arXiv subjects

D. J. Saikia

Publications and source records attributed to D. J. Saikia.

At least 19 recordsLinked to original sources

arXiv2026

A Gamma-ray Emitting Head-Tail Radio Galaxy Swimming Through the Hot Medium of the Merging Cluster Abell 3627

Head-tail radio sources are jetted active galactic nuclei (AGN) moving through the dense intracluster medium of galaxy clusters. They are extremely rare in the gamma-ray sky, likely because they are usually observed at large viewing angles. Therefore, every new gamma-ray detection of head-tail radio galaxies allows us to probe the origin of the high-energy emission in this unique class of AGN. Here we report, for the first time, the association of ESO~0137-G007 (z=0.016), a head-tail radio galaxy located at the outskirts of the merging cluster Abell 3627, with the gamma-ray source FL16Y~J1615.5$-$6034. The motion of the galaxy through the dense cluster environment and subsequent ram pressure and/or intracluster medium turbulence have led to the formation of a $>$500 kpc-long jet exhibiting a twisted, wiggly radio morphology. Its optical spectrum, obtained with the 4.1 m SOAR telescope, is devoid of emission lines, indicating that the underlying accretion activity is radiatively inefficient. From the measured line-of-sight stellar velocity dispersion, we estimate the mass of the central supermassive black hole to be $(3.36\pm1.32)\times10^9$ \Msun. We briefly discuss several radiative models to explain the observed broadband spectral energy distribution of ESO~0137$-$G007. We conclude that deeper multi-wavelength observations of this unique gamma-ray emitting head-tail radio galaxy will enable us to better understand the interaction of the jet with the hot cluster environment, thus setting the stage for the research of this class of AGN with the upcoming Square Kilometer Array.

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arXiv2026

The Quasar Main Sequence of Super-Eddington NLSy1 Galaxies

The quasar main sequence, or Eigenvector 1 (EV1), describes the optical diversity of active galactic nuclei (AGN), with Narrow-Line Seyfert 1 (NLSy1) galaxies anchoring the high-accretion end. Recent discoveries of overly massive black holes in the early Universe highlight the need to study low-redshift, low-mass super-Eddington accretors as analogs of rapid black hole growth. We map a population of 18,749 NLSy1 galaxies identified in the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1) onto the EV1 plane to determine whether they represent a distinct population of super-accretors. We compare the spectral properties of the DESI DR1 NLSy1 sample with the SDSS DR17 NLSy1 catalog. We extract key parameters, including the broad H-beta full width at half maximum (FWHM) and Fe II strength (R4570). To evaluate their accretion states, we derive single-epoch virial black hole masses using an Fe II strength-dependent scaling relation and an Eddington rate-dependent fundamental plane, both of which confirm their high accretion rates. The DESI DR1 NLSy1 population shows a consistent shift toward the extreme end of the EV1 parameter space, possessing strong Fe II emission (median R4570 = 0.97 +/- 0.36), similar to the SDSS sample (median R4570 = 0.97 +/- 0.34). Furthermore, considering both calibrations, DESI and SDSS sources harbor low-mass black holes (median log black hole mass ~6.81-6.85). About 50% of the NLSy1 galaxies exhibit super-Eddington accretion (log Eddington ratio > 0). The unprecedented sensitivity of DESI has unveiled a large population of low-mass, super-Eddington accreting sources. These intriguing EV1 objects struggle to process luminous accretion flows, naturally producing the observed intense Fe II emission. This unique sample provides a rich statistical dataset of low-redshift super-Eddington accretors for understanding early-Universe black hole growth.

