arXiv subjects

Arti Goyal

Publications and source records attributed to Arti Goyal.

At least 19 recordsLinked to original sources

arXiv2026

Size measurements and characterization of the astrophysical properties of multiple-component radio-AGNs in the ROGUE I catalog

We present hand-curated size measurements for a sample of 2,002 multiple-component radio AGNs in the Radio sources associated with Optical Galaxies and having Unresolved or Extended morphologies I (ROGUE I) catalog. The sources span total angular sizes of $\sim 5^{\prime\prime}$-1,100$^{\prime\prime}$ which translates to projected linear sizes $\sim$10 kpc- $\sim$2 Mpc across 0.01$\leq$z$\leq$0.54. About 10% of the sample are compact ($\leq$60 kpc) while $\sim$3% are giant radio sources ($\geq$700 kpc). Roughly 34\% are associated with galaxy clusters, and 16\% exhibit an arm-length asymmetry ratio $\geq$2. The cluster association fractions are comparable across Fanaroff-Riley (FR) I, II, and hybrid type morphological classes. Arm-length asymmetries occur at similar rates in FR I and hybrid classes but are about a factor of 2-4 less common in FR II, supporting the view that their jets are more stable and collimated, and thus less prone to disruption in dense cluster environments. In contrast, bent-angle sources (wide- and narrow-angle tails) show arm-length asymmetries about a factor of four less frequently than cluster-associated sources, suggesting that asymmetries are smoothed out by the local intracluster medium conditions. The mean galaxy number densities of FR I, II, hybrid, wide-angle, narrow-angle, and head-tail sources are comparable, supporting the dominant role of local intra-cluster medium conditions in shaping bent morphologies. Radio power-linear size tracks for FR IIs show that the population is dominated by AGNs with jet kinetic powers $\leq$10$^{38}$ erg/s, comprising both young (10 Myr) and old (100 Myr) AGNs. We also compare z, angular and linear sizes, core and total radio luminosities, optical magnitudes, black hole, and stellar masses across the morphological classes.

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arXiv2025

Origin of the double-peaked narrow emission-lines in the optical spectra of X-shaped Radio Galaxies

We investigate X-shaped radio galaxies (XRGs) with optical double-peaked narrow emission lines (DPNELs) as potential hosts of dual or binary supermassive black holes (SMBHs). Using available optical spectra from the SDSS and DESI surveys, we identify 55 DPNEL sources among a sample of 187 XRGs. We find that the occurrence of [O III] DPNELs in XRGs is $\sim$30 %, which is substantially higher than the $\sim$1 % observed in the general galaxy population. Using optical (involving {[O III]$\lambda$5007}, H$\alpha$, H$\beta$, {[N II]$\lambda$,6584}, and {[S II]$\lambda\lambda$6716,6731}) and mid-infrared diagnostic diagrams, we found that both components are predominantly AGN-powered, implying a $\sim$95 % probability of hosting dual AGN systems. In comparison, a stellar-mass-, color-, and redshift-matched control sample of non-XRG DPNEL galaxies shows a lower dual AGN fraction, which depends strongly on radio luminosity, rising from ~25 % in radio-undetected to ~54 % in radio-detected galaxies, and reaching ~95 % in radio-bright XRGs and FR-II radio galaxies. Interestingly, more than 30 % of DPNEL XRGs have companion galaxies, supporting a merger-driven origin for these systems. Finally, we analyze parsec-scale radio structure in 7 XRGs using VLBA observations at 1.4, 4.3, and 7.6 GHz, resolving the core in only one source. Nevertheless, flat radio spectral indices, AGN-like emission-line properties, and radio-optical VLBA-Gaia positional offsets found in 5 cases together support the interpretation that XRGs are strong dual/binary AGN candidates.

