The Sun, our host star, plays a fundamental role in astrophysics due to its vicinity. To understand the processes taking place in the outermost layers of the Sun, but also, indirectly, in its interior, we conduct high spectral and spatial resolution, high cadence observations of diagnostically important spectral lines. The intensity and the polarization of the electromagnetic radiation in the observed spectral lines are a result of the joint action of absorption, emission, and scattering processes, further altered by the Zeeman and Hanle effects. Spectropolarimetric inversions are the techniques devised for the inference of the physical parameters in the solar atmosphere through the interpretation of spectral line shapes and line polarization. This review aims to provide a pedagogical overview of the necessary radiative transfer formalism and cutting-edge techniques, and to help interested readers enter the field and use some of the concepts in their own research. The secondary aim is a critical assessment of the emerging directions for the development of spectropolarimetric inversions, namely, to discuss the friction between physical realism, numerical efficiency, and modeling of the instrumental effects.
We present two new radio-continuum images from the Australian Square Kilometre Array Pathfinder (ASKAP) POlarisation Sky Survey of the Universe's Magnetism (POSSUM) survey in the direction of the Small Magellanic Cloud (SMC). The two new source lists produced from these images contain 36,571 radio continuum sources observed at 944 MHz and 15,227 sources at 1367 MHz, with beam sizes of 14.''5x12.''2 and 8.''7x8.''2, respectively. We used the Aegean software to create these point source catalogues, and together with the previously published Meer-Karoo Array Telescope (MeerKAT) point source catalogue, we estimated spectral indices for the whole population of radio point sources in common. By cross-matching our ASKAP catalogues with the MeerKAT catalogue, we found 21,442 and 12,654 point sources in common for 944 MHz and 1367 MHz, respectively, within a 2'' region. This point source catalogue will help to further our knowledge of the SMC and highlights the power of the new generation of telescopes such as ASKAP in studying different galactic populations.
Coronal holes (CHs) play an important role in solar physics. They contribute to geomagnetic storms. The emission of charged particles into interplanetary space influences the space weather. CHs, being an important part of the solar activity, have a significant influence on Earth's climate. So, detection of CH regions is a significant task. Many attempts have already been made in this regard. In this work, we are proposing a new method for automatic detection of CH regions using a deep learning technique. We used Supervised Intensity Thresholding with Distance Transform Clustering and Connected Component Labeling (SITDTCCCL) to find out Regions of Interest (ROI) on 193 A & ring; images collected from the Solar Dynamics Observatory (SDO). The state-of-the-art deep learning method (YOLO v8) has shown excellent performance in detection of CH regions with scores of evaluation matrices such as F1 score 95%, Precision 97.1%, mAP50 98.1%, and True Positive Rate
This study explores a Kantowski-Sachs cosmological model within the framework of Modified gravity, utilizing an interacting field as the energy source. The interacting field comprises a linear combination of electromagnetic, massless scalar, and charged perfect fluid components. Our analysis encompasses four distinct scenarios: perfect fluid, disordered radiation, dust fluid, and dark energy. By establishing a relationship between metric potentials, we solved the field equations and investigated pressure and density profiles using the equation of state. A comprehensive examination of cosmological and dynamical parameters was also conducted.
The purpose of this study is to investigate the relation between binary asteroids and mean motion resonances (MMRs). For more than 700 asteroids from two catalogues, the Johnston Archive (Johnston 2024) and the Gaia DR3 VizieR list of binary candidates from Liberato et al. (2024), we applied a resonance identification algorithm, treating all planetary perturbations. Our results showed that the presence of binary asteroids in MMRs largely depends on their dynamical class. The highest percentage, more than 30%, is found in the Trans-Neptunian region where most of these objects have exhibited resonant librations longer than 10 Myr. For the main-belt asteroid pairs, this percentage is about 10-12%. Contrary to expectations, the more unstable region populated with NEOs, showed a higher percentage of resonant pairs (above 17%), but with temporal resonant captures. These results could indicate that the mean motion resonances, particularly the stronger ones, could play a role in the evolution and formation of binary systems. Finally, we highlight that in the present paper, 82 resonant binary asteroids are newly identified.
