arXiv subjects

Amit Kumar Mandal

Publications and source records attributed to Amit Kumar Mandal.

At least 19 recordsLinked to original sources

arXiv2026

Accretion-disk sizes in two quasars with interferometrically resolved broad-line regions at $z=2.3$ and $z=4.0$

We present the first direct comparison between accretion-disk (AD) and broad-line region (BLR) sizes in quasars at z > 2, combining continuum reverberation mapping with interferometric BLR constraints from GRAVITY and GRAVITY+. Using medium-band photometric monitoring with the MPG/ESO 2.2 m telescope, we measure inter-band continuum lags in SDSS J092034.17+065718.0 at z = 2.33 (J0920) and SMSS J052915.80-435152.0 at z = 3.96 (J0529), among the most luminous quasars known (Lbol ~ 10^48 erg s^-1). These are currently the only two quasars at z > 2 with spatially resolved BLRs and dynamical black-hole masses from interferometry. We detect significant continuum lags in both quasars, increasing monotonically with wavelength. The inferred UV disk sizes are RAD = 4.35 +0.78/-0.91 light-days for J0529 and RAD = 3.15 +0.50/-0.48 light-days for J0920. For J0920, accreting at lambda_Edd ~ 7-20, the disk size is consistent with standard thin-disk expectations despite its super-Eddington regime. For J0529, the disk size agrees with thin-disk predictions using the single-epoch black-hole mass, but implies disk inflation by a factor of a few if the GRAVITY+ dynamical mass, an order of magnitude lower, is adopted. UV continuum disk sizes therefore provide an independent physical scale constraining black-hole mass and accretion-rate models, particularly where BLR kinematics are dominated by outflows. The interferometric BLR sizes reveal pronounced radial hierarchies, with RBLR/RAD ~ 270 for J0529 (Hbeta) and ~115 for J0920 (Halpha). The successful lag detections at Lbol ~ 10^48 erg s^-1 show that continuum reverberation mapping remains feasible for the most luminous systems, opening a path to larger samples with surveys such as the Vera C. Rubin Observatory's LSST.

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arXiv2026

HALO II: Constraining Hubble constant $H_{0}$ through continuum delay fitting of Fairall 9

The Hubble tension remains one of the most significant unresolved problems in modern cosmology. A key question is whether it may arise from underestimated systematic uncertainties in the different measurement techniques. In this context, new independent methods are of exceptional importance. We therefore pursue a novel approach to determining the Hubble constant, $H_{0}$ based on continuum time delay and spectral energy distribution (SED) modeling in active galactic nuclei (AGNs). Unlike conventional techniques, this method is entirely independent of the cosmic distance ladder and does not require cross-calibration against other distance indicators. As a result, it enables a direct determination of $H_{0}$, free from the arbitrary normalizations that often affect indirect measurements. We conducted a dedicated monitoring campaign of the Seyfert galaxy Fairall 9 and further developed the {\tt H0RIZON-AGN} model to interpret the resulting observations. The model incorporates the effects of radiation reprocessing in the surrounding cold accretion disk, enabling a more realistic description of the observed continuum delays. Through the simultaneous modeling of the continuum lag-spectrum and the broadband SED of Fairall 9, we derived a Hubble constant of $H_{0}=72.4_{-3.7}^{+3.4} \, \rm km \, s^{-1} \, Mpc^{-1}$. Achieving a measurement precision of approximately 5% from a single source demonstrates the considerable potential of this method for independent determinations of the Hubble constant. Our determination of $H_{0}$ is broadly consistent, within the current uncertainties, with both early- and late-Universe measurements. Future applications of the method to larger datasets, particularly those provided by the Vera Rubin Observatory, are expected to reduce the uncertainty to below 1%, thereby establishing this approach as a powerful independent probe of the Hubble tension.

