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We present the objectives, implementation, and preliminary results of the first Bulgarian polar astronomy project, "Impact of Solar Activity on Ionospheric Dynamics and High-Energy Particle Fluxes over Antarctica" (2024─2025), conducted at the Bulgarian Antarctic Base on Livingston Island. The study aims to investigate solar-terrestrial interactions, focusing on solar flares, coronal mass ejections, and their effects on ionospheric disturbances, geomagnetic field variations, and radiation levels. Four synchronized experiments were deployed: (1) HF radio spectrography for direct solar burst observations (50-1000 MHz), (2) VLF monitoring of ionospheric D-layer disturbances via long-range ground- based transmitters, (3) magnetometric measurements of local geomagnetic field variations, and (4) radiation dosimetry using passive detectors. Data analysis is ongoing, with plans for long-term monitoring to improve space weather forecasting and mitigate impacts on satellite communications. This initiative establishes Bulgaria's capacity for cutting-edge polar astrophysical research, supported by collaborations between the Institute of Astronomy and NAO, Technical University-Sofia, the Nikola Vaptsarov Naval Academy, and international partners.

The energy dissipation and particle acceleration mechanisms within relativistic jets remain fundamental questions in active galactic nucleus research. In this paper, we present a comprehensive 13 yr (2012─2025) multiwavelength study of the broad-line radio galaxy 3C 120, utilizing Fermi Large Area Telescope (γ rays), All-Sky Automated Survey for Supernovae (optical), and high-resolution Very Long Baseline Array (15 and 43 GHz) monitoring. Cross-correlation analyses reveal that γ-ray flares lead radio emission by <inline-formula> <mml:math><mml:mn>11.0</mml:mn><mml:msubsup><mml:mrow><mml:mn>8</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.88</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>4.03</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula> months at 15 GHz and <inline-formula> <mml:math><mml:mn>8.2</mml:mn><mml:msubsup><mml:mrow><mml:mn>7</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>5.55</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>3.45</mml:mn></mml:mrow></mml:msubsup></mml:math> </inline-formula> months at 43 GHz. This frequency-dependent temporal hierarchy positions the high-energy dissipation zone upstream of the radio core, corroborating the opacity-driven core-shift effect. By tracking the parsec-scale jet morphology during major γ-ray flaring epochs, we demonstrate that radio outbursts systematically coincide with compact core brightening, pronounced surges in polarized flux, abrupt electric vector position angle rotations, and the subsequent ejection of superluminal knots. Ultimately, our internal radio correlations suggest that jet dynamics are governed by a dual mechanism: long-term kinematic and flux baseline variations are geometrically modulated by a secular jet precession, while rapid, highly energetic polarimetric bursts are driven by short-lived internal shocks propagating down the jet channel.

The formation and evolution of galaxies during the Cosmic Dawn is a topic of current interest in extragalactic astrophysics. The physical processes involved are highly non-linear and multi-scale, and their understanding therefore requires numerical simulations. One such process is the energetic feedback from stellar systems on their surroundings, in particular that produced by X-ray binaries. With the aim of incorporating this feedback into simulations, the present work analyses the performance of the hydrodynamical cosmological code SWIFT, characterising its scalability and identifying an optimal configuration for its task-based parallelism. Using this configuration, we run a reference simulation employing the EAGLE physics model and study the properties of the resulting galaxies. The results are consistent with previous work and will be compared with those obtained once the new feedback process is included.

The magnetorotational instability can generate strong, turbulent substructure within magnetized shear flows. The efficacy of the mechanism as a function of microphysical aspects of the fluid, such as stratification and diffusivity, has been explored extensively. One aspect that has not been studied thus far, however, is whether the instability can also operate in solids. Motivated by the possibility that solid regions within planets or degenerate stars may rotate differentially with respect to liquid or gaseous layers during some phase of their life, we examine the extent to which elasticity suppresses the instability. A simplified, plane-parallel analysis reveals that only in cases where the flow is strongly sheared, such that the magnetic tension that would result from the instability in a liquid exceeds the shear modulus of the elastic cavity, can magnetic growth occur. In the context of dynamical tides in binary neutron-star mergers, this implies that the magnetic field can be amplified in the crust prior to coalescence only if the star boasts a spin frequency of <inline-formula><mml:math><mml:mo>≳</mml:mo><mml:mn>300</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>Hz</mml:mi></mml:math></inline-formula>. If viscous heating weakens the crystalline structure prior to resonance, the required spin frequency is reduced.

