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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., Comment: 12 pages, aa.cls style; accepted for publication in A&A

We present and analyse the WEBT multifrequency observations of 3C 454.3 in the 2007-2008 observing season, including XMM-Newton observations and near-IR spectroscopic monitoring, and compare the recent emission behaviour with the past one. In the optical band we observed a multi-peak outburst in July-August 2007, and other faster events in November 2007 - February 2008. During these outburst phases, several episodes of intranight variability were detected. A mm outburst was observed starting from mid 2007, whose rising phase was contemporaneous to the optical brightening. A slower flux increase also affected the higher radio frequencies, the flux enhancement disappearing below 8 GHz. The analysis of the optical-radio correlation and time delays, as well as the behaviour of the mm light curve, confirm our previous predictions, suggesting that changes in the jet orientation likely occurred in the last few years. The historical multiwavelength behaviour indicates that a significant variation in the viewing angle may have happened around year 2000. Colour analysis reveals a complex spectral behaviour, which is due to the interplay of different emission components. All the near-IR spectra show a prominent Halpha emission line, whose flux appears nearly constant. The analysis of the XMM-Newton data indicates a correlation between the UV excess and the soft-X-ray excess, which may represent the head and the tail of the big blue bump, respectively. The X-ray flux correlates with the optical flux, suggesting that in the inverse-Compton process either the seed photons are synchrotron photons at IR-optical frequencies or the relativistic electrons are those that produce the optical synchrotron emission. The X-ray radiation would thus be produced in the jet region from where the IR-optical emission comes., Comment: 10 pages, 12 figures (7 included in the text, 5 in GIF format), accepted for publication in A&A

We report the detection by the AGILE (Astro-rivelatore Gamma a Immagini LEggero) satellite of an intense gamma-ray flare from the source AGL J1511-0909, associated with the powerful quasar PKS 1510-089, during ten days of observations from 23 August to 1 September 2007. During the observation period, the source was in optical decrease following a flaring event monitored by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). The simultaneous gamma-ray, optical, and radio coverage allows us to study the spectral energy distribution and the theoretical models based on the synchrotron and inverse Compton (IC) emission mechanisms. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector and the hard X-ray imager Super-AGILE sensitive in the 30 MeV - 50 GeV and 18 - 60 keV bands, respectively. Between 23 and 27 August 2007, AGILE detected gamma-ray emission from PKS 1510-089 when this source was located about 50 degrees off-axis, with an average flux of (270 +/- 65) x 10^{-8} photons cm^{-2} s^{-1} for photon energy above 100 MeV. In the following period, 28 August - 1 September, after a satellite re-pointing, AGILE detected the source at about 35 degrees off-axis, with an average flux (E > 100 MeV) of (195 +/- 30) x 10^{-8} photons cm^{-2} s^{-1}. No emission was detected by Super-AGILE, with a 3-sigma upper limit of 45 mCrab in 200 ksec. The spectral energy distribution is modelled with a homogeneous one-zone synchrotron self Compton (SSC) emission plus contributions by external photons: the SSC emission contributes primarily to the X-ray band, whereas the contribution of the IC from the external disc and the broad line region match the hard gamma-ray spectrum observed., Comment: 5 pages, 5 figures, accepted for publication in A&A