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arXiv2026

Tracing the Cosmic web across Cosmic time through SKA observations of radio galaxies

The Square Kilometre Array will transform studies of the cosmic web by tracing radio galaxies (RGs) and star-forming systems across cosmic time with unprecedented sensitivity, angular resolution, frequency coverage, and survey speed. Powered by accreting supermassive black holes, RGs are not only signposts of AGN feedback but also incisive probes of their environments, from dense clusters to the low-density intergalactic medium. Their lobes, magnetic fields, and energy outflows encode the thermal and non-thermal histories of the surrounding gas, offering diagnostics of IGM pressure, particle ageing, and magnetisation over megaparsec scales. With its broad frequency coverage (50 MHz-15 GHz), microJy to sub-microJy continuum sensitivity, and wide field of view, the SKA will detect vast radio-source populations across broad ranges of redshift and environment. Measurements of source size, morphology, spectral ageing, radio power, polarisation, and Faraday rotation will reveal how the environment regulates jet propagation and lobe evolution, how radio plasma heats and magnetises the intracluster and intergalactic media, and how early AGN activity influences galaxy growth and star formation in protoclusters. Combined with host identifications, spectroscopic redshifts, and optical, infrared, X-ray, Sunyaev--Zel'dovich, and cosmic-web catalogues, SKA observations will place RGs within their three-dimensional large-scale environments. This chapter presents a framework for using RGs to trace and probe the cosmic web, from nearby filaments and clusters to high-redshift protoclusters, and to test how environment, magnetic fields, feedback, and gas dynamics shape radio-galaxy evolution, protocluster assembly, and star formation across cosmic time.

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arXiv2026

SMA Observations of the Archetypal Compact Steep-spectrum Radio Source 3C 303.1

AGN (active galactic nuclei) feedback plays a crucial role in shaping galaxy evolution. Numerical simulations show that the kinetic energy transfer efficiency from radio jets to interstellar medium (ISM) is relatively high, suggesting a significant role of radio jets in the feedback. We investigate AGN feedback on cold gas in the compact steep-spectrum (CSS) radio source 3C 303.1. CSS sources are largely young radio galaxies evolving through dense environments of their host galaxies. This early evolutionary phase likely represents a critical stage in which the radio source has maximum impact on host galaxy evolution. 3C 303.1 is the only CSS source so far showing alignment in optical, X-ray and UV with the radio structure, making it an interesting source to investigate jet-feedback. We present continuum and spectral line observations of the J=3-2 transition of carbon monoxide ($^{12}$CO) of 3C 303.1, obtained with the Submillimeter Array. We detect continuum emission at 221.1 and 271.2 GHz. We do not detect the $^{12}$CO(J=3-2) line and derive an upper limit on the molecular gas mass of $\sim 2.3 \times 10^{9}\ \mathrm{M_\odot}$. The gas to-dust mass (G/D) ratio is found to be at the lower end of typical Galactic values, and the star-formation rate (SFR) derived is moderate but declining likely due to a recent quenching event. Therefore, $^{12}$CO(J=3-2) non-detection, relatively low G/D ratio, and the moderate but declining SFR point to the shock-heating and/or removal of CO gas by AGN, consistent with the AGN feedback signatures observed in 3C 303.1.

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arXiv2026

Simulating megaparsec-scale jets of radio galaxies: Magneto-hydrodynamics of jets reaching 5 Mpc

Extragalactic jets have long prompted the question of how far relativistic outflows can extend, with some radio sources reaching 5 - 7 Mpc in length. These great extents motivate investigations into their ages, propagation dynamics, stability, and impact on the environment. We perform 3D high-resolution numerical simulations of two jet configurations involving continuous injection at different powers propagating in low-density regions of the cosmos (static and laminar), investigating the conditions for jet collimation versus disruption at extreme scales. We show that the combined effects of higher jet thrust (enhanced kinetic power), improved collimation (suppression of transverse distortions), and magnetic stabilization (strengthened poloidal field) can sustain a laterally confined flow, enabling such a jet to reach 5 Mpc in just 15 Myr (injecting a total energy of $2.3 \times 10^{61}$ erg into the environment). In contrast, a jet lacking these conditions dissipates more rapidly, forming lobe-like morphologies and reaching only $\sim 3$ Mpc over $\sim35$ Myr (injecting total energy of $8.1 \times 10^{60}$ erg). Pinch and kink MHD instabilities are identified as the primary drivers of transverse distortions; their suppression allows the persistence of a fast spine alongside a slower, dissipative head (location of maximum environmental interaction). We find that the jet-head propagation shows two regimes: one with speed $\sim0.5 c$; the other with speed from $\sim 0.2 c$ to $\sim 0.05 c$. We consider a proxy of synchrotron emission and find that radiation is concentrated in regions of enhanced compression and magnetic amplification, primarily near the first recollimation shock (producing a bright radio spot) and at the jet-head interaction zone (producing the radio termination lobe). Such jets facilitate the transport of substantial energy and magnetic flux into underdense cosmic regions.

