IIA DEBIKA CHOWDHURYCross-correlation between the curvature perturbations and magnetic fields in pure ultraslow roll inflation
Motivated by the aim of producing significant number of primordial black holes, over the past few years, there has been a considerable interest in examining models of inflation involving a single, canonical field, that permit a brief period of ultraslow roll. Earlier, we had examined inflationary magnetogenesis—achieved by breaking the conformal invariance of the electromagnetic action through a coupling to the inflaton—in situations involving departures from slow roll. We had found that a transition from slow roll to ultraslow roll inflation can lead to a strong blue tilt in the spectrum of the magnetic field over small scales and also considerably suppress its strength over large scales. In this work, we consider the scenario of pure ultraslow roll inflation and show that scale invariant magnetic fields can be obtained in such situations with the aid of a nonconformal coupling function that depends on the kinetic energy of the inflaton. Apart from the power spectrum, an important probe of the primordial magnetic fields are the three-point functions, specifically, the cross-correlation between the curvature perturbations and the magnetic fields. We calculate the three-point cross-correlation between the curvature perturbations and the magnetic fields in pure ultraslow roll inflation for the new choice of the nonconformal coupling function. In particular, we examine the validity of the consistency condition that is expected to govern the three-point function in the squeezed limit and comment on the wider implications of the results we obtain.
IIA SANTANU MONDALExploring the disc-jet scenario in 3C 273 using simultaneous XMM-Newton and NuSTAR observations
The well-studied active galactic nucleus (AGN) 3C 273 displays characteristics of both jetted-AGNs and Seyfert galaxies, which makes it an excellent source to study the disc-jet connection in AGNs.
Aims. We aim to investigate the disc-jet scenario in 3C 273 using broad-band (0.3–78 keV) X-ray spectra from XMM-Newton and NuSTAR.
Methods. We used simultaneous XMM-Newton and NuSTAR observations of 3C 273 carried out between 2012 and 2024. The 0.3–78 keV X-ray spectra were first fitted with a simple power law (PL) and then with the accretion-ejection-based JeTCAF model. The JeTCAF model accounts for emission from the jet, which extends up to the sonic surface. In this framework, a reflection hump above 10 keV can also arise due to the bulk motion Comptonization of coronal photons by the jet.
Results. We find that the simple PL did not provide a good fit, leaving significant residuals at energies below 1.5 keV. All the spectra were fitted well by the JeTCAF model. The weighted-averaged black hole mass of (7.77 ± 0.30) × 108 M⊙ obtained from the JeTCAF model is comparable with the previous estimates based on reverberation mapping observations and accretion disc models.
Conclusions. The 0.3–78 keV X-ray emission of 3C 273 can be fit by the accretion-ejection-based model in which the corona and the jet on top of it make significant contributions to the X-ray flux. The Doppler boosting factor estimated from the jet flux ranges from 1.6 to 2.2, consistent with the lower limit from the literature.
IIA STALIN, C. SStudy of giant radio galaxies using spectroscopic observations from the Himalayan Chandra Telescope
We present the results of spectroscopic observations of host galaxies of eleven candidate giant radio galaxies (GRGs), powered by active galactic nuclei (AGNs), conducted with the 2-m Himalayan Chandra Telescope (HCT). The primary aim of these observations, performed with the Hanle Faint Object Spectrograph Camera (HFOSC), was to secure accurate spectroscopic redshifts, enabling precise calculations of their projected linear sizes. Based on these measurements, we confirm all eleven sources as giants, with linear sizes ranging from 0.7 to 2.9 Mpc, including ten GRGs and one giant radio quasar (GRQ). One of the GRGs shows evidence of a potential AGN jet-driven ionised outflow, extending up to ∼12 kpc, which, if confirmed, would represent a rarely observed feature. Two of the confirmed GRGs exceed 2 Mpc in size, which are relatively rare examples of GRGs. The redshifts of the host galaxies span 0.09323 ≤ z ≤ 0.41134. Using the obtained spectroscopic data, we characterised their AGN states based on the optical emission line properties. To complement these observations, archival radio and optical survey data were utilised to characterise their large-scale radio morphology and estimate projected linear sizes, arm-length ratios, flux densities, luminosities, and core dominance factors. These results provide new insights into the properties of GRGs and form a critical foundation for further detailed studies of their environments, AGN activity, and evolution using future high-sensitivity optical and radio datasets.
