Coulomb collisions are believed to be a fundamental process governing the thermodynamic equilibrium of solar wind plasmas, yet their role within the Alfv & eacute;nic slow solar wind remains poorly understood. In this study, we carry out a statistical analysis of proton core parameters measured by the Helios spacecraft to examine how its temperature anisotropy (T-perpendicular to/T-parallel to) varies with collisional age for three types of solar wind, namely, fast wind, Alfv & eacute;nic slow wind, and non-Alfv & eacute;nic slow wind. Here, T-perpendicular to and T-parallel to denote the proton temperatures perpendicular and parallel to the ambient magnetic field. Consistent with previous findings, we confirm that Coulomb collisions play a negligible role in the fast wind but strongly regulate the thermodynamics of non-Alfv & eacute;nic slow wind. Remarkably, however, the Alfv & eacute;nic slow wind displays two distinct regimes: a weakly collisional regime characterized by significant temperature anisotropy, T-perpendicular to/T-parallel to> 1, and a collisionally regulated regime characterized by T-perpendicular to/T-parallel to approximate to 1. In addition, a close examination of the radial evolution of several well-defined Alfv & eacute;nic slow wind streams indicates that their thermodynamic behavior is strongly governed by several key plasma parameters, notably the solar wind speed. This in turn implies that the origin of the streams-either the cores of coronal holes or their over-expanded edges-determines the dominant processes governing the dynamics and thermodynamics of the Alfv & eacute;nic slow wind.
We describe spectroscopic tomographic weak lensing measurements (spectrotomography) for two rich clusters of galaxies, A1767 and A2065, based on extensive spectroscopy and Subaru/Hyper Suprime-Cam (HSC) imaging. These detections represent the first use of spectrotomography based on archival Subaru/HSC imaging. The measurements depend only on galaxies with spectroscopic redshifts reported here. The approach cleanly separates cluster members from the background and suppresses systematics that may be introduced by the use of photometric background redshifts. We detect the tomographic shear signals at 3.1 sigma (A1767) and 3.5 sigma (A2065). The shear signal amplitudes are consistent with the cluster dynamical (caustic) masses, and they scale appropriately with source redshift. However, comparison with the first spectrophotometric detection of A2029 based on DECam imaging reveals some subtle potential systematic issues in deriving the shear signal for the relatively bright background galaxies used in the analysis. These issues may be important for understanding more extensive future applications of spectrotomography based on further Subaru imaging, as well as Euclid and LSST data. The total of three spectrophotometric detections (A1767, A2029, and A2065) sets the stage for broader application of the technique for unbiased cluster weak lensing mass determinations and potentially for a geometric cosmological test that is independent of other methods.
Most barred galaxies exhibit symmetric structures. However, recent studies have shown that a subset of barred galaxies exhibits lopsided morphologies. To quantify their occurrence and investigate their physical origins, we analyze barred galaxies in the IllustrisTNG TNG50 simulation. We select 519 clearly barred galaxies in their stellar mass maps out of 770 barred galaxies from the TNG50-1 catalog. We classify the bar morphologies into four subgroups-'Lopsided', 'Perturbed', 'Symmetric', and 'Indeterminate'-and perform a comparative analysis of their physical properties. We find that galaxies hosting asymmetric bars ('Lopsided' and 'Perturbed') tend to have higher gas densities around the bar region, enhanced star formation activity, and more recent bar-formation epochs than galaxies with symmetric bars. However, the factor that most consistently distinguishes the four subgroups is the stellar mass distribution of the host galaxy, and there appears to be no physical correlation with bar size. These findings suggest that asymmetric bars form preferentially in less massive galaxies and may evolve into symmetric bars over time through secular processes. However, this conclusion should be considered with caution, as the fraction of asymmetric bars in the TNG50 simulation is systematically higher than that observed in the local universe.
