Blazar 3C Research Articles

Two Repeated Transient Quasiperiodic Oscillations in the γ-Ray Emission from the Blazar 3C 279

In this work, we report for the first time two repeated transient quasiperiodic oscillations (QPOs) in the γ-ray light curve of the TeV blazar 3C 279. We search for the periodicity in the light curve and estimate its confidence level using the weighted wavelet Z-transform, the Lomb–Scargle periodogram, and the REDFIT techniques. The main results are as follows: (1) a QPO of ∼33 days (>2.5σ) is found during the flare of 117 days (MJD 55008–55125) from 2009 June to November. Interestingly, the same QPO (∼39 days) reappeared in the flaring duration from MJD 59430 to 59585, with the confidence level of >4σ. (2) Another transient QPO of ∼91 days with a significance of >3.8σ is found during a period with 455 days (MJD 58430–58985) from 2019 February to 2020 May. Under the premise of considering the QPOs reported in the literature, the QPO of ∼40 days is repeated three times and the QPO of ∼91 days is repeated twice. We discuss several physical models explaining the QPOs of this blazar. Our study may suggest that the two QPOs originate from the twin jets of the binary black holes at the center of 3C 279. The repeated occurrence of QPOs of a similar scale strongly supports the geometric scenario of a blob spiraling within the jet. Furthermore, the hypothesis of a sheath in the jet may also be a potential explanation for the repetitive γ-ray flare patterns observed in the light curve.

Optical variability of the blazar 3C 371: From minute to year timescales

The BL Lac object 3C 371 was observed by the Transiting Exoplanet Survey Satellite (TESS) for approximately a year, between July 2019 and July 2020, with an unmatched two-minute imaging cadence. In parallel, the Whole Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing campaign, providing three years of continuous optical monitoring between 2018 and 2020. These datasets allow for a thorough investigation of the variability of the source. The goal of this study is to evaluate the optical variability of 3C 371. Taking advantage of the remarkable cadence of TESS data, we aim to characterize the intra-day variability (IDV) displayed by the source and identify its shortest variability timescale. With this estimate, constraints on the size of the emitting region and black hole mass can be calculated. Moreover, WEBT data are used to investigate long-term variability (LTV), including in terms of the spectral behavior of the source and the polarization variability. Based on the derived characteristics, we aim to extract information on the origin of the variability on different timescales. We evaluated the variability of 3C 371 by applying the variability amplitude tool, which quantifies variability of the emission. Moreover, we employed common tools, such as ANOVA (ANalysis Of VAariance) tests, wavelet and power spectral density (PSD) analyses to characterize the shortest variability timescales present in the emission and the underlying noise affecting the data. We evaluated the short- and long-term color behavior to understand to understand its spectral behavior. The polarized emission was analyzed, studying its variability and possible rotation patterns of the electric vector position angle (EVPA). Flux distributions of the IDV and LTV were also studied with the aim being to link the flux variations to turbulent and/or accretion-disk-related processes. Our ANOVA and wavelet analyses reveal several entangled variability timescales. We observe a clear increase in the variability amplitude with increasing width of the time intervals evaluated. We are also able to resolve significant variations on timescales of as little as sim 0.5 hours. The PSD analysis reveals a red-noise spectrum with a break at IDV timescales. The spectral analysis shows a mild bluer-when-brighter (BWB) trend on long timescales. On short timescales, mixed BWB, achromatic and redder-when-brighter (RWB) signatures can be observed. The polarized emission shows an interesting slow EVPA rotation during the flaring period where a simple stochastic model can be excluded as the origin with a 3sigma significance . The flux distributions show a preference for a Gaussian model for the IDV, and suggest it may be linked to turbulent processes, while the LTV is better represented by a log-normal distribution and may have a disk-related.

