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Papers by Arkadipta Sarkar

Research paper thumbnail of Transient quasi-periodic oscillations at γ-rays in the TeV blazar PKS 1510-089

Monthly Notices of the Royal Astronomical Society

We present periodicity search analyses on the γ-ray light curve of the TeV blazar PKS 1510-089 ob... more We present periodicity search analyses on the γ-ray light curve of the TeV blazar PKS 1510-089 observed by the Fermi Large Area Telescope. We report the detection of two transient quasi-periodic oscillations: A 3.6-d QPO during the outburst in 2009 that lasted five cycles (MJD 54906–54923); and a periodicity of 92 d spanning over 650 d from 2018 to 2020 (MJD 58200–58850), which lasted for seven cycles. We employed the Lomb–Scargle periodogram, Weighted Wavelet Z-transform, redfit, and the Monte Carlo light-curve simulation techniques to find any periodicity and the corresponding significance. The 3.6-d QPO was detected at a moderate significance of ∼3.5σ, while the detection significance of the 92-d QPO was ∼7.0σ. We explore a few physical models for such transient QPOs including a binary black hole system, precession of the jet, a non-axisymmetric instability rotating around the central black hole near the innermost stable circular orbit, the presence of quasi-equidistant magnetic ...

Research paper thumbnail of Long-term Variability and Correlation Study of the Blazar 3C 454.3 in the Radio, NIR, and Optical Wavebands

The Astrophysical Journal

We performed a long-term optical (B, V, R bands), infra-red (J and K bands) and radio band (15, 2... more We performed a long-term optical (B, V, R bands), infra-red (J and K bands) and radio band (15, 22, 37 GHz band) study on the flat spectrum radio quasar, 3C 454.3, using the data collected over a period of more than 8 years (MJD 54500-57500). The temporal variability, spectral properties and inter-waveband correlations were studied by dividing the available data into smaller segments with more regular sampling. This helped us constrain the size and the relative locations of the emission regions for different wavebands. Spectral analysis of the source revealed the interplay between the accretion disk and jet emission. The source predominantly showed a redder-when-brighter trend, though we observed a bluer-when-brighter trend at high flux levels which could be signatures of particle acceleration and radiative cooling. Significant correlations with near-zero lag were seen between various optical/infrared bands, indicating that these emission regions are co-spatial. Correlations with a time lag of about 10-100 days are seen between optical/infra-red and radio bands indicating these emissions arise from different regions. We also observe the DCF peak lag change from year to year. We try to explain these differences using a curved jet model where the different emission regions have different viewing angles resulting in a frequency dependent Doppler factor. This variable Doppler factor model explains the variability timescales and the variation in DCF peak lag between the radio and optical emissions in different segments. Lags of 6-180 days are seen between emissions in various radio bands, indicating a core-shift effect.

Research paper thumbnail of Multiwavelength study of the quiescent states of six brightest flat-spectrum radio quasars detected by Fermi-LAT

Monthly Notices of the Royal Astronomical Society

The regular monitoring of flat-spectrum radio quasars (FSRQs) in γ-rays by Fermi-LAT (Large Area ... more The regular monitoring of flat-spectrum radio quasars (FSRQs) in γ-rays by Fermi-LAT (Large Area Telescope) since past 12 yr indicated six sources who exhibited extreme γ-ray outbursts crossing daily flux of 10−5 photons cm−2 s−1. We obtained nearly simultaneous multiwavelength data of these sources in radio to γ-ray waveband from OVRO (Owens Valley Radio Observatory), Steward Observatory, SMARTS (Small and Moderate Aperture Research Telescope System), Swift-UVOT (UV/Optical Telescope), Swift-XRT (X-ray Telescope), and Fermi-LAT. The time-averaged broad-band spectral energy distributions (SEDs) of these sources in quiescent states were studied to get an idea about the underlying baseline radiation processes. We modelled the SEDs using one-zone leptonic synchrotron and inverse Compton emission scenario from broken power-law electron energy distribution inside a spherical plasma blob, relativistically moving down a conical jet. The model takes into account inverse Compton scattering o...

