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Full list of authors: Jorstad, S. G.; Marscher, A. P.; Raiteri, C. M.; Villata, M.; Weaver, Z. R.; Zhang, H.; Dong, L.; Gomez, J. L.; Perel, M., V; Savchenko, S. S.; Larionov, V. M.; Carosati, D.; Chen, W. P.; Kurtanidze, O. M.; Marchini, A.; Matsumoto, K.; Mortari, F.; Aceti, P.; Acosta-Pulido, J. A.; Andreeva, T.; Apolonio, G.; Arena, C.; Arkharov, A.; Bachev, R.; Bonnoli, G.; Borman, G. A.; Bozhilov, V; Carnerero, M., I; Damljanovic, G.; Ehgamberdiev, S. A.; Elsasser, D.; Frasca, A.; Gabellini, D.; Grishina, T. S.; Gupta, A. C.; Hagen-Thorn, V. A.; Hallum, M. K.; Hart, M.; Hasuda, K.; Hemrich, F.; Hsiao, H. Y.; Ibryamov, S.; Irsmambetova, T. R.; Ivanov, D., V; Joner, M. D.; Kimeridze, G. N.; Klimanov, S. A.; Knoett, J.; Kopatskaya, E. N.; Kurtanidze, S. O.; Kurtenkov, A.; Kuutma, T.; Larionova, E. G.; Leonini, S.; Lin, H. C.; Lorey, C.; Mannheim, K.; Marino, G.; Minev, M.; Mirzaqulov, D. O.; Morozova, D. A.; Nikiforova, A. A.; Nikolashvili, M. G.; Ovcharov, E.; Papini, R.; Pursimo, T.; Rahimov, I; Reinhart, D.; Sakamoto, T.; Salvaggio, F.; Semkov, E.; Shakhovskoy, D. N.; Sigua, L. A.; Steineke, R.; Stojanovic, M.; Strigachev, A.; Troitskaya, Y., V; Troitskiy, I. S.; Tsai, A.; Valcheva, A.; Vasilyev, A. A.; Vince, O.; Waller, L.; Zaharieva, E.; Chatterjee, R., Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales1,2,3. This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare4 in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet5, plasma instabilities6,7 or orbital motion in an accretion disc7,8. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref. 9). BL Lac, the prototype of a subclass of blazars10, is powered by a 1.7 × 108 MSun (ref. 11) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref. 12)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities6 near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image. © 2022 Springer Nature Limited., The research reported here is based on work supported in part by US National Science Foundation grants AST-2108622 and AST-2107806, and NASA Fermi GI grants 80NSSC20K1567, 80NSSC21K1917 and 80NSSC21K1951; by Shota Rustaveli National Science Foundation of Georgia under contract FR-19-6174; by the Bulgarian National Science Fund of the Ministry of Education and Science under grants DN 18-10/2017, DN 18-13/2017, KP-06-H28/3 (2018), KP-06-H38/4 (2019) and KP-06-KITAJ/2 (2020), and by National RI Roadmap Project D01-383/18.12.2020 of the Ministry of Education and Science of the Republic of Bulgaria; by JSPS KAKENHI grant #19K03930 of Japan; by the Ministry of Education, Science and Technological Development of the Republic of Serbia (contract 451-03-9/2021-14/200002) and observing grant support from the Institute of Astronomy and Rozhen NAO BAS through the bilateral joint research project ‘Gaia Celestial Reference Frame (CRF) and fast variable astronomical objects’; by the Agenzia Spaziale Italiana (ASI) through contracts I/037/08/0, I/058/10/0, 2014-025-R.0, 2014-025-R.1.2015 and 2018-24-HH.0 to the Italian Istituto Nazionale di Astrofisica (INAF). H.Z. is supported by the NASA Postdoctoral Program at Goddard Space Flight Center, administered by ORAU. M.V.P. is partially supported by the Russian Foundation for Basic Research grant 20-02-00490. G.B. acknowledges support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and from the Spanish ‘Ministerio de Ciencia e Innovacíon’ (MICINN) through grant PID2019-107847RB-C44. M.D.J. thanks the Brigham Young University Department of Physics and Astronomy for continued support of the extragalactic monitoring programme under way at the West Mountain Observatory. R.C. thanks ISRO for support under the AstroSat archival data utilization programme and BRNS for support through a project grant (sanction no. 57/14/10/2019-BRNS). The measurements at the Hans Haffner Observatory, Hettstadt, Germany, were supported by Baader Planetarium, Mammendorf, Germany. This study was based (in part) on observations conducted using the 1.8-m Perkins Telescope Observatory (PTO) in Arizona, USA, which is owned and operated by Boston University. These results made use of the Lowell Discovery Telescope (LDT) at Lowell Observatory. Lowell Observatory is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy, and operates the LDT in partnership with Boston University, the University of Maryland and the University of Toledo. This paper is partly based on observations made with the IAC-80 operated on the island of Tenerife by the Instituto de Astrofisica de Canarias in the Spanish Observatorio del Teide and on observations made with the LCOGT telescopes, one of whose nodes is located at the Observatorios de Canarias del IAC on the island of Tenerife in the Observatorio del Teide. This paper is partly based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The VLBA is an instrument of the NRAO, USA. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around the world. In mid 2021 the source showed its historical maximum, with R = 11.14. The light curves display many episodes of intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet. We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations in flux density and those in polarisation degree, with a time delay of about 13 h. This suggests a common physical origin. The overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time scales greater than several days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly induced by kink instabilities in the jet, can explain variability on shorter time scales. The latter scenario could also account for the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more closely aligned with our line of sight and the time scales are shortened by relativistic effects., Comment: 15 pages, 16 figures, submitted to MNRAS