Characterization of High-polarization Stars and Blazars with DIPOL-1 at Sierra Nevada Observatory

We report here the performance and first results of the new multiband optical polarimeter DIPOL-1, installed at the Sierra Nevada Observatory 90 cm T90 telescope (SNO, Granada, Spain). DIPOL-1 is equipped with a plane parallel calcite plate and λ/2 retarder for modulating the intensity of two perpendicularly polarized beams, and a high readout speed CMOS camera that allows for fast, time-dense coverage. We characterize the performance of this instrument through a series of tests on zero- and high-polarization standard stars. The instrumental polarization in the Nasmyth focus was well determined, with a very stable contribution of 4.0806% ± 0.0014% in the optical R band. For bright high-polarization standards (m R < 8) we reach precisions <0.02% in polarization degree and 0.°1 in polarization angle for exposures of 2–4 min. The polarization properties of these stars have been constrained, providing more recent results also about possible variability for future studies of some of the most used calibrators. Moreover, we have tested the capability of observing much fainter objects, in particular through blazar observations, where we reach a precision of <0.5%−0.6% and <0.°5 for faint targets (m R ∼ 16.5) with exposures of ∼1 hr. For brighter targets (m R ∼ 14.5−15), we can aim for time-dense observations with errors <0.2%−0.4% and <1°−1.°5 in 5–20 min. We have successfully performed a first campaign with DIPOL-1, detecting significant polarized emission of several blazars, with special attention to the highest ever polarization degree measured from blazar 3C 345 at ∼32%.

Lost in the curve: Investigating the disappearing knots in blazar 3C 454.3

One of the most well-known extragalactic sources in the sky, quasar 3C 454.3, shows a curved parsec-scale jet that has been exhaustively monitored with very-long-baseline interferometry (VLBI) over the recent years. In this work, we present a comprehensive analysis of four years of high-frequency VLBI observations at 43 GHz and 86 GHz, between 2013–2017, in total intensity and linear polarization. The images obtained from these observations enabled us to study the jet structure and the magnetic field topology of the source on spatial scales down to 4.6 parsec in projected distance. The kinematic analysis reveals the abrupt vanishing of at least four new superluminal jet features in a characteristic jet region (i.e., region C), which is located at an approximate distance of 0.6 milliarcsec from the VLBI core. Our results support a model in which the jet bends, directing the relativistic plasma flow almost perfectly toward our line of sight, co-spatially with the region where components appear to stop.

Multiband cross-correlated radio variability of the blazar 3C 279

ABSTRACT We present the results of our study of cross-correlations between long-term multiband observations of the radio variability of the blazar 3C 279. More than a decade (2008–2022) of radio data were collected at seven different frequencies ranging from 2 to 230 GHz. The multiband radio light curves show variations in flux, with the prominent flare features appearing first at higher-frequency and later in lower-frequency bands. This behaviour is quantified by cross-correlation analysis, which finds that the emission at lower-frequency bands lags that at higher-frequency bands. Lag versus frequency plots are well fit by straight lines with negative slope, typically ∼−30 day GHz−1. We discuss these flux variations in conjunction with the evolution of bright moving knots seen in multiepoch Very Long Baseline Array maps to suggest possible physical changes in the jet that can explain the observational results. Some of the variations are consistent with the predictions of shock models, while others are better explained by a changing Doppler beaming factor as the knot trajectory bends slightly, given a small viewing angle to the jet.

Constraining X-Ray Variability of the Blazar 3C 273 Using XMM-Newton Observations over Two Decades

Blazars exhibit relentless variability across diverse spatial and temporal frequencies. The study of long- and short-term variability properties observed in the X-ray band provides insights into the inner workings of the central engine. In this work, we present timing and spectral analyses of the blazar 3C 273 using the X-ray observations from the XMM-Newton telescope covering the period from 2000 to 2020. The methods of timing analyses include estimation of fractional variability, long- and short-term flux distribution, rms–flux relation, and power spectral density analysis. The spectral analysis include estimating a model-independent flux hardness ratio and fitting the observations with multiplicative and additive spectral models such as power law, log-parabola, broken power law, and blackbody. The blackbody represents the thermal emission from the accretion disk, while the other models represent the possible energy distributions of the particles emitting synchrotron radiation in the jet. During the past two decades, the source flux changed by a factor of three, with a considerable fractional variability of 27%. However, the intraday variation was found to be moderate. Flux distributions of the individual observations were consistent with a normal or log-normal distribution, while the overall flux distribution including all observations appears to be rather multimodal and of a complex shape. The spectral analyses indicate that a log-parabola added to a blackbody gives the best fit for most of the observations. The results indicate a complex scenario in which the variability can be attributed to the intricate interaction between the disk/corona system and the jet.

