IIA STALIN, C. SDiscovery of Changing-look Behavior in AGN NGC 3822: A Long-term Multiwavelength Study
We present a comprehensive long-term multiwavelength study of the active galactic nucleus NGC 3822, based on 17 yr (2008─2025) of X-ray, ultraviolet (UV), and optical observations. The data set includes observations from Swift, XMM-Newton, and NuSTAR, the Very Large Telescope, and the Himalayan Chandra Telescope. Our multiwavelength light-curve analysis reveals flux variations across X-ray to optical/UV bands, with an increased variability amplitude at shorter wavelengths. X-ray spectral analysis indicates the presence of intrinsic absorption during the 2016 and 2022 observations; however, this absorption disappeared before and after these epochs. The presence and absence of the absorber are attributed to clouds moving in and out of the line of sight. During the long-term monitoring period, the bolometric luminosity of the source varies between (1.32 and 17) × 1043 erg s−1. Optical spectroscopic monitoring reveals changing-look (CL) behavior in NGC 3822, characterized by the appearance and disappearance of broad emission lines (BELs). These CL transitions are associated with changes in the Eddington ratio rather than changes in the obscuration. The BELs appear only when the Eddington ratio is relatively high (∼3.8 × 10−3) and disappear when it drops to a lower value (∼0.9 × 10−3).
IIA ANNAPURNI SUBRAMANIAMA kinematic and structural study of young open clusters in the Milky Way galaxy using Gaia DR3 catalogue
We aim to identify the cluster members, estimate cluster properties, study the dynamical state of the clusters as a function of mass, trace the existence of dynamical effects in massive stars, and check for spatial patterns of members in young clusters. We studied 14 young open clusters located within 1 kpc using the data from Gaia DR3 with the membership estimated using the GMM method. The cluster parameters, such as age, distance, metallicity, and extinction were estimated by fitting PARSEC isochrones to the CMDs. These clusters are found to have ages between 6 and 90 Myr, located between 334 and 910 pc, covering a mass range of 0.13–13.77 M⊙. In five of these clusters, stars from F to M spectral type show increasing velocity dispersion, a signature for dynamical relaxation. We detect high proper motion for B and A-type stars, possible walkaway stars in the other five clusters, Alessi Teutsch 5, ASCC 16, ASCC 21, IC 2395, and NGC 6405. We demonstrate the existence of mass-dependent velocity dispersion in young clusters, suggestive of dynamical relaxation. The typical range of transverse velocity dispersion is found to be 0.40–0.70 km s-1 for young clusters.
IIA ANWESH KUMAR MISHRAOn-axis afocal telescopes as framework for CubeSat based astronomical imagers and slit-less spectrographs
CubeSats present unique opportunities for observational astronomy in the modern era. They are useful in observing difficult-to-access wavelength regions and long-term monitoring of interesting astronomical sources. However, conventional telescope designs are not necessarily the best fit for the restricted envelope of a CubeSat. Additionally, fine-pointing stability on these platforms is difficult due to the low mass of the spacecraft, and special allocations within the optical design are needed to achieve stable pointing. We propose afocal telescope designs as the framework to realise imagers and low-resolution spectrographs on CubeSat platforms.These designs help reduce the number of components in the optical chain and aim to improve throughput and sensitivity compared to conventional designs. Additionally, they also provide a fine steering mechanism within a collimated beam section. Fine beam steering within the collimated beam section avoids issues of image degradation due to out-of-plane rotation of the image plane or offset in the rotation axis of the mirror. This permits using simple and mostly off-the-shelf tip-tilt mirrors for beam steering. The designs discussed here also allow for a standard telescope design to be used in many instrument types; thus reducing the complexity as well as the development time and cost. The optical design, performance, and SNR estimations of these designs,along with some interesting science cases, are discussed. Several practical aspects in implementation, such as guiding, tolerancing, choice of detectors, vibration analysis, and laboratory test setups, are also presented.