The study of magnetohydrodynamic (MHD) waves in the solar atmosphere has received considerable attention due to their potential role in solar coronal heating and acceleration of the solar wind. In this work, a numerical investigation of wave propagation and energy transport in an idealized one-dimensional (1-D) cylindrical solar flux tube is presented using MHD theory and numerical simulation. The study focuses on a comparative analysis of compressible longitudinal (acoustic) waves and incompressible torsional Alfv & eacute;n waves under controlled boundary and driving conditions, respectively. The acoustic wave evolution is examined using reflecting and open upper boundary conditions in the presence of an explicit viscous damping term. Whereas, torsional Alfv & eacute;n dynamics are studied for three distinct driver configurations: a localized Gaussian wave packet, a spatially sinusoidal initial perturbation, and a boundary-driven configuration without an initial perturbation. The findings reveal impulsive torsional events and mechanisms that generate or transfer power to higher frequencies are the most promising pathways for Alfv & eacute;nic energy to reach coronal heights, while compressible acoustic waves are efficient at heating lower layers but unlikely, by themselves, to supply the bulk of coronal heating without additional (magnetic) processes.
We present the first public data release (DR1) of the KMTNet Synoptic Survey of Southern Sky (KS4). This deep, wide-field imaging survey covers a southern footprint of -85(degrees) < Decl. < -28.8(degrees) in the B, V , R, and I bands using a network of three 1.6-m telescopes. Although primarily designed to secure reference imaging for gravitational wave counterpart identification, DR1 delivers science-ready data for similar to 4,000 deg(2) to enable a broad range of astrophysical research. The release includes deep co-added images reaching median 5 sigma depths of 22.0-23.5 AB mag. It is accompanied by two source catalogs containing over 200 million sources with SNR > 5: an I-band-selected forced-photometry catalog optimized for consistent colors, and a band-merged catalog offering enhanced completeness. Validation demonstrates robust data quality, characterized by mean astrometric offsets of +0.054 +/- 0.129 arcsec in RA and -0.015 +/- 0.120 arcsec in Dec relative to Gaia DR3. Photometric uniformity for point sources is maintained within +/- 0.03 mag relative to Gaia XP for 97.5-99.8% of the footprint across all four bands. A key advantage of KS4 is its uniform and contiguous spatial coverage. It extends to fainter magnitudes than other uniform surveys while filling irregular gaps in existing deep datasets. All data products are publicly available via the CDS and NOIRLab's Astro Data Lab.
Low-surface-brightness (LSB) structures provide critical insights into the hierarchical formation of galaxies and galaxy clusters. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) is designed to detect such diffuse features through deep, wide-field optical imaging with a surface brightness reaching similar to 30 mag arcsec-2. To interpret the observational data expected from KDRIFT, we have developed the Galaxy Replacement Technique (GRT), an N-body simulation framework optimized for tracing the gravitational evolution of stellar components. The GRT works by inserting high-resolution galaxy models, including a dark matter (DM) halo and stellar disk, in place of multiple low-resolution DM halos in the base N-body cosmological simulation. It allows us to achieve very high mass (mstar = 5.4 & times;104 M circle dot h-1) and spatial resolution (10 pc h-1) with shorter computation time compared to full hydrodynamic cosmological simulations. Therefore, this technique is particularly well-suited for studying LSB structures, with a surface brightness reaching similar to 31 mag arcsec-2. In this paper, we present the motivation and methodology of the GRT, summarize key results from previous studies, and highlight its synergy with K-DRIFT observations. We further discuss planned science cases using the GRT, aiming to build a theoretical basis for interpreting LSB features in various environments.