We present the results of extensive multi-waveband monitoring of the blazar 3C~279 between 1996 and 2007 at X-ray energies (2-10 keV), optical R band, and 14.5 GHz, as well as imaging with the Very Long Baseline Array (VLBA) at 43 GHz. In all bands the power spectral density corresponds to "red noise" that can be fit by a single power law over the sampled time scales. Variations in flux at all three wavebands are significantly correlated. The time delay between high and low frequency bands changes substantially on time scales of years. A major multi-frequency flare in 2001 coincided with a swing of the jet toward a more southerly direction, and in general the X-ray flux is modulated by changes in the position angle of the jet near the core. The flux density in the core at 43 GHz--increases in which indicate the appearance of new superluminal knots--is significantly correlated with the X-ray flux. We decompose the X-ray and optical light curves into individual flares, finding that X-ray leads optical variations (XO) in 6 flares, the reverse occurs in 3 flares (OX), and there is essentially zero lag in 4 flares. Upon comparing theoretical expectations with the data, we conclude that (1) XO flares can be explained by gradual acceleration of radiating electrons to the highest energies; (2) OX flares can result from either light-travel delays of the seed photons (synchrotron self-Compton scattering) or gradients in maximum electron energy behind shock fronts; and (3) events with similar X-ray and optical radiative energy output originate well upstream of the 43 GHz core, while those in which the optical radiative output dominates occur at or downstream of the core., Accepted for publication in ApJ. 50 pages (including 23 figures and 8 tables)