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arXiv2026

Consistency between X-ray and UV-Optical reverberation measurements in NGC 5548

The hard X-ray$-$emitting hot corona is a key component of active galaxies. Constraints on the hot corona height can be derived from reverberation studies in both the X-ray and optical bands. X-ray reverberation (X-ray$-$RM) studies often imply a very low corona height, whereas UV/optical reverberation mapping (photometrcic continuum$-$RM) typically points to a much larger one. To reconcile this discrepancy, we examine the constraints provided by both methods for the same source. We adopt a uniform methodology using the {\tt KYNSED} and {\tt KYNXiltr} codes within a consistent modeling framework for reverberation mapping, applicable across both the X-ray and UV-optical spectral and time domains. We select the source NGC 5548, for which the necessary observational data are available in the literature. We carry out our analysis for NGC 5548, a source with extensive reverberation mapping data obtained independently in the X-ray and UV-optical bands across different epochs. Our results hint for a substantial discrepancy between the global parameters required to reproduce the X-ray and those needed to fit the UV-optical reverberation signals. In particular, the mismatch in the inferred black hole mass and accretion rate presents a significant challenge for interpreting the observed time delays within a unified reflection-based framework. Our unified reflection-based modeling sheds light on X-ray and UV-optical variability of NGC 5548, but discrepancies in black hole mass, accretion rate, and corona properties might imply fundamental challenges to a self-consistent model. However, future analyses leveraging extended X-ray dataset with improved treatment of absorption and variability coherence are crucial to obtaining more robust constraints.

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arXiv2026

Ultraviolet variability in Radio-Loud Active Galactic Nuclei observed by UVIT onboard AstroSat

Radio-loud active galactic nuclei (AGN) are among the most luminous objects in the Universe, emitting radiation from low-energy radio waves to high energy $\gamma$-rays. They are well known to exhibit flux variations at nearly all accessible wavelengths. However, their variability properties in the ultraviolet (UV) band remain relatively less explored compared to other wavebands. Here, we present the results of a systematic investigation of the UV flux and spectral variability characteristics of 24 radio-loud AGN spanning the redshift range 0.018 $\le$ $z$ $\le$ 2.218. The sample comprises 17 BL Lac objects, 6 flat spectrum radio quasars (FSRQs) and one radio-loud narrow line Seyfert 1 galaxy. We used observations obtained with the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat during its first ten years of operation, covering both the far-UV (FUV; 1300$-$1800 \AA) and near-UV (NUV; 2000$-$3000 \AA) bands. Of the 24 sources analysed, 18 showed significant UV variability on hour timescales. We found a bluer when brighter (BWB) spectral trend in two sources: the FSRQ CTA 102 and the BL Lac PKS 0447$-$439. The observed UV variability in our sample of radio-loud AGN, together with the BWB trend detected in these two sources, supports a scenario in which the hour timescale UV variations are driven by intrinsic processes within their relativistic jets.

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arXiv2026

Detection of time delay between UV and X-ray variability in Mrk 1044 using AstroSat observations

Active galactic nuclei are known to exhibit flux variations across the entire electromagnetic spectrum. Among these, correlations between UV/optical and X-ray flux variations serve as a key diagnostics for understanding the physical connection between the accretion disk and the corona. In this work, we present the results of analysis of ultraviolet (UV) and X-ray flux variations in the narrow line Seyfert 1 galaxy Mrk 1044. Simultaneous observations in the far-UV band (FUV: 1300$-$1800 \AA) and the X-ray band (0.5$-$7 keV) obtained during 31 August $-$ 8 September 2018 with the Ultraviolet Imaging Telescope and the Soft X-ray Telescope onboard \textit{AstroSat} were used for this study. Significant flux variability was detected in both FUV and X-ray bands. The fractional root mean square variability amplitude ($F_{\rm var}$) was found to be 0.036 $\pm$ 0.001 in the FUV band and 0.384 $\pm$ 0.004 in the X-ray band. To explore potential time lag between the two bands, cross-correlation analysis was performed using both the interpolated cross-correlation function (ICCF) and just another vehicle for estimating lags in nuclei (JAVELIN) methods. Results from both approaches are consistent within 2$\sigma$ uncertainty, indicating that X-ray variations lead the FUV variations, with measured lags of 2.25$\pm$0.05 days (ICCF) and $2.35_{-0.01}^{+0.02}$ days (JAVELIN). This is the first detection of a time delay between UV and X-ray variations in Mrk 1044. The observed UV lag supports the disk reprocessing scenario, wherein X-ray emission from the corona irradiates the accretion disk, driving the observed UV variability.