We present new radio-continuum observations of the Large Magellanic Cloud (LMC) supernova remnant (SNR) N206, which we give the name "Goat's Eye". Goat's Eye contains an interior radio structure that is likely a pulsar wind nebula (PWN), which we analyse in further detail. We use new radio observations from the Australia Telescope Compact Array (ATCA) telescope, as well as several archival radio observations, to calculate spectral indices, and find a steep spectral index for the whole SNR (alpha = -0.60 +/- 0.02), and a flatter spectral index for the PWN (alpha = -0.16 +/- 0.03). We also measure the polarisation and magnetic field properties of the PWN. Previously reported as a linear structure, the new observations show an unusual "zig-zag"-like structure, visible in radio-continuum total intensity, linear polarisation, and magnetic field orientations. The origin of this zig-zag structure is unclear, but we propose some origin scenarios that will require further observations to differentiate between.
We present a new selected sample of 69 Galactic supernova remnants (SNRs) for calibration of the radio Sigma - D relation at 1 GHz. Calibrators with the most reliable distances were selected through an extensive literature search. The calibration is performed using kernel smoothing of the chosen sample of calibrators in the Sigma - D plane and an orthogonal offsets fitting procedure. We use the obtained calibration to derive the distances to 164 Galactic SNRs and 27 new detected SNRs/SNR candidates with none or poor distance estimates. The analysis given in this paper confirms the expected predictions from our previous papers that the kernel smoothing method is more reliable for SNR distance calibration than the orthogonal offset fitting method, except for the distance determinations of the very low brightness SNRs.
We present a radio-continuum detection of the well-known Wolf-Rayet star WR40 at 943.5 MHz using observations from the Evolutionary Map of the Universe (EMU) survey. We find that the shell surrounding WR40, known as RCW 58, has a flux density of 158.9 +/- 15.8 mJy and the star itself is 0.41 +/- 0.04 mJy. The shell size is found to be 9'x 6', which matches well with the shell in Hoy and is similarly matched to the shell at 22 mu m in infrared. Using Gaia data, we derive a linear size of 7.32(+/- 0.34) x 4.89(+/- 0.23) pc at a distance of 2.79 +/- 0.13 kpc. We use the previous Australia Telescope Compact Array (ATCA) observations at 8.64, 4.80, and 2.4 GHz to determine a spectral index of WR40, which is estimated to be oy= 0.80 +/- 0.11, indicating that the emission from the star is thermal.
Photometric analysis of KIC 9272276 (K9272), a low-mass contact binary system, is presented. We find it to be an example of extreme low mass ratio system that satisfies the current theoretical criteria for a potential red-nova progenitor. V-band photometry of the system was modelled using the Wilson-Devenney code and we find the system to be in marginal contact with a mass ratio of 0.081. The estimated mass of the primary component is one solar mass and the theoretical instability mass ratio range, accounting for both metallicity and age, is well above the modelled mass ratio at 0.09-0.101. We find no evidence of a potential third light contamination and discuss other findings confirming our analysis.
The Stark broadening parameters, widths, and shifts, for spectral lines within the 45 N II multiplets, were calculated for collisions with electrons, protons and helium ions, by using the semiclassical perturbation method. The obtained data were compared with other theoretical calculations. They are of interest for interpretation, analysis, and synthesis of stellar spectra and for modelling, diagnostics, and investigation of stellar plasma.
This study investigates reliability of the black hole mass (MBH) estimation using broad emission lines in spectra of the Type 1 Active Galactic Nuclei (AGN) from the Sloan Digital Sky Survey (SDSS). Masses derived from the broad H alpha and H beta emission lines are compared with the stellar velocity dispersion (sigma & lowast;). The results show that the MBH-sigma & lowast; correlation strengthens in spectral sub-samples characterized by broader H beta, red asymmetry in H beta profiles, high continuum luminosity, and absence of outflows. These findings suggest that H alpha and H beta provide more accurate black hole mass estimates for objects with such spectral properties.
