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We report results from our ongoing project MOMO (Multiwavelength Observations and Modelling of OJ 287). In this latest publication of a sequence, we combine our Swift UVOT–XRT and Effelsberg radio data (2.6–44 GHz) between 2019 and 2022.04 with public SMA data and gamma-ray data from the Fermi satellite. The observational epoch covers OJ 287 in a high state of activity from radio to X-rays. The epoch also covers two major events predicted by the binary supermassive black hole (SMBH) model of OJ 287. Spectral and timing analyses clearly establish: a new UV–optical minimum state in 2021 December at an epoch where the secondary SMBH is predicted to cross the disc surrounding the primary SMBH; an overall low level of gamma-ray activity in comparison to pre-2017 epochs; the presence of a remarkable, long-lasting UV–optical flare event of intermediate amplitude in 2020–2021; a high level of activity in the radio band with multiple flares; and particularly a bright, ongoing radio flare peaking in 2021 November that may be associated with a gamma-ray flare, the strongest in 6 yr. Several explanations for the UV–optical minimum state are explored, including the possibility that a secondary SMBH launches a temporary jet, but the observations are best explained by variability associated with the main jet. [ABSTRACT FROM AUTHOR]

We carried out a pilot campaign of radio and optical band intra-day variability (IDV) observations of five blazars (3C66A, S5 0716+714, OJ287, B0925+504 and BL Lacertae) on 2015 December 18-21 by using the radio telescope in Effelsberg (Germany) and several optical telescopes in Asia, Europe and America. After calibration, the light curves from both 5 GHz radio band and the optical R band were obtained, although the data were not smoothly sampled over the sampling period of about four days. We tentatively analyse the amplitudes and time-scales of the variabilities, and any possible periodicity. The blazars vary significantly in the radio (except 3C66A and BL Lacertae with only marginal variations) and optical bands on intra- and inter-day time-scales, and the source B0925+504 exhibits a strong quasi-periodic radio variability. No significant correlation between the radio- and optical-band variability appears in the five sources, which we attribute to the radio IDV being dominated by interstellar scintillation whereas the optical variability comes from the source itself. However, the radioand optical-band variations appear to be weakly correlated in some sources and should be investigated based on well-sampled data from future observations. [ABSTRACT FROM AUTHOR]

We present the results of a broad-band radio-to-GeV observing campaign organized to get a better understanding of the radiation processes responsible for the γ-ray flares observed in 3C 279. The total intensity and polarization observations of the source were carried out between 2013 December 28 and 2014 January 03 using the Fermi-Large Area Telescope, Swift-XRT, Swift-UVOT, and Korean VLBI Network telescopes. A prominent flare observed in the optical/near-UV passbands was found to be correlated with a concurrent γ-ray flare at a confidence level > 95 per cent, which suggests a co-spatial origin of the two. Moreover, the flaring activity in the two regimes was accompanied by no significant spectral variations. A peak in the X-ray light curve coincides with the peaks of the fractional polarization curves at 43 and 86 GHz radio bands. No prominent variation was noticed for the total intensity and the electric vector position angle observations at radio bands during this period. We noticed a possible hint of steepening of the radio spectrum with an increase in percentage polarization, which suggests that the radio polarization variations could be simply due to a spectral change. In a simple scenario, the correlated optical/γ-ray flares could be caused by the same population of emitting particles. The coincidence of the increase in radio polarization with the X-ray flux supports the picture that X-rays are produced via inverse-Compton scattering of radio photons. The observed fractional variability for the γ-ray flare ~0.23 does not exceed that in the optical regime, which is inconsistent with what we usually observe for 3C 279; it could be due to different dependencies of the magnetic field and the external radiation field energy density profiles along the jet. [ABSTRACT FROM AUTHOR]

