arXiv subjects

Alok C. Gupta

Publications and source records attributed to Alok C. Gupta.

At least 19 recordsLinked to original sources

arXiv2026

The October 2022 flare in OJ 287 and the mass of its primary black hole

The bright blazar OJ~287 has demonstrated a sequence of flares, which are well explained by a quasi-Keplerian orbit model. The flares are associated with the impact of the secondary on the accretion disk of the primary. The orbit must precess in order to produce the correct sequence of flares, and from the precession rate we calculate the mass of the primary. This precession rate gives the mass of the primary $M_{BH} = (18.35\pm0.05) \times 10^9 M_{\odot}$. Two kinds of flares have been identified: direct flares from the impacts, and tidal flares arising from an increased accretion flow into the jet. The precession rate and the primary black hole mass may be independently determined from both sets of flares; the tidal flare of October 2022 was recommended for an intense campaign for this reason. This paper describes these observations over a wide spectral range. We show that the October 2022 flare fits the expectations for a tidal flare and thus supports the earlier determination of the mass of the binary black hole system in OJ 287. The mass of the primary may also be deduced from secondary indicators such as the correlation with the hydrogen line strength and the black hole mass. These studies require that the mass is above $M_{BH} \sim 10^{10} M_{\odot}$, but do not specify the value more exactly.

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arXiv2026

The Intermediate-Mass Black Hole Reverberation Mapping Project: Scientific Overview and Sample Characteristics

Recent discoveries with the James Webb Space Telescope of massive black holes at high redshift have highlighted fundamental questions about black hole seed formation and the coevolution of black holes with their host galaxies. Because the initial seed population cannot yet be observed directly, nearby intermediate-mass black holes provide a complementary fossil record of black hole formation and early growth. Motivated by this opportunity, we present the Intermediate-Mass Black Hole Reverberation Mapping (IMBH-RM) project and construct a homogeneous Sloan Digital Sky Survey sample of active broad-line IMBHs by uniformly reanalyzing literature candidates with consistent spectral decomposition and black hole mass estimation. Our sample contains 192 reliable IMBH candidates at $z\lesssim0.3$ with $\log(M_{\rm BH}/M_\odot)<6$, including four particularly compelling sources with $\log(M_{\rm BH}/M_\odot)<5$. The primary goal of IMBH-RM is to obtain reliable black hole masses from direct measurements and characteristic sizes of the broad-line region and accretion disk for a carefully selected subsample. These measurements will provide robust low-mass anchors for calibrating single-epoch black hole mass estimates and extending black hole--galaxy scaling relations into the IMBH regime. By building a statistically meaningful reverberation-mapped sample spanning $10^4-10^6\,M_\odot$, we aim to constrain the local IMBH mass distribution and place observational constraints on competing black hole seed formation scenarios. The future Multi-Channel Imager aboard the Chinese Space-station Survey Telescope provides a particularly promising platform for achieving these goals.

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arXiv2026

Constraints on the Intranight Optical Variability of Intermediate-Mass Black Hole Candidates

Intermediate-mass black holes (IMBHs) provide a unique regime for studying accretion variability at the low-mass end of the black hole population, yet their intranight optical variability (INOV) remains poorly constrained. We present a systematic investigation of INOV in an optically selected sample of IMBH candidates using high-cadence observations from the Zwicky Transient Facility (ZTF). From a parent sample of 1,447 broad H$\alpha$-selected candidates, we identify 64 IMBH candidates (median $f_{\mathrm{AGN}}\sim0.06$) with 163 intranight monitoring sessions. Apparent INOV signals identified by conventional ZTF PSF-fit photometry are largely associated with seeing-dependent changes in the relative contributions of compact nuclear and extended host components, which can mimic intrinsic short-timescale variability. In contrast, no robust INOV is detected with difference-image analysis. An ensemble structure function spanning $\Delta t\sim0.003$--$1600$ days reveals long-term variability in a small subsample of sources, whereas intrinsic variability remains unresolved at intranight timescales. Monte Carlo simulations further show that ZTF-like single-night monitoring has a low INOV recovery probability ($\sim1.2%$) for the variability amplitudes inferred from the long-term analysis. The recovery probability is primarily controlled by source brightness, AGN contribution, intrinsic variability amplitude, and photometric precision. These results demonstrate that the absence of detected INOV does not imply the absence of rapid accretion variability, but can reflect the limited detectability of low-amplitude signals under current observing capabilities. Our findings highlight the importance of robust photometric methodologies for future high-cadence variability studies of low-mass accreting black holes.