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arXiv2025

Spatially-resolved spectro-photometric SED Modeling of NGC 253's Central Molecular Zone I. Studying the star formation in extragalactic giant molecular clouds

Studying the interstellar medium in nearby starbursts is essential for understanding the physical mechanisms driving these objects, thought to resemble young star-forming galaxies. This study aims to analyze the physical properties of the first spatially-resolved multi-wavelength SED of an extragalactic source, spanning six decades in frequency (from near-UV to cm wavelengths) at an angular resolution of 3$^{\prime\prime}$ (51 pc at the distance of NGC,253). We focus on the central molecular zone (CMZ) of NGC,253, which contains giant molecular clouds (GMCs) responsible for half of the galaxy's star formation. We use archival data, spanning optical to centimeter wavelengths, to compute SEDs with the GalaPy and CIGALE codes for validation, and analyze stellar optical spectra with the \textsc{starlight} code. Our results show significant differences between central and external GMCs in terms of stellar and dust masses, star formation rates (SFRs), and bolometric luminosities. We identify the best SFR tracers as radio continuum bands at 33 GHz, radio recombination lines, and the total infrared luminosity (L$_{\rm IR}$; 8-1000$μ$m), as well as 60$μ$m IR emission. BPT and WHAN diagrams indicate shock signatures in NGC~253's nuclear region, associating it with AGN/star-forming hybrids, though the AGN fraction is negligible ($\leq$7.5%). Our findings show significant heterogeneity in the CMZ, with central GMCs exhibiting higher densities, SFRs, and dust masses compared to external GMCs. We confirm that certain centimeter photometric bands can reliably estimate global SFR at GMC scales.

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arXiv2024

Radio-only and Radio-to-far-ultraviolet Spectral Energy Distribution Modeling of 14 ULIRGs: Insights into the Global Properties of Infrared Bright Galaxies

We present detailed spectral energy distribution (SED) modeling of 14 local ultraluminous infrared galaxies (ULIRGs) with outstanding photometric data from the literature covering the ultraviolet--infrared (FIR) and radio bands ($\sim$50 MHz to $\sim$30 GHz). We employ the CIGALE SED fitting code to model the ultraviolet--FIR--radio SED. For the radio-only SED modeling, we use the UltraNest package, leveraging its nested sampling algorithm. Combining the results from our previous study on 11 luminous infrared galaxies (LIRGs), we discuss the global astrophysical properties of a sample of 25 starburst galaxies (z $<$ 0.5). Their radio spectra are frequently characterized by bends and turnovers, with no indication of ULIRGs exhibiting more complicated SEDs than LIRGs despite showing more signs of interactions. Including radio measurements in the CIGALE modeling constrained the dust luminosity and star formation rate (SFR) estimates by more than 1 order of magnitude better than previously reported for starburst galaxies. We show that total and nonthermal radio luminosity at 1.4 and 4.8 GHz frequencies can be good estimators of recent SFRs for all LIRGs and those ULIRGS with an insignificant influence of active galactic nuclei. A weaker but still significant correlation is observed between radio SFRs at 1.4 GHz and old (averaged over 100 Myr) SFRs based on SED modeling, indicative of multiple episodes of starburst activity during their lifetime. The thermal radio luminosity at 4.8 GHz is a better tracer of recent star formation than the thermal luminosity at 1.4 GHz. Statistically, our modeled nonthermal radio spectral indices do not significantly correlate with redshift, stellar mass, SFR, specific SFR, and dust mass.

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arXiv2022

Optical variability of eight FRII-type quasars with 13-yr photometric light curves

We characterize the optical variability properties of eight lobe-dominated radio quasars (QSOs): B2 0709$+$37, FBQS J095206.3$+$235245, PG 1004$+$130, [HB89] 1156$+$631, [HB89] 1425$+$267, [HB89] 1503$+$691, [HB89] 1721$+$343, 4C $+$74.26, systematically monitored for a duration of 13 years since 2009. The quasars are radio-loud objects with extended radio lobes that indicate their orientation close to the sky plane. Five of the eight QSOs are classified as giant radio quasars. All quasars showed variability during our monitoring, with magnitude variations between 0.3 and 1 mag for the least variable and the most variable QSO, respectively. We performed both structure function (SF) analysis and power spectrum density (PSD) analysis for the variability characterization and search for characteristic timescales and periodicities. As a result of our analysis, we obtained relatively steep SF slopes ($α$ ranging from 0.49 to 0.75) that are consistent with the derived PSD slopes ($\sim$2--3). All the PSDs show a good fit to single power law forms, indicating a red-noise character of variability between $\sim$13 years and weeks timescales. We did not measure reliable characteristic timescales of variability from the SF analysis which indicates that the duration of the gathered data is too short to reveal them. The absence of bends in the PSDs (change of slope from $\geq$1 to $\sim$0) on longer timescales indicates that optical variations are most likely caused by thermal instabilities in the accretion disk.