Research paper thumbnail of Multi-waveband quasi-periodic oscillations in the light curves of blazar CTA 102 during its 2016-2017 optical outburst

Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlyin... more Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlying emission process. This type of flux modulation hints at periodic physical processes that result in emission. CTA 102, a flat spectrum radio quasar at z=1.032, has displayed significant activity since 2016. The multi-waveband light curve of CTA 102 shows signs of quasi-periodic oscillations during the 2016-2017 flare. Our goal is to quantify the presence of any periodicity in the emitted flux during the mentioned period and to explore the causes that can give rise to it. Techniques such as the Lomb-Scargle periodogram and weighted wavelet z-transform were employed to observe the power emitted at different frequencies. To quantify the significance of the dominant period, Monte-Carlo techniques were employed considering an underlying smooth bending power-law model for the power spectrum. In addition, the light curve was modeled using an autoregressive process to analytically obtain the si...

Research paper thumbnail of Quasi-periodic Oscillation and Evidence of a Curved Jet in the Blazar 3C 454.3

We report the detection of a quasi-periodic oscillation (QPO) in light curves of the blazar 3C 45... more We report the detection of a quasi-periodic oscillation (QPO) in light curves of the blazar 3C 454.3. Periodic flux modulation was detected simultaneously in both gamma\gammagamma--ray and optical wavebands with a dominant period of sim47\sim 47sim47 days. This period has a significance of over 4.2sigma4.2\sigma4.2sigma in the \textit{Fermi} gamma\gammagamma--ray waveband and around 2.4sigma2.4\sigma2.4sigma in the optical V band. This QPO lasted for over 450 days (from MJD 56800 to 57250) resulting in over nine observed periods which is among the highest number of periods ever detected in a blazar light curve. The lower significance of the dominant period in optical wavebands is attributed to the absence of optical data for a number of QPO cycles due to the daytime transit of the source. We explore several physical models to explain the origin of this transient quasi-periodic modulation in the observed flux with a month-like period. These scenarios include a binary black hole system, a hotspot orbiting close to the innermost stable ...

Research paper thumbnail of Temporal and spectral study of PKS B1222 + 216 flares in 2014

We report on a temporal and spectral study of a flat-spectrum radio quasar, PKS B1222 + 216, in a... more We report on a temporal and spectral study of a flat-spectrum radio quasar, PKS B1222 + 216, in a flare state to get insight into the acceleration and emission mechanisms inside the jet. It is one of the brightest and highly active blazars in the MeV–GeV regime. The long-term multiwaveband light curves of this object showed flaring activity in 2014, with two distinct flares. The work presented here includes the study of flux-index variation, flare fitting, and hardness ratio, and the spectral modelling of X-ray and γ-ray data. The flux-index correlation found in the MeV–GeV regime indicates a ‘softer when brighter’ feature. The modelling of γ-ray light curves suggests that low-energy particles initiate both the flares, followed by the injection of high-energy particles. The short rise time indicates the presence of Fermi first-order acceleration. A single-zone leptonic model is used to fit the multiwaveband spectral energy distributions generated for both flares. The spectral energy...

Research paper thumbnail of A possible γ-ray quasi-periodic oscillation of ∼314 days in the blazar OJ 287

We report the detection of a probable gamma\gammagamma-ray quasi-periodic oscillation (QPO) of around 314... more We report the detection of a probable gamma\gammagamma-ray quasi-periodic oscillation (QPO) of around 314 days in the monthly binned 0.1 -- 300 GeV gamma\gammagamma-ray {\it Fermi}-LAT light curve of the well known BL Lac blazar OJ 287. To identify and quantify the QPO nature of the gamma\gammagamma-ray light curve of OJ 287, we used the Lomb-Scargle periodogram (LSP), REDFIT, and weighted wavelet z-transform (WWZ) analyses. We briefly discuss possible emission models for radio-loud active galactic nuclei (AGN) that can explain a gamma\gammagamma-ray QPO of such a period in a blazar. Reports of changes in the position of quasi-stationary radio knots over a yearly timescale as well as a strong correlation between gamma-ray and mm-radio emission in previous studies indicate that the signal is probably associated with these knots.

