arXiv subjects

Shubham Kishore

Publications and source records attributed to Shubham Kishore.

15 recordsLinked to original sources

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

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

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 $2.53\sigma$ 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

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 $\mu$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, $\delta\sim5$, 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

Multi-band optical variability of the blazar OP 313 in the outburst state during 2024-2025

We present the analysis results of flux and spectral variability of the blazar OP 313 across intra-night to short-term timescales using BVRI photometric data, gathered over 25 nights from Nov 2024 to May 2025, using two optical telescopes in ARIES, India. The source was in an outburst state during this period. We searched for intraday variations (IDV), using two powerful statistical tests: the Power Enhanced F-test and the Nested ANOVA test. The source displayed IDV in the R band for five of the ten nights, yielding a duty cycle of 34$\%$. During the entire monitoring of the source, it showed variations of over two mag in all B, V, R, and I data bands. We obtained a variability timescale for a variable light curve, giving us an upper limit for the size of the emission region. We generated optical SEDs of the blazar for these 25 nights, fitted a power law of form $(F_\nu \propto \nu^{-\alpha_{o}})$ and found the weighted mean spectral index to be 1.471$\pm$0.004. An analysis of the color-magnitude diagram shows that, contrary to the redder-when-brighter (RWB) trend typically observed in FSRQs, this source exhibits a bluer-when-brighter (BWB) trend on short-term variability (STV) timescales - a behavior more commonly associated with BL Lac object. We explore potential physical mechanisms responsible for the observed spectral variability.

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arXiv2024

Optical Continuum Reverberation Mapping of a Candidate IMBH in a Nearby Seyfert 1 Galaxy

To investigate the short-term variability and determine the size of the optical continuum emitting region of intermediate-mass black holes (IMBHs), we carried out high-cadence, multiband photometric monitoring of a Seyfert 1 galaxy J0249-0815 across two nights, together with a one-night single-band preliminary test. The presence of the broad Ha component in our target was confirmed by recent Paloma spectroscopic observations, 23 years after the Sloan Digital Sky Survey, ruling out the supernovae origin of the broad Ha line. The photometric experiment was primarily conducted utilizing four-channel imagers MuSCAT 3 and 4 mounted on 2 m telescopes within the Las Cumbres Observatory Global Telescope Network. Despite the expectation of variability, we observed no significant variation (<1.4%) on timescales of 6 to 10 hr. This nondetection is likely due to substantial host galaxy light diluting the subtle active galactic nucleus (AGN) variability. Additionally, we cannot rule out that the target was in a relatively quiescent state without intranight variability during our monitoring, owing to the stochastic nature of AGN variations. To enhance the possibility of detecting subtle variability signals and lag in future IMBH reverberation campaigns, it may be beneficial to select targets with a higher AGN-to-host flux ratio, and conduct dual-band preliminary tests and tailored simulations.

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arXiv2024

Evidence of jet activity from the secondary black hole in the OJ287 binary system

We report the study of a huge optical intraday flare on November 12, 2021, at 2 am UT, in the blazar OJ287. In the binary black hole model it is associated with an impact of the secondary black hole on the accretion disk of the primary. Our multifrequency observing campaign was set up to search for such a signature of the impact, based on a prediction made eight years earlier. The first I-band results of the flare have already been reported by \cite{2024ApJ...960...11K}. Here we combine these data with our monitoring in the R-band. There is a big change in the R-I spectral index by $1.0\pm0.1$ between the normal background and the flare, suggesting a new component of radiation. The polarization variation during the rise of the flare suggests the same. The limits on the source size place it most reasonably in the jet of the secondary black hole. We then ask why we have not seen this phenomenon before. We show that OJ287 was never before observed with sufficient sensitivity on the night when the flare should have happened according to the binary model. We also study the probability that this flare is just an oversized example of intraday variability, using the Krakow-dataset of intense monitoring between 2015 and 2023. We find that the occurrence of a flare of this size and rapidity is unlikely. In the Appendix, we give the full orbit-linked historical light curve of OJ287 as well as the dense monitoring sample of Krakow.

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arXiv2024

Rapid Optical Flare in the Extreme TeV Blazar 1ES 0229+200 on Intraday Timescale with TESS