[Abridged] We report on a multiwavelength observation of the blazar 3C 454.3 (which we dubbed "crazy diamond") carried out on November 2007 by means of the astrophysical satellites AGILE, INTEGRAL, Swift, the WEBT Consortium, and the optical-NIR telescope REM. 3C 454.3 is detected at a $\sim 19-\sigma$ level during the 3-week observing period, with an average flux above 100 MeV of $F_{\rm E>100MeV} = (170 \pm 13) \times 10^{-8}$ \phcmsec. The gamma-ray spectrum can be fit with a single power-law with photon index $\Gamma_{\rm GRID} = 1.73 \pm 0.16$ between 100 MeV and 1 GeV. We detect significant day-by-day variability of the gamma-ray emission during our observations, and we can exclude that the fluxes are constant at the 99.6% ($\sim 2.9 \sigma$) level. The source was detected typically around 40 degrees off-axis, and it was substantially off--axis in the field of view of the AGILE hard X-ray imager. However, a 5-day long ToO observation by INTEGRAL detected 3C 454.3 at an average flux of about $F_{\rm 20-200 keV} = 1.49 \times 10^{-3}$ \phcmsec with an average photon index of $\Gamma_{\rm IBIS} = 1.75 \pm 0.24$ between 20--200 keV. Swift also detected 3C 454.3 with a flux in the 0.3--10 keV energy band in the range $(1.23-1.40) \times 10^{-2}$ \phcmsec{} and a photon index in the range $\Gamma_{\rm XRT} = 1.56-1.73$. In the optical band, both WEBT and REM show an extremely variable behavior in the $R$ band. A correlation analysis based on the entire data set is consistent with no time-lags between the gamma-ray and the optical flux variations. Our simultaneous multifrequency observations strongly indicate that the dominant emission mechanism between 30 MeV and 30 GeV is dominated by inverse Compton scattering of relativistic electrons in the jet on the external photons from the broad line region., Comment: Accepted for publication in ApJ. Abridged Abstract. 37 pages, 14 Figures, 3 Tables

We report the gamma-ray activity from the intermediate BL Lac S5 0716+714 during observations acquired by the AGILE satellite in September and October 2007. These detections of activity were contemporaneous with a period of intense optical activity, which was monitored by GASP-WEBT. This simultaneous optical and gamma-ray coverage allows us to study in detail the light curves, time lags, gamma-ray photon spectrum, and Spectral Energy Distributions (SEDs) during different states of activity. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector (GRID) and the hard X-ray imager (Super-AGILE), which are sensitive to the 30 MeV-50 GeV and 18-60 keV energy ranges, respectively. Observations were completed in two different periods, the first between 2007 September 4-23, and the second between 2007 October 24-November 1. Over the period 2007 September 7-12, AGILE detected gamma-ray emission from the source at a significance level of 9.6-sigma with an average flux (E>100 MeV) of (97 +/- 15) x 10^{-8} photons cm^{-2} s^{-1}, which increased by a factor of at least four within three days. No emission was detected by Super-AGILE for the energy range 18-60 keV to a 3-sigma upper limit of 10 mCrab in 335 ksec. In October 2007, AGILE repointed toward S5 0716+714 following an intense optical flare, measuring an average flux of (47 +/- 11) x 10^{-8} photons cm^{-2} s^{-1} at a significance level of 6.0-sigma. The gamma-ray flux of S5 0716+714 detected by AGILE is the highest ever detected for this blazar and one of the most intense gamma-ray fluxes detected from a BL Lac object. The SED of mid-September appears to be consistent with the synchrotron self-Compton (SSC) emission model, but only by including two SSC components of different variabilities., Comment: 5 pages, 5 figures, accepted by A&A Letters 18/8/2008

We report the gamma-ray activity from the Intermediate BL Lac S5 0716+714 during 2007 September-October observations by the AGILE satellite, coincident with a period of intense optical activity of the source monitored by GASP-WEBT. AGILE observed the source with its two co-aligned imagers, the Gamma-Ray Imaging Detector (GRID) and the hard X-ray imager (Super-AGILE) sensitive in the energy range 30 MeV-50 GeV and 18-60 keV respectively, in two different periods: the first between 4 and 23 September 2007, the second between 24 October and 1 November 2007. Over the period 7-12 September, AGILE detected gamma-ray emission from the source at a significance level of 9.6-sigma with an average flux (E>100 MeV) of (97+/-15) x 10^{-8} photons cm^{-2} s^{-1}, increasing by a factor of at least four within three days. No emission was detected by Super-AGILE in the energy range 18-60 keV, with a 3-sigma upper limit of 10 mCrab in 335 ksec. The gamma-ray flux of S5 0716+714 detected by AGILE is the highest ever detected for this blazar and one of the most intense gamma-ray fluxes detected from a BL Lac object. The Spectral Energy Distribution (SED) of mid-September seems to be consistent with the synchrotron self-Compton (SSC) emission model, but only by including two SSC components with different variability. In October 2007 AGILE repointed toward S5 0716+714 following an intense optical flare, measuring an average flux of (47+/-11) x 10^{-8} photons cm^{-2} s^{-1} at a significance level of 6.0-sigma. The gamma-ray flux during both AGILE pointings appears to be highly variable on timescales of 1 day., Comment: 8 pages, 4 figures, "Workshop on Blazar Variability across the Electromagnetic Spectrum", 2008 April 22-25, Palaiseau, France