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arXiv2025

Standardizing reverberation-mapped H$\beta$ active galactic nuclei using time-averaged radius$-$luminosity relations with 5100\,\AA\,, broad H$\beta$, and narrow \OIII\ luminosities

Active galactic nuclei (AGN) have been studied as alternate probes in cosmology due to their large and stable luminosities and broad redshift range. Previously it was shown that higher-redshift AGN that were reverberation-mapped (RM) using broad Mg\,\textsc{ii} and C\,\textsc{iv} lines are standardizable and yield weak cosmological constraints that are consistent with those from better-established probes. In contrast, lower-redshift AGN that were reverberation-mapped using the broad H$\beta$ line exhibited tensions with the standard cosmological model, in particular they preferred currently decelerating cosmological expansion. Here we study the standardizability of a homogeneous RM H$\beta$ sample of $\sim 100$ AGN (over redshifts $0.00308 \leq z \leq 0.8429$), whose H$\beta$ time delays and three luminosity tracers (at 5100\,\AA\,, broad H$\beta$, and narrow [O\,\textsc{iii}]) are averaged over several epochs. We find that this averaged sample is standardizable using three $R-L$ relations. While for luminosities corresponding to 5100\,\AA\, and the broad H$\beta$ line the cosmological constraints prefer currently decelerating cosmological expansion, the cosmological parameters for the narrow [O\,\textsc{iii}] luminosity are more consistent with those from better-established probes and they are in agreement with currently accelerating cosmological expansion. This demonstrates for the first time that narrow-line region [O\,\textsc{iii}] can be utilized for AGN standardization and cosmological constraints. Selecting proper photoionizing flux proxies for the broad-line region is thus crucial in studies of RM AGN standardizability.

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arXiv2025

HALO I: Photometric continuum reverberation mapping of Fairall 9

We investigate the origin of inter-band continuum time delays in active galactic nuclei (AGNs) to study the structure and properties of their accretion disks. We aim to measure the inter-band continuum time delays through photometric monitoring of Seyfert galaxy Fairall 9 to construct the lag-spectrum. Additionally, we explain the observed features in the Fairall 9 lag-spectrum and discuss the potential drivers behind them, based on our newly collected data from the Obserwatorium Cerro Murphy (OCM) telescope. We initiated a long-term, continuous AGN photometric monitoring program in 2024, titled 'Hubble constant constraints through AGN Light curve Observations' (HALO) using intermediate and broad band filters. Here, we present the first results from HALO, focusing on photometric light curves and continuum time-delay measurements for Fairall 9. To complement these observations and extend the wavelength coverage of the lag-spectrum, we also reanalyzed archival Swift light curves and spectroscopic data available in the literature. Using HALO and Swift light curves, we measured inter-band continuum delays to construct the lag-spectrum of Fairall 9. Excess lags appear in the $u$ and $U$ bands (Balmer continuum contamination) and in the $I$ band (Paschen jump/dust emission from the torus). Overall, the lag-spectrum deviates significantly from standard disk model predictions. We find that inter-band delays deviate from the power-law, $\tau_{\lambda} \propto \lambda^{\beta}$ due to BLR scattering, reprocessing, and dust contributions at longer wavelengths. Power-law fits are therefore not well suited for characterizing the nature of the time delays.