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arXiv2026

Revisiting neutrino event epochs for the blazar PKS 0735+178 with TESS

We present here the results of the optical light curve variability analysis of the blazars PKS 0735+178, in weeks-scale flare state, observed in three sectors with the Transiting Exoplanet Survey Satellite (TESS). The TESS observations in this study coincide with a well-known neutrino emission phase detected with four different neutrino observatories at multiple epochs in a narrow time window. We segmented the rising and decaying parts of the flare and individually analyzed their flux distribution, excess variance, variability timescale, and the power spectral density (PSD). The source displayed an elevated excess variance of ~25%, with a multi-modal flux distribution (coherent in the rising and distorted in the decaying phase). The variability timescale analysis highlights a much faster decay than the rising scale, and the PSDs depict a nominal change in the power spectral slope. We discuss a likely connection in the optical variations and the neutrino events, and briefly explain a possible physical scenario for the observed optical flux behavior in view of previously discovered radio-band results.

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arXiv2026

22 Newly Identified Repeating Changing-look AGNs and Evidence for Extreme Broad-line Region Breathing

Changing-look active galactic nuclei (CL AGNs) show the appearance or disappearance of broad emission lines on timescales of years. Among them, the repeating CL (RCL) AGNs may provide a clear clue for our understanding of the CL transitions because the same nucleus crosses some physical boundaries more than once. We search for RCL AGNs in known CL-AGN samples using long-term multi-band light curves, and selected 34 candidates for spectroscopic follow-up. We confirm 25 RCL AGNs, including 22 newly identified cases. Properties of these RCL AGNs are analyzed. The observed rest-frame intervals of the second transitions are mostly 3--4 yr, while the variations of the optical light curves suggest that some transitions might occur on timescales of several months. The latest spectra show that the on/off states correspond to higher/lower Eddington-ratio, in the expected direction relative to the parent CL-AGN samples. As seven RCL AGNs are well covered by nearly continuous single-band light curves, their on/off states can be found to follow multi-year optical excursions, and their Eddington ratios vary consistently with the photometric changes. We also find that the H$\beta$-only transitions occur at higher Eddington ratios than the transitions involving both H$\alpha$ and H$\beta$, suggesting a line-dependent Broad-Line-Region (BLR) visibility threshold. These results support a picture in which different accretion-flow processes drive reversible changes in the central ionizing emissions, while the observed RCL transitions are produced by the BLR breathing across line-dependent visibility thresholds.

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arXiv2026

Detection of Quasiperiodic Oscillations in the Blazar PKS 0735+178 with TESS

We report here the detection of signatures of a quasiperiodic oscillation (QPO) and a short flare in the optical light curve of the blazar PKS 0735+178, observed in two sectors, 71 and 72, spanning around 49 days with the Transiting Exoplanet Survey Satellite. The modest flare in sector 71 lasted ~4.3 days and appears as a combination of two sub-flares. In sector 72, a transient QPO with a period ~11.2 hours is detected at local and global significance levels of 4.11$\sigma$ and 3.06$\sigma$, respectively. We used weighted wavelet z transform, Lomb-Scargle periodogram, and phase dispersion minimization analysis techniques to look for and confirm the QPO feature. We also performed a segment-wise statistical inspection of these light curves and discuss here possible mechanisms that could explain the observed flux behavior.

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arXiv2026

Multi-band optical photometric variability of the blazar OJ 287 from 2015 to 2025

We present the most densely sampled multi-band optical photometric observations of the peculiar BL Lacertae object OJ 287 from 2015 to 2025 with a focus on its optical activity on diverse timescales. We present a total of 2296, 10927, 11484, and 2982 data points in B, V, R, and I bands, respectively. The densely sampled observations allow us to keep track of the source evolution that it has exhibited since the start of the predicted major optical flaring activity at the end of 2015. The study reveals clear and persistent bluer when brighter trends in both the long-term and short-term variations. Different bands were cross-correlated with discrete correlation functions, which peak at zero lag, implying co-spatial emission. Using eight optical spectra in the low flux states of OJ 287 taken from 2017 October 21 to 2017 November 22, from Steward Observatory, we estimate the central black hole mass to be at least 3.89 $\times \ \rm{10}^{9} \ \rm{M}_{\odot}$ from the [O III] line width. The emission mechanism of the binary black hole blazar, and its possible implication in various aspects of multi-messenger astronomy are briefly discussed.