We present a comprehensive pipeline developed for image processing of the KMTNet Synoptic Survey of the Southern Sky (KS4) Data Release 1. This pipeline encompasses several key processes, including data quality assurance, astrometry, photometric zero-point (ZP) calibration, bad pixel masking, image stacking, and difference image analysis (DIA). The astrometric solutions were validated by cross-matching with the Gaia EDR3 catalog, achieving sub-pixel astrometric accuracy (<0.4 arcsec). To ensure spatial consistency, we divided each image into multiple subsections and confirmed that astrometric accuracy was maintained even at the edges. We performed a two-stage photometric calibration. Initial ZP solutions were computed for each individual image frame using the APASS DR9 and SkyMapper DR3 catalogs. Subsequently, we corrected residual spatial variations in the stacked images using Gaia XP photometry. This procedure yielded a 5 sigma depth of 22-23ABmag across theBV RI bands, with root-meansquare errors of approximately 0.03 mag when referenced to Gaia stars in the magnitude range of 14-19 mag. The processed KS4 images span over 4,000 deg(2) of the southern sky, providing reference images suitable for DIA. This publicly available pipeline also supports real-time processing of newly acquired images, enabling prompt transient detection. We demonstrate its effectiveness through successful applications in gravitational-wave follow-up observations.
In this study, we explore the solar differential rotation using recurrent sunspots observed by space-borne instruments from August 1996 to June 2025. To avoid systematic errors, the differential rotation profiles are calculated taking into account both Earth's elliptical orbit and the inclination of the solar rotation axis to the ecliptic. We have found that the equatorial rotation rate first increases and then decreases in Solar Cycles 23, 24, and 25, which can be interpreted as torsional oscillations with a period of approximately 11 years. When comparing the angular rotation rates at the solar maximum and minimum, the latitudinal gradient of rotations at solar minima exhibits significant cycle-to-cycle variation. During the descending phase, the latitudinal gradient of rotations changes significantly between solar cycles. The latitudinal gradient of rotations in the northern hemisphere is comparable across solar cycles, whereas that in the southern hemisphere displays significant modulations across solar cycles. In terms of the Z & uuml;rich sunspot classification system, the equatorial rotation rate is higher and the differential rotation is stronger for J-type groups than for H-type groups. It is also attempted to investigate the dependence on the order of successive passages, revealing that although the equatorial rotation rates for the first and second passages are similar, the differential rotation for the second passage appears significant. This is indicative of less rigid rotation during the second passage. To conclude, we point out that the Sun appears to rotate more differentially in the case that the solar magnetic activity is relatively weaker, when comparing cases of weak and strong solar activity.
We present the Solar system Objects Light curve Observatory (SOLO), a wide-field, high-cadence optical survey system designed to obtain absolutely calibrated asteroid light curves, converted to the Gaia G-band photometric system, in support of the SPHEREx Solar System Object Catalog (SSOC). SOLO was installed at the Sierra Remote Observatories (SRO) in California, USA, in July 2025 and is optimized for continuous, multi-night monitoring of asteroid brightness variations. We describe the system configuration, remote operation, and data reduction pipeline, and evaluate its optical and photometric performance using commissioning data. SOLO achieves stable photometric calibration across the 11.6 deg(2) field of view and reaches a 10-sigma limiting magnitude of G similar to 17.5 for a 180 sec exposure. Sample asteroid light curves obtained over multiple nights demonstrate consistent absolute photometry at the same rotational phase, validating the estimated performance. Finally, we outline the planned operational use of SOLO in connection with NASA's SPHEREx mission. Full science operations of SOLO are scheduled to begin in January 2026. Using these data, we aim to obtain on the order of 10(3) absolutely calibrated asteroid light curves per year in the Gaia G band, which will be used to support the construction and scientific utilization of the SPHEREx SSOC.
Low-surface-brightness (LSB) structures serve as evidence of the intricate mass assembly of galaxies, and dedicated studies of these structures promise profound insights into the evolutionary history of galaxies. Furthermore, delving into the properties of star formation (SF) in the LSB regime can broaden our understanding of SF activity in regions characterized by low surface gas density, thereby shedding light on fundamental cosmic processes. However, systematic uncertainties may hamper the exploration of the LSB universe by limiting detectable SB levels. Indeed, despite dedicated advancements in telescope and observing techniques over decades, achieving ultra-deep photometric depths in optical wavelengths remains a formidable challenge. To overcome this challenge and explore the LSB universe that we have yet to see, we have been developing a novel telescope called K-DRIFT. This paper outlines the telescope's specifications and describes various LSB features we aim for, explicitly focusing on nearby individual galaxies. To further advance the capabilities of the K-DRIFT survey, focused on LSB detection, we present several feasible research topics that utilize other survey data together and discuss the role of LSB observation in understanding the evolution of galaxies.