A Close Binary Supermassive Black Hole Model for the Galaxy 3C 273

The data from the last 60 years on the programs of long-term multi-frequency monitoring of active galactic nucleus (AGN) 3C 273 were analyzed. A model is proposed for finding the parameters of close binary systems (CBSs) from supermassive black holes (SMBHs), including a harmonic analysis of observational data series obtained in the optical and radio ranges. The purpose of this research was to show that in the absence of optical information on AGNs, only radio data can be used and the necessary information on the physical objects can be obtained. Regarding the example of the blazar 3C 273, the following parameters were obtained: the masses of the companions; their orbital characteristics, such as the speeds of movement in orbits; the reserves of the kinetic energy of the system; and others. It was found that AGN 3C 273 can be a very massive binary system at the stage of evolution close to merging. Based on the obtained parameters, the characteristics of the gravitational waves (GWs) of this system, its lifetime before the merger, and the possible observation of 3C 273 using gravitational wave detectors were considered.

Double SSA spectrum and magnetic field strength of the FSRQ 3C 454.3

ABSTRACT We present the results of a radio multifrequency ($\rm 3{-}340~GHz$) study of the blazar 3C 454.3. After subtracting the quiescent spectrum corresponding to optically thin emission, we found two individual synchrotron self-absorption (SSA) features in the wide-band spectrum. The one SSA had a relatively low turnover frequency (νm) in the range of $\rm 3{-}37~GHz$ (lower νm SSA spectrum, LSS), and the other one had a relatively high νm of $\rm 55{-}124~GHz$ (higher νm SSA spectrum, HSS). Using the SSA parameters, we estimated B-field strengths at the surface where optical depth τ = 1. The estimated B-field strengths were $\rm \gt 7$ and $\rm \gt 0.2~mG$ for the LSS and HSS, respectively. The LSS-emitting region was magnetically dominated before the 2014 June γ-ray flare. The quasi-stationary component (C), ∼0.6 mas apart from the 43 -GHz radio core, became brighter than the core with decreasing observing frequency, and we found that component C was related to the LSS. A decrease in jet width was found near component C. As a moving component, K14 approached component C, and the flux density of the component was enhanced while the angular size decreased. The high intrinsic brightness temperature in the fluid frame was obtained as TB, int ≈ (7.0 ± 1.0) × 1011 K from the jet component after the 2015 August γ-ray flare, suggesting that component C is a high-energy emitting region. The observed local minimum of jet width and re-brightening behaviour suggest a possible recollimation shock in component C.

Correlation between Emission-line Luminosity and Gamma-Ray Dominance in the Blazar 3C 279

Despite numerous studies, the origin of the γ-ray emission from blazars is still debated, in particular whether it is produced by leptonic or hadronic processes. In this study, we are testing the leptonic scenario for the flat-spectrum radio quasar 3C 279, assuming that the γ-ray emission is generated by inverse Compton scattering of external target photons from the broad-line region (IC-BLR scenario). For this purpose we use a 10 yr data set of the source consisting of the optical spectroscopy data from the Steward Observatory blazar monitoring program and Fermi Large Area Telescope γ-ray data. We search for a possible correlation between the Compton dominance and the emission-line luminosity using the discrete correlation function analysis. As a result, we find no significant correlation between these two quantities at any time lag value, while the emission-line luminosity displays a moderate correlation with the γ-ray flux at a zero time lag. We also reveal that the optical synchrotron continuum flux shows a pronounced correlation with the γ-ray flux, and therefore we interpret these results within the leptonic IC-BLR scenario where the variations in Compton dominance are primarily induced by changes in the magnetic field, rather than in the emission-line luminosity.