IIA ARCHANA SOAMB-fields and Dust in Interstellar Filaments Using Dust Polarization (BALLAD-POL). III. Grain Alignment and Disruption Mechanisms in G34.43+0.24 Using Polarization Observations from JCMT/POL-2
Polarization of starlight and thermal dust emission due to aligned non-spherical grains helps us to trace magnetic field (B-field) morphology in molecular clouds and to study grain alignment mechanisms. In this work, we study grain alignment and disruption mechanisms in a filamentary infrared dark cloud G34.43+0.24 using thermal dust polarization observations from JCMT/POL-2 at 850 μm. We study three regions: the North harboring the MM3 core, the Center harboring the MM1 and MM2 cores, and the South harboring no core. We find the decrease in polarization fraction P with increasing total intensity and gas column density, known as polarization hole. To disentangle the effect of magnetic field tangling on the polarization hole, we estimate the polarization angle dispersion function. We find depolarizations in the North and Center regions are due to a decrease in the net alignment efficiency of grains, but in the South region, the effect of magnetic field tangling is significant to cause depolarization. To test whether the radiative torque (RAT) mechanism can reproduce the observational data, we calculate minimum alignment and disruption sizes of grains using RAT theory, and our study finds that the RAT alignment (RAT-A) mechanism can explain the depolarizations in the North and Center regions where the B-field tangling effect is less important, except for core regions. We find hints of RAT disruption (RAT-D) in the core regions of MM3 in the North, and MM1 and MM2 in the Center. We also find that the high P value of around 8%–20% in the outer regions of the filament can potentially be explained by the magnetically enhanced RAT alignment mechanism.
IIA MRIDWEEKA SINGHA study in scarlet I. Photometric properties of a sample of intermediate-luminosity red transients
We investigate the photometric characteristics of a sample of intermediate-luminosity red transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. Our goal is to provide a stepping stone in the path to reveal the physical origin of such events, thanks to the analysis of the datasets collected.
Methods. We present the multi-wavelength photometric follow-up of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd, and AT 2019udc. Through the analysis and modelling of their spectral energy distribution and bolometric light curves, we inferred the physical parameters associated with these transients.
Results. All four objects display a single-peaked light curve which ends in a linear decline in magnitudes at late phases. A flux excess with respect to a single blackbody emission is detected in the infrared domain for three objects in our sample, a few months after maximum. This feature, commonly found in ILRTs, is interpreted as a sign of dust formation. Mid-infrared monitoring of NGC 300 2008OT-1 761 days after maximum allowed us to infer the presence of ∼10−3–10−5 M⊙ of dust, depending on the chemical composition and the grain size adopted. The late-time decline of the bolometric light curves of the considered ILRTs is shallower than expected for 56Ni decay, hence requiring an additional powering mechanism. James Webb Space Telescope observations of AT 2019abn prove that the object has faded below its progenitor luminosity in the mid-infrared domain, five years after its peak. Together with the disappearance of NGC 300 2008OT-1 in Spitzer images seven years after its discovery, this supports the terminal explosion scenario for ILRTs. With a simple semi-analytical model we tried to reproduce the observed bolometric light curves in the context of a few solar masses ejected at few 103 km s−1 and enshrouded in an optically thick circumstellar medium.
IIA MRIDWEEKA SINGHA study in scarlet II. Spectroscopic properties of a sample of intermediate-luminosity red transients
We investigate the spectroscopic characteristics of intermediate-luminosity Red Transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. Our goal is to provide a stepping stone in the path to unveiling the physical origin of these events based on the analysis of the collected datasets.