The extreme TeV blazar 1ES 0229+200 is a high-frequency-peaked BL Lacertae object. It has not shown intraday variability in extensive optical and X-ray observations. Nor has it shown any significant variability on any measurable timescale in the 1-100 GeV energy range over a 14-year span, but variations in the source flux around its average are present in the energy range above 200 GeV. We searched for intraday optical variability in 1ES 0229+200 as part of an ongoing project to search for variability and quasi-periodic oscillations in the high-cadence, nearly uniformly sampled optical light curves of blazars provided by the Transiting Exoplanet Survey Satellite (TESS). 1ES 0229+200 was monitored by TESS in its Sectors 42, 43 and 44. We analysed the data of all these sectors both with the TESS provided lightkurve software and the eleanor reduction pipeline. We detected a strong, essentially symmetric flare that lasted ~6 hours in Sector 42. We fit the flare's rising and declining phases to exponential functions. We also analysed the light curve of Sector 42 using the Lomb-Scargle periodogram (LSP) and continuous auto-regressive moving average (CARMA) methods. The Sector 42 light curve displayed in the present work provides the first evidence of a strong, rapid, short-lived optical flare on the intraday timescale in 1ES 0229+200. The variability timescale of the flare provides the upper limit for the size of the emission region to be within (3.3\pm0.2 - 8.3\pm0.5)x10^{15}cm. Away from the flare, the slope of the periodogram's power spectrum is fairly typical of many blazars (α<2), but the nominal slopes for the flaring regions are very steep (α~4.3), which may indicate the electron distribution undergoes a sudden change. We discuss possible emission mechanisms that could explain this substantial and rapid flare.

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arXiv2023

Optical intra-day variability of the blazar S5 0716+714

We present an extensive recent multi-band optical photometric observations of the blazar S5 0716+714 carried out over 53 nights with two telescopes in India, two in Bulgaria, one in Serbia, and one in Egypt during 2019 November -- 2022 December. We collected 1401, 689, 14726, and 165 photometric image frames in B, V, R, and I bands, respectively. We montiored the blazar quasi-simultaneously during 3 nights in B, V, R, and I bands; 4 nights in B, V, and R; 2 nights in V, R, and I; 5 nights in B and R; and 2 nights in V and R bands. We also took 37 nights of data only in R band. Single band data are used to study intraday flux variability and two or more bands quasi-simultaneous observations allow us to search for colour variation in the source. We employ the power-enhanced F-test and the nested ANOVA test to search for genuine flux and color variations in the light curves of the blazar on intraday timescales. Out of 12, 11, 53, and 5 nights observations, intraday variations with amplitudes between ~3% and ~20% are detected in 9, 8, 31 and 3 nights in B, V, R, and I bands, respectively, corresponding to duty cycles of 75%, 73%, 58% and 60%. These duty cycles are lower than those typically measured at earlier times. On these timescales color variations with both bluer-when-brighter and redder-when-brighter are seen, though nights with no measurable colour variation are also present. We briefly discuss possible explanations for this observed intraday variability.

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arXiv2023

Rapid Optical Flares in the Blazar OJ 287 on Intraday Timescales with TESS

We have analyzed the optical light curves of the blazar OJ 287 obtained with the Transiting Exoplanet Survey Satellite (TESS) over about 80 days from 2021 October 13 to December 31, with an unprecedented sampling of 2 minutes. Although significant variability has been found during the entire period, we have detected two exceptional flares with flux nearly doubling and then nearly tripling over 2 days in the middle of 2021 November. We went through the light curves analysis using the excess variance, generalized Lomb-Scargle periodogram, and Continuous Auto-Regressive Moving Average (CARMA) methods, and estimated the flux halving/doubling timescales. The most probable shortest variability timescale was found to be 0.38 days in the rising phase of the first flare. We briefly discuss some emission models for the variability in radio-loud active galactic nuclei that could be capable of producing such fast flares.

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arXiv2023

Intra-night optical flux and polarization variability of BL~Lacertae during its 2020 $-$ 2021 high state

In this work, we report the presence of rapid intra-night optical variations in both -- flux and polarization of the blazar BL Lacertae during its unprecedented 2020--2021 high state of brightness. The object showed significant flux variability and some color changes, but no firmly detectable time delays between the optical bands. The linear polarization was also highly variable in both -- polarization degree and angle (EVPA). The object was observed from several observatories throughout the world, covering in a total of almost 300 hours during 66 nights. Based on our results, we suggest, that the changing Doppler factor of an ensemble of independent emitting regions, travelling along a curved jet that at some point happens to be closely aligned with the line of sight can successfully reproduce our observations during this outburst. This is one of the most extensive variability studies of the optical polarization of a blazar on intra-night timescales.

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arXiv2022

Detection of Quasi-Periodic Oscillations in the Blazar S4 0954+658 with TESS

We report the detection of several quasi-periodicities around 0.6--2.5 days in the optical emission of the blazar S4 0954+658. The source was observed by the Transiting Exoplanet Survey Satellite (TESS) in six sectors and it showed these features in all but one of them, with a QPO of 1.52 days apparently present in portions of four of them. We used the generalized Lomb-Scargle periodogram method to search for significant signals and we confirmed them using a weighted wavelet transform for time-frequency domain analyses. We discuss several possible explanations for these rapid quasi-periodic variations and suggest that an origin in the innermost part of the accretion disk is most likely. Within this framework, we provide estimates for the mass of the black hole at the core of this blazar.

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