We introduce a native tensor-based framework for foreground mitigation in 21\,cm intensity mapping (IM), utilizing the Oriented Singular Value Decomposition (O-SVD) algorithm. While 21\,cm IM is a powerful probe of the large-scale structure of the Universe, its efficacy is severely limited by astrophysical foregrounds that are orders of magnitude brighter than the cosmological signal. Traditional mitigation strategies often necessitate flattening multidimensional data cubes into two-dimensional matrices, a process that potentially compromises the intrinsic spatial-spectral correlations by treating distinct spatial pixels as independent samples. By treating multi-frequency sky maps and angular power spectra as third-order tensors, the O-SVD method performs decomposition directly on the multilinear manifold, preserving the underlying physical topology and leveraging the distinct coherence properties of astrophysical foregrounds across different dimensions. We demonstrate the performance and versatility of the O-SVD framework through its application to high-fidelity simulations from the SKA Science Data Challenge 3a (SDC3a) and real-world observational data from the Tianlai Cylinder Pathfinder Array. Our results indicate that O-SVD provides a robust and universal approach for foreground subtraction, achieving high-fidelity signal recovery while offering superior performance compared to conventional matrix-based Singular Value Decomposition (SVD) methods.

We examine the entropy balance of black hole mergers in the presence of quantum gravity corrections described by the Generalized Uncertainty Principle (GUP). Focusing on the coalescence of two non-spinning Schwarzschild black holes with unequal masses, we analyze the thermodynamic evolution from the initial binary system to the final merger remnant, explicitly accounting for energy loss through gravitational-wave emission. Using a logarithmically corrected entropy motivated by a quadratic GUP, we derive a generalized entropy change as a function of the gravitational-wave efficiency and the mass ratio of the merging components. The classical area theorem is recovered in the appropriate limit, while the GUP correction introduces a quantum-modified entropy bound that constrains the maximum allowed gravitational-wave energy emission. This bound is most restrictive for nearly equal-mass mergers, indicating an enhanced sensitivity of symmetric systems to quantum gravity effects.

The dark matter content of ultra-diffuse galaxies is the subject of considerable debate. Stellar streams, which form when a host galaxy tidally strips stars from an orbiting stellar system, provide a powerful technique to constrain the dark matter content of external galaxies. The stripped stars form long, thin leading and trailing tidal arms that persist for billions of years. Stellar streams from globular clusters are particularly sensitive probes of dark matter halos and substructure. Globular cluster streams are expected to exist in a variety of host galaxy types, but so far, they have only been observed in the Milky Way. We present evidence for the first extragalactic globular cluster stellar stream, identified in deep Hubble Space Telescope imaging of the ultra-diffuse galaxy, UGC9050-Dw1. The stream's morphology, colour, and apparent association with a compact source support the globular cluster progenitor interpretation observationally, and we reproduce the observed surface brightness with simulated globular cluster stellar populations. We use generative stream modelling, which fits dynamical models directly to the stream morphology, to constrain the mass of the progenitor and present the first stream-based halo constraint for an ultra-diffuse galaxy. The stream models point to a globular cluster origin and suggest a massive dark matter host halo. By extending the reach of globular cluster stream analysis to external galaxies, this work opens a new chapter in dark matter science.

The supermassive black hole at the center of the nearby radio galaxy M87 (M87*) is a prime target for studying black hole physics. Spatially resolved spectral measurements on event-horizon scales can reveal the origin of the emission and probe the plasma and gravitational environment in the immediate vicinity of the black hole. Here, we present an analysis of spectral properties based on nearly simultaneous high-resolution images at 3.5 mm (86 GHz) and 1.3 mm (230 GHz), obtained in 2018 with the Global Millimeter VLBI Array (GMVA) including ALMA and the Greenland Telescope, and the Event Horizon Telescope (EHT). We obtain the first spatially resolved spectral-index map ($S_ν\propto ν^α$) within the compact region ($\leq 100\,μ$as). We further detect a robust radial gradient with a modest rise in the inner $\lesssim 20~μ$as (slightly inside the 1.3 mm ring), followed by a systematic decline at larger radii. The spectral index transitions from positive to negative values near $\sim 30~μ$as, close to the 3.5 mm ring radius, consistent with frequency-dependent synchrotron opacity in the innermost accretion flow. These results provide new observational constraints that can help discriminate between models of the horizon-scale emission and the launching of relativistic jets in M87*.

Blazars constitute the main population of active galactic nuclei detected in $\gamma$-rays by the Fermi-LAT satellite. However, around 34\% of the sources in its catalog remain unidentified due to their large positional uncertainty, complicating the association with optical counterparts. To address this issue, a method has been proposed to identify BL Lac-type blazars based on their X-ray properties, using Swift-XRT observations and the analysis of light curves, spectra, and hardness ratios. In this work, we empirically evaluate the effectiveness of this method through comparisons with recent optical classifications. The results show that most candidates selected from their X-ray emission were confirmed as BL Lac blazars, representing 78,94\% of the analyzed sample. This value increases to 84,75\% when more restrictive positional criteria are applied. This study demonstrates that the X-ray band is an effective tool for identifying blazars among unclassified $\gamma$-ray sources and supports future spectroscopic follow-up campaigns.