IIA SUDHANSHU BARWAYInvestigating the bulge morphology of dual AGN host galaxies from the GOTHIC survey
We present a structural analysis of bulges in dual active galactic nuclei (AGNs) host galaxies. Dual AGNs arise in galaxy mergers where both supermassive black holes are actively accreting. The AGNs are typically embedded in compact bulges, which appear as luminous nuclei in optical images. Galaxy mergers can result in bulge growth, often via star formation. The bulges can be discy (pseudo-bulges), classical bulges, or belong to elliptical galaxies. Using Sloan Digital Sky Survey Data Release 18 gri images and GALFIT modelling, we performed 2D decomposition for 131 dual AGNs bulges (comprising 61 galaxy pairs and 3 galaxy triplets) identified in the GOTHIC survey. We derived Sersic ´ indices, luminosities, masses, and scale lengths of the bulges. Most bulges (105/131) are classical, with Sersic ´ indices lying between n = 2 and n = 8. Among these, 64 per cent are elliptical galaxies, while the remainder are classical bulges in disc galaxies. Only ∼20 per cent of the sample exhibits pseudo-bulges. Bulge masses span 1.5 × 109 M to 1.4 × 1012 M, with the most massive systems being ellipticals. Galaxy-type matching shows that elliptical–elliptical and elliptical–disc mergers dominate over disc–disc mergers. At least one galaxy in two-thirds of the dual AGN systems is elliptical and only ∼30 per cent involve two disc galaxies. Although our sample is limited, our results suggest that dual AGNs preferentially occur in evolved, red, quenched systems, which typically form via major mergers. They are predominantly hosted in classical bulges or elliptical galaxies rather than star-forming disc galaxies.
IIA SHANTIKUMAR, N. SClassification of global aerosol types and its radiative effects using Aerosol Robotic Network (AERONET) data
The present study performed classification global aerosols based on particle linear depolarization ratio (PLDR) and single scattering albedo (SSA) provided from AErosol RObotic NETwork (AERONET) Version 3.0 and Level 2.0 inversion products of 171 AERONET sites located in six continents. Current methodology could distinguish effectively between dust and non-dust aerosols using PLDR and SSA. These selected sites include dominant aerosol types such as, pure dust (PD), dust dominated mixture (DDM), pollution dominated mixture (PDM), very weakly absorbing (VWA), strongly absorbing (SA), moderately absorbing(MA), and weakly absorbing (WA). Biomass-burning aerosols which are associated with black carbon are assigned as combinations of WA, MA and SA. The key important findings show the sites in the Northern African region are predominantly influenced by PD, while south Asian sites are characterized by DDM as well as mixture of dust and pollution aerosols. Urban and industrialized regions located in Europe and North American sites are characterized by VWA, WA, and MA aerosols. Tropical regions, including South America, South-east-Asia and southern African sites which prone to forest and biomass-burning, are dominated by SA aerosols. The study further examined the impacts by radiative forcing for different aerosol types. Among the aerosol types, SA and VWA contribute with the highest (30.14 ± 8.04 Wm‑2) and lowest (7.83 ± 4.12 Wm‑2) atmospheric forcing, respectively. Consequently, atmospheric heating rates are found to be highest by SA (0.85 K day‑1) and lowest by VWA aerosols (0.22 Kday‑1). The current study provides a comprehensive report on aerosol optical, micro-physical and radiative properties for different aerosol types across six continents.
IIA MUTHU PRIYAL, VNear-Sun Characteristics of a CME Inferred from Observations in the 5303 Å Coronal Emission Line
Using observations of the solar corona in the 5303 Å (Fe XIV) emission line with the Visible Emission Line Coronagraph (VELC) on board Aditya-L1, we estimated the electron density (Ne), thermal energy (ECME), and mass (MCME) of a coronal mass ejection (CME) very close to the Sun at heliocentric distance r ≈ 1.06R⊙. The corresponding values are Ne ≈ 3.7 × 108 cm−3, ECME ≈ 9.4 × 1028 erg, and MCME ≈ 2.7 × 1014 g, respectively. The procedure adopted suggests a possibility to understand the properties of the CMEs in the visible wavelength range, particularly during the temporal phase close to their onset. The widths and Doppler velocities of the line are nearly constant in the VELC field of view (FoV ≈ 1.06–1.50R⊙). The values are ≈0.85 Å and ≈+2 km s−1, respectively.
IIA YOGESH CHANDOLAInvestigating the impacts of AGN activities on dwarf galaxies with FAST H I observations
We present the results of H I line observations towards 26 active galactive nucleus (AGN)-hosting and one star-forming dwarf galaxies ( M∗<10 9.5M⊙) with the 19-beam spectral line receiver of FAST at 1.4 GHz. Our FAST observed targets are combined with other AGN-hosting dwarf galaxies covered in the ALFALFA footprint to form a more comprehensive sample. Utilizing the information from optical surveys, we further divide them into isolated and accompanied subsamples by their vicinity of nearby massive galaxies. We compare the H I gas abundance and star-forming rate (SFR) between the subsamples to assess the role of internal and external processes that may regulate the gas content in dwarf galaxies. As a result, we find that AGNs are more commonly identified in accompanied dwarf galaxies than in their isolated counterparts. Meanwhile, AGN-hosting dwarf galaxies have slightly but significant lower H I mass fraction relatively to the non-AGN control sample in accompanied dwarf galaxies. On the other hand, we find a decreasing SFR in AGN-hosting dwarf galaxies towards denser environments, as well as an extremely low incidence of quenched isolated dwarfs within both AGN and non-AGN subsamples. These results indicate that although these AGNs could potentially regulate the gas reservoir of dwarf galaxies; environmental effects are likely the dominant quenching mechanism in the low-mass universe.