Methods. We present the extensive optical and near-infrared (NIR) spectroscopic monitoring of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. First we focus on the evolution of the most prominent spectral features observed in the low-resolution spectra. We then present a more detailed description of the high-resolution spectrum collected for NGC 300 2008OT-1 with the Very Large Telescope equipped with UVES. Finally, we describe our analysis of late-time spectra of NGC 300 2008OT-1 and AT 2019ahd through comparisons with both synthetic and observed spectra.
Results. Balmer and Ca lines dominate the optical spectra, revealing the presence of slowly moving circumstellar medium (CSM) around the objects. The line luminosity of Hα, Hβ, and Ca II NIR triplet presents a double peaked evolution with time, possibly indicative of interaction between fast ejecta and the slow CSM. The high-resolution spectrum of NGC 300 2008OT-1 reveals a complex circumstellar environment, with the transient being surrounded by a slow (∼30 km s−1) progenitor wind. At late epochs, optical spectra of NGC 300 2008OT-1 and AT 2019ahd show broad (∼2500 km s−1) emission features at ∼6170 Å and ∼7000 Å which are unprecedented for ILRTs. We find that these lines originate most likely from the blending of several narrow lines, possibly of iron-peak elements.
IIA ANUPAMA, G. CMultiwavelength analysis of AT 2023sva: a luminous orphan afterglow with evidence for a structured jet
We present multiwavelength analysis of ZTF23abelseb (AT 2023sva), an optically discovered fast-fading ( mag in d), luminous ( mag), and red ( mag) transient at with accompanying luminous radio emission. AT 2023sva does not possess a -ray burst (GRB) counterpart to an isotropic equivalent energy limit of erg, determined through searching -ray satellite archives between the last non-detection and first detection, making it the sixth example of an optically discovered afterglow with a redshift measurement and no detected GRB counterpart. We analyse AT 2023sva’s optical, radio, and X-ray observations to characterize the source. From radio analyses, we find the clear presence of strong interstellar scintillation (ISS) 72 d after the initial explosion, allowing us to place constraints on the source’s angular size and bulk Lorentz factor. When comparing the source sizes derived from ISS of orphan events to those of the classical GRB population, we find orphan events have statistically smaller source sizes. We also utilize Bayesian techniques to model the multiwavelength afterglow. Within this framework, we find evidence that AT 2023sva possesses a shallow power-law structured jet viewed slightly off-axis () just outside of the jet’s core opening angle (). We determine this is likely the reason for the lack of a detected GRB counterpart, but also investigate other scenarios. AT 2023sva’s evidence for possessing a structured jet stresses the importance of broadening orphan afterglow search strategies to a diverse range of GRB jet angular energy profiles, to maximize the return of future optical surveys.
IIA SAHU, D. KSN 2021foa: the bridge between SN IIn and Ibn
We present the long-term photometric and spectroscopic analysis of a transitioning SN IIn/Ibn from –10.8 d to 150.7 d post V-band maximum. SN 2021foa shows prominent He i lines comparable in strength to the H line around peak, placing SN 2021foa between the SN IIn and SN Ibn populations. The spectral comparison shows that it resembles the SN IIn population at pre-maximum, becomes intermediate between SNe IIn/Ibn, and at post-maximum matches with SN IIn 1996al. The photometric evolution shows a precursor at –50 d and a light curve shoulder around 17 d. The peak luminosity and colour evolution of SN 2021foa are consistent with most SNe IIn and Ibn in our comparison sample. SN 2021foa shows the unique case of an SN IIn where the narrow P-Cygni in H becomes prominent at 7.2 d. The H profile consists of a narrow (500–1200 km s) component, intermediate width (3000–8000 km s) and broad component in absorption. Temporal evolution of the H profile favours a disc-like CSM geometry. Hydrodynamical modelling of the light curve well reproduces a two-component CSM structure with different densities ( r– r), mass-loss rates (10–10 M yr) assuming a wind velocity of 1000 km s and having a CSM mass of 0.18 M. The overall evolution indicates that SN 2021foa most likely originated from an LBV star transitioning to a WR star with the mass-loss rate increasing in the period from 5 to 0.5 yr before the explosion or it could be due to a binary interaction.