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arXiv2022

Low frequency radio continuum imaging and SED modeling of 11 LIRGs: radio-only and FUV to radio bands

We present the detailed analysis of 11 local luminous infrared galaxies (LIRGs) from ultraviolet through far-infrared to radio ($\sim$70 MHz to $\sim$15 GHz) bands. We derive the astrophysical properties through spectral energy distribution (SED) modeling using the Code Investigating GALaxy Emission (CIGALE) and UltraNest codes. The radio SEDs include our new observations at 325 and 610 MHz from the GMRT and the measurements from public archives. Our main results are (1) radio SEDs show turnovers and bends, (2) the synchrotron spectral index of the fitted radio spectra ranges between $-$0.5 and $-$1.7, and (3) the infrared luminosity, dust mass, dust temperature, stellar mass, star-formation rates (SFRs) and AGN fraction obtained from CIGALE falls in the range exhibited by galaxies of the same class. The ratio of 60$μ$m infrared and 1.4 GHz radio luminosity, the 1.4 GHz thermal fraction, and emission measure range between 2.1 and 2.9, 0.1% and 10%, 0.02 and 269.5$\times$10$^{6}$ cm$^{-6}$ pc, respectively. We conclude that the turnovers seen in the radio SEDs are due to free-free absorption; this is supported by the low AGN fraction derived from the CIGALE analysis. The decomposed 1.4 GHz thermal and nonthermal radio luminosities allowed us to compute the star formation rate (SFR) using scaling relations. A positive correlation is observed between the SFR$_{IR}$ obtained 10 Myr ago (compared to 100 Myr ago) and 1.4 GHz radio (total and nonthermal) because similar synchrotron lifetimes are expected for typical magnetic field strengths observed in these galaxies ($\approx$50$μ$G).

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arXiv2022

Multiwavelength variability power spectrum analysis of the blazars 3C 279 and PKS 1510-089 on multiple timescales

We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV\,$γ$-ray light curves covering decades/years to days/minutes timescales for the blazars 3C 279 and PKS 1510-089. The PSDs are modeled as single power-laws, and the best-fit spectral shape is derived using the `power spectral response' method. With more than ten years of data obtained with weekly/daily sampling intervals, most of the PSDs cover ~2-4 decades in temporal frequency; moreover, in the optical band, the PSDs cover ~6 decades for 3C 279 due to the availability of intranight light curves. Our main results are the following: (1) on timescales ranging from decades to days, the synchrotron and the inverse Compton spectral components, in general, exhibit red-noise (slope ~2) and flicker-noise (slope ~1) type variability, respectively; (2) the slopes of $γ$-ray variability PSDs obtained using a 3-hr integration bin and a 3-weeks total duration exhibit a range between ~1.4 and ~2.0 (mean slope = 1.60$\pm$0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales $\leq$100\, days at $γ$-ray frequencies as compared to longer wavelengths, in general (except between $γ$-ray and optical frequencies for PKS 1510$-$089); (4) the normalization of intranight optical PSDs for 3C\,279 appears to be a simple extrapolation from longer timescales, indicating a continuous (single) process driving the variability at optical wavelengths; (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS\,1510$-$089.

