arXiv subjects

Paul J. Wiita

Publications and source records attributed to Paul J. Wiita.

At least 55 records · Page 3Linked to original sources

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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arXiv2022

X-ray Intraday Variability of HBL Blazars with XMM-Newton

We present an extensive study on the X-ray intraday variability of ten TeV-emitting high synchrotron peaked blazars (HBLs): 1ES 0229+200, 1ES 0414+009, PKS 0548-322, 1ES 1101-232, 1H 1219+301, H 1426+428, Mrk 501, 1ES 1959+650, PKS 2005-489, and 1ES 2344+514 made with twenty-five XMM-Newton pointed observations during its operational period. Intraday variability has been estimated in three energy bands: soft (0.3--2 keV), hard (2--10 keV) and total (0.3--10 keV). Although seven out of these ten TeV HBLs exhibited some intraday variability at three-sigma levels no major variations exceeding six percent were detected. We explored the spectral properties of the sample by extracting the hardness ratio from the soft and hard bands; no significant variations in the hardness ratio were observed in any source. We performed power spectral density analyses on the variable light-curves by fitting power-laws, yielding slopes lying in the range from 1.11 to 2.93 for different HBLs. We briefly discuss possible emission mechanisms and carry out rough estimates for magnetic fields, electron Lorentz factors and emission region sizes for seven of these HBLs.

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arXiv2022

The Twin Radio galaxy TRG J104454+354055

We report observations of a bright dumb-bell system of galaxies ($z =0.162$) with the upgraded Giant Metrewave Radio Telescope ($u$GMRT), which show that each member of this gravitationally bound pair of galaxies hosts bipolar radio jets extended on 100 kiloparsec scales. Only two cases of such radio morphology have been reported previously, both being dumb-bell systems, as well. The famous first example, 3C 75, was discovered 4 decades ago, and the second case was discovered 3 decades ago. This implies that such `Twin-Radio-Galaxies' (TRGs) are an exceedingly rare phenomenon. As in the case of its two senior cousins, the bi-polar radio jets of the present TRG (J104454+354055) exhibit strong wiggles and are edge-darkened (Fanaroff-Riley class I). However, there are important differences, too. For instance, the jets in the present TRG do not merge and, moreover, show no signs of distortion due to an external crosswind. This makes the present TRG a much neater laboratory for studying the physics of (sideway) colliding jets of relativistic plasma. This TRG has a Wide-Angle-Tail (WAT) neighbour hosted by another bright galaxy belonging to the same group, which appears to be moving towards the TRG.

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arXiv2022

Detection of a quasi-periodic oscillation in the optical light curve of the remarkable blazar AO 0235+164

We present a long term optical $R$ band light curve analysis of the gravitationally lensed blazar AO 0235+164 in the time span 1982 - 2019. Several methods of analysis lead to the result that there is a periodicity of ~8.13 years present in these data. In addition, each of these five major flares are apparently double-peaked, with the secondary peak following the primary one by ~2 years. Along with the well known system, OJ 287, our finding constitutes one of the most secure cases of long term quasi-periodic optical behaviour in a blazar ever found. A binary supermassive black hole system appears to provide a good explanation for these results.

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arXiv2022

Long Term Multi-band Near Infra-Red Variability of the Blazar OJ 287 during 2007--2021

We present the most extensive and well-sampled long-term multi-band near-infrared (NIR) temporal and spectral variability study of OJ 287, considered to be the best candidate binary supermassive black hole blazar. These observations were made between December 2007 and November 2021. The source underwent ~ 2 -- 2.5 magnitude variations in the J, H, and Ks NIR bands. Over these long-term timescales there were no systematic trends in either flux or spectral evolution with time or with the source's flux states. However, on shorter timescales, there are significant variations in flux and spectra indicative of strong changes during different activity states. The NIR spectral energy distributions show diverse facets at each flux state, from the lowest to the highest. The spectra are, in general, consistent with a power-law spectral profile (within 10%) and many of them indicate minor changes (observationally insignificant) in the shift of the peak. The NIR spectra generally steepens during bright phases. We briefly discuss these behaviors in the context of blazar emission scenarios/mechanisms, OJ 287's well-known traditional behavior, and implications for models of the source central engine invoked for its long-term optical semi-periodic variations.