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arXiv2026

The Emerging Population of High-energy Emitting Radio Galaxies

High-energy emission from radio galaxies provides a unique laboratory to study the connection between accretion, jet formation, and particle acceleration in active galactic nuclei (AGN). The recent detection of $γ$-ray emission from misaligned radio galaxies - including Compact Symmetric Objects (CSOs), FR0, FRI/II, and even Giant Radio Galaxies (GRGs) - has shown that efficient particle acceleration is not limited to blazars, but occurs throughout the full radio-loud AGN population. This finding supports a unifying framework where leptonic synchrotron, synchrotron self-Compton (SSC), and external inverse-Compton (EIC) processes coexist across multiple spatial scales, from the inner jet and corona to the extended lobes, possibly with a hadronic contribution in dense environments. The Square Kilometre Array (SKA) will be pivotal in advancing this field. SKA1-Low will detect and characterize diffuse, low-surface-brightness emission tracing aged plasma and jet duty cycles. SKA1-Mid will enable high-resolution spectral and polarimetric studies of compact jets and nuclear regions, while SKA-VLBI will connect parsec- to kiloparsec-scale structures, identifying the exact sites of high-energy dissipation. In synergy with forthcoming high-energy missions such as NewAthena and CTAO, SKA will provide the first spatially resolved, multi-scale view of particle acceleration and energy release in misaligned AGN, unveiling the physical link between the central engine and its large-scale feedback on the host galaxy evolution.

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arXiv2026

Narrow-Line Seyfert 1 Galaxies in the Dark Energy Spectroscopic Instrument Data Release 1

Narrow-line Seyfert 1 (NLSy1) galaxies are peculiar active galactic nuclei (AGN) known to exhibit a variety of intriguing observational features from low-frequency radio waves to high-energy $γ$~rays. As of now, NLSy1 catalogs are primarily based on optical spectroscopic observations from the Sloan Digital Sky Survey (SDSS). Here we report, for the first time, a new catalog of NLSy1 galaxies using the high-quality optical spectroscopic observations made public in the first data release of the Dark Energy Spectroscopic Instrument (DESI). We performed a detailed spectral decomposition of more than 71,000 optical spectra of AGN not included in the SDSS catalog and located at $z<0.9$. From this sample, we identify 18749 objects as NLSy1 galaxies for the first time. We also supplement the NLSy1 catalog with a sample of broad-line Seyfert 1 galaxies. The NLSy1 galaxies identified in the DESI data tend to have slightly higher bolometric luminosities and lower black hole masses (though with large dispersions), leading to the higher Eddington ratios than those of the SDSS-NLSy1 sample matched in redshifts and absolute $B$-band magnitudes. Moreover, the fraction of DESI-NLSy1 galaxies detected in the radio, X-ray, and $γ$-ray catalogs was found to be lower than that of SDSS-NLSy1 sources. We conclude that deeper multiwavelength investigations of these enigmatic AGN will help unravel the low-luminosity end of the NLSy1 population. The catalog has been made available at https://www.ucm.es/blazars/seyfert and Zenodo https://doi.org/10.5281/zenodo.20484681.