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arXiv2025

Spectroscopic Reverberation Mapping for SARM: The Case of Mrk 1048 and Mrk 618

Robust extragalactic distance measurements are crucial for resolving the persistent discrepancy in the value of the Hubble constant (H$_0$)). Active Galactic Nuclei (AGNs), through their compact and variable broad-line regions (BLRs), enable the determination of geometric distances when reverberation mapping (RM) is combined with spectroastrometry(SA). We report results from a spectroscopic RM campaign (October 2022 to March 2023) targeting two narrow-line Seyfert 1 galaxies, Mrk 1048 and Mrk 618, using 3.6-m DOT and 2-m HCT. High-cadence spectro-photometric monitoring was carried out using onboard instruments such as ADFOSC, HFOSC, and TANSPEC, resulting in well-sampled continuum and emission line light curves. The observed fractional variability ($F_{\mathrm{var}}$) ranged from 4% to 14% across the $g$-band, H$\beta$, and H$\alpha$ light curves. The time lags were measured using the interpolated cross-correlation function (ICCF), PyI$^{2}$CCF, and \textsc{JAVELIN} methods. In the rest frame, the ICCF analysis yields H$\beta$ lags of $10.5^{+2.6}_{-4.2}$ days for Mrk 1048 and $10.2^{+3.4}_{-2.9}$ days for Mrk 618, while the corresponding H$\alpha$ lags are $18.7^{+5.3}_{-5.4}$ and $14.4^{+4.6}_{-10.5}$ days, respectively. The emission-line widths, measured from the rms spectra using $\sigma_{\mathrm{line}}$, give virial black hole mass estimates of $6.3^{+2.0}_{-2.1} \times 10^7\,M_\odot$ for Mrk 1048 and $1.2^{+0.4}_{-0.6} \times 10^7\,M_\odot$ for Mrk 618. These results will serve as a basis for absolute geometric distance calibration when combined with VLTI/GRAVITY spectro-astrometric measurements, thereby contributing to the development of AGNs as standardizable cosmological probes.

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arXiv2025

Probing dust torus radius--luminosity relation: An WISE view

We present measurements of the dusty torus sizes of 51 active galactic nuclei (AGNs) with a redshift of $z<$ 0.8. Our analysis utilizes about 16 years of optical photometric data of 146 AGNs from various time-domain surveys, including ASAS-SN, CRTS, and ZTF, along with 14 years of infrared data in the $W$1 ($\sim$ 3.4 $\mu$m) and $W$2 ($\sim$ 4.6 $\mu$m) bands obtained from the Wide-Field Infrared Survey Explorer (WISE). The estimated dust torus size ranges from 1000 to 3000 days, using both the cross-correlation analysis and lightcurve modeling through `MICA'. The measured lag has been corrected by $(1+z)^{-0.37}$, to account for cosmological time dilation and the torus temperature-gradient scaling. We conduct a linear regression analysis for both the $W$1 and $W$2 bands to examine the radius--luminosity ($R$--$L_{BOL}$) relationship under two conditions: one where the slope is fixed at 0.5 and one where it is allowed to vary. For the fixed slope of 0.5, we find the ratio of R$_{\mathrm{BLR}}$: R$_{W1}$: R$_{W2}$ to be 1: 9: 12, indicating that the torus lies outside the BLR and that its size increases with wavelength. Furthermore, we determine the relationship between torus size and L$_{BOL}$, yielding best-fit slopes of $0.413\pm0.047$ for the $W$1 band and $0.397\pm0.058$ for the $W$2 band. Both slopes are shallower than predicted by the dust radiation equilibrium model. Furthermore, our findings indicate that the torus size systematically decreases as the Eddington ratio increases, a trend that can be explained by the self-shadowing effects of slim disks.

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arXiv2025

Scaling Relations of the Dusty Torus with Luminosity and the Broad-Line Region

We measure and compare the size of the dusty torus with active galactic nucleus (AGN) luminosity and the size of the broad-line region (BLR), using a sample of 182 AGNs with the best H$\beta$ lag measurements. After correcting for accretion-disk contamination, torus sizes are determined from the time lags of the Wide-field Infrared Survey Explorer W1 and W2 band light curves relative to the optical band variability based on the interpolated cross-correlation function (ICCF) analysis and the Multiple and Inhomogeneous Component Analysis. We find that the torus size from the W1-band (W2-band) tightly correlates with the 5100~\AA\ continuum luminosity with an intrinsic scatter of 0.15-0.16 dex and the best-fit slope of $0.35 \pm 0.03$ ($0.33 \pm 0.03$), which is clearly shallower than the expected 0.5 slope from the sublimation radius-luminosity relation. We find a moderate negative trend that higher Eddington AGNs tend to have smaller torus sizes than expected from the best-fit, suggesting the Eddington ratio plays a role in flattening the torus size-luminosity relation. By comparing the torus size with the H$\beta$ reverberation time lag for a subsample of 67 AGNs, we find that the torus size is a factor of $\sim 10$ and $\sim 14$ larger than the BLR size, respectively for W1 and W2 bands. The torus size based on the W1 (W2) band correlates with the BLR size with the best-fit slope of $1.28 \pm 0.16$ ($1.10 \pm 0.15$), which is comparable but slightly steeper than a linear correlation.