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arXiv2022

X-ray Intraday Variability and Power Spectral Density Profiles of the Blazar 3C 273 with XMM-Newton during 2000-2021

We present X-ray intraday variability and power spectral density (PSD) analyses of the longest 23 pointed XMM-Newton observations of the blazar 3C 273 that were taken during 2000 -- 2021. These good time intervals contained between 5 and 24.6 hours of data. Variability has been estimated in three energy bands: 0.2 -- 2 keV (soft), 2 -- 10 keV (hard), and 0.2 -- 10 keV (total). Nine of the 23 observations exhibited some variability, though no major variations exceeding 5 per cent were detected. Typical timescales for variability were $\sim 1$ ks. For those variable light curves we find that a power-law model provides good fits to each PSD, with most of the slopes between $-1.7$ and $-2.8$. Although no variations of hardness ratio could be measured in any individual observation, an anti-correlation in flux and hardness ratio is found in long term data that indicates a harder when brighter trend. Our flux and spectral analyses indicate that both particle acceleration and synchrotron cooling processes make an important contribution to the emission from this blazar.

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arXiv2022

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

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

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arXiv2021

Transient Quasi-Periodic Oscillations at $γ$-rays in the TeV Blazar PKS 1510-089

We present periodicity search analyses on the $γ$-ray lightcurve of the TeV blazar PKS 1510-089 observed by the \textit{Fermi} Large Area Telescope. We report the detection of two transient quasi-periodic oscillations: a 3.6-day QPO during the outburst in 2009 that lasted five cycles (MJD 54906--54923); and a periodicity of 92 days spanning over 650 days 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 lightcurve simulation techniques to find any periodicity and the corresponding significance. The 3.6-day QPO was detected at a moderate significance of $\sim$3.5$σ$, while the detection significance of the 92-day QPO was $\sim$7.0$σ$. 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 islands inside the jet, and a geometric model involving a plasma blob moving helically inside a curved jet.

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arXiv2021

A double-period oscillation signal in millimeter emission of the radio galaxy NGC 1275

The nearby Seyfert type galaxy NGC 1275 contains a bright radio nucleus at its center, revealed through high-spatial resolution imaging to be the source of the jets emanating from the galaxy. Coincident with the emergence of a new component C3 in the nucleus since 2005, flux densities from NGC 1275, at least at radio, millimeter (mm), and gamma-ray frequencies, had been increasing up through 2017 and leveled off afterwards. We analyze the long-term light curves of the nucleus that span the rising trend to 2015 July, and find a pair of approximately year-long quasi-periodic oscillations, with periods of $P_l\simeq 345$\,d and $P_h\simeq 386$\,d respectively, in emission at 1.3-mm wavelength. We discuss the case that there would be a long precession period $P_{\rm prec}\simeq 9$\,yr, causing the appearance of $P_h$ that is slightly higher than $P_l$. The accretion disk around the central supermassive black hole (SMBH) would be precessing at $P_{\rm prec}$, induced by either the Lense-Thirring effect or the existence of a companion SMBH. In the two scenarios, $P_l$ would be the jet wobbling timescale or the SMBH binary period respectively. The finding, which could be verified through high-spatial resolution mm imaging, would not only identify the nature of the jet variation but also help reveal the full features of the galaxy.