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., Comment: 5 pages, 4 figures, accepted for publication in Astronomy & Astrophysics Letters

AIMS: Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy gamma-rays, through the investigation of the prototypical blazar PKS 2155-304, which shows complex behaviour. METHODS: We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. RESULTS: We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with Gamma_2 ~ 2.4 harder than Gamma_1 ~ 2.6 and a break at about 3.5 keV. This suggests that the soft X-rays (E < 3.5 keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays (E > 3.5 keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour., Comment: 5 pages, 3 figures. A&A Letters, Accepted for publication

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., Comment: 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 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

In September 2005, the observing program of the Whipple 10 m gamma-ray telescope was redefined to be dedicated almost exclusively to AGN monitoring. Since then the five Northern Hemisphere blazars that had already been detected at Whipple are monitored routinely each night that they are visible. Thanks to the efforts of a large number of multiwavelength collaborators, the first year of this program has been very successful. We report here on the analysis of Markarian 421 observations taken from November, 2005 to May, 2006 in the gamma-ray, X-ray, optical and radio bands., Comment: 4 pages; contribution to the 30th International Cosmic Ray Conference, Merida, Mexico, July 2007

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., Comment: 9 pages, 7 figures, accepted for publication in A&A

We present the results from a study of the long-term optical spectral variations of BL Lacertae, using the long and well-sampled B and R-band light curves of the Whole Earth Blazar Telescope (WEBT) collaboration, binned on time intervals of 1 day. The relation between spectral slope and flux (the spectrum gets bluer as the source flux increases) is well described by a power-law model, although there is significant scatter around the best-fitting model line. To some extent, this is due to the spectral evolution of the source (along well-defined loop-like structures) during low-amplitude events, which are superimposed on the major optical flares, and evolve on time scales of a few days. The "bluer-when-brighter" mild chromatism of the long-term variations of the source can be explained if the flux increases/decreases faster in the B than in the R band. The B and R-band variations are well correlated, with no significant, measurable delays larger than a few days. On the other hand, we find that the spectral variations lead those in the flux light curves by ~ 4 days. Our results can be explained in terms of Doppler factor variations due to changes in the viewing angle of a curved and inhomogeneous emitting jet., Comment: 7 pages, 5 figures, accepted for publication in A&A

At the end of July 2006, the blazar PKS 2155-304 (z=0.116) underwent a strong outburst observed at TeV energies by HESS (up to 17 Crab flux level at E > 200 GeV). The Swift satellite followed the evolution of the source for about one month. The data analysis - reported in another paper - has shown that, despite the violent activity at TeV energies, the synchrotron energy distribution increased in normalization, but only with a small shift in frequency. In the present work, we fit the broad-band spectrum with a log-parabolic model, to search for indications of intrinsic curvature, which in turn is usually interpreted as a signature of energy-dependent acceleration mechanisms of electrons., Comment: 2 pages, 1 figure, 1 Table. Proceedings of the First GLAST Symposium, to be published

We present all the publicly available data, from optical/UV wavelengths (UVOT) to X-rays (XRT, BAT), obtained from Swift observations of the blazar PKS 2155-304, performed in response to the rapid alert sent out after the strong TeV activity (up to 17 Crab flux level at E > 200 GeV) at the end of July 2006. The X-ray flux increased by a factor of 5 in the 0.3-10 keV energy band and by a factor of 1.5 at optical/UV wavelengths, with roughly one day of delay. The comparison of the spectral energy distribution built with data quasi-simultaneous to the TeV detections shows an increase of the overall normalization with respect to archival data, but only a