Research paper thumbnail of Multiwaveband quasi-periodic oscillation in the blazar 3C 454.3

Monthly Notices of the Royal Astronomical Society

We report the detection (>4σ) of a quasi-periodic oscillation (QPO) in the gamma-ray light cur... more We report the detection (>4σ) of a quasi-periodic oscillation (QPO) in the gamma-ray light curve of 3C 454.3 along with a simultaneous marginal QPO detection (>2.4σ) in the optical light curves. Periodic flux modulations were detected in both of these wavebands with a dominant period of ∼47 d. The gamma-ray QPO lasted for over 450 d (from MJD 56800 to 57250), resulting in over nine observed cycles which is among the highest number of periods ever detected in a blazar light curve. The optical light curve was not well sampled for almost half of the gamma-ray QPO span due to the daytime transit of the source, which could explain the lower significance of the optical QPO. Autoregressive Integrated Moving Average (ARIMA) modelling of the light curve revealed a significant, exponentially decaying, trend in the light curve during the QPO, along with the 47 d periodicity. We explore several physical models to explain the origin of this transient quasi-periodic modulation and the overa...

Research paper thumbnail of Multi-waveband quasi-periodic oscillations in the light curves of blazar CTA 102 during its 2016–2017 optical outburst

Astronomy & Astrophysics

Context. Quasi-periodic fluctuations in the light curves of blazars can provide insight into the ... more Context. Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlying emission process. This type of flux modulation hints at periodic physical processes that result in emission. CTA 102, a flat spectrum radio quasar at a redshift of 1.032, has displayed significant activity since 2016. The multi-waveband light curve of CTA 102 shows signs of quasi-periodic oscillations during the 2016–2017 flare. Aims. Our goal is to rigorously quantify the presence of any possible periodicity in the emitted flux during the mentioned period and to explore the possible causes that can give rise to it. Methods. Techniques such as the Lomb-Scargle periodogram and weighted wavelet z-transform were employed to observe the power emitted at different frequencies. To quantify the significance of the dominant period, Monte-Carlo techniques were employed to consider an underlying smooth bending power-law model for the power spectrum. In addition, the light curve was mode...

Research paper thumbnail of Transient quasi-periodic oscillations at γ-rays in the TeV blazar PKS 1510-089

Monthly Notices of the Royal Astronomical Society

We present periodicity search analyses on the γ-ray light curve of the TeV blazar PKS 1510-089 ob... more We present periodicity search analyses on the γ-ray light curve of the TeV blazar PKS 1510-089 observed by the Fermi Large Area Telescope. We report the detection of two transient quasi-periodic oscillations: A 3.6-d QPO during the outburst in 2009 that lasted five cycles (MJD 54906–54923); and a periodicity of 92 d spanning over 650 d from 2018 to 2020 (MJD 58200–58850), which lasted for seven cycles. We employed the Lomb–Scargle periodogram, Weighted Wavelet Z-transform, redfit, and the Monte Carlo light-curve simulation techniques to find any periodicity and the corresponding significance. The 3.6-d QPO was detected at a moderate significance of ∼3.5σ, while the detection significance of the 92-d QPO was ∼7.0σ. We explore a few physical models for such transient QPOs including a binary black hole system, precession of the jet, a non-axisymmetric instability rotating around the central black hole near the innermost stable circular orbit, the presence of quasi-equidistant magnetic ...

Research paper thumbnail of Long-term Variability and Correlation Study of the Blazar 3C 454.3 in the Radio, NIR, and Optical Wavebands