Binary white dwarfs (BWDs) represent one of the most common systems of compact objects in the Universe and are expected to be the main sources of low-frequency gravitational waves (GW). The upcoming Laser Interferometer Space Antenna (LISA) mission will open a new observational window, by accessing a low-frequency ( $10^{-4}$ Hz to $10^{-1}$ Hz) regime invisible to ground-based detectors, including the signal from BWDs across the Milky Way (MW) and nearby galaxies. In this work, we use the rapid population synthesis code \textsc{sevn} to simulate BWD populations, exploring different evolutionary parameters, compositions, and metallicities. We characterize their properties including mass distribution, orbital periods, and formation timescales, and compare our results with observational data. We aim to study their detectability in a realistic galactic environment by integrating these populations into a MW-like galaxy model from \textsc{tng50} simulations, allowing us to build spatial distributions of these systems and identify regions with higher probabilities for GW detection. Our results provide a theoretical framework for interpreting LISA's future observations and for advancing our understanding of compact binary populations in the Universe.

Motivated by [Phys. Rev. D. 107, 064019 (2023)], we develop a unified and model-independent framework for the spectroscopy of photon frequency shifts in generic static, spherically symmetric spacetimes. Working with a line element characterized by three arbitrary radial metric functions, we derive exact expressions for the conserved quantities of massive and massless probes, the conditions for circular timelike geodesics, the local-emission-angle-dependent photon impact parameter, and the corresponding local redshift and blueshift branches measured by distant static observers. The formalism is further extended to include the line-of-sight peculiar motion of the source and the local propagation of photons in a nonmagnetized cold plasma, thereby identifying the gravitational, orbital, and dispersive factors entering the frequency-shift signal under the stated assumptions. We also show that, in vacuum, the same geometric structures governing orbital spectroscopy determine the photon sphere and the shadow impact parameter whenever an external null critical orbit is present. To make the framework suitable for deformed compact-object models, we construct a perturbative expansion around Schwarzschild geometry up to second order in a dimensionless deformation parameter, obtaining explicit corrections to the orbital energy, angular momentum, emitter four-velocity, photon impact parameter, and the vacuum and plasma frequency shifts. We apply the formalism to regular black holes from nonlinear electrodynamics, the Simpson--Visser black-bounce spacetime, and the Fisher--Janis--Newman--Winicour--Wyman geometry. These examples show that the same local spectroscopic language can be used across regular-black-hole, black-bounce, wormhole, and scalar-supported horizonless sectors.

A detailed analysis of high-resolution spectra in the optical and near-infrared range of the central zone of the proto-planetary nebula Hen 3-1475, obtained in 2006 and 2024, is conducted. The spectrum primarily revealed stellar emission of H Balmer and Paschen series, lines of iron (Fe I, Fe II, and [Fe II]), and other significant lines of He I, Ca II, [Ca II], and O I. Several multiplets of Fe II, the H Balmer series, Ca II H and K, Na I D and He I lines exhibit P-Cygni profiles and bump-like features, which are clear indicators of strong stellar winds. The terminal velocity of the winds was calculated, reaching values up to ~ 800 km/s. [Fe II] and Fe I lines were used as gas density indicators. In these observations, no photoionized nebular lines of planetary nebulae were identified, suggesting that the central star has not reached the temperature required to photoionize the nebula. A comparison between the observations from 2006 and 2024 reveals some few significant differences, being the most important the probably ejection of a new jet emerging from the central zone and the detection of a broad component adjacent to H α, probably originating from emission associated with a jet at 700 km/s. High-spectral resolution observations of the knot NW1 obtained on August 2024, are also presented. This knot shows a typical spectrum of shocked gas. The emission of this knot has changed, presently not showing [O III] 5007 and other ionized lines that were reported in 2006.

We performed a deep study of the open cluster Trumpler 11, located in the southwestern region of the Carina Nebula, using Gemini South GMOS spectroscopy in multi-object mode. We determined the physical parameters and spectral type of 24 late B-type stars, characterized the reddening across the cluster field, and obtained accurate estimates of the cluster's distance and age. This spectroscopic analysis was complemented with {\sl GAIA DR3} photometry and {\sl TESS} light curves. By comparing the properties and evolutionary status of Trumpler 11 with those of nearby open clusters, we explored the large-scale structure and star- formation history of the Carina-Sagittarius arm.