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arXiv2021

Optical variability power spectrum analysis of blazar sources on intranight timescales

We report the first results of a systematic investigation to characterize blazar variability power spectral densities (PSDs) at optical frequencies using densely sampled (5--15 minutes integration time), high photometric accuracy ($\lesssim$0.2--0.5\%) R-band intranight light curves, covering timescales ranging from several hours to $\sim$15\,minutes. Our sample consists of 14 optically bright blazars, including nine BL Lacertae objects (BL Lacs) and five flat-spectrum radio quasars (FSRQs) which have shown statistically significant variability during 29 monitoring sessions. We model the intranight PSDs as simple power--laws and derive the best-fit slope along with uncertainty using the `power spectral response' method. Our main results are the following: (1) on 19 out of 29 monitoring sessions, the intranight PSDs show an acceptable fit to simple power-laws at the rejection confidence $\leq$ 90\%; (2) for these 19 instances, the PSD slopes show a large range from 1.4 to 4.0, consistent with statistical characters of red (slope$\sim$2) and black (slope$\geq$3) noise stochastic processes; (3) the average PSD slopes for the BL Lacs and FSRQs are indistinguishable from one another; (4) the normalization of intranight PSDs for individual blazar sources which were monitored on more than one occasion turns out to be consistent with one another with a few exceptions. The average PSD slope, 2.9$\pm$0.3 (1$σ$ uncertainty) is steeper than the red-noise type character of variability found on longer timescales (many decades to days), indicative of a cutoff in the variability spectrum on timescales around a few days at the synchrotron frequencies of the emission spectrum.

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arXiv2020

Identifying radio active galactic nuclei among radio-emitting galaxies

Basing our analysis on ROGUE I, a catalog of over 32,000 radio sources associated with optical galaxies, we provide two diagnostics to select the galaxies where the radio emission is due to an active galactic nucleus (AGN). Each of these diagnostics can be applied independently. The first one, dubbed MIRAD, compares the flux $F_{W3}$ in the $W3$ mid-infrared band of the WISE telescope, with the radio flux at 1.4 GHz, $\Frad$. MIRAD requires no optical spectra. The second diagnostic, dubbed DLM, relates the 4000 Å break strength, $D_{\rm n}(4000)$, with the radio luminosity per unit stellar mass. The DLM diagram has already been used in the past, but not as standalone. For these two diagrams, we propose simple, empirical dividing lines that result in the same classification for the objects in common. These lines correctly classify as radio-AGN 99.5 percent of the extended radio sources in the ROGUE~I catalog, and as star-forming (SF) galaxies 98--99 percent of the galaxies identified as such by their emission line ratios. Both diagrams clearly show that radio AGNs are preferentially found among elliptical galaxies and among galaxies hosting the most massive black holes. Most of the radio sources classified as radio-AGNs in the MIRAD or DLM diagrams are either optically weak AGNs or retired galaxies.

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arXiv2020

Blazar variability power spectra from radio up to TeV photon energies: Mrk 421 and PKS 2155-304

We present the results of the power spectral density (PSD) analysis for the blazars Mrk\,421 and PKS\,2155$-$304, using good-quality, densely sampled light curves at multiple frequencies, covering 17 decades of the electromagnetic spectrum, and variability timescales from weeks up to a decade. The data were collected from publicly available archives of observatories at radio from OVRO, optical and infrared (B, V, R, I, J, H, and K-bands), X-rays from the {\it Swift} and the {\it Rossi} X-ray Timing Explorer, high and very high energy $γ-$rays from the {\it Fermi} and Very Energetic Radiation Imaging Telescope Array System as well as the High Energy Stereoscopic System. Our results are: (1) the power-law form of the variability power spectra at radio, infra-red and optical frequencies have slopes $\sim$1.8, indicative of random-walk type noise processes; (2) the power-law form of the variability power spectra at higher frequencies, from X-rays to very high energy \,$γ$-rays, however, have slopes $\sim$1.2, suggesting a flicker noise type process; (3) there is significantly more variability power at X-rays, high and very high energy $γ$-rays on timescales $\lesssim$ 100 days, as compared to lower energies. Our results do not easily fit into a simple model, in which a single compact emission zone is dominating the radiative output of the blazars across all the timescales probed in our analysis. Instead, we argue that the frequency-dependent shape of the variability power spectra points out a more complex picture, with highly inhomogeneous outflow producing non-thermal emission over an extended, stratified volume.