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arXiv2026

LeMMINGs VII: 5 GHz, 50 mas e-MERLIN observations of a statistically complete sample of nearby AGN

We present 5 GHz e-MERLIN radio images at 50 mas resolution of the nuclear regions of the Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies survey (LeMMINGs), the deepest statistically complete radio-band survey of the local Universe (<120 Mpc), consisting of 280 galaxies spanning all morphological and nuclear types. We detect nuclear radio emission above a median 5 sigma threshold of 0.33 mJy beam^-1 in 68 of 280 sources (24 percent), with core luminosities in the range 10^35 to 10^41.9 erg s^-1. The radio emission is attributed to active galactic nuclei, circumnuclear star formation, or, in the case of NGC 3690, a tidal disruption event. The brightest radio nuclei, with brightness temperatures >=10^6 K, reside in optically active galaxies such as LINERs and Seyferts. The detection rate for inactive systems (H II and absorption-line galaxies), which may host low-luminosity active galactic nuclei, is 8 percent. Most detections (78 percent) are compact (<10 pc), while the remaining 22 percent show extended jet-like features up to 380 pc. Compared to the 1.5 GHz LeMMINGs data, the 5 GHz observations provide superior resolution and spatial filtering, resolving out large-scale structures and isolating genuine nuclear emission. Our results suggest that low-luminosity active galactic nuclei are the primary manifestation of black hole activity in the local Universe in the form of compact jets and cores, with a preference for early-type hosts. The two LeMMINGs campaigns indicate that up to 30 percent of the local galaxy population hosts a radio-active nucleus, highlighting the necessity of high-resolution, high-sensitivity imaging for uncovering nuclear emission at the lowest luminosities.

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arXiv2026

Stratification of the AGN-Driven multi-phase outflows in the dwarf Seyfert galaxy NGC 4395

We present a multi-wavelength study of nuclear outflows in the nearby dwarf Seyfert galaxy NGC~4395, which hosts an intermediate-mass black hole. Using \textit{JWST}/NIRSpec and MIRI IFU spectroscopy (1.66--28.6~$μ$m), together with ALMA and Gemini/GMOS data, we probe the ionised and molecular gas on parsec scales. The JWST nuclear spectra reveal 134 emission lines, including H\,\textsc{i}, He, numerous fine-structure lines, H$_2$ rotational/ro-vibrational transitions, and several PAH bands. Modelling of the H$_2$ rotational lines reveals three warm/hot molecular components ($T\!\approx\!580$, 1480, and 2900~K), along with a cold ($<50$~K) phase traced by ALMA CO(2--1). Outflow signatures are detected in cold and warm/hot molecular gas, in H\,\textsc{i}, and in 36 fine-structure lines spanning ionisation potentials of 7.6--300~eV. Ionised outflow velocities range from 127 to 716~km\,s$^{-1}$, with blueshifted and redshifted components consistent with a stratified biconical geometry. The cold molecular gas shows a mass outflow rate nearly 1--2 orders of magnitude larger than that of the warm/hot molecular and ionised phases. The kinetic coupling efficiency is 0.003--0.12\% for the coronal-line gas and 0.4--1.4\% for the H\,\textsc{i} outflow, indicating that only the low-ionisation gas significantly impacts the surrounding ISM. Outflow velocity and the fraction of flux in the outflowing component increase with ionisation potential, implying that the most highly ionised gas originates closest to the AGN and is most efficiently accelerated.

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arXiv2026

A Serendipitous NuSTAR Detection of a Giant Radio Source Harboring an Obscured Active Galactic Nucleus

Giant radio sources (GRSs) harbor the Universe's largest structures generated by individual galaxies, with projected source sizes exceeding 700 kpc. These enigmatic objects have been mainly studied at radio frequencies, and their physical properties in the high-energy domain are poorly understood. Here we present the results of a multiwavelength study focused on NuSTAR J112829+5831.8 (J1128+5831), the only known GRS serendipitously detected with the Nuclear Spectroscopic Telescope Array. Being located in proximity to the famous interacting galaxy system, Arp 299, J1128+5831 has been serendipitously observed also by the Chandra X-ray Observatory, Hubble Space Telescope, and XMM-Newton satellites. From radio observations with the Low Frequency Array, the NRAO VLA Sky Survey and the Very Large Array Sky Survey, we have determined that J1128+5831 has an overall steep radio spectrum ($α=-0.86$; $F_ν\proptoν^α$) and a low core dominance ($C_{\rm D}=-2.4$, in log-scale), indicating the source to be viewed at large angles. From the X-ray spectral analysis, we found J1128+5831 to harbor an obscured active galactic nucleus (AGN) with neutral hydrogen column density exceeding $10^{23}$ cm$^{-2}$. Its optical spectrum, taken with the Dark Energy Spectroscopic Instrument, exhibits prominent narrow emission lines but lacks broad components, thus confirming J1128+5831 to be a Type 2 AGN powered by a radiatively efficient accreting system. Overall, the broadband properties of J1128+5831 are consistent with those observed for the general GRS population.