The blazar AO 0235+164 was claimed to show a quasi-periodic behaviour in the radio and optical bands. Moreover, an extra emission component contributing to the UV and soft X-ray flux was detected, whose nature is not yet clear. A predicted optical outburst was observed in late 2006/early 2007. We here present the radio-to-optical WEBT light curves during the outburst, together with UV data acquired by Swift in the same period. We found the optical outburst to be as strong as the big outbursts of the past: starting from late September 2006, a brightness increase of 5 mag led to the outburst peak in February 19-21, 2007. We also observed an outburst at mm and then at cm wavelengths, with an increasing time delay going toward lower frequencies during the rising phase. Cross-correlation analysis indicates that the 1 mm and 37 GHz flux variations lagged behind the R-band ones by about 3 weeks and 2 months, respectively. These short time delays suggest that the corresponding jet emitting regions are only slightly separated and/or misaligned. In contrast, during the outburst decreasing phase the flux faded contemporaneously at all cm wavelengths. This abrupt change in the emission behaviour may suggest the presence of some shutdown mechanism of intrinsic or geometric nature. The behaviour of the UV flux closely follows the optical and near-IR one. By separating the synchrotron and extra component contributions to the UV flux, we found that they correlate, which suggests that the two emissions have a common origin., Comment: 9 pages, 7 figures, in press for Astronomy and Astrophysics

We present the results of optical (R band) photometric and polarimetric monitoring and Very Long Baseline Array (VLBA) imaging of the blazar S4 0954+658, along with Fermi and gamma;-ray data during a multi-waveband outburst in 2011 March-April. After a faint state with a brightness level R ~17.6 mag registered in the first half of January 2011, the optical brightness of the source started to rise and reached ~14.8 mag during the middle of March, showing flare-like behavior. The most spectacular case of intranight variability was observed during the night of 2011 March 9, when the blazar brightened by ~0.7 mag within ~7 hours. During the rise of the flux the position angle of optical polarization rotated smoothly over more than 300$\deg$. At the same time, within 1$\sigma$ uncertainty a new superluminal knot appeared with an apparent speed of 19.0$\pm$0.3 c. We have very strong evidence for association of this knot with the multi-waveband outburst in 2011 March-April. We also analyze the multi-frequency behavior of S4 0954+658 during a number of minor outbursts from August 2008 to April 2012. We find some evidence of connections between at least two more superluminal ejecta and near-simultaneous optical flares.

Some results of the observational investigation of four candidates for polar-ring galaxies (PRGs) are reported.

The quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum. We present the results of low-energy multifrequency monitoring by the GASP project of the WEBT consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. In the optical-UV band, several results indicate that there is a contribution from a QSO-like emission component, in addition to both variable and polarised jet emission. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R(QSO) ~ 17.85 - 18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and \gamma-ray flux apparently fades in time, likely because of an increasing optical to \gamma-ray flux ratio. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor \delta. Under the hypothesis that in the period 2008-2011 all the \gamma-ray and optical variability on a one-week timescale were due to changes in \delta, this would range between ~ 7 and ~ 21. If the variability were caused by changes in the viewing angle \theta\ only, then \theta\ would go from ~ 2.6 degr to ~ 5 degr. Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model., Comment: 19 pages, 23 figures, in press for Astronomy and Astrophysics

Since 2001, the radio quasar 3C 454.3 has undergone a period of high optical activity, culminating in the brightest optical state ever observed, during the 2004-2005 outburst. The Whole Earth Blazar Telescope (WEBT) consortium has carried out several multifrequency campaigns to follow the source behaviour. The GLAST-AGILE Support Program (GASP) was born from the WEBT to provide long-term continuous optical-to-radio monitoring of a sample of gamma-loud blazars, during the operation of the AGILE and GLAST (now known as Fermi GST) gamma-ray satellites. The main aim is to shed light on the mechanisms producing the high-energy radiation, through correlation analysis with the low-energy emission. Thus, since 2008 the monitoring task on 3C 454.3 passed from the WEBT to the GASP, while both AGILE and Fermi detected strong gamma-ray emission from the source. We present the main results obtained by the GASP at optical, mm, and radio frequencies in the 2008-2009 season, and compare them with the WEBT results from previous years. An optical outburst was observed to peak in mid July 2008, when Fermi detected the brightest gamma-ray levels. A contemporaneous mm outburst maintained its brightness for a longer time, until the cm emission also reached the maximum levels. The behaviour compared in the three bands suggests that the variable relative brightness of the different-frequency outbursts may be due to the changing orientation of a curved inhomogeneous jet. The optical light curve is very well sampled during the entire season, which is also well covered by the various AGILE and Fermi observing periods. The relevant cross-correlation studies will be very important in constraining high-energy emission models., Comment: 5 pages, 5 figures, to be published in A&A (Letters)