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arXiv2025

Standardizing a larger, higher-quality, homogeneous sample of reverberation-mapped H$\beta$ active galactic nuclei using the broad-line region radius-luminosity relation

We present a high-quality, homogeneous sample of 157 H$\beta$ reverberation-mapped active galactic nuclei (RM AGNs) spanning redshifts $0.00308 \leq z \leq 0.8429$, which is approximately 3.8 times larger than the previously available high-quality homogeneous sample. Using the broad-line region radius$-$luminosity relation ($R-L$), which involves the broad H$\beta$ line time delay and the monochromatic luminosity at 5100\,\AA\,, we show that the sample is standardizable by using six spatially flat and nonflat cosmological models. The inferred cosmological model parameters are consistent within 2$\sigma$ uncertainties with those from better established baryon acoustic oscillation and Hubble parameter measurements, with the exception of two nonflat models that are ruled out by other data. The $R-L$ relation slope is found to be flatter ($\gamma=0.428 \pm 0.025$ in the flat $\Lambda$CDM model) than the slope expected from a simple photoionization model as well as the slope found previously for the smaller homogeneous sample. In addition, we find a mild dependence of H$\beta$ $R-L$ relation parameters as well as its intrinsic scatter on the Eddington ratio by comparing the $R-L$ relations for low- and high-accreting equal-sized subsamples. A future analysis of a larger homogeneous sample containing a broader range of luminosities and Eddington ratios is necessary to confirm the standardizability of H$\beta$ AGNs.

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arXiv2025

Seoul National University AGN Monitoring Project. V. Velocity-resolved H-beta Reverberation Mapping and Evidence of Kinematics Evolution

We present velocity-resolved reverberation lags of H-beta for 20 active galactic nuclei (AGNs) from the Seoul National University AGN Monitoring Project. We detect unambiguous velocity-resolved structures in 12 AGNs, among which eight objects exhibit symmetric structures, two objects show inflow-like characteristics, and two objects display outflow-like signatures. For two AGNs, we successfully measure the velocity-resolved lags in different years, revealing evidence of evolving broad-line region (BLR) kinematics. By combining our sample with the literature velocity-resolved lags, we find that the symmetric velocity-resolved lags are the most common (40%) type among this sample. The frequency of inflow kinematics is also notable (20%), while outflow kinematics are less common (11%). Our sample significantly expands the previous velocity-resolved reverberation mapping sample in the high-luminosity regime, enabling us to constrain BLR kinematics across a large dynamic range of luminosity.

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arXiv2025

The size of the continuum emission region and its scaling relations with active galactic nucleus luminosity and the broad-line region size

We present a continuum lag analysis for a sample of 37 relatively high-luminosity active galactic nuclei (AGNs) from the Seoul National University AGN Monitoring Project (SAMP), utilizing the light curve data in $B$ and $V$ bands from SAMP and in $g,r,i$ bands from the Zwicky Transient Facility. We find that the inter-band lags ($\tau$) increase with wavelength (i.e., $\tau \propto \lambda^{\sim 4/3}$) as prescribed by the standard disk model (SSD), suggesting consistency with the "lamp-post" reprocessing model. We report that the size of the continuum emitting region (CER) normalized at 2500 {\AA} ($R_{2500}$) is a factor of $\sim$5 (i.e, $0.69\pm0.04$ dex) larger than predicted by SSD. By combining our new measurements with the re-measurements of the literature sample, we report a correlation between $R_{2500}$ and AGN continuum luminosity as $R_{2500} \, \propto \, L_{5100}^{0.58\pm0.03}$, which suggests that the observed continuum could be composed of both the disk emission and the diffuse emission from the broad line region (BLR). The size of CER shows a tight relation with the size of H$\beta$ BLR with a sublinear slope (i.e., $R_{\text{BLR}} \, \propto \, R_{\text{2500}}^{0.87\pm0.07}$) and a scatter of 0.29 dex. This empirical relation offers a promising method for estimating single-epoch black hole masses, once established over a large dynamic range of AGN luminosity.