IIA SAILI KESHRIUnveiling the kinematics of a central region in the triple-AGN host NGC 7733-7734 interacting group
We present a detailed study of the interacting triple active galactic nuclear system, NGC 7733-34, focusing on stellar kinematics, ionised gas characteristics, and star formation within the central region and stellar bars of both galaxies. We performed a comprehensive analysis using archival data from MUSE, HST/ACS, and DECaLS, complemented with observations from UVIT and IRSF. We identified a disc-like bulge in both NGC 7733 and NGC 7734 through a 2D decomposition. A central nuclear structure, with a semi-major axis of ∼1.113 kpc, was detected in NGC 7733 via a photometric and kinematic analysis, confirmed by the strong anticorrelation between V/σ and h3, indicative of circular orbits in the centre. NGC 7734 lacks a distinct nuclear structure. The presence of a disc-like bulge results in an anti-correlation between V/σ and h3, along with diffuse light. However, it does show higher central velocity dispersion, possibly attributed to an interaction with a smaller clump, which is likely a fourth galaxy within the system. Both galaxies demonstrate ongoing star formation, evidenced by FUV and Hα observations. NGC 7734 shows recent star formation along its bar, while NGC 7733 experiences bar quenching. The star formation rate (SFR) analysis of NGC 7734 reveals that the bar region’s SFR dominates the galaxy’s overall SFR. Conversely, in NGC 7733, the lack of star formation along the bar and the presence of a Seyfert 2 active galactic nucleus (AGN) at the galaxy centre suggest the possibility of link among both. However, this would not affect the galaxy’s overall star formation. Our findings provide valuable insights into the stellar and gas kinematics, star formation processes, and AGN feedback mechanisms in interacting galaxies hosting stellar bars.
IIA RAVI JOSHIBaryonic ecosystem in galaxies (BEINGMgII): I. host galaxies of ultra-strong Mg II absorbers in the Subaru Hyper Suprime-Cam Survey
Context. We study the galaxies hosting ultra-strong Mg II (USMgII) absorbers at small impact parameters of ∼2″ (5‑20 kpc) spanning a redshift range of 0.4 ≤ z ≤ 1.7 using deep high-resolution images from the Hyper Suprime-Cam Subaru Strategic Survey and spectra from the Sloan Digital Sky Survey (SDSS). Aims. Our aim is to explore the physical origin of the USMgII absorbers and characterize the associated galaxies. Methods. We performed a galaxy spectral energy distribution (SED) fitting using optical and near-IR multiband data to identify potential absorber host galaxies. Further, we searched for the [O II] nebular emission line from absorber galaxies in the SDSS fiber spectra. Results. From a total of 418 USMgII absorbers with W2796 ≥ 3 Å along 412 quasar sight lines, we detected 50 galaxies based on [O II] λλ3727, 3729 nebular emission detected at the ≥2σ level. Utilizing the [O II] emission from the stacked spectrum and employing the best-fit galaxy SED template, we further identified 86 galaxies, leading to a total of 136 bona fide USMgII galaxies. With a prerequisite of having a minimum of four HSC passbands available, we found a detection rate of ∼38% at an average impact parameter of 11.4 kpc. We find that galaxies hosting USMgII systems are typically star-forming main sequence galaxies, with 21% exhibiting a starburst nature. The non-zero [O II] emission along the "clear" sight lines, with no stellar counterpart, indicates that the USMgII absorbers may likely emanate from the unseen faint galaxies near the quasar. The USMgII absorbers preferentially align along the major and minor axes of the galaxy, which suggests that they originate in the disk or large-scale wind. We show that the distribution of W2796 as a function of the impact parameter indicates a discernible radial dependence for the "disk" and "wind" subsets, with the observed large scatter in W2796 potentially attributed to large-scale outflows. The quasar sight line hosting USMgII systems show a factor of three higher galaxy surface density at impact parameters of ≲50 kpc, highlighting the multiple pathways that give rise to USMgII absorption.