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arXiv2020

Catalogue with visual morphological classification of 32,616 radio galaxies with optical hosts

We present the catalogue of Radio sources associated with Optical Galaxies and having Unresolved or Extended morphologies I (ROGUE I). It was generated by cross-matching galaxies from the Sloan Digital Sky Survey Data Release 7 (SDSS DR 7) as well as radio sources from the First Images of Radio Sky at Twenty Centimetre (FIRST) and the National Radio Astronomical Observatory VLA Sky Survey (NVSS) catalogues. We created the largest handmade catalogue of visually classified radio objects and associated with them optical host galaxies, containing 32,616 galaxies with a FIRST core within 3 arcsec of the optical position. All listed objects possess the good quality SDSS DR 7 spectra with the signal-to-noise ratio $>$10 and spectroscopic redshifts up to $z=0.6$. The radio morphology classification was performed by a visual examination of the FIRST and the NVSS contour maps overlaid on a DSS image, while an optical morphology classification was based on the 120 arcsec snapshot images from SDSS DR 7. The majority of radio galaxies in ROGUE I, i.e. $\sim$ 93%, are unresolved (compact or elongated), while the rest of them exhibit extended morphologies, such as Fanaroff-Riley (FR) type I, II, and hybrid, wide-angle tail, narrow-angle tail, head-tail sources, and sources with intermittent or reoriented jet activity, i.e. double-double, X-shaped, and Z-shaped. Most of FR IIs have low radio luminosities, comparable to the luminosities of FR Is. Moreover, due to visual check of all radio maps and optical images, we were able to discover or reclassify a number of radio objects as giant, double-double, X-shaped, and Z-shaped radio galaxies. The presented sample can serve as a database for training automatic methods of identification and classification of optical and radio galaxies.

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arXiv2019

Radio sources associated with Optical Galaxies and having Unresolved or Extended morphologies (ROGUE). I. A catalog of SDSS galaxies with FIRST core identifications

We present the catalog of Radio sources associated with Optical Galaxies and having Unresolved or Extended morphologies I (ROGUE~I), consisting of 32,616 spectroscopically selected galaxies. It is the largest handmade catalog of this kind, obtained by cross-matching galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 7 and radio sources from both the First Images of Radio Sky at Twenty Centimetre (FIRST) survey and the NRAO VLA Sky Survey \textit{without imposing a limit to the radio flux densities}. The catalog provides a \textit{visual} classification of radio and optical morphologies of galaxies presenting a FIRST core within 3\arcsec\ of the optical position. The radio morphological classification is performed by examining the radio-optical overlays of linear sizes equal to 1 Mpc at the source distance, while the 120\arcsec\ image snapshots from the SDSS database are used for optical classification. The results of our search are: (i) single-component unresolved and elongated, radio sources constitute the major group in the ROGUE I catalog ($\sim$90%), and $\sim$8% exhibiting {\it extended} morphologies, (ii) samples of 269, 730, and 115 Fanaroff-Riley (FR) type I, II, and hybrid galaxies, respectively, are presented (iii) we report 55 newly discovered giant/possible giant, 16 double-double, 9 X-shaped, and 25 Z-shaped radio sources, (iv) on the optical front, most galaxies have elliptical morphologies ($\sim$62%) while spirals form the second major category ($\sim$17%) followed by distorted ($\sim$ 12%) and lenticular ($\sim$7%) morphologies, (v) division between the FR I and the FR~II sources in the radio-optical luminosity plane is blurred, in tune with recent studies.