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arXiv2024

Standardizing reverberation-mapped H$\alpha$ and H$\beta$ active galactic nuclei using radius--luminosity relations involving monochromatic and broad H$\alpha$ luminosities

We test the standardizability of a homogeneous sample of 41 lower-redshift ($0.00415\leq z \leq 0.474$) active galactic nuclei (AGNs) reverberation-mapped (RM) using the broad H$\alpha$ and H$\beta$ emission lines. We find that these sources can be standardized using four radius$-$luminosity ($R-L$) relations incorporating H$\alpha$ and H$\beta$ time delays and monochromatic and broad H$\alpha$ luminosities. Although the $R-L$ relation parameters are well constrained and independent of the six cosmological models considered, the resulting cosmological constraints are weak. The measured $R-L$ relations exhibit slightly steeper slopes than predicted by a simple photoionization model and steeper than those from previous higher-redshift H$\beta$ analyses based on larger datasets. These differences likely reflect the absence of high-accreting sources in our smaller, lower-redshift sample, which primarily comprises lower-accreting AGNs. The inferred cosmological parameters are consistent within 2$\sigma$ (or better) with those from better-established cosmological probes. This contrasts with our earlier findings using a larger, heterogeneous sample of 118 H$\beta$ AGNs, which yielded cosmological constraints differing by $\gtrsim 2\sigma$ from better-established cosmological probes. Our analysis demonstrates that sample homogeneity$-$specifically, the use of a consistent time-lag determination method$-$is crucial for developing RM AGNs as a cosmological probe.

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arXiv2024

Simultaneous X-ray and optical polarization observations of the blazar Mrk 421

We present near-simultaneous X-ray and optical polarization measurements in the high synchrotron peaked (HSP) blazar Mrk 421. The X-ray polarimetric observations were carried out using {\it Imaging X-ray Polarimetry Explorer} ({\it IXPE}) on 06 December 2023. During {\it IXPE} observations, we also carried out optical polarimetric observations using 104cm Sampurnanand telescope at Nainital and multi-band optical imaging observations using 2m Himalayan Chandra Telescope at Hanle. From model-independent analysis of {\it IXPE} data, we detected X-ray polarization with degree of polarization ($\Pi_X$) of 8.5$\pm$0.5\% and an electric vector position angle ($\Psi_X$) of 10.6$\pm$1.7 degrees in the 2$-$8 keV band. From optical polarimetry on 06 December 2023, in B, V, and R bands, we found values of $\Pi_B$ = 4.27$\pm$0.32\%, $\Pi_V$= 3.57$\pm$0.31\%, and $\Pi_R$= 3.13$\pm$0.25\%. The value of $\Pi_B$ is greater than that observed at longer optical wavelengths, with the degree of polarization suggesting an energy-dependent trend, gradually decreasing from higher to lower energies. This is consistent with that seen in other HSP blazars and favour a stratified emission region encompassing a shock front. The emission happening in the vicinity of the shock front will be more polarized due to the ordered magnetic field resulting from shock compression. The X-ray emission, involving high-energy electrons, originates closer to the shock front than the optical emission. The difference in the spatial extension could plausibly account for the observed variation in polarization between X-ray and optical wavelengths. This hypothesis is further supported by the broadband spectral energy distribution modeling of the X-ray and optical data.