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arXiv2021

Multi-band Variability of the TeV Blazar PG 1553+113 with XMM-Newton

We present variability analyses of twenty pointed XMM-Newton observations of the high energy peaked TeV blazar PG 1553+113 taken during 2010 to 2018. We found intraday variability in the total X-ray energy range (0.3 -- 10 keV) in 16 out of 19 light curves or a duty cycle of ~84%. A discrete correlation function analysis of the intraday light curves in the soft and hard X-ray bands peaks on zero lag, showing that the emission in hard and soft bands are co-spatial and emitted from the same population of leptons. Red-noise dominates the power spectral density (PSD) of all the LCs although the PSDs have a range of spectral slopes from -2.36 to -0.14. On longer timescales, the optical and UV variability patterns look almost identical and well correlated, as are the soft and hard X-ray bands, but the optical/UV variations are not correlated to those in the X-ray band, indicating that the optical/UV and X-ray emissions are emitted by two different populations of leptons. We briefly discuss physical mechanisms which may be capable of explaining the observed flux and spectral variability of PG 1553+113 on these diverse timescales.

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arXiv2021

X-ray Intraday Variability of the TeV Blazar PKS 2155-304 with Suzaku During 2005-2014

We have examined 13 pointed observations of the TeV emitting high synchrotron peak blazar PKS 2155-304, taken by the Suzaku satellite throughout its operational period. We found that the blazar showed large-amplitude intraday variabilities in the soft (0.8 - 1.5 keV) and the hard (1.5 - 8.0 keV) bands in the light curves. Spectral variability on intraday timescales is estimated using the hardness ratio. The blazar usually becomes harder when brighter and vice versa, following the typical behavior of high synchrotron peak blazars. The power spectral density (PSD) analyses of 11 out of 13 light curves in the total energy (0.8 - 8.0 keV) are found to be red-noise dominated, with power-law spectral indices that span a large range, from -2.81 to -0.88. Discrete correlation function analyses of all the 13 light curves between the soft and the hard bands show that they are well correlated and peak at, or very close to, zero lag. This indicates that the emissions in soft and hard bands are probably cospatial and emitted from the same population of leptons. Considering fluxes versus variability timescales, we found no correlation on intraday timescales, implying that X-ray emission from PKS 2155-304 is not dominated by simple changes in the Doppler factor. We briefly discuss the most likely emission mechanisms responsible for the observed flux and spectral variabilities and place constraints on magnetic field strength and Lorentz factors of the electrons emitting the X-rays in the most likely scenario.

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arXiv2021

Quasi-Periodic Oscillations in the long term radio light curves of the blazar AO 0235+164

We present time series analyses of three-decade long radio observations of the BL Lacertae object AO 0235+164 made at the University of Michigan Radio Astronomical Observatory operating at three central frequencies of 4.8 GHz, 8.0 GHz and 14.5 GHz. We detected a quasi-periodic oscillation of $\sim$965 days in all three frequency bands in the light curve of the effectively simultaneous observations, along with strong signals at $\sim$1950 d, $\sim$1350 d, and $\sim$660 d. The periodicity is analyzed with three methods: Data Compensated Discrete Fourier Transform, Generalized Lomb-Scargle Periodogram and Weighted Wavelet Z-transform. These methods are chosen as they have different analysis approaches toward robust measurement of claimed periodicities. The QPO at $965\pm 50$ days is found to be significant (at least $3.5σ$) and is persistent throughout the observation for all three radio frequencies, and the others, which may be harmonics, are comparably significant in at least the 8.0 GHz and 14.5 GHz bands. We briefly discuss plausible explanations for the origin of such long and persistent periodicity.

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arXiv2020

Multiwaveband Quasi--periodic Oscillation in the Blazar 3C 454.3

We report the detection ($>4σ$) of a Quasi-Periodic Oscillation (QPO) in the $γ$-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 $\sim 47$ days. The $γ$-ray QPO lasted for over 450 days (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 $γ$-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 $47$ days periodicity. We explore several physical models to explain the origin of this transient quasi-periodic modulation and the overall trend 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 circular orbit of the supermassive black hole, and precessing jets. We conclude that the most likely scenario involves a region of enhanced emission moving helically inside a curved jet. The helical motion gives rise to the QPO and the curvature ($\sim 0.05^{\circ}$ pc$^{-1}$) of the jet is responsible for the observed trend in the light curve.