We present SQUIDPOL, a low-cost, multi-channel optical imaging polarimeter that performs simultaneous linear polarization measurements using a rotating half-wave plate, a non-polarizing beam splitter, and four wire-grid filters. We show that the multilayer dielectric coating of the off-the-shelf non-polarizing beam splitter introduces different phase delays to the s- and p-polarized components, which introduces polarization-dependent systematics that can bias polarimetric measurements if left uncorrected. We quantify this effect for both transmitted and reflected beams and incorporate a correction scheme into the data-analysis pipeline. On-sky validation demonstrates stable and reproducible performance, achieving a polarization accuracy of sigma(P) similar to 0.15% for bright polarimetric standard stars (V similar to 2-3 mag). Mounted on the 60-cm Ritchey-Chretien telescope (focal length of 4200 mm, f/7) at the Pyeonchang Observatory of Seoul National University, SQUIDPOL provides an effective common field of view of 13.5 ' & times; 8.2 ' with a pixel scale of 0.45 '' pixel(-1) and supports standard B, V , R-C, and I-C filters.
We determine the galaxy luminosity function of cluster galaxies in the nearby galaxy cluster Abell 2199 (A2199), focusing on the faint-end slope down to Mr similar to -14.5. To achieve this, we augment the existing dataset by adding redshift data from our deep MMT/Hectospec survey and from the Dark Energy Spectroscopic Instrument (DESI), significantly improving the spectroscopic completeness down to rpetro,0 = 20.8 within the central 30 ' region. The resulting luminosity function is well described by a Schechter function with a characteristic magnitude M & lowast; = -21.30 +/- 0.27 and a faint-end slope cx = -1.23 +/- 0.05. This faint-end slope is consistent with those measured in the nearby Coma and Virgo clusters and in a cluster from the TNG50 cosmological simulation, and is slightly shallower than that of field galaxies. These findings indicate that the previously claimed steep faint-end upturn (with cx similar to -2) in nearby galaxy clusters is not supported. Instead, they indicate that environmental processes in dense cluster cores do not seem to trigger the formation or survival of low-mass galaxies, thereby preventing a steep faint-end upturn in the luminosity function.
We present the result from a comprehensive laboratory and on-sky characterization of the commercial spectrograph system consisting of a PIXIS 1300BX charge-coupled device (CCD) camera and an IsoPlane 320A spectrograph as part of the preparation of the forthcoming all-sky spectroscopic survey of nearby galaxies (A-SPEC). In the laboratory, we have quantified readout noise, dark current, gain, and full-well capacity via bias, dark, and photon transfer curve analysis at all acquisition modes. To do that, we have developed a gradient correction technique to address row-dependent signal gradients in the image, which are caused by the shutter-less condition of our CCD camera test setup. The technique successfully reproduces the values in the manufacturer specifications. We also have measured quantum efficiency exceeding 80% from 400-800 nm and greater than or similar to 90% between 450-750 nm, with sub-second persistence decay, making it ideal for rapid, multi-object spectroscopy. Using a set of diffraction gratings (150, 300, and 600 gr mm-1), we have evaluated the spatial separability of multiple spectra and spectral resolution. We have conducted a test observation with this spectrograph system at the Seoul National University Astronomical Observatory (SAO) 1 m telescope and successfully demonstrated its capability of multi-object spectroscopy with moderate resolution of R approximate to 600-2600. We release all Python codes for the test and recipes to facilitate further instrument evaluations.