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arXiv2026

X-ray Spectral Properties of Four Classical TeV Blazars using Simultaneous Observations from NICER and NuSTAR

We present a detailed study of X-ray spectral properties observed in 4 classical TeV (tera-electron volt) photon-emitting high synchrotron-peaked BL Lacertae objects using the simultaneous data of NICER and NuSTAR satellites. We analyzed 13 spectra in total from four BL Lacertae objects: Mrk 421, Mrk 501, PG 1553+113, and PKS 2155-304. We fitted all the spectra using the absorbed Log-Parabolic (LP) model first. While 7 spectra were fitted well using the absorbed LP model, we observed that 4 spectra of Mrk 421 and 2 spectra of Mrk 501 were not fitted satisfactorily using the absorbed LP model. The investigation of the flux states of the sources revealed that Mrk 421 was in a moderate to low-flux state during the 4 epochs and Mrk 501 was in a low-flux state during the 2 epochs. We concluded that there was a contribution from the disk in these 6 spectra. The moderate to low-flux state can justify the contribution of disk emission in the X-ray spectra. In the case of 4 spectra of Mrk 421, we observed a Gaussian feature between 1.42 and 1.70 keV.

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arXiv2026

Extreme color-magnitude variability: connection to changing-look AGNs

Context. Changing-look active galactic nuclei (CL-AGNs) challenge the unified model of AGNs and offer key insights into the physics of the accretion processes of super-massive black holes. While systematic spectroscopic comparisons have successfully identified large samples of CL-AGNs, photometric selection based on variability features provides an efficient alternative. Methods. We use the colour--magnitude (CM) variability method to continue our identification of the CL transition in AGNs, which utilizes the slope ($k$) of the CM variations to identify strong bluer-when-brighter behavior, while the variation amplitudes in optical and mid-infrared bands are also considered. The candidates thus selected from the Type-2 AGNs given in the Sloan Digital Sky Survey catalog are spectroscopically observed using the 3.6-m DOT and the 2-m HCT. Results. We confirm seven turn-on CL-AGNs among 12 candidates. Comparing them with both the general AGN populations and the spectroscopically identified CL-AGN sample, the CL-AGNs showed larger optical and MIR variations and $k$ values. The extreme CM variabilities of these sources (with optical magnitude changes $>$ 0.9) occurred recently. For four sources, flare-like brightening episodes were temporally associated with the turn-on transitions within 3--7 years, suggesting that these flares may trace short-timescale accretion enhancement, central brightening, and BLR re-illumination. Conclusions. The extreme CM variability serves as a highly efficient criterion for finding CL-AGNs. The properties of the CL-AGNs thus found suggest that they may represent AGNs at a pivotal state, which likely occur CL transitions due to enhanced accretion activity, while the cause of the accretion activity, determined to have a time scale of several years, remains to be investigated.

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arXiv2026

Detection of optical quasi-periodic oscillation in the blazar 3C 454.3

We analyzed 19 years of $R$-band data of the blazar 3C 454.3 from the Whole Earth Blazar Telescope (WEBT) archive, along with new data from its members and from public archives such as those provided by the Small and Moderate Aperture Research Telescope System (SMARTS) and the Steward Observatory projects to search for quasi-periodic oscillations (QPOs). We detected a QPO of $\sim$ 433 days using Lomb-Scargle periodogram, which lasted from MJD 54980--58450 as detected by the weighted wavelet Z-transform technique, making it one of the most persistent QPOs ever detected in the optical regime. The phase dispersion minimization technique was also performed to further validate this QPO claim. We detected this signal at a global significance of $2.53σ$ across all methodologies. To explain the observed QPO, we have considered both models focused on the accretion disk around the super-massive black hole (SMBH), and those based purely on jet emissions. Plausible jet-based models involve a shock moving down the jet in a helical magnetic field, whereas the SMBH models could involve Lense-Thirring effect-induced jet precession or dual jets in a binary SMBH system. We introduce a novel approach to distinguish genuine QPOs from spurious signals arising from annual seasonal gaps, a common limitation of ground-based observations.