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arXiv2025

Identification of Large-Scale (>100 kpc) Radio Jets in Narrow-Line Seyfert 1 Galaxies

Powerful, large-scale relativistic jets are usually associated with massive, old elliptical galaxies. This paradigm has recently been challenged by the identification of narrow-line Seyfert 1 (NLSy1) galaxies, thought to be young active galactic nuclei with low-mass black holes, harboring relativistic jets. Among them, sources hosting $>$100 kpc radio jets are extremely rare. Here, we report the discovery of large-scale, double-lobed radio structures in 33 NLSy1s with the projected linear size of at least 100 kpc from a recently published catalog of 22656 NLSy1 galaxies. These 33 include 29 confirmed double-lobed sources and 4 candidates whose radio structure requires further study. We suggest that their low black hole masses are unlikely to be due to their small angles of inclination to the line of sight. These enigmatic sources were identified by examining the radio observations taken with the Faint Images of the Radio Sky at Twenty centimeters, Very Large Array Sky Survey, Low Frequency Array, and Rapid ASKAP Continuum Survey. Among them, the NLSy1 source J1318+2626 ($z=0.62$) is found to host a radio jet with the projected linear size of 4.3 Mpc, making it the only NLSy1 galaxy hosting a Mpc-scale radio jet known as of today. We conclude that future observations of NLSy1 sources with the next generation of sensitive telescopes may reveal a much larger population harboring large-scale jets, thus providing crucial clues on their origin, propagation, and interaction with the ambient environment.

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arXiv2025

Probing the formation of megaparsec-scale giant radio galaxies II. Continuum & polarization behavior from MHD simulations

The persistence of radiative signatures in giant radio galaxies remains a frontier topic of research, with contemporary telescopes revealing intricate features that require investigation. This study aims to examine the emission characteristics of simulated GRGs, and correlate them with their underlying 3D dynamical properties. Sky-projected continuum and polarization maps at 1 GHz were computed from five 3D-RMHD simulations by integrating the synthesized emissivity data along the line of sight, with the integration path chosen to reflect the GRG evolution in the sky plane. The emissivities were derived from these RMHD simulations, featuring FR-I and FR-II jets injected from different locations of the large-scale environment. The jet-cocoon morphologies are strongly shaped by the triaxiality of the environment, resulting in features like wings and asymmetric cocoons, thereby making morphology a crucial indicator of GRG formation mechanisms. The decollimation of the bulk flow in GRG jets gives rise to intricate cocoon features, most notably filamentary structures-magnetically dominated threads with lifespans of a few Myr. High-jet-power cases frequently display enhanced emission zones at mid-cocoon distances (alongside warmspots around the jet-head), contradicting the interpretations of the GRG as a restarting source. In such cases, examining the lateral intensity variation of the cocoon may reveal the source's state, with a gradual decrease in emission suggesting a low-active stage. This study highlights that applying a simple radio power-jet power relation to a statistical GRG sample is unfeasible, as it depends on growth conditions of individual GRGs. Effects such as inverse-Compton CMB cooling and matter entrainment significantly impact the long-term emission persistence of GRGs. The diminishing fractional polarization with GRG evolution reflects increasing turbulence in the cocoon.