The exact location of the γ-ray emitting region in blazars is still controversial. In order to attack this problem we present first results of a cross-correlation analysis between radio (11 cm to 0.8 mm wavelength, F-GAMMA programme) and γ-ray (0.1–300 GeV) ∼3.5 yr light curves of 54 Fermi-bright blazars. We perform a source stacking analysis and estimate significances and chance correlations using mixed source correlations. Our results reveal: (i) the first highly significant multiband radio and γ-ray correlations (radio lagging γ rays) when averaging over the whole sample, (ii) average time delays (source frame: 76 ± 23 to 7 ± 9 d), systematically decreasing from cm to mm/sub-mm bands with a frequency dependence τr, γ(ν) ∝ ν−1, in good agreement with jet opacity dominated by synchrotron self-absorption, (iii) a bulk γ-ray production region typically located within/upstream of the 3 mm core region (τ3mm, γ = 12 ± 8 d), (iv) mean distances between the region of γ-ray peak emission and the radio ‘τ = 1 photosphere’ decreasing from 9.8 ± 3.0 pc (11 cm) to 0.9 ± 1.1 pc (2 mm) and 1.4 ± 0.8 pc (0.8 mm), (v) 3 mm/γ-ray correlations in nine individual sources at a significance level where one is expected by chance (probability: 4 × 10−6), (vi) opacity and ‘time lag core shift’ estimates for quasar 3C 454.3 providing a lower limit for the distance of the bulk γ-ray production region from the supermassive black hole (SMBH) of ∼0.8–1.6 pc, i.e. at the outer edge of the broad-line region (BLR) or beyond. A 3 mm τ = 1 surface at ∼2–3 pc from the jet base (i.e. well outside the ‘canonical BLR’) finally suggests that BLR material extends to several parsec distances from the SMBH. [ABSTRACT FROM AUTHOR]

ABSTRACT We report results from a one-week multiwavelength campaign to monitor the BL Lacertae object (BL Lac) S5 0716+714 (on 2009 December 9-16). Nine ground-based telescopes at widely separated longitudes and one space-based telescope aboard the Swift satellite collected optical data. Radio data were obtained from the Effelsberg and Urumqi observatories and X-ray data from Swift. In the radio bands, the source shows rapid [∼(0.5-1.5) d] intraday variability with peak amplitudes of up to ∼10 per cent. The variability at 2.8 cm leads by about 1 d the variability at 6 and 11 cm. This time lag and more rapid variations suggest an intrinsic contribution to the source's intraday variability at 2.8 cm, while at 6 and 11 cm, interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ∼0.8 mag were detected in the V, R and I bands. The X-ray data (0.2-10 keV) do not reveal significant variability on a 4 d time-scale, favouring reprocessed inverse Compton over synchrotron radiation in this band. The characteristic variability time-scales in radio and optical bands are similar. A quasi-periodic variation of 0.9-1.1 d in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical bands are discussed. The lack of a strong radio-optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical) and is consistent with a high jet opacity and a compact synchrotron component peaking at ≃100 GHz in an ongoing very prominent flux-density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution, including γ-ray data from the Fermi satellite. We obtain lower limits for the relativistic Doppler boosting of δ ≥ 12-26, which for a BL Lac-type object is remarkably high. [ABSTRACT FROM AUTHOR]

ABSTRACT We report a search for linear polarization in the active galactic nucleus (AGN) 3C 84 (NGC 1275) at observed frequencies of 239 and 348 GHz, corresponding to rest-frame frequencies of 243 and 354 GHz. We collected polarization data with the IRAM Plateau de Bure Interferometer via Earth rotation polarimetry. We do not detect linear polarization. Our analysis finds 3σ upper limits on the degree of polarization of 0.5 and 1.9 per cent at 239 and 348 GHz, respectively. We regard the influence of Faraday conversion as marginal, leading to expected circular polarizations ≲0.3 per cent. Assuming depolarization by a local Faraday screen, we constrain the rotation measure, as well as the fluctuations therein, to be ≳ 106 rad m−2. From this we estimate line-of-sight magnetic field strengths of ≳100 μG. Given the physical dimensions of 3C 84 and its observed structure, the Faraday screen appears to show prominent small-scale structure, with ΔRM ≳ 106 rad m−2 on projected spatial scales ≲1 pc. [ABSTRACT FROM AUTHOR]