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arXiv2026

X-Ray Spectral Variability of the TeV HBL Blazar PG 1553+113 with XMM-Newton

We present an extensive X-ray spectral variability study of the TeV photon-emitting high-energy-peaked BL Lacertae object PG 1553+113, using the data from EPIC-PN camera of XMM-Newton, which observed the source during its operational period from Sep 2001 to Nov 2024. X-ray spectra in this energy range, $0.6-7.0$ keV, were fitted with absorbed Power-law (PL) and absorbed Log-Parabola (LP) models. We found with 99$\%$ confidence that 14 of them were fit well by LP models having parameters in the range $α\simeq2.13-2.80$, and $β\simeq0.04-0.18$, one spectrum favours a LP model with $β<0$, while simple PL models with $Γ\simeq2.53-2.69$ were sufficient to describe the X-ray spectra of the remaining 15. Two of these 30 observations showed strong signatures of an additional inverse Compton component, while one showed weaker indications. On fitting joint Optical Monitor and EPIC-PN data with LP models, we found synchrotron peaks in the energy range of $ν_s\simeq4.59-48.61$ eV. This indicates that the spectral evolution is probably caused by variations in particle acceleration or cooling conditions within the jet.

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arXiv2026

X-Ray Intraday Variability of the Blazar OJ 287 Observed with XMM-Newton

We present X-ray intraday variability, cross-correlated variability, and power spectrum density analysis of the binary black hole blazar candidate OJ 287. The X-ray pointed observations of the source were carried out on eight occasions by the EPIC-pn camera on board the XMM-Newton satellite from November 2005 to November 2022. These good time intervals range between 3.6 hours and 24.1 hours. Three energy bands -- 0.2-2 keV (soft), 2-10 keV (hard), and 0.2-10 keV (total) -- have been used to estimate variability. Low amplitude variations are observed in 4, 5, and 6 light curves in soft, hard, and total energy bands, respectively. Only two observation IDs has shown variation in the all energy bands. The discrete correlation function of the light curves in soft and hard energy bands peaks at zero lag, suggesting that the emission in both bands was cospatial and came from the same population of leptons. Red noise dominates the power spectral densities of variable light curves. According to our flux and spectrum investigations, both particle acceleration and synchrotron cooling mechanisms contribute significantly to the emission from this blazar.

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arXiv2026

Detection of quasi-periodic oscillations in the 37 GHz radio light curve of the blazar Ton 599 during 1990-2020

Blazars are a subclass of radio-loud active galactic nuclei (AGNs) that display strong multi-wavelength variability on diverse timescales ranging from years down to minutes. In the last 1.5 decades, there have been occasional detections of quasi-periodic oscillations in several blazars in their time series data. We search for quasi-periodic oscillations (QPOs) in the 37 GHz radio band light curve of the flat-spectrum radio quasar Ton~599 made at the RT-22 radio telescope in Simeiz, Crimea, from 1990 to 2020. To identify and quantify the QPO nature of this radio light curve of Ton 599, we used the Lomb-Scargle periodogram (LSP), REDFIT, and weighted wavelet Z-transform (WWZ) analyses. We report the detection of a likely QPO of about 2.4 years in the 37 GHz radio light curves of Ton 599. We briefly discuss possible emission models for radio-loud active galactic nuclei that could explain such QPOs with periods of a few years.

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arXiv2026

Variability Study and Searching for QPOs with day-like periods in the blazar S5 0716+714 with TESS

Using an unprecedented cadence of 30 minutes provided by the Transiting Exoplanet Survey Satellite (TESS), we have examined the optical light curves (LCs) of the blazar S5 0716+714 obtained from its Sectors 40, 47, and 53 over a period of about 75 days. This source exhibited flux variability in each of those sectors, reaching a maximum variability amplitude of 5.6%. The power spectral density (PSD) shapes were tested with a simple power law and two distinct bending power laws and were found to be better fit by bending power laws than simple power laws for all but one of the segments. To look for any periodicities in these LCs, we used weighted wavelet Z (WWZ) transform analysis and generalized Lomb-Scargle periodograms (LSPs). We identified one possible quasi-periodic oscillation (QPO) signature in a portion of sector 40 (period of ~6.5 h), having ~95% global significance. A statistical approach to assess the light curves involving continuous autoregressive moving average (CARMA) was implemented, and the light curves were found to follow more complex processes than the simplest and typical damped random walk process. We briefly discuss the statistical properties of the light curves along with the general variability features and physical processes that could cause these types of fluctuations.