The Astrophysical Journal

We performed a long-term optical (B, V, R bands), infra-red (J and K bands) and radio band (15, 2... more We performed a long-term optical (B, V, R bands), infra-red (J and K bands) and radio band (15, 22, 37 GHz band) study on the flat spectrum radio quasar, 3C 454.3, using the data collected over a period of more than 8 years (MJD 54500-57500). The temporal variability, spectral properties and inter-waveband correlations were studied by dividing the available data into smaller segments with more regular sampling. This helped us constrain the size and the relative locations of the emission regions for different wavebands. Spectral analysis of the source revealed the interplay between the accretion disk and jet emission. The source predominantly showed a redder-when-brighter trend, though we observed a bluer-when-brighter trend at high flux levels which could be signatures of particle acceleration and radiative cooling. Significant correlations with near-zero lag were seen between various optical/infrared bands, indicating that these emission regions are co-spatial. Correlations with a time lag of about 10-100 days are seen between optical/infra-red and radio bands indicating these emissions arise from different regions. We also observe the DCF peak lag change from year to year. We try to explain these differences using a curved jet model where the different emission regions have different viewing angles resulting in a frequency dependent Doppler factor. This variable Doppler factor model explains the variability timescales and the variation in DCF peak lag between the radio and optical emissions in different segments. Lags of 6-180 days are seen between emissions in various radio bands, indicating a core-shift effect.

Research paper thumbnail of Multiwavelength study of the quiescent states of six brightest flat-spectrum radio quasars detected by Fermi-LAT

Monthly Notices of the Royal Astronomical Society

The regular monitoring of flat-spectrum radio quasars (FSRQs) in γ-rays by Fermi-LAT (Large Area ... more The regular monitoring of flat-spectrum radio quasars (FSRQs) in γ-rays by Fermi-LAT (Large Area Telescope) since past 12 yr indicated six sources who exhibited extreme γ-ray outbursts crossing daily flux of 10−5 photons cm−2 s−1. We obtained nearly simultaneous multiwavelength data of these sources in radio to γ-ray waveband from OVRO (Owens Valley Radio Observatory), Steward Observatory, SMARTS (Small and Moderate Aperture Research Telescope System), Swift-UVOT (UV/Optical Telescope), Swift-XRT (X-ray Telescope), and Fermi-LAT. The time-averaged broad-band spectral energy distributions (SEDs) of these sources in quiescent states were studied to get an idea about the underlying baseline radiation processes. We modelled the SEDs using one-zone leptonic synchrotron and inverse Compton emission scenario from broken power-law electron energy distribution inside a spherical plasma blob, relativistically moving down a conical jet. The model takes into account inverse Compton scattering o...

Research paper thumbnail of Multi-waveband quasi-periodic oscillations in the light curves of blazar CTA 102 during its 2016-2017 optical outburst

Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlyin... more Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlying emission process. This type of flux modulation hints at periodic physical processes that result in emission. CTA 102, a flat spectrum radio quasar at z=1.032, has displayed significant activity since 2016. The multi-waveband light curve of CTA 102 shows signs of quasi-periodic oscillations during the 2016-2017 flare. Our goal is to quantify the presence of any periodicity in the emitted flux during the mentioned period and to explore the causes that can give rise to it. Techniques such as the Lomb-Scargle periodogram and weighted wavelet z-transform were employed to observe the power emitted at different frequencies. To quantify the significance of the dominant period, Monte-Carlo techniques were employed considering an underlying smooth bending power-law model for the power spectrum. In addition, the light curve was modeled using an autoregressive process to analytically obtain the si...

Research paper thumbnail of Quasi-periodic Oscillation and Evidence of a Curved Jet in the Blazar 3C 454.3

We report the detection of a quasi-periodic oscillation (QPO) in light curves of the blazar 3C 45... more We report the detection of a quasi-periodic oscillation (QPO) in light curves of the blazar 3C 454.3. Periodic flux modulation was detected simultaneously in both gamma\gammagamma--ray and optical wavebands with a dominant period of sim47\sim 47sim47 days. This period has a significance of over 4.2sigma4.2\sigma4.2sigma in the \textit{Fermi} gamma\gammagamma--ray waveband and around 2.4sigma2.4\sigma2.4sigma in the optical V band. This QPO lasted for over 450 days (from MJD 56800 to 57250) resulting in over nine observed periods which is among the highest number of periods ever detected in a blazar light curve. The lower significance of the dominant period in optical wavebands is attributed to the absence of optical data for a number of QPO cycles due to the daytime transit of the source. We explore several physical models to explain the origin of this transient quasi-periodic modulation in the observed flux with a month-like period. These scenarios include a binary black hole system, a hotspot orbiting close to the innermost stable ...