Time variability of the core-shift effect in the blazar 3C 454.3

Measuring and inferring the key physical parameters of jets in active galactic nuclei (AGN) requires high-resolution very long baseline interferometry (VLBI) observations. Using VLBI to measure a core-shift effect is a common way of obtaining estimates of the jet magnetic field strength, a key parameter for understanding jet physics. The VLBI core is typically identified as the bright feature at the upstream end of the jet, and the position of this feature changes with the observed frequency, rcore ∝ ν−1/kr. Due to the variable nature of AGN, flares can cause variability of the measured core shift. In this work, we investigated the time variability of the core-shift effect in the luminous blazar 3C 454.3. We employed a self-referencing analysis of multi-frequency (5, 8, 15, 22−24, and 43 GHz) Very Long Baseline Array (VLBA) data covering 19 epochs from 2005 to 2010. We found significant core-shift variability ranging from 0.27 to 0.86 milliarcsec between 5 GHz and 43 GHz. These results confirm the core-shift variability phenomenon observed previously. Furthermore, we also found time variability of the core-shift index, kr, which was typically below one, with an average value of 0.85 ± 0.08 and a standard deviation of 0.30. Values of kr below one were found during flaring and quiescent states. Our results indicate that the commonly assumed conical jet shape and equipartition conditions do not always hold simultaneously. Even so, these conditions are typically assumed when deriving magnetic field strengths from core-shift measurements, which can lead to unreliable results if kr significantly deviates from unity. Therefore, it is necessary to verify that kr = 1 actually holds before using core-shift measurements and the equipartition assumption to derive physical conditions in the jets. When kr = 1 epochs are selected in the case of 3C 454.3, the magnetic field estimates are consistent, even though the core shift varies significantly with time. Subsequently, we estimated the magnetic flux in the jet of 3C 454.3 and found that the source is in the magnetically arrested disc state, which agrees with earlier studies. Finally, we found a good correlation of the core position with the core flux density, rcore ∝ Score0.7, which is consistent with increased particle density during the flares.

Photometric Monitoring of Blazar 3C 66A with the Yunnan University Astronomical Observatory 1 m Telescope

3C 66A is one of our first batches of photometric monitoring objects with the 1 m optical telescope at Yunnan University Astronomical Observatory. In the present work, the observational campaign was performed from 2021 November 1 to 2022 February 27 in the Johnson–Morgan system V and R bands. The average magnitudes in each band were mag and mag. The overall variability amplitudes were ΔV = 0.ͫ74, Amp = 70.27% and ΔR = 0.ͫ72, Amp = 68.56%, respectively. Most of the intraday variabilities (IDVs) occurred in 2021 December and 2022 February. The minimal rise/decay timescale was about 6 minutes (5.82 ± 2.74 minutes and 6.18 ± 2.81 minutes on 2022 February 11, 6.99 ± 3.70 minutes and 6.17 ± 2.91 minutes on 2022 February 12). Durations of these rapid variabilities were from 11.99 to 179.67 minutes. The discrete correlation function analyses between V and R bands showed significantly correlated variability. Color index analysis of IDVs showed that the spectrums do not change with variabilities.

Catching profound optical flares in blazars

ABSTRACT Flaring episodes in blazars represent one of the most violent processes observed in extra-galactic objects. Studies of such events shed light on the energetics of the physical processes occurring in the innermost regions of blazars, which cannot otherwise be resolved by any current instruments. In this work, we present some of the largest and most rapid flares captured in the optical band in the blazars 3C 279, OJ 49, S4 0954+658, TXS 1156+295, and PG 1553+113. The source flux was observed to increase by nearly ten times within a time-scale of a few weeks. We applied several methods of time series analysis and symmetry analysis. Moreover, we also performed searches for periodicity in the light curves of 3C 279, OJ 49 and PG 1553+113 using the Lomb–Scargle method and found plausible indications of quasi-periodic oscillations (QPOs). In particular, the 33- and 22-day periods found in 3C 279, i.e. a 3:2 ratio, are intriguing. These violent events might originate from magnetohydrodynamical instabilities near the base of the jets, triggered by processes modulated by the magnetic field of the accretion disc. We present a qualitative treatment as the possible explanation for the observed large amplitude flux changes in both the source-intrinsic and source-extrinsic scenarios.