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arXiv2024

Spectrophotometric reverberation mapping of Intermediate-mass black hole NGC 4395

Understanding the origins of massive black hole seeds and their co-evolution with their host galaxy requires studying intermediate-mass black holes (IMBHs) and estimating their mass. However, measuring the mass of these IMBHs is challenging due to the high spatial resolution requirement. A spectrophotometric reverberation monitoring is performed for a low-luminosity Seyfert 1 galaxy NGC 4395 to measure the size of the broad line region (BLR) and black hole mass. The data were collected using the 1.3-m Devasthal fast optical telescope (DFOT) and 3.6-m Devasthal optical telescope (DOT) at ARIES, Nainital, over two consecutive days in March 2022. The analysis revealed strong emission lines in the spectra and light curves of merged 5100{\AA} spectroscopic continuum flux ($f_{\mathrm{5100}}$) with photometric continuum V-band and H$\alpha$, with fractional variabilities of 6.38\% and 6.31\% respectively. In comparison to several previous studies with lag estimation $<$ 90 minutes, our calculated H$\alpha$ lag supersedes by $125.0^{+6.2}_{-6.1}$ minutes using ICCF and {\small JAVELIN} methods. The velocity dispersion ($\sigma_{\mathrm{line}}$) of the broad line clouds is measured to be $544.7^{+22.4}_{-25.1}$ km s$^{-1}$, yielding a black hole mass of $\sim$ $2.2^{+0.2}_{-0.2}\times 10^{4}M_{\mathrm{\odot}}$ and an Eddington ratio of 0.06.

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arXiv2024

Ultraviolet flux and spectral variability study of blazars observed with UVIT/AstroSat

Blazars, the peculiar class of active galactic nuclei (AGN), are known to show flux variations across the accessible electromagnetic spectrum. Though they have been studied extensively for their flux variability characteristics across wavelengths, information on their ultraviolet (UV) flux variations on time scales of hours is very limited. Here, we present the first UV flux variability study on intraday time scales of a sample of 10 blazars comprising 2 flat spectrum radio quasars (FSRQs) and 8 BL Lacertae objects (BL Lacs). These objects, spanning a redshift (z) range of 0.034 <= z <= 1.003, were observed in the far-UV (FUV: 1300 - 1800 \textÅ) and near-UV (NUV: 2000 - 3000 \textÅ) wavebands using the ultraviolet imaging telescope on board AstroSat. UV flux variations on time scales of hours were detected in 9 sources out of the observed 10 blazars. The spectral variability analysis showed a bluer-when-brighter trend with no difference in the UV spectral variability behavior between the studied sample of FSRQs and BL Lacs. The observed UV flux and spectral variability in our sample of both FSRQs and BL Lacs revealed that the observed UV emission in them is dominated by jet synchrotron process.

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arXiv2024

Revisiting the dust torus size-luminosity relation based on a uniform reverberation mapping analysis

We investigate the torus size -- luminosity relation of Type 1 AGNs based on the reverberation-mapping analysis using the light curves of the optical continuum and the IR continuum obtained with the W1 and W2-bands of the Wide-field Infrared Survey Explorer (WISE) survey. The final sample consists of 446 and 416 AGNs, respectively, for W1 and W2-band light curves, covering a large dynamic range of bolometric luminosity from $10^{43.4}$ to $10^{47.6}$ $erg \, s^{-1}$, which show reliable lag measurements based on our quality assessment analysis. After correcting for the accretion disk contamination in the observed IR flux, we constrain the torus size ($R_{dust}$) and AGN bolometric luminosity ($L_{bol}$) relationship with the best-fit slope of 0.39 (0.33) for the W1- (W2-) band, which is shallower than expected from the dust radiation equilibrium model. By combining the previous K-band lag measurements, we find that the measured torus size depends on the observed wavelength of the dust radiation, as $R_{dust,K}:R_{dust,W1}:R_{dust,W2}$ = 1.0:1.5:1.8 ($R_{dust} \, \propto \, λ^{0.80}$) at $L_{bol}$ = $10^{46} \, erg \, s^{-1}$, confirming a stratified structure of the torus, where wavelength-dependent emissions originate from distinct regions of the torus. By investigating the deviation from the best-fit torus size -- luminosity relation, we find a moderate correlation between the offset from the $R_{dust}$--$L_{bol}$ relation and Eddington ratio. This suggests a possible influence of the Eddington ratio on the observed flattening of the $R_{dust}$--$L_{bol}$ relationship.

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