IIA DIPANKAR BANERJEEObserved Joy’s Law of Bipolar Magnetic Region Tilts at Emergence Supports the Thin Flux Tube Model
Bipolar sunspots, or more generally, bipolar magnetic regions (BMRs), are the dynamic magnetic regions that appear on the solar surface and are central to solar activity. One striking feature of these regions is that they are often tilted with respect to the equator, and this tilt increases with the latitude of appearance, popularly known as Joy’s law. Although this law has been examined for over a century through various observations, its physical origin is still not established. An attractive theory that has been put forward behind Joy’s law is the Coriolis force acting on the rising flux tube in the convection zone, which has been studied using the thin flux tube model. However, observational support for this theory is limited. If the Coriolis force is the cause of the tilt, then we expect BMRs to hold to Joy’s law at their initial emergence on the surface. By automatically identifying the BMRs over the last two solar cycles from high-resolution magnetic observations, we robustly capture their initial emergence signatures on the surface. We find that from their appearance, BMRs exhibit tilts consistent with Joy’s law. This early tilt signature of BMRs suggests that the tilt is developed underneath the photosphere, driven by the Coriolis force and helical convection, as predicted by the thin flux tube model. Considerable scatter around Joy’s law observed during the emergence phase, which reduces in the postemergence phase, reflects the interaction of the vigorous turbulent convection with the rising flux tubes in the near-surface layer.
IIA LABANYA K. GUHAH I 21 cm Absorption Associated with Foreground Galaxies on Top of Quasars
A systematic search for H I 21 cm absorption in quasar─galaxy pairs (QGPs) provides a powerful means to map the distribution of cold gas around high-redshift star-forming galaxies. Fiber spectroscopy of high-redshift quasars enables the serendipitous detection of foreground star-forming galaxies at extremely small impact parameters, forming a unique subset of QGPs known as galaxies on top of quasars (GOTOQs). In this study, we present results from a pilot upgraded Giant Metrewave Radio Telescope survey of three GOTOQs, where we achieved a remarkable 100% detection rate of H I 21 cm absorption. By combining our findings with the existing literature, we establish that GOTOQs constitute a distinct population in terms of H I 21 cm absorption, with significantly higher detection rates than those observed in damped Lyα-based or metal absorption-based searches. For the GOTOQs, we find a strong correlation between the line-of-sight reddening and the H I 21 cm optical depth, characterized by ∫τdv(kms−1)=13.58−2.35+2.75E(B−V)+0.68−1.27+1.06 , consistent with the Milky Way sightlines. We also show that the H I 21 cm optical depth declines with the impact parameter, and find a tentative trend for the H I 21 cm detection rates to also decline with the impact parameter. With upcoming wide-field spectroscopic surveys expected to substantially expand the catalog of known GOTOQs, the success of this pilot survey lays the foundation for constructing a statistically significant sample of intervening H I 21 cm absorbers.
IIA JUDHAJEET BASUSurvival of the Accretion Disk in LMC Recurrent Nova 1968-12a: UV–X-Ray Case Study of the 2024 Eruption
We report on UV and X-ray observations of the 2024 eruption of the recurrent nova LMCN 1968-12a, a rapidly recurring extragalactic system with a ∼4.3 yr recurrence period and a massive white dwarf. The eruption was discovered on 2024 August 1.8 by Swift, and subsequently monitored using AstroSat’s UVIT and Soft X-Ray Telescope, along with Swift's UVOT and X-Ray Telescope. The multiwavelength light curves reveal a rapid UV– optical decline, followed by a plateau phase exhibiting 1.26 day modulations consistent with the orbital period. The supersoft X-ray emission, which emerged by day 5, exhibited a double peak, suggesting variable obscuration that could be due to an inhomogeneous nova ejecta or due to a nova superremnant along the line of sight. Timeresolved X-ray spectroscopy shows a blackbody component with T ≈ 106 K. The spectral energy distributions obtained concurrently in the UV, peaking at T ≈ 20,000 K and with a source radius ∼2–3 R⊙, are inconsistent with emission from the secondary star or nova photosphere alone. Instead, the UV emission is attributed to an irradiated accretion disk that survived the eruption. The persistent UV plateau and its temperature suggest that the accretion disk was not completely disrupted and resumed activity within days, consistent with recent findings in other rapidly recurring novae such as U Sco and M31N 2008-12a.