Multiwavelength study of blazar 4C + 01.02 during its long-term flaring activity in 2014–2017

ABSTRACT We conducted a detailed long-term spectral and temporal study of flat spectrum radio quasar 4C + 01.02, by using the multiwavelength observations from Fermi-LAT, Swift-XRT, and Swift-UVOT. The 2-d bin γ-ray light curve in the 2014–2017 active state displays 14 peak structures with a maximum integral flux $(\rm \mathit{ E} \gt 100 \ MeV)$ of $\rm (2.5 \pm 0.2) \times 10^{-6}\ ph\ cm^{-2}\ s^{-1}$ at MJD 57579.1, which is approximately 61 times higher than the base flux of $\rm (4.1 \pm 0.3) \times 10^{-8}\ ph\ cm^{-2}\ s^{-1}$, calculated by averaging the flux points when the source was in quiescent state. The shortest γ-ray variability of 0.66 ± 0.08 d is observed for the source. The correlation study between γ-ray spectral index and flux suggests that the source deviates from the usual trend of harder when brighter feature shown by blazars. To understand the likely physical scenario responsible for the flux variation, we performed a detailed broad-band spectral analysis of the source by selecting different flux states from the multiwavelength light curve. A single zone leptonic model was able to reproduce the broad-band spectral energy distribution (SED) of each state. The parameters of the model in each flux state are determined using a χ2 fit. We observed that the synchrotron, synchrotron-self-Compton (SSC), and External-Compton (EC) processes produce the broad-band SED under varied flux states. The adjoining contribution of the seed photons from the broad-line region (BLR) and the IR torus for the EC process are required to provide adequate fits to the GeV spectrum in all the chosen states.

Multiwavelength Variability Power Spectrum Analysis of the Blazars 3C 279 and PKS 1510–089 on Multiple Timescales

Abstract We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV γ-ray light curves covering timescales of decades/years to days/minutes 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 10 yr 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 3 weeks total duration exhibit a range between ∼1.4 and ∼2.0 (mean slope = 1.60 ± 0.70), consistent within errors with the slope on longer timescales; (3) comparisons of fractional variability indicate more power on timescales ≤100 days at γ-ray frequencies compared to longer wavelengths, in general (except between the γ-ray and optical wavelengths 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; and (5) the emission at optical/infrared wavelengths may involve a combination of disk and jet processes for PKS 1510−089.

X-ray intraday variability and power spectral density profiles of the blazar 3C 273 with XMM–Newton during 2000–2021

ABSTRACT 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 contain between 5 and 24.6 h 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). 9 of the 23 observations exhibited some variability, though no major variations exceeding 5 per cent were detected. Typical time-scales for variability were ∼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 (HR) could be measured in any individual observation, an anticorrelation in flux and HR 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.

Investigation of the Jets of the Blazar 3C 279 with Korean VLBI Network (KVN) 22–129 GHz Observations

We present analysis results of Korean VLBI Network (KVN) four-band data for the highly variable blazar 3C 279. We measured the 22, 43, 86, and 129 GHz flux densities and spectral indices of the source using contemporaneous data taken over 5.6 years. We used the discrete correlation function to investigate correlations between the radio emission properties and those measured in the optical (2 × 1014 – 1.5 × 1015 Hz), X-ray (0.3–10 keV), and gamma-ray (0.1–300 GeV) bands. We found a significant correlation between the radio spectral index and gamma-ray flux without a time delay and interpreted the correlation using an extended jet scenario for blazar emission.