The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) is a pioneering instrument designed to explore low-surface-brightness (LSB) phenomena. This white paper presents a compelling set of science cases that showcase K-DRIFT's unique capabilities in unraveling the mysteries of intracluster light (ICL) and other LSB components within galaxy clusters. Exploring the origin of ICL in galaxy clusters and comparing the spatial distributions of ICL and dark matter will offer new insights into galaxy cluster dynamics. Moreover, investigating LSB objects in galaxy clusters, such as LSB structures in the brightest cluster galaxies, ultra-diffuse galaxies, and tidal features, will enhance our understanding of galaxy evolution within the cluster environment. We present our strategies for addressing scientific queries, encompassing LSB observation and analysis techniques, specialized simulations, and machine-learning approaches. Additionally, we examine the potential synergies between K-DRIFT and other ongoing and forthcoming multi-wavelength surveys. This white paper advocates for the recognition and support of K-DRIFT as a dedicated tool for advancing our understanding of the universe's subtlest phenomena.
Low-surface-brightness (LSB) structures play a crucial role in understanding galaxy evolution by providing significant insights into galaxy interactions, the histories of mass assembly, and the distribution of dark matter. Nevertheless, their inherently faint nature, coupled with observational difficulties such as stray light interference and variations in the sky background, has significantly impeded comprehensive studies of LSB features. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) project aims to address these observational challenges by developing off-axis freeform three-mirror telescopes and observational strategies specifically designed for LSB imaging surveys. The first generation of K-DRIFT (G1) has been successfully completed, and the forthcoming survey, scheduled to commence shortly, is expected to yield novel insights into the LSB universe. This paper outlines the scientific motivations of the project, discusses the technical challenges encountered, highlights the innovative solutions devised, and describes the future trajectory of the K-DRIFT.
This paper is intended to demonstrate how to apply an inversion method to one of the observed solar active regions (ARs) generally having complex magnetic structure. The method was developed in our previous work for solving an inverse problem to derive physical properties of a subsurface magnetic field from surface magnetic field evolution. We handled two key issues in the application of the method to the observed AR, one of which is to reconstruct three-dimensional magnetic structure of the AR, while the other is to select a single magnetic flux tube responsible for an X-class flare observed in this AR by isolating closed magnetic-loop structure from the reconstructed AR structure. The isolation of the loop structure is essential for a proper evaluation of unsigned magnetic flux and relative magnetic helicity, which are used to solve the inverse problem. By refining data of the so-called force-free alpha, we also derived the principal field line representing the axis of the selected flux tube in the AR, which could contribute to a quantitative classification of ARs having full of variety in size, magnetic field strength, and magnetic field configuration.
In this study, we have studied properties of solar, interplanetary, and geomagnetic indices for the period from 1976 to 2024 in view of active days and spotless days. Specifically, we have compared the characteristics of monthly spotless days in percent around a solar minimum period for each solar cycle. In addition, we have revisited the slope of the linear relationship between monthly mean sunspot numbers and monthly number of active days and further explored similar relationship with the F10.7 index, solar flare index, mean magnetic field of the Sun, Kp, Ap, Dst, AE indices, as well as the magnitude of interplanetary magnetic field with components of the interplanetary magnetic field. As a result, we have found that the distribution of monthly number of spotless days has a skewed shape in common, yet varies from one solar cycle to another implying that the number of spotless days around the solar minimum can be used as a predictor of the maximum level of solar activity. It is also found that the slope of the linear relationship in solar parameters tends to become steeper as the maximum sunspot number is large. Unlike solar parameters, results from interplanetary and geomagnetic indices reveal a quite dissimilar behavior. We conclude that the study in terms of active (spotless) days assures combination of various solar and interplanetary indices is required to find a more geoeffective parameter.
