Research Highlights
ARTICLES | IIA

Although the differential rotation rate on the solar surface has long been studied using optical and extreme ultraviolet (EUV) observations, associating these measurements with specific atmospheric heights remains challenging due to the temperature dependent emission of tracers observed in EUV wavelengths. Radio observations, being primarily influenced by coherent plasma processes and/or thermal bremsstrahlung, offer a more height-stable diagnostic and thus provide an independent means to test and validate rotational trends observed at other EUV wavelengths. Aims. We aim to characterise the differential rotation profile of the upper chromosphere using cleaned solar full-disc 17 GHz radio imaging from the Nobeyama Radioheliograph spanning a little over two solar cycles (1992–2020). Methods. A tracer-independent method based on automated image correlation was employed on daily full-disc 17 GHz radio maps. This method determines the angular velocities in 16 latitudinal bins of 15◦ each by maximising the 2D cross-correlation of overlapping image segments. Results. The best-fit parameters for the differential rotation profile are A = 14.520 ± 0.006◦ /day, B= –1.443 ± 0.099◦ /day, and C = –0.433 ± 0.267◦ /day. These results suggest that the upper chromosphere rotates significantly faster than the photosphere at all latitudes, with a relatively flatter latitudinal profile. We also observed a very weak anti-correlation, ρs = −0.383 (94.73%), between the equatorial rotation rate and solar activity. Conclusions. Our findings reaffirm the potential of radio observations to probe the dynamics of the solar chromosphere with reduced height ambiguity. The overlap of the equatorial rotation rate (A) found in this study with that for 304 Å in the EUV regime lends additional support to the view that the equatorial rotation rates increase with height above the photosphere. Future coordinated studies at wavelengths with better-constrained height formation will be crucial for further understanding the complex dynamics of the solar atmosphere

Read more

Galaxies undergoing ram-pressure stripping develop gaseous tails that can extend several kiloparsecs outside the galaxy disc. Under favourable conditions, star formation can occur in the stripped tail, but there are cases where it does not happen, and this can be attributed to several factors, including the properties of the intracluster medium and different stages of stripping, although a clear consensus has not yet been reached. We used FUV and Hα imaging from the GASP survey to investigate how different stages of stripping affect star formation properties in the tail and disc of 13 galaxies undergoing stripping. FUV imaging provides a direct opportunity to study star formation, unlike Hα, which is an indirect tracer and can have other mechanisms responsible for the emission. The 13 galaxies have different stripping strengths, as identified from the MUSE integral field spectroscopy. The star-forming knots in the disc and tails show a good correspondence between the measured FUV and Hα flux. This is especially true for strong and extreme cases of stripping, which have developed extended ionised gaseous tails featuring clumpy structures. The mechanism behind the Hα emission on the tails of these regions, which correlates well with FUV emission, is photoionisation caused by young massive stars. Optical emission line ratio maps enable us to understand the emission mechanism, which can be attributed to star formation, LINER activity, or a combination of both phenomena and AGN. The star-forming regions in the emission line maps correspond well to the areas with significant FUV flux in these galaxies. FUV imaging reveals that six galaxies exhibit minimal star formation in their tails. In two cases, star formation is limited to the central regions, and their discs are truncated. In galaxies with truncated discs, star formation is confined to a smaller region on the disc, as indicated by the FUV flux, compared to Hα. Galaxies with strong stripping, marked by significant FUV and Hα emission along their tails, are undergoing recent star formation and are likely recent infalls. In contrast, galaxies with truncated discs confine star formation to the centre, likely because they have completed a cluster crossing that depleted most of their outer gaseous disc. Galaxies with little FUV flux along their tails show unresolved Hα emission, particularly in the extended Hα tail where no FUV emission is present. The unresolved Hα emission along the tail may be the result of processes other than star formation.