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arXiv2025

Possibility of Month-scale Quasi-periodic Oscillations in the Gamma-ray Light Curve of OP 313

In this work, we report evidence suggesting the potential future detection of a month-scale quasi-periodic oscillation (QPO) in the gamma-ray light curve of OP 313. We analysed almost 16.8 years of Fermi-LAT gamma-ray data and applied the Bayesian block method to the monthly-binned light curve. We identified four high-flux states and investigated the possibility of a QPO in the fourth high-flux state (MJD 59482-60832). Using the Weighted Wavelet Z-transform (WWZ) and Lomb-Scargle Periodogram (LSP) methods, we find tentative evidence for a month-scale QPO; however, its detection significance is limited by the small number of observed cycles. With a sufficiently long data set, the QPO may be detected with higher significance in the future. We further explored possible physical origins of this potential QPO and examined several models. We found that a curved-jet model can explain the observed behaviour.

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arXiv2025

Identifying the secondary jet in the RadioAstron image of OJ~287

The 136 year long optical light curve of OJ~287 is explained by a binary black hole model where the secondary is in a 12 year orbit around the primary. Impacts of the secondary on the accretion disk of the primary generate a series of optical flares which follow a quasi-Keplerian relativistic mathematical model. The orientation of the binary in space is determined from the behavior of the primary jet. Here we ask how the jet of the secondary black hole projects onto the sky plane. Assuming that the jet is initially perpendicular to the disk, and that it is ballistic, we follow its evolution after the Lorentz transformation to the observer's frame. Since the orbital speed of the secondary is of the order of one-tenth of the speed of light, the result is a change in the jet direction by more than a radian during an orbital cycle. We match the theoretical jet line with the recent 12 $μ$as-resolution RadioAstron map of OJ~287, and determine the only free parameter of the problem, the apparent speed of the jet relative to speed of light. It turns out that the Doppler factor of the jet, $δ\sim5$, is much lower than in the primary jet. Besides following a unique shape of the jet path, the secondary jet is also distinguished by a different spectral shape than in the primary jet. The present result on the spectral shape agrees with the huge optical flare of 2021 November 12, also arising from the secondary jet.

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arXiv2025

A Disk-Originated 329-day Quasi-Periodic Oscillation in the Seyfert 1 Galaxy J1626+5120

The Seyfert 1 galaxy J1626+5120 is estimated to host a $10^8 M_{\odot}$ black hole (BH) accreting at Eddington ratio $\dot{m}_{\text{Edd}} \approx 0.043$. Its long-term multi-band light curve data show flicker-like variations, but in a well-sampled $g$-band light curve, we are able to determine a $\simeq 329$\,d quasi-periodic oscillation (QPO) at a $\sim$4.53$σ$ significance. Six optical spectra were obtained for the source, three of which were taken by us. The spectra show that the variations were mainly because of flux changes blueward of 4000\,Å. We also analyze X-ray and ultraviolet (UV) data obtained with {\it the Neil Gehrels Swift Observatory (Swift)}, which targeted the source in the past two years. X-ray and UV emissions of the source show variations correlated with optical. Time lags of four UV bands and four optical bands are determined with respect to the X-ray emission, which are consistent with a continuum reprocessing disk model. These properties point out a disk origin for the QPO, likely due to Lense-Thirring (LT) precession of the accretion flow at $\sim$20 gravitational radii of the BH. This QPO could be a key case linking sub-year long QPOs in jets, which have more cases reported, to LT precession.

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arXiv2025

AT2022sxl: A Candidate Repeating Tidal Disruption Event in Possible Association with Two High-Energy Neutrino Events

We report a candidate repeating tidal disruption event (TDE), AT2022sxl, found from large-field optical survey data. Two flares with a separation time of $\sim$7.2\,yr between the two optical peaks are observed. Related mid-infrared (MIR) flares, with delay times of $\sim$200\,day are also seen. We analyze two optical spectra of the TDE source, onenear the optical peak of the second flare from the Transient Name Server and one at the quiescent flux level after the second flare. The latter was taken by us with the 10.4-m Gran Telescopio Canarias. Comparing the features of the two spectra, we identify that the host is likely a composite galaxy at redshift 0.23 and the TDE event, probably an H+He type, mainly powered broad components in the emission lines of H$α$, H$β$, and He~I $λ$5876. More interestingly, we find that two Bronze-type neutrino events, detected by the IceCube neutrino observatory, match the TDE in position and the second flare, especially the delayed MIR flare, in time. We discuss the MIR luminosity properties of the currently reported (candidate) neutrino-emitting TDEs and suggest that luminous MIR emission is a prerequisite for neutrino production in TDEs.

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