This study introduces a refined method for accurately determining the Invariant Point (IVP) of Very Long Baseline Interferometry (VLBI) antennas through constrained optimization, emphasizing the critical role of axis-offset sign conditions. Optical surveying techniques, commonly used to determine IVPs, inherently involve measurement limitations and observational errors, which may lead to biased estimations of the Azimuth (AZ) and Elevation (EL) axes. To mitigate these biases, we implement physical geometric constraints, including equal-radius conditions for target circles and equal inter-circle distances to ensure consistency across multiple Azimuth positions. Our method specifically incorporates a novel approach for determining the axis-offset sign, which significantly influences VLBI delay estimations. To validate the effectiveness of our method, we conducted numerical simulations using a virtual model with a predefined IVP and axis configuration. Realistic measurement noise was introduced to generate synthetic observational data. Simulation results clearly show that our constrained optimization approach substantially reduces bias and variance in IVP estimation compared to the geometric method proposed in our previous work. Specifically, the proposed method reduced the 3D RMSE of IVP estimation by approximately 40% (e.g., from 1.239 mm to 0.736 mm at 50 m observation distance) and the Interquar tile Range (IQR) Error Norm by over 60% (e.g., from 0.949 mm to 0.343 mm at 50 m observation distance). The proposed method's explicit handling of axis-offset sign conditions demonstrates originality and practical applicability, providing robust and reliable antenna reference point determination. Furthermore, we successfully applied this refined method to the Korean VLBI Network (KVN) Pyeongchang VLBI antenna, demonstrating practical effectiveness in operational geodetic VLBI environments. This advancement contributes to enhanced precision in International Terrestrial Reference Frame (ITRF) realization through improved VLBI station positioning.
Recent advances in millimeter-and submillimeter-wave astronomy demand heterodyne receivers that simultaneously provide wide intermediate-frequency (IF) bandwidth and low receiver noise temperature. In this work, we present the design, fabrication, and experimental validation of a wide-IF (8-16 GHz) superconductor-insulator-superconductor (SIS) mixer for the 150 GHz band. The device employs a series array of four Nb/AlOx/Nb SIS junctions with a target RF embedding impedance near the optimum impedance of 31 ohm, and it is integrated with a pentagonal probe and a WR 6.5 waveguide for RF broadband coupling. Full-circuit simulations conducted in SUPeRMIX were used to define the layout, and the performance of the fabricated mixer was evaluated in a 4 K test cryogenic receiver using hot/cold-load measurements over the 8-16 GHz IF band in double-sideband (DSB) mode. At a local oscillator (LO) frequency of 154 GHz, the mixer exhibits a maximum DSB conversion gain of 6 dB with a corresponding receiver noise temperature (T-rx) similar to 29 K. Across the IF band, T-rx averages 43 K with a range of 25-76 K. These results establish the developed mixer as a promising candidate for next-generation wide-IF astronomical receivers.
The wide-angle Polarimetric Camera (PolCam) onboard South Korea's first lunar orbiter, Danuri, is a pioneering instrument designed to conduct the first global polarimetric and high-phase-angle survey of the Moon. Precise geometric calibration is critical for this mission, particularly due to PolCam's highly oblique viewing geometry, which introduces significant topographic distortion. We present a comprehensive on-orbit geometric calibration that relies on 160,256 tie points derived from matching features between PolCam images and the well-orthorectified global map of the Kaguya Multiband Imager (MI). This dataset allows us to address two fundamental challenges: (1) the accurate reconstruction of the observation time for each line of an observation strip via a simple linear model, and (2) the refinement of the precise camera model, geometric model for PolCam optics. Our optimization method for these two challenges transforms the 2D image coordinates of identified features into 3D lunar coordinates and minimizes the reprojection error against the reference coordinates provided by the Kaguya MI map. From the refined observation time and camera model, we compute the precise longitude, latitude, and elevation of each pixel of an observed image. These estimated 3D coordinates are then used to generate orthorectified images, the final product of the geometric calibration. The resulting calibration achieves a geometric precision comparable to that of previous lunar orbiters and establishes the foundational framework necessary to produce geometrically-corrected data products of PolCam.