This study presents new, high-quality, optical photometric observations of three W UMa type contact binaries. The analysis of the corresponding light curves is made using Djurasevic's inverse problem method. To explain the light curve asymmetries and variations, we used the Roche model that involved regions containing spots on the components. The fundamental parameters of these systems were derived, including the mass ratios determined using the q-search method. Hypotheses involving active surface regions, such as dark spots on the primary and secondary components, or bright spots in the neck region due to magnetic activity or continuous mass transfer between components, were examined to explain the varying amplitudes of maxima and nearly equal minima depths in the light curves. Using the derived orbital and physical parameters, three-dimensional models were constructed for various orbital phases.
We present a preliminary study exploring whether the stellar orbital circularity of simulated galaxies, available from precomputed catalogs in the IllustrisTNG project, can be used as a proxy for a broad morphological classification. We focus on the publicly available "Stellar Circularities, Angular Momenta, Axis Ratios" catalog, which enables a simple kinematic decomposition of the stellar component into disk and spheroid subsystems. By validating this approach against the more detailed five-component kinematic decomposition in TNG50, we confirm that the circularity-based disk fraction correlates most strongly with the thin disk, while the bulge fraction broadly represents the combined contribution of classical bulges and stellar halos. We then apply this decomposition to galaxies in the TNG100 simulation at redshift z = 0 and identify a data-motivated threshold of Fdisk = 0:25 to distinguish early- and late-type galaxies. This threshold, being lower than the commonly adopted value of 0:4, better captures the diversity of disk-dominated systems and avoids excluding galaxies with moderately prominent disks. Additionally, we identify irregular or morphologically complex systems based on galaxies with low total disk and spheroid mass fractions. Using this classification, we recover a morphology-density relation that is broadly consistent with observations: the late-type galaxies dominate in the field, while the early-type galaxies are the most prevalent morphological type in clusters. Our results demonstrate that stellar circularity alone can serve as an accessible and computationally efficient morphological proxy. We also discuss the potential for this classification to support machine learning efforts as a baseline or training set for future morphological studies.
We present a high-resolution radio-continuum view and multi-frequency analysis of the Large Magellanic Cloud (LMC) Supernova Remnant (SNR) J0450.4-7050, which we nicknamed Veliki. These high-resolution observations reveal a larger extent than previously measured, making J0450.4-7050 one of the largest SNRs that we know of. Additionally, we observe a higher than expected radio surface brightness and an unusually at spectral index (? = -0:26 ? 0:02), with little spectral variation over the remnant. We observe a bright H? shell indicating significant cooling over the remnant, but also an excess of [OIII] on the eastern shock front. We investigate several theoretical scenarios to explain the emission and radio evolution of J0450.4-7050 in the context of the LMC environment, and determine that this is most likely an older, predominantly radiative, SNR with a higher shock compression ratio, which gives a flatter non-thermal spectrum in combination with thermal (bremsstrahlung) emission contribution.
The atmosphere of hypergiant rho Cas is investigated by the method of atmospheric model. Effective temperature and the surface of gravity are determined by comparing the observed and theoretical values of photometrical indices [c(1)], Q, and equivalent widths of Balmer lines: T (eff )= 5450 +/- 200K, log g=0.1 +/- 0.2. The microturbulence parameter is evaluated as xi t = 10 +/- 1 km/s, based on studies of FeII lines. The chemical composition of the star is determined. In the atmosphere of rho Cas the C turned out to be a deficit, N, and Na in excess, other investigated elements practically display solar abundance.
In the paper by Arbutina and Wadhwa (2024), due to an error at the production stage, axes designations are incorrect or missing in Figs. 2-5. Link to the corrected article 10.2298/SAJ2408001A
The extremes of emission-line and continuum variability of Active Galactic Nuclei (AGN) are unique probes of the physics and geometry of the central engine. This review provides an overview of the most extreme cases of continuum and optical emission-line variability of AGN and the proposed interpretations. We also point out remaining challenges in the identification of changing-look (CL) AGN and discuss future prospects. This includes the need for identification of larger samples of CL narrow-line Seyfert 1 galaxies and CL LINERs at opposite ends of the accretion regime. In the second part, evidence for semi-periodic variability of broad lines and continuum emission, and its possible interpretation by the presence of binary SMBHs is addressed. Most recent results from the project MOMO are presented which monitors densely the best-known binary SMBH candidate OJ 287. In the last few years, the results from this project have ruled out the leading binary model and clearly established the need for new binary modeling in an entirely different parameter regime and based on the actually measured (primary) SMBH mass of 108 M⊙. Ongoing and near-future time-domain surveys, and first gravitational wave detections of single systems, will play an important role in advancing this frontier in astrophysics.