Read more

The Vainu Bappu Telescope (VBT) is a 2.34-m reflector, primarily supported on-axis field of view, offering high-resolution and low-to-medium resolution spectroscopic observations in its prime and Cassegrain configurations. This study presents the design and fabrication of a compact, lightweight, three-element wide-field corrector (WFC) utilizing three spherical lenses to cover a polychromatic wavelength range over a 30' FoV at prime focus. The WFC design was optimized using ZEMAX, ensuring precision in aberrations, tolerances, and atmospheric dispersion. The fabricated lenses met stringent tolerances, with a ±1 mm deviation in radius of curvature and ±2 mm deviation in center thickness. A mechanical mount was developed to integrate all the WFC lenses, and wavefront error testing for the WFC system was performed using ZYGO interferometry, yielding a Wavefront Error of 0.05 λ. Laboratory performance tests were designed and conducted using a dedicated setup with achromatic lenses and 100 μm fiber-coupled polychromatic light source showed a deviation of 0.1 pixel on-axis and 0.5 pixel at the extreme off-axis field compared to the ZEMAX design, demonstrating that the optical performance of WFC is with minimal aberrations across the entire FoV. The successful integration of the WFC at the VBT prime focus will increase the FoV, enabling the multi-fiber, multi-spectrograph setup in 30' field that will facilitate both OMR and Echelle spectrograph to be used on the same night along with the addition of new multi-object spectrograph and an integral field unit instrument. This will mark a significant upgrade for the VBT, broadening its research potential, and expanding its observational versatility.

Read more

The Gaia DR3 has provided a large sample of more than 6.6 million quasar candidates with high completeness but low purity. Previous work on the CatNorth quasar candidate catalog has shown that including external multiband data and applying machine learning methods can efficiently purify the original Gaia DR3 quasar candidate catalog and improve the redshift estimates. In this paper, we extend the Gaia DR3 quasar candidate selection to the Southern Hemisphere using data from SkyMapper, CatWISE, and Visible and Infrared Survey Telescope for Astronomy surveys. We train an XGBoost classifier on a unified set of high-confidence stars and spectroscopically confirmed quasars and galaxies. For sources with available Gaia BP/RP spectra, spectroscopic redshifts are derived using a pretrained convolutional neural network (RegNet). We also train an ensemble photometric redshift estimation model based on XGBoost, TabNet, and FT-Transformer, achieving a root mean square error of 0.2256 and a normalized median absolute deviation of 0.0187 on the validation set. By merging CatSouth with the previously published CatNorth catalog, we construct the unified all-sky CatGlobe catalog with nearly 1.9 million sources at G < 21, providing a comprehensive and high-purity quasar candidate sample for future spectroscopic and cosmological investigations.

Read more

Significant variability in broad emission line strengths of active galactic nuclei (AGN) over months to years has been observed, often accompanied by intrinsic continuum changes. Such spectral variability challenges the traditional AGN classification scheme, which attributes differences between type 1 and type 2 to geometrical effects, as transitions between these types occur on timescales shorter than viscous ones. In this work, using the cloudy photoionization simulations, we investigated the response of the major emission line fluxes, in the optical/UV and hard X-ray bands, to changes in the intensity and shape of the continuum emission of the AGN under two scenarios: (i) changes in the X-ray power law while keeping disk emission fixed, and (ii) broadband continuum variations. We demonstrate that broad-line region (BLR) line fluxes are insensitive to X-ray power-law changes alone. Considering a well-studied case of the changing-look (CL) AGN Mrk 1018, which exhibits variations in the intrinsic disk emission, as well as the X-ray power law, our simulations reproduce observed brightening and dimming trends of the BLR emission. Moreover, we show that the highly ionized Fe Kα X-ray flux, primarily produced by the H-like and He-like ions of Fe, strongly depends on the X-ray strength of the intrinsic spectral energy distribution. These findings suggest that the origin of highly ionized Fe Kα emission is in the coronal part of the accretion disk and that the CL phenomenon can be triggered by intrinsic changes in the accretion properties of AGN.