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arXiv2025

The Detection of Teraelectronvolt Radiation from a Flat Spectrum Radio Quasar

The very high-energy (VHE; $>$100 GeV) radiation carries the signatures of the matter-energy interaction in some of the most extreme astrophysical environments. Considering broad emission line blazars, i.e., flat spectrum radio quasars (FSRQs), the dense photon fields surrounding the relativistic jet can prohibit the particle population from accelerating to very high energies and producing VHE radiation. They can also possibly make the environment opaque for the VHE $γ$ rays due to $γγ$ pair production, thus explaining the paucity of VHE-detected FSRQs and non-detection of TeV radiation ($>$1 TeV) from them. Here we report, for the first time, a $>$7$σ$ detection of an FSRQ, S5 1027+74 ($z=0.123$), in the VHE band, including the first ever detection of TeV emission from an object of this class, using the Fermi Large Area Telescope observations. Its $γ$-ray spectrum covering the 100 MeV to 2 TeV band revealed a prominent spectral break with a flat, rising shape above $\sim$10 GeV, a feature never detected from other VHE-detected FSRQs. The radio-to-$γ$-ray spectral energy distribution of S5 1027+74 provides strong evidence of a third bump peaking at multi-TeV energies. These enigmatic findings imply that FSRQ jets can accelerate particles to extremely high energies and provide tantalizing clues about the complex radiative environment of relativistic jets.

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arXiv2025

Fermi-Large Area Telescope Detection of Very High Energy (>100 GeV) Emission from Misaligned Jetted Active Galactic Nuclei

The detection of very-high-energy (VHE; $>$100 GeV) $γ$-ray radiation from misaligned jetted Active Galactic Nuclei (AGN) challenges the emission models that primarily explain VHE emissions from beamed AGN, i.e., blazars. Using over 16 years of \textit{Fermi}-Large Area Telescope (\textit{Fermi}-LAT) Pass 8 data in the energy range 0.1$-$2 TeV, we systematically explore the VHE emission from a recently published sample of 160 radio galaxies. We identify 12 sources detected at $>4σ$ confidence level (test statistic or TS$>$16), including nine with TS$>$25 and two Fanaroff-Riley type II objects. This detected sample includes seven out of eight previously known VHE objects. Two radio galaxies are detected in the VHE band for the first time, and we identify three promising candidates with 16$<$TS$<$25. Additionally, 13 objects are identified as candidate VHE emitters with at least one VHE photon detected with the \textit{Fermi}-LAT. These findings expand the sample of known VHE-emitting radio galaxies, whose multiwavelength follow-up observations can help provide insights into the emission mechanisms, jet physics, and the contribution of misaligned AGN to the extragalactic $γ$-ray background.

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arXiv2025

MeerKAT 1.3 GHz Observations of the Wide Angle Tail Radio Galaxy J1712$-$2435

We present full polarization MeerKAT images of the wide-angle tail, giant radio galaxy J1712$-$2435 at 1.3 GHz with 7.\asec5 resolution and an RMS sensitivity of 8 $μ$Jy beam$^{-1}$. Due to the angular proximity to the Galactic Center (l=359.6$^\circ$, b=+8.5$^\circ$) the immediate environment is not well understood but there are massive clusters nearby. Emission can be traced over an extent of 34.\amin6 which at the redshift of 0.024330 corresponds to a projected length of 1.02 Mpc. The inner jets are quite straight but then bend and completely decollimate into extended plumes nearly orthogonal to the initial jet directions at a projected distance of approximately 100 kpc. The nearly unity brightness ratio of the inner jets suggest that they are orientated within a few degrees of the plane of the sky. The 1400 MHz power is 3.9$\times 10^{24}$ W Hz$^{-1}$, somewhat below the FRI/FRII divide. The total power emitted is estimated to be 5.6$\times 10^{41}$ erg sec$^{-1}$ over the range 10 MHz to 100 GHz. The source dynamics are modeled with magneto-hydrodynamics simulations; the result is a rough reproduction of the source's radio morphology / appearance. This study further highlights the merit of alternative scenarios, calling for future observational and numerical efforts.