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arXiv2020

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 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 significance of the dominant period. Lastly, the light curve was modeled using a generalized autoregressive integrated moving average process to check whether introducing a periodic component results in a statistically preferable model. Significant, simultaneous quasi-periodic oscillations (QPOs) were observed in the $γ$-ray and optical fluxes of blazar CTA 102 during its 2016-2017 flare. The periodic flux modulation had a dominant period of $\sim$ 7.6 days and lasted for $\sim$ 8 cycles (MJD 57710-57770). All of the methods used point toward significant ($>4σ$) quasi-periodic modulation in both $γ$-ray and optical fluxes. Several possible models were explored while probing the origin of the periodicity, and by extension, the 2016--2017 optical flare. The best explanation for the detected QPO appears to be a region of enhanced emission, moving helically inside the jet.

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arXiv2020

A Possible $γ$-ray Quasi-periodic Oscillation of $\sim$ 314 days in the Blazar OJ 287

We report the detection of a probable $γ$-ray quasi-periodic oscillation (QPO) of around 314 days in the monthly binned 0.1 -- 300 GeV $γ$-ray {\it Fermi}-LAT light curve of the well known BL Lac blazar OJ 287. To identify and quantify the QPO nature of the $γ$-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 $γ$-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.

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arXiv2020

Catching butterflies in the sky: Extended catalog of winged or X-shaped radio sources from the latest FIRST data release

We present a catalog of 290 "winged" or X-shaped radio galaxies (XRGs) extracted from the latest (2014 December 17) data release of the "Very Large Array Faint Images of the Radio Sky at Twenty centimeter." We have combined these radio images with their counterparts in the TIFR GMRT sky survey at 150 MHz, in an attempt to identify any low surface brightness radio emission present in these sources. This has enabled us to assemble a sample of 106 "strong" XRG candidates and 184 "probable" XRG candidates whose XRG designation needs to be verified by further observations. The present sample of 290 XRG candidates is almost twice as large as the number of XRGs currently known. Twenty-five of our 290 XRG candidates (9 "strong" and 16 "probable") are identified as quasars. Double-peaked narrow emission lines are seen in the optical spectra of three of the XRG candidates (two "strong" and one "probable"). Nearly 90% of the sample is located in the FR II domain of the Owen-Ledlow diagram. A few of the strong XRG candidates have a rather flat radio spectrum (spectral index alpha flatter than -0.3) between 150 MHz and 1.4 GHz, or between 1.4 and 5 GHz. Since this is not expected for lobe-dominated extragalactic radio sources (like nearly all known XRGs), these sources are particularly suited for follow-up radio imaging and near-simultaneous measurement of the radio spectrum.

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arXiv2020

Optical Variability of the TeV Blazar 1ES 0806+524 on Diverse Timescales

We report the results of our optical (VRI) photometric observations of the TeV blazar 1ES 0806$+$524 on 153 nights during 2011-2019 using seven optical telescopes in Europe and Asia. We investigated the variability of the blazar on intraday as well as on long-term timescales. We examined eighteen intraday light curves for flux and color variations using the most reliable power-enhanced F-test and the nested ANOVA test. Only on one night was a small, but significant, variation found, in both $V$ band and $R$ band light curves. The $V-R$ color index was constant on every one of those nights. Flux density changes of around 80 % were seen over the course of these eight years in multiple bands. We found a weighted mean optical spectral index of 0.639$\pm$0.002 during our monitoring period by fitting a power law ($F_ν \propto ν^{-α}$) in 23 optical ($VRI$) spectral energy distributions of 1ES 0806$+$524. We discuss different possible mechanisms responsible for blazar variability on diverse timescales.

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arXiv2019

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

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

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