A Shock-in-Jet Synchrotron Mirror Model for Blazars

Reinhard Schlickeiser has made groundbreaking contributions to various aspects of blazar physics, including diffusive shock acceleration, the theory of synchrotron radiation, the production of gamma-rays through Compton scattering in various astrophysical sources, etc. This paper, describing the development of a self-consistent shock-in-jet model for blazars with a synchrotron mirror feature, is therefore an appropriate contribution to a Special Issue in honor of Reinhard Schlickeiser’s 70th birthday. The model is based on our previous development of a self-consistent shock-in-jet model with relativistic thermal and non-thermal particle distributions evaluated via Monte-Carlo simulations of diffusive shock acceleration, and time-dependent radiative transport. This model has been very successful in modeling spectral variability patterns of several blazars, but has difficulties describing orphan flares, i.e., high-energy flares without a significant counterpart in the low-frequency (synchrotron) radiation component. As a solution, this paper investigates the possibility of a synchrotron mirror component within the shock-in-jet model. It is demonstrated that orphan flares result naturally in this scenario. The model’s applicability to a recently observed orphan gamma-ray flare in the blazar 3C279 is discussed and it is found that only orphan flares with mild (≲ a factor of 2–3) enhancements of the Compton dominance can be reproduced in a synchrotron-mirror scenario, if no additional parameter changes are invoked.

Long-term multi-wavelength variations of Fermi blazar 3C 279

Long-term optical, X-ray and γ-ray data of blazar 3C 279 have been compiled from Swift-XRT, RXTE-PCA, Fermi-LAT, SMARTS and literature. The source exhibits strong variability on long timescales. From the 1980s until now, the optical R band light curve spans more than 32 yr, and a possible 5.6-yr-long quasi-periodic variation component has been found in it. The optical spectral behavior has been investigated. In the optical band, the mean spectral index is –1.71. The source exhibits an obvious special spectral behavior. In the low state, the source manifests a clear bluer-when-brighter behavior in the sense that the optical spectrum turns harder (flatter) when the brightness increases. While in the high state, the optical spectrum is stable, which means the source spectral index does not vary with brightness. The correlation analysis has been performed among optical, X-ray and γ-ray energy bands. The result indicates that the variations of γ-ray and X-ray bands are well correlated without time delay on the timescale of days, and their variations exhibit weak correlations with those of the optical band. The variations, especially outbursts, are simultaneous, but the magnitude of variations is disproportionate. The detailed analysis reveals that the main outbursts exhibit strong correlations in different γ-ray, X-ray and optical bands.

Time-dependent lepto-hadronic modelling of the emission from blazar jets with SOPRANO: the case of TXS 0506 + 056, 3HSP J095507.9 + 355101, and 3C 279

ABSTRACT The observation of a very high-energy neutrino by IceCube (IceCube-170922A) and its association with the flaring blazar TXS 0506 + 056 provided the first multimessenger observations of blazar jets, demonstrating the important role of protons in their dynamics and emission. In this paper, we present SOPRANO (https://www.amsdc.am/soprano), a new conservative implicit kinetic code that follows the time evolution of the isotropic distribution functions of protons, neutrons, and the secondaries produced in photo-pion and photo-pair interactions, alongside with the evolution of photon and electron/positron distribution functions. SOPRANO is designed to study leptonic and hadronic processes in relativistic sources such as blazars and gamma-ray bursts. Here, we use SOPRANO to model the broadband spectrum of TXS 0506 + 056 and 3HSP J095507.9 + 355101, which are associated with neutrino events, and of the extreme flaring blazar 3C 279. The SEDs are interpreted within the guise of both a hadronic and a hybrid model. We discuss the implications of our assumptions in terms of jet power and neutrino flux.

Long-term multiband monitoring of blazar 3C 66A: Evidence of the two distinct states with different baseline flux

ABSTRACT Blazar variability can be described as flaring activities on a wide range of time-scales over a baseline flux level. It is important to detect and distinguish baseline flux changes from long-term flare variations, since the former may reflect state transitions caused by a secular change in bulk properties such as the bulk Lorentz factor or the viewing angle. We report such a transition observed in the 11 yr Fermi γ-ray light curve of the blazar 3C 66A, where the baseline flux of ∼1.8 × 10−7 ph cm−2 s−1 persisted for three years and then changed over a month time-scale to ∼0.8 × 10−7 ph cm−2 s−1 and remained in that level for the next eight years. Moreover, there is evidence for a similar shift in baseline flux in the optical band. Broad-band spectral energy distribution modelling for the two activity states reveals that the baseline flux change is consistent with an overall decrease in the Doppler factor, which in turn implies an increase in the jet inclination angle by ∼1° or a decrease in the bulk Lorentz factor by ∼25 per cent. We discuss the implication of such a variation occurring on a month time-scale.