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arXiv2019

Multiband optical flux density and polarization microvariability study of optically bright blazars

We present the results of flux density, spectral index, and polarization intra-night monitoring studies of a sample of eight optically bright blazars, carried out by employing several small to moderate aperture (0.4\,m to 1.5\,m diameter) telescopes fitted with CCDs and polarimeters located in Europe, India, and Japan. The duty cycle of flux variability for the targets is found to be $\sim 45$ percent, similar to that reported in earlier studies. The computed two-point spectral indices are found to be between 0.65 to 1.87 for our sample, comprised of low- and intermediate frequency peaked blazars, with one exception; they are also found to be statistically variable for about half the instances where `confirmed' variability is detected in flux density. In the analysis of the spectral evolution of the targets on hourly timescale, a counter-clockwise loop (soft-lagging) is noted in the flux-spectral index plane on two occasions, and in one case a clear spectral flattening with the decreasing flux is observed. In our data set, we also observe a variety of flux-polarization degree variability patterns, including instances with a relatively straightforward anti-correlation, correlation, or counter-clockwise looping. These changes are typically reflected in the flux-polarization angle plane: the anti-correlation between the flux and polarization degree is accompanied by an anti-correlation between the polarization angle and flux, while the counter-clockwise flux-PD looping behaviour is accompanied by a clockwise looping in the flux-polarization angle representation. We discuss our findings in the framework of the internal shock scenario for blazar sources.

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arXiv2018

Identification of blazar candidates behind Small and Large Magellanic Clouds

We report the identification of blazar candidates behind the Magellanic Clouds. The objects were selected from the Magellanic Quasars Survey (MQS), which targeted the entire Large Magellanic Cloud (LMC) and 70\% of the Small Magellanic Cloud (SMC). Among the 758 MQS quasars and 898 of unidentified (featureless spectra) objects, we identified a sample of 44 blazar candidates, including 27 flat spectrum radio quasars and 17 BL Lacertae objects, respectively. All the blazar candidates from our sample were identified with respect to their radio, optical, and mid-infrared properties. The newly selected blazar candidates possess the long-term, multi-colour photometric data from the Optical Gravitational Lensing Experiment, multi-colour mid-infrared observations, and archival radio data for one frequency at least. In addition, for nine of them the radio polarization data are available. With such data, these objects can be used to study the physics behind the blazar variability detected in the optical and mid-infrared bands, as a tool to investigate magnetic field geometry of the LMC and SMC, and as an exemplary sample of point like sources most likely detectable in $γ$-ray range with the newly emerging Cherenkov Telescope Array.

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arXiv2017

Multi-wavelength variability study of the classical BL Lac object PKS 0735+178 on timescales ranging from decades to minutes

We present the results of our power spectral analysis for the BL Lac object PKS 0735+178 utilizing the Fermi-LAT survey at high-energy $γ$-rays, several ground-based optical telescopes, and single-dish radio telescopes operating at GHz frequencies. The novelty of our approach is that, by combining long-term and densely sampled intra-night light curves in the optical regime, we were able to construct for the first time the optical power spectrum of the blazar for a time domain extending from 23 years down to minutes. Our analysis reveals that: (i) the optical variability is consistent with a pure red noise, for which the power spectral density can well be approximated by a single power-law throughout the entire time domain probed; (ii) the slope of power spectral density at high-energy $γ$-rays ($\sim 1$), is significantly flatter than that found at radio and optical frequencies ($\sim 2$) within the corresponding time variability range; (iii) for the derived power spectra we did not detect any low-frequency flattening, nor do we see any evidence for cut-offs at the highest frequencies down to the noise floor levels due to measurement uncertainties. We interpret our findings in terms of a model where the blazar variability is generated by the underlying single stochastic process (at radio and optical frequencies), or a linear superposition of such processes (in the $γ$-ray regime). Along with the detailed PSD analysis, we also present the results of our extended (1998-2015) intra-night optical monitoring program and newly acquired optical photo-polarimetric data for the source.

