC, Warsaw)