ABSTRACT NRAO 530 is an optically violent variable source and has been studied with multi-epoch multifrequency high-resolution very long baseline interferometry (VLBI) observations. NRAO 530 was monitored with the Very Long Baseline Array (VLBA) at three frequencies (22, 43 and 86 GHz) on 10 consecutive days in 2007 May during observations of the Galactic Centre (Sgr A*). Furthermore, analysis of archival data of NRAO 530 at 15 GHz over the last 10 years allows us to study its detailed jet kinematics. We identified the compact component located at the southern-end of the jet as the VLBI core, consistent with previous studies. The 10-d monitoring data at the three high frequencies were shown to produce high-quality and self-consistent measurements of the component positions, from which we detected for the first time a two-dimensional frequency-dependent position shift. In addition, the repeated measurements also permit us to investigate the interday flux density and structure variability of NRAO 530. We find that it is more variable for the inner jet components than those further out. We obtained apparent velocities for eight jet components with βapp ranging from 2 to 26 c. Accordingly, we estimated physical jet parameters with the minimum Lorentz factor of 14 and Doppler factors in the range of 14-28 (component f). The changes in the morphology of NRAO 530 were related to the motion of separate jet components with the most pronounced changes occurring in the regions close to the core. For NRAO 530, we estimated a position angle swing of 34 per year for the entire inner jet (components d and e). The non-ballistic motion and change of jet orientation make this source another prominent example of a helical and possibly 'swinging' jet. [ABSTRACT FROM AUTHOR]

We report the results from a 6-yr, multi-epoch very long baseline interferometry monitoring of the Seyfert galaxy NGC 3079. We have observed NGC 3079 during eight epochs between 1999 and 2005 predominantly at 5 GHz, but covering the frequency range of 1.7 to 22 GHz. Using our data and observations going back to 1985, we find that the separation of two of the three visible nuclear radio components underwent two decelerations. At the time of these decelerations, the flux density of one of the components increased by factors of 5 and 2, respectively. We interpret these events as a radio jet component undergoing compression, possibly as a result of a collision with interstellar medium material. This interpretation strongly supports the existence of jets surrounded by a clumpy medium of dense clouds within the first few parsec from the central engine in NGC 3079. Moreover, based on recently published simulations of jet interactions with clumpy media, this scenario is able to explain the nature of two additional regions of ageing synchrotron material detected at the lower frequencies as by-products of such interactions, and also the origin of the kpc-scale super-bubble observed in NGC 3079 as the result of the spread of the momentum of the jets impeded from propagating freely. The generalization of this scenario provides an explanation why jets in Seyfert galaxies are not able to propagate to scales of kpc as do jets in radio-loud AGN. [ABSTRACT FROM AUTHOR]

The BL Lacertae object S5 1803+784 has been monitored with very long baseline interferometry at in 43 epochs between 1986.21 and 1993.95. The motivation of this work is to obtain statistically meaningful data with which to study the short-term structural variability in the source on monthly time-scales. We present a detailed analysis of the structural evolution seen in the milliarcsecond-scale jet by way of model-fitting results. Within the simplest identification scenario, the jet can be described as a bent chain of seven jet components within 0.2- and 3-mas separation from the core. New components seem to emerge almost regularly (every 2 yr) from the core. Three jet components approach the brightest and so-called ‘stationary’ component (at ∼1.4 mas at 8.4 GHz) with apparent superluminal motion of 8–11 c. In this paper, we show that the ‘stationary’ component (under the assumption that this is the brightest jet component in each epoch) oscillates, and we discuss several possible explanations. We show that a reconfinement shock can easily reproduce the observed most recent oscillation of the ‘stationary’ component. We present evidence that the average jet ridge line is significantly curved and discuss the possibility that the jet components follow a helically bent path. [ABSTRACT FROM AUTHOR]