Research paper thumbnail of Temporal and spectral study of PKS B1222 + 216 flares in 2014

We report on a temporal and spectral study of a flat-spectrum radio quasar, PKS B1222 + 216, in a... more We report on a temporal and spectral study of a flat-spectrum radio quasar, PKS B1222 + 216, in a flare state to get insight into the acceleration and emission mechanisms inside the jet. It is one of the brightest and highly active blazars in the MeV–GeV regime. The long-term multiwaveband light curves of this object showed flaring activity in 2014, with two distinct flares. The work presented here includes the study of flux-index variation, flare fitting, and hardness ratio, and the spectral modelling of X-ray and γ-ray data. The flux-index correlation found in the MeV–GeV regime indicates a ‘softer when brighter’ feature. The modelling of γ-ray light curves suggests that low-energy particles initiate both the flares, followed by the injection of high-energy particles. The short rise time indicates the presence of Fermi first-order acceleration. A single-zone leptonic model is used to fit the multiwaveband spectral energy distributions generated for both flares. The spectral energy...

Research paper thumbnail of A possible γ-ray quasi-periodic oscillation of ∼314 days in the blazar OJ 287

We report the detection of a probable gamma\gammagamma-ray quasi-periodic oscillation (QPO) of around 314... more We report the detection of a probable gamma\gammagamma-ray quasi-periodic oscillation (QPO) of around 314 days in the monthly binned 0.1 -- 300 GeV gamma\gammagamma-ray {\it Fermi}-LAT light curve of the well known BL Lac blazar OJ 287. To identify and quantify the QPO nature of the gamma\gammagamma-ray light curve of OJ 287, we used the Lomb-Scargle periodogram (LSP), REDFIT, and weighted wavelet z-transform (WWZ) analyses. We briefly discuss possible emission models for radio-loud active galactic nuclei (AGN) that can explain a gamma\gammagamma-ray QPO of such a period in a blazar. Reports of changes in the position of quasi-stationary radio knots over a yearly timescale as well as a strong correlation between gamma-ray and mm-radio emission in previous studies indicate that the signal is probably associated with these knots.

Research paper thumbnail of Multiwaveband quasi-periodic oscillation in the blazar 3C 454.3

Monthly Notices of the Royal Astronomical Society

We report the detection (>4σ) of a quasi-periodic oscillation (QPO) in the gamma-ray light cur... more We report the detection (>4σ) of a quasi-periodic oscillation (QPO) in the gamma-ray light curve of 3C 454.3 along with a simultaneous marginal QPO detection (>2.4σ) in the optical light curves. Periodic flux modulations were detected in both of these wavebands with a dominant period of ∼47 d. The gamma-ray QPO lasted for over 450 d (from MJD 56800 to 57250), resulting in over nine observed cycles which is among the highest number of periods ever detected in a blazar light curve. The optical light curve was not well sampled for almost half of the gamma-ray QPO span due to the daytime transit of the source, which could explain the lower significance of the optical QPO. Autoregressive Integrated Moving Average (ARIMA) modelling of the light curve revealed a significant, exponentially decaying, trend in the light curve during the QPO, along with the 47 d periodicity. We explore several physical models to explain the origin of this transient quasi-periodic modulation and the overa...

Research paper thumbnail of Multi-waveband quasi-periodic oscillations in the light curves of blazar CTA 102 during its 2016–2017 optical outburst

Astronomy & Astrophysics

Context. Quasi-periodic fluctuations in the light curves of blazars can provide insight into the ... more Context. Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlying emission process. This type of flux modulation hints at periodic physical processes that result in emission. CTA 102, a flat spectrum radio quasar at a redshift of 1.032, has displayed significant activity since 2016. The multi-waveband light curve of CTA 102 shows signs of quasi-periodic oscillations during the 2016–2017 flare. Aims. Our goal is to rigorously quantify the presence of any possible periodicity in the emitted flux during the mentioned period and to explore the possible causes that can give rise to it. Methods. Techniques such as the Lomb-Scargle periodogram and weighted wavelet z-transform were employed to observe the power emitted at different frequencies. To quantify the significance of the dominant period, Monte-Carlo techniques were employed to consider an underlying smooth bending power-law model for the power spectrum. In addition, the light curve was mode...