We present radio-to-optical data taken by the WEBT, supplemented by VLBA and RXTE observations, of 3C 279. Our goal is to use this extensive database to draw inferences regarding the physics of the relativistic jet. We assemble multifrequency light curves with data from 30 ground-based observatories and the space-based instruments, along with linear polarization vs. time in the optical R band. In addition, we present a sequence of 22 images (with polarization vectors) at 43 GHz at resolution 0.15 milliarcsec, obtained with the VLBA. We analyse the light curves and polarization, as well as the spectral energy distributions at different epochs, corresponding to different brightness states. The IR-optical-UV continuum spectrum of the variable component corresponds to a power law with a constant slope of -1.6, while in the 2.4-10 keV X-ray band it varies in slope from -1.1 to -1.6. The steepest X-ray spectrum occurs at a flux minimum. During a decline in flux from maximum in late 2006, the optical and 43 GHz core polarization vectors rotate by ~300 degrees. The continuum spectrum agrees with steady injection of relativistic electrons with a power-law energy distribution of slope -3.2 that is steepened to -4.2 at high energies by radiative losses. The X-ray emission at flux minimum comes most likely from a new component that starts in an upstream section of the jet where inverse Compton scattering of seed photons from outside the jet is important. The rotation of the polarization vector implies that the jet contains a helical magnetic field that extends ~20 pc past the 43 GHz core., 12 pages, aa.cls style; accepted for publication in A&A

Since the CGRO operation in 1991-2000, one of the primary unresolved questions about the blazar gamma-ray emission has been its possible correlation with the low-energy (in particular optical) emission. To help answer this problem, the Whole Earth Blazar Telescope (WEBT) consortium has organized the GLAST-AGILE Support Program (GASP) to provide the optical-to-radio monitoring data to be compared with the gamma-ray detections by the AGILE and GLAST satellites. This new WEBT project started in early September 2007, just before a strong gamma-ray detection of 0716+714 by AGILE. We present the GASP-WEBT optical and radio light curves of this blazar obtained in July-November 2007, about various AGILE pointings at the source. We construct NIR-to-UV spectral energy distributions (SEDs), by assembling GASP-WEBT data together with UV data from the Swift ToO observations of late October. We observe a contemporaneous optical-radio outburst, which is a rare and interesting phenomenon in blazars. The shape of the SEDs during the outburst appears peculiarly wavy because of an optical excess and a UV drop-and-rise. The optical light curve is well sampled during the AGILE pointings, showing prominent and sharp flares. A future cross-correlation analysis of the optical and AGILE data will shed light on the expected relationship between these flares and the gamma-ray events., 5 pages, 5 figures, to be published in A&A (Letters); revised to match the final version (changes in Fig. 5 and related text)

The quasar-type blazar 3C 454.3 underwent a phase of high activity in summer and autumn 2007, which was intensively monitored in the radio-to-optical bands by the Whole Earth Blazar Telescope (WEBT). The gamma-ray satellite AGILE detected this source first in late July, and then in November-December 2007. In this letter we present the multifrequency data collected by the WEBT and collaborators during the second AGILE observing period, complemented by a few contemporaneous data from UVOT onboard the Swift satellite. The aim is to trace in detail the behaviour of the synchrotron emission from the blazar jet, and to investigate the contribution from the thermal emission component. Optical data from about twenty telescopes have been homogeneously calibrated and carefully assembled to construct an R-band light curve containing about 1340 data points in 42 days. This extremely well-sampled optical light curve allows us to follow the dramatic flux variability of the source in detail. In addition, we show radio-to-UV spectral energy distributions (SEDs) at different epochs, which represent different brightness levels. In the considered period, the source varied by 2.6 mag in a couple of weeks in the R band. Many episodes of fast (i.e. intranight) variability were observed, most notably on December 12, when a flux increase of about 1.1 mag in 1.5 hours was detected, followed by a steep decrease of about 1.2 mag in 1 hour. The contribution by the thermal component is difficult to assess, due to the uncertainties in the Galactic, and possibly also intrinsic, extinction in the UV band. However, polynomial fitting of radio-to-UV SEDs reveals an increasing spectral bending going towards fainter states, suggesting a UV excess likely due to the thermal emission from the accretion disc., 5 pages, 4 figures, accepted for publication in Astronomy & Astrophysics Letters