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arXiv2015

Discovery of a Highly Polarized Optical Microflare in the Blazar S5 0716+714 During 2014 WEBT Campaign

The occurrence of low-amplitude flux variations in blazars on hourly timescales, commonly known as microvariability, is still a widely debated subject in high-energy astrophysics. Several competing scenarios have been proposed to explain such occurrences, including various jet plasma instabilities leading to the formation of shocks, magnetic reconnection sites, and turbulence. In this letter we present the results of our detailed investigation of a prominent, five-hour-long optical microflare detected during recent WEBT campaign in 2014, March 2-6 targeting the blazar 0716+714. After separating the flaring component from the underlying base emission continuum of the blazar, we find that the microflare is highly polarized, with the polarization degree $\sim (40-60)\%$$\pm (2-10)\%$, and the electric vector position angle $\sim (10 - 20)$deg$\pm (1-8)$deg slightly misaligned with respect to the position angle of the radio jet. The microflare evolution in the $(Q,\,U)$ Stokes parameter space exhibits a looping behavior with a counter-clockwise rotation, meaning polarization degree decreasing with the flux (but higher in the flux decaying phase), and approximately stable polarization angle. The overall very high polarization degree of the flare, its symmetric flux rise and decay profiles, and also its structured evolution in the $Q-U$ plane, all imply that the observed flux variation corresponds to a single emission region characterized by a highly ordered magnetic field. As discussed in the paper, a small-scale but strong shock propagating within the outflow, and compressing a disordered magnetic field component, provides a natural, though not unique, interpretation of our findings.

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arXiv2014

Low-frequency High-resolution Radio Observations of the TeV-emitting Blazar SHBLJ001355.9-185406

In the framework of the unification scheme of radio-loud active galactic nuclei, BL Lac objects and quasars are the beamed end-on counterparts of low-power (FRI) and high-power (FRII) radio galaxies, respectively. Some BL Lacs have been found to possess the FRII-type large-scale radio morphology, suggesting that the parent population of BL Lacs is a mixture of low- and high-power radio galaxies. This seems to apply only to `low frequency-peaked' BL Lacs, since all the `high frequency-peaked' BL Lacs studied so far were shown to host exclusively the FRI-type radio jets. While analyzing the NVSS survey maps of the TeV detected BL Lacs, we have however discovered that the high frequency-peaked object SHBL J001355.9-185406 is associated uniquely with the one-sided, arcmin-scale, and edge-brightened jet/lobe-like feature extending to the south-west from the blazar core. In order to investigate in detail the large-scale morphology of SHBL J001355.9-185406, we have performed low-frequency and high-resolution observations of the source at 156, 259 and 629 MHz using the Giant Metrewave Radio Telescope. Our analysis indicates that no diffuse arcmin-scale emission is present around the unresolved blazar core, and that the arcmin-scale structure seen on the NVSS map breaks into three distinct features unrelated to the blazar, but instead associated with background AGN. The upper limits for the extended radio halo around the TeV-emitting BL Lac object SHBL J001355.9-185406 read as < 10% - 1% at $156-629$ MHz. The fact that the integrated radio spectrum of the unresolved blazar core is flat down to 156 MHz indicates that a self-similar character of the jet in the source holds up to relatively large distances from the jet base.

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arXiv2013

Improved characterisation of intra-night optical variability of prominent AGN classes

The incidence of intra-night optical variability (INOV) is known to to differ significantly among different classes of powerful active galactic nuclei (AGN). A number of statistical methods have been employed in the literature for testing the presence of INOV in the light curves, sometimes leading to discordant results. In this paper we compare the INOV characteristics of six prominent classes of AGN, as evaluated using three commonly used statistical tests, namely the $χ^2-$test, the modified $C-$test and the $F-$test, which has recently begun to gain popularity. The AGN classes considered are: radio-quiet quasars (RQQs), radio-intermediate quasars (RIQs), lobe-dominated quasars (LDQs), low optical polarization core-dominated quasars (LPCDQs), high optical polarization core-dominated quasars (HPCDQs), and TeV blazars. Our analysis is based on a large body of AGN monitoring data, involving 262 sessions of intra-night monitoring of a total 77 AGN, using 1-2 metre class optical telescopes located in India. In order to compare the usefulness of the statistical tests, we have also subjected them to a `sanity check' by comparing the number of false positives yielded by each test with the corresponding statistical prediction. The present analysis is intended to serve as a benchmark for future INOV studies of AGN of different classes.

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