Read more

The interaction between the solar wind and Martian crustal magnetic fields plays a significant role in shaping electron precipitation in the ionosphere, particularly on the nightside. Using five years of data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, this study explores how magnetic topology influences the distribution of ionizing electrons, quantified through the electron impact ionization frequency (EIIF). Data from MAVEN’s Solar Wind Electron Analyzer (SWEA) and magnetometer (MAG) were analyzed, with strong and weak crustal field regions defined at observation altitudes using a crustal field model. As expected, the results demonstrate that magnetic topology critically influences the altitude distribution of EIIF on the nightside. However, variations in EIIF across the terminator reveal dynamic and novel patterns. In open and draped topologies, EIIF decreases gradually from the dayside, across the terminator, and onto the nightside. Conversely, closed topologies exhibit a sharper decrease in EIIF across the terminator, characterized by strong altitude dependence. This is attributed to day-to-night transport along cross-terminator closed field lines. Below 600 km on the nightside, EIIF in closed topologies is up to two orders of magnitude lower than in open and draped topologies due to the shielding effect of crustal fields, with stronger shielding observed at lower altitudes. Above 240 km, EIIF in open and draped topologies shows no dependence on crustal field strength and weak dependence on solar wind dynamic pressure (Psw). In contrast, the EIIF in closed topologies shows a strong dependence on magnetic field strength and a weak dependence on Psw. These findings provide critical insights into the role of magnetic topology in electron precipitation, aiding the parameterization of electron dynamics in global circulation models of the Martian ionosphere.

Read more

Aerosol optical and radiative properties over the Leh Ladakh, located in the climate sensitive Hindu Kush Himalayan region are examined using one year measurements from a newly installed sky radiometer from July 2023 to June 2024. Columnar aerosol optical depth (AOD), at 500 nm exhibits a distinct seasonal variation with peak during spring (0.104 ± 0.041) and minimum in autumn (0.077 ± 0.031), while Extinction Angstrom Exponent (EAE) is found to be maximum in winter (1.041 ± 0.205) and minimum (0.658 ± 0.225) in spring. The highest EAE in winter is complemented with the highest AOD_Fine-mode (0.071 ± 0.034) among the seasons which may be associated with the combustion of firewood, charcoal, kerosene or other fuels used to burn to bear the cold winter months, when temperature varied between −15 and −25 ◦C. Seasonal variation of Single Scattering Albedo (SSA) at 500 nm varied from 0.905 ± 0.055 (winter) to 0.942 ± 0.061 (summer), while the lowest value of SSA during winter is complemented with more absorbing aerosols during winter season. The dominance of fine mode aerosols during the winter seasons is also complemented by the bi-modal volume size distribution. However, the station is experiencing a negligible contribution of dust-aerosols (1%), followed by 18% mixed aerosols and 16% non-absorbing aerosols. The current observing site is dominated by absorbing aerosols which contributed by highly absorbing (49%), moderately absorbing (9%) and slightly absorbing (6%) during the one year of study period. The absorbing aerosols are characterized by high values of AOD_Fine-mode (0.061 ± 0.028), imaginary refractive-index (0.014 ± 0.009), EAE (0.932 ± 0.240) and low value of SSA_Total (0.872 ± 0.070). On the other hand, the non-absorbing aerosols are characterized by high values of AOD_Total (0.088 ± 0.040), SSA_Total (0.991 ± 0.016), SSA_Coarse-mode (0.978 ± 0.038), and low value of imaginary refractive-index (∼0.001). The estimated aerosol radiative forcing (ARF) reveals the high heating rates during spring (0.09 K day−1) among the seasons, while the heating rate is found to be highest by absorbing aerosols (0.08 K day−1) among the aerosol types.