The quasar 3C279 was the target of an extensive multiwavelength monitoring campaign from January through April 2006, including an optical-IR-radio monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we focus on the results of the WEBT campaign. The source exhibited substantial variability of optical flux and spectral shape, with a characteristic time scale of a few days. The variability patterns throughout the optical BVRI bands were very closely correlated with each other. In intriguing contrast to other (in particular, BL Lac type) blazars, we find a lag of shorter- behind longer-wavelength variability throughout the RVB ranges, with a time delay increasing with increasing frequency. Spectral hardening during flares appears delayed with respect to a rising optical flux. This, in combination with the very steep IR-optical continuum spectral index of ~ 1.5 - 2.0, may indicate a highly oblique magnetic field configuration near the base of the jet. An alternative explanation through a slow (time scale of several days) acceleration mechanism would require an unusually low magnetic field of < 0.2 G, about an order of magnitude lower than inferred from previous analyses of simultaneous SEDs of 3C279 and other FSRQs with similar properties., Accepted for publication in ApJ

The blazar 3C 454.3 underwent an unprecedented optical outburst in spring 2005. This was first followed by a mm and then by a cm radio outburst, which peaked in February 2006. We report on follow-up observations by the WEBT to study the multiwavelength emission in the post-outburst phase. XMM-Newton observations on July and December 2006 added information on the X-ray and UV fluxes. The source was in a faint state. The radio flux at the higher frequencies showed a fast decreasing trend, which represents the tail of the big radio outburst. It was followed by a quiescent state, common at all radio frequencies. In contrast, moderate activity characterized the NIR and optical light curves, with a progressive increase of the variability amplitude with increasing wavelength. We ascribe this redder-when-brighter behaviour to the presence of a "little blue bump" due to line emission from the broad line region, which is clearly visible in the source SED during faint states. Moreover, the data from the XMM-Newton OM reveal a rise of the SED in the UV, suggesting the existence of a "big blue bump" due to thermal emission from the accretion disc. The X-ray spectra are well fitted with a power-law model with photoelectric absorption, possibly larger than the Galactic one. However, the comparison with previous X-ray observations would imply that the amount of absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum presents a curvature, which may depend on the X-ray brightness. In this case, two scenarios are possible., 9 pages, 7 figures, accepted for publication in A&A

In spring 2005 the blazar 3C 454.3 was observed in an unprecedented bright state from the near-IR to the hard X-ray frequencies. A mm outburst peaked in June-July 2005, and it was followed by a flux increase at high radio frequencies. In this paper we report on multifrequency monitoring by the WEBT aimed at following the further evolution of the outburst in detail. In particular, we investigate the expected correlation and time delays between the optical and radio emissions in order to derive information on the variability mechanisms and jet structure. A comparison among the light curves at different frequencies is performed by means of visual inspection and discrete correlation function, and the results are interpreted with a simple model taking into account Doppler factor variations of geometric origin. The high-frequency radio light curves show a huge outburst starting during the dimming phase of the optical one and lasting more than 1 year. The first phase is characterized by a slow flux increase, while in early 2006 a major flare is observed. The lower-frequency radio light curves show a progressively delayed and fainter event, which disappears below 8 GHz. We suggest that the radio major peak is not physically connected with the spring 2005 optical one, but it is actually correlated with a minor optical flare observed in October-November 2005. This interpretation involves both an intrinsic and a geometric mechanism. The former is represented by disturbances travelling down the emitting jet, the latter being due to the curved-jet motion, with the consequent differential changes of viewing angles of the different emitting regions., Comment: 5 pages, 3 figures, to be published in A&A (Letters)