In this paper, we present a pipeline written in Python v3.9 that performs reduction of CCD images, astrometric calibration, and photometric measurements for observations made with the 1.4m Milankovic telescope, installed at the Astronomical Station Vidojevica in Serbia. The main driver for writing such a multipurpose pipeline lies in its intended use once the dedicated observations of transient objects with the Milankovic telescope start as the follow-up observations contributing to the Vera Rubin Observatory scientific goals, but not limited to this specific need, since other follow-up observations had been conducted and some are still ongoing. The pipeline is run via the command line using different positional and optional arguments depending on the required operations (reduction, astrometry, photometry). There are also command-line arguments that allow the user to choose how to perform a certain operation (mode), or whether to use more PC processors to execute them. By default, only bias, dark, and flat-field corrections are performed, while other operations and modes are optional. In the paper, we will describe in detail each of the above mentioned operations, as well as the use of positional/optional arguments. We will also describe in detail the output FITS files that can be used to analyze the pipeline performance, as well as the final output tables with photometric measurements. Finally, we compare the pipeline performance with IRAF package procedures to test and validate the results.
Nowadays it is strongly believed that in a large number of spiral galaxies there are regular magnetic fields. Their existence is proved by measurements of the Faraday rotation of polarization plane of electromagnetic waves which are received on modern radio telescopes. The theoretical description of magnetic fields generation is connected with the mean field dynamo mechanism. It is based on joint action of the alpha- effect and differential rotation. At the first stage of evolution, magnetic fields enlarge exponentially with the growth rate described by the largest eigenvalue of the corresponding differential operator. If the field becomes comparable with the equipartition value, the nonlinear terms become more important, and they are connected with saturation of the field growth. The nonlinear system of equations has several stationary points, and some of them are stable. According to the contrast structures theory for a quite small turbulent viscosity and some initial conditions, in different regions there will be magnetic fields of opposite directions, and they will be divided by narrow transition layers. Such features, for example, characterize the magnetic field of the Milky Way. Most of the existing works describe reversals for quite moderate distances from the galaxy center. However, it was shown previously that the magnetic field can principally be generated also in outer parts of galaxies, which are situated at distances of more than 10 kpc from the rotation axis. It is interesting to describe the appearance of reversals in such parts of galaxies. In this work we have studied the dynamo action in outer parts of the Milky-Way-like galaxies. It is shown that it is possible to generate opposite magnetic fields there. We have used random initial conditions to obtain spatial reversals. Because of a large volume of computations, they were carried out by using graphics processing units (GPUs).
Contact binaries are close binary systems in which both components fill their inner Roche lobes so that the stars are in direct contact, and in potential mass and energy exchange. The most common such systems of low mass are the so-called W UMa-type. In the last few years, there has been a growing interest of the astronomical community in stellar mergers, primarily due to the detection of gravitational waves (mergers of black holes and neutron stars), but also because of an alternative model for the type Ia supernovae (merger of two white dwarfs), which are again particularly important in cosmology where they played a significant role in the discovery of dark energy and the accelerated expansion of the Universe. In that sense, contact systems of W UMa type with extremely low mass ratio are especially interesting because there are indications that, in their case too, stars can merge and possibly form fast-rotating stars such as FC Com stars and the blue-stragglers, and (luminous) red novae such as V1309 Sco. Namely, the previous theoretical research has shown that in the cases when the orbital angular momentum of the system is only about three times larger than the rotational angular momentum of the primary, a tidal Darwin's instability occurs, the components can no longer remain in synchronous rotation, orbit continue to shrink fast, and they finally merge into a single star. The above stability condition for contact systems can be linked to a specific critical mass ratio below which we expect a system to be unstable. We give an overview of this condition and show how it can be used to identify potential mergers. Finally, we discuss a number of known extreme mass ratio binaries from the literature and consider prospects for future research on this topic.