Read more

Our understanding of large-scale radio jets in merger systems has been drastically improved in the era of the Very Large Array, Very Long Baseline Array/European VLBI Network, upgraded Giant Metrewave Radio Telescope, and MeerKAT. Twin radio galaxies (TRGs) are rare interacting galaxy pairs where both supermassive black holes host kiloparsec-scale bipolar radio jets. Only recently was a third TRG discovered, and it shows significantly different jet morphologies than the previous two. Due to both the extreme paucity and complexity of such systems, the launching of their jets as well as their mutual interaction during the propagation through the ambient medium are not well understood. We have performed three-dimensional hydrodynamic simulations to study the bipolar jets in the third TRG, J104454+354055. Our study indicates that the precession of mutually tilted bipolar jets originating from the two galactic nuclei separated by tens of kiloparsecs and propagating at low velocities can explain the observed morphologies. The simulated jet precession timescales are short compared to the overall dynamical timescale of the jets, and could originate from Lense–Thirring effect in the accretion disks. This approach to understanding TRG jet dynamics could also be applied to other TRG systems with similar helical morphologies that may be discovered in the upcoming era of the Square Kilometre Array and its pathfinder surveys.

Read more

We explore the diagnostic potential of the Hα line for probing the chromospheric magnetic field using a realistic 3D radiative magnetohydrodynamic (rMHD) model. The Stokes profiles of the Hα line are synthesized through full 3D radiative transfer under the field-free approximation, alongside the Ca II 8542 Å and Fe I 6173 Å lines for comparison. The line-of-sight (LOS) magnetic fields are inferred using the weak-field approximation for the Hα and Ca II 8542 Å lines, while the Fe I 6173 Å line is analyzed through Milne-Eddington inversion techniques. The comparison between the inferred LOS magnetic field maps and the magnetic fields in the rMHD model revealed that the Hα line core primarily probes the chromospheric magnetic field at log 500 = −5.7, which corresponds to higher layers than the Ca II 8542 Å line core, which is most sensitive to conditions at log 500 = −5.1. On average, the Stokes V profiles of the Hα line core form 500 km higher than those of the Ca II 8542 Å line core. The Hα polarization signals persist after adding noise, and with noise at the level of 10−3 Ic, most simulated magnetic structures remain visible. These findings suggest that spectropolarimetric observations of the Hα line can provide complementary insights into the stratification of the magnetic field at higher altitudes, especially when recorded simultaneously with widely used chromospheric diagnostics such as the Ca II 8542 Å line.

Read more

We analysed high-resolution mid-infrared spectra of 78 well-known Herbig Ae/Be (HAeBe) stars using Spitzer InfraRedSpectrograph data, focusing on the detection of [Ne II] and [Ne III] emission lines as indicators of ionized outflows or disc winds. Emission from [Ne II] at 12.81 μm or [Ne III] at 15.55 μm was identified in 25 sources, constituting the largest sample of HAeBe stars with these detected lines. Our analysis revealed a higher detection frequency of [Ne II] in sources with lower relative accretion luminosity (Lacc/L∗ < 0.1), suggesting a connection to the disc dispersal phase. We examined correlations between neon lines and various spectral features and investigated [Ne III]-to-[Ne II] line flux ratios to explore potential emission mechanisms. Neon emission is predominantly observed in Group I sources (75 per cent), where their flared disc geometry likely contributes to the observed emission, potentially originating from the irradiated disc atmosphere. Interestingly, we also find that Group II sources exhibit a higher median relative [Ne II] line luminosity (L[Ne II]/L∗), suggesting enhanced photoevaporation rates possibly associated with their more settled disc structures. However, larger samples and higher-resolution spectra are required to confirm this trend definitively. The high detection rate of the [Fe II] and [S III] lines, commonly associated with EUV-dominated regions, alongside a [Ne III]-to-[Ne II] emission ratio greater than 0.1 in sources where both lines detected, suggests that EUVradiation is the primary driver of neon emission in our sample.

Read more