Aims. A huge multiwavelength campaign targeting the blazar AO 0235+164 was organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the variability properties of the source. Methods. Monitoring observations were carried out at cm and mm wavelengths, and in the near-IR and optical bands, while three pointings by the XMM-Newton satellite provided information on the X-ray and UV emission. Results. We present the data acquired during the second observing season, 2004-2005, by 27 radio-to-optical telescopes. The ∼2600 data points collected allow us to trace the low-energy behaviour of the source in detail, revealing an increased near-IR and optical activity with respect to the previous season. Increased variability is also found at the higher radio frequencies, down to ∼15 GHz, but not at the lower ones. While the X-ray (and optical) light curves obtained during the XMM-Newton pointings reveal no significant short-term variability, the simultaneous intraday radio observations with the 100 m telescope at Effelsberg show flux-density changes at 10.5 GHz, which are more likely due to a combination of intrinsic and extrinsic processes. Conclusions. The radio (and optical) outburst predicted to peak around February-March 2004 on the basis of the previously observed 5-6 yr quasi-periodicity did not occur. The analysis of the optical light curves reveals now a longer characteristic time scale of variability of ∼8 yr, which is also present in the radio data. The spectral energy distributions corresponding to the XMM-Newton observations performed during the WEBT campaign are compared with those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray spectra can be described in terms of an extra component, which appears also when the source is faint through a hard UV spectrum and a curvature of the X-ray spectrum. Finally, there might be a correlation between the X-ray and optical bright states with a long time delay of about 5 yr, which would require a geometrical interpretation., La lista completa de autores que integran el documento puede consultarse en el archivo., Facultad de Ciencias Astronómicas y Geofísicas

The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was R = 12.0, which represents the most luminous quasar state thus far observed (M_B ~ -31.4). In order to follow the emission behaviour of the source in detail, a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope (WEBT). Continuous optical, near-IR and radio monitoring was performed in several bands. ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005. The historical radio and optical light curves show different behaviours. Until about 2001.0 only moderate variability was present in the optical regime, while prominent and long-lasting radio outbursts were visible at the various radio frequencies, with higher-frequency variations preceding the lower-frequency ones. After that date, the optical activity increased and the radio flux is less variable. This suggests that the optical and radio emissions come from two separate and misaligned jet regions, with the inner optical one acquiring a smaller viewing angle during the 2004-2005 outburst. Moreover, the colour-index behaviour (generally redder-when-brighter) during the outburst suggests the presence of a luminous accretion disc. A huge mm outburst followed the optical one, peaking in June-July 2005. The high-frequency (37-43 GHz) radio flux started to increase in early 2005 and reached a maximum at the end of our observing period (end of September 2005). VLBA observations at 43 GHz during the summer confirm the, 7 pages, 4 figures, to be published in A&A

Context: Being dominated by non-thermal emission from aligned relativistic jets, blazars allow us to elucidate the physics of extragalactic jets, and, ltimately, how the energy is extracted from the central black hole in radio-loud active galactic nuclei. Aims: Crucial information is provided by broad-band spectral energy distributions (SEDs), their trends with luminosity and correlated multi-frequency variability. With this study we plan to obtain a database of contemporaneous radio-to-optical spectra of a sample of blazars, which are and will be observed by current and future high-energy satellites. Methods: Since December 2004 we are performing a monthly multi-frequency radio monitoring of a sample of 35 blazars at the antennas in Medicina and Noto. Contemporaneous near-IR and optical observations for all our observing epochs are organised. Results: Until June 2006 about 4000 radio measurements and 5500 near-IR and optical measurements were obtained. Most of the sources show significant variability in all observing bands. Here we present the multi-frequency data acquired during the first eighteen months of the project, and construct the SEDs for the best-sampled sources., Comment: 16 pages, 3 large figures, accepted for publication in A&A

A multiwavelength campaign to observe the BL Lac object AO 0235+16 (z = 0.94) was set up by the Whole Earth Blazar Telescope (WEBT) collaboration during the observing seasons 2003-2004 and 2004-2005, involving radio, near-IR and optical photometric monitoring, VLBA monitoring, optical spectral monitoring, and three pointings by the XMM-Newton satellite. Here we report on the results of the first season, which involved the participation of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the first XMM-Newton pointing, which occurred on January 18-19, 2004. Unpublished data from previous epochs were also collected (from 5 optical-NIR and 3 radio telescopes), in order to fill the gap between the end of the period presented in Raiteri et al. (2001) and the start of the WEBT campaign. The contribution of the southern AGN, 2 arcsec distant from the source, is taken into account. It is found to especially affect the blue part of the optical spectrum when the source is faint. In the optical and near-IR the source has been very active in the last 3 years, although it has been rather faint most of the time, with noticeable variations of more than a magnitude over a few days. In contrast, in the radio bands it appears to have been "quiescent" since early 2000. The major radio (and optical) outburst predicted to peak around February-March 2004 (with a six month uncertainty) has not occurred yet. When comparing our results with the historical light curves, two different behaviours seem to characterize the optical outbursts: only the major events present a radio counterpart. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at z = 0.524; the energy index in the 0.2-10 keV range is well constrained: α = 0.645 ± 0.028 and the 1 keV flux density is 0.311 ± 0.008 μJy. The analysis of the X-ray light curves reveals that no significant variations occurred during the pointing. In contrast, simultaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ∼2-3% flux decrease in 6-7 h, which, if intrinsic, would imply a brightness temperature well above the Compton limit and hence a lower limit to the Doppler factor δ ≳ 46. We construct the broad-band spectral energy distribution of January 18-19, 2004 with simultaneous radio data from Effelsberg, optical data from the Nordic Optical Telescope (NOT), optical-UV data from the Optical Monitor onboard XMM-Newton, and X-ray data by the EPIC instruments. Particular care is taken to correct data for extinction due to both the Milky Way and the z = 0.524 absorber. The resulting SED suggests the existence of a bump in the UV spectral region., La lista completa de autores que integran el documento puede consultarse en el archivo., Facultad de Ciencias Astronómicas y Geofísicas, Instituto Argentino de Radioastronomía

BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present UBVRI light curves obtained by theWEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ∼15 600 observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a fewday time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ∼0.1 bluerwhen- brighter slope in the B − R versus R plane; a steeper slope of ∼0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ∼1.1. Doppler factor variations on a “convex” spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism. Reig Torres, Pablo, Pablo.Reig@uv.es

The analysis of multicolor observations of BL Lac obtained in the 1997 and 1999 outbursts shows that in both cases the spectral energy distribution of the variable source was unchanged in the course of outburst. The sources have power-law spectra with slightly different spectral indices (the spectrum was flatter in the more powerful outburst of 1997). The variable source is most probably of synchrotron nature.

The results are given of polarimetric and photometric observations of BL Lacertae-type object OJ 287 for 1972–76. These, and all other data available from the literature, are used in a comparative analysis of polarimetric and photometric properties of the object. The variations of time-scales from several years to several hours are noted. The variability is caused by the flaring up and fading of separate sources (hot spots) of polarized (synchrotron) radiation. The existence of a preferable direction of polarization (θ0=80°) is an indication of a stable magnetic field. It may be used as an argument in favour of the single-body hypothesis of Lacertids. The behaviour of OJ 287 during a 6-day interval may be explained by intensity changes of a single source with constant parameters of polarization. It is found that, for this source,p=42.8%, θ0=101°. The night variations of brightnes and parameters of polarization limit the dimensions of the source responsible for this variability (R