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arXiv2025

Radio Morphology of Gamma-ray Sources -- II. Giant Radio Galaxies

Giant radio sources, including galaxies and quasars (hereafter GRGs), are active galactic nuclei (AGN) hosting relativistic jets with source sizes exceeding the projected length of 0.7 Mpc. They are crucial to understanding the evolution of radio sources and their interaction with the surrounding environment. Some of these enigmatic objects, e.g., NGC 315, have also been reported as gamma-ray emitters. Since GRGs are thought to be aligned close to the plane of the sky, they are invaluable targets to explore the radiative mechanisms responsible for the observed gamma-ray emission. We have carried out a systematic search of gamma-ray emitting GRGs using sensitive low-resolution radio surveys, such as by Low Frequency Array, NRAO VLA Sky Survey, and Rapid ASKAP Continuum Survey, and considering the fourth data release of the fourth Fermi-Large Area Telescope gamma-ray source (4FGL-DR4) catalog. By carefully inspecting the radio maps of all AGN included in the 4FGL-DR4 catalog, we have identified 16 gamma-ray emitting GRGs, including 8 of them being reported as GRGs for the first time. Some of their observed parameters, e.g., core dominance, appeared to differ from that found for the non-gamma-ray detected GRG population, possibly due to the relatively small viewing angle of the gamma-ray emitting jet. The observed gamma-ray properties of these objects were found to be similar to non-GRG gamma-ray emitting misaligned AGN. We conclude that the origin of the gamma-ray emission could be similar in both source populations.

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arXiv2025

The long-term optical flux variations of Compact Symmetric Objects

Compact Symmetric Objects (CSOs) are a distinct category of jetted active galactic nuclei (AGN) whose optical variability characteristics have not been well investigated. We present here the results of our investigation on the optical flux and colour variability properties of a bona fide sample of 38 CSOs. We used the g-, r- and i-bands data from the Zwicky Transient Facility survey that spans a duration of about 5 years. We also considered a comparison sub-sample of blazars that includes 5 flat spectrum radio quasars and 12 BL Lac objects with redshifts and g-band magnitudes similar to the limited sub-sample of 9 CSOs. These two sub-samples of AGN, chosen for this comparative study of their long-term optical variability, represent different orientations of their relativistic jets with respect to the observer. We found that both CSOs and blazars exhibit optical flux variations, although variability of CSOs is lower than that of blazars. The observed variability in both CSOs and blazars is attributed to the relativistic jets and the increased optical variations in blazars relative to CSOs are likely due to beaming effects. CSOs and blazars exhibit similar colour variations, with both of them showing a bluer when brighter trend. Such a colour variability pattern is expected due to processes associated with their relativistic jets.

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arXiv2025

Warm Ionized Gas Outflows in Active Galactic Nuclei: What Causes it?

The driving force behind outflows, often invoked to understand the correlation between the supermassive black holes powering active galactic nuclei (AGN) and their host galaxy properties, remains uncertain. We provide new insights into the mechanisms that trigger warm ionized outflows in AGN, based on findings from the MaNGA survey. Our sample comprises 538 AGN with strong [OIII]$λ$5007 emission lines, of which 197 are detected in radio and 341 are radio-undetected. We analyzed [OIII]$λ$5007 line in summed spectra, extracted over their central 500$\times$500 pc$^2$ region. The calculated Balmer 4000 $Å$ break, D$_n$4000 is larger than 1.45 for $\sim$95$\%$ of the sources indicating that the specific star-formation rate in their central regions is smaller than 10$^{-11.5}$ yr$^{-1}$, pointing to evidence of negative AGN feedback suppressing star-formation. Considering the whole sample, radio-detected sources show an increased outflow detection rate (56$\pm$7\%) compared to radio-undetected sources (25$\pm$3\%). They also show higher velocity, mass outflow rate, outflow power and outflow momentum rate. We noticed a strong correlation between outflow characteristics and bolometric luminosity in both samples, except that the correlation is steeper for the radio-detected sample. Our findings suggest (a) warm ionized outflows are prevalent in all types of AGN, (b) radiation from AGN is the primary driver of these outflows, (c) radio jets are likely to play a secondary role in enhancing the gas kinematics over and above that caused by radiation, and (d) very low star-formation in the central regions of the galaxies, possibly due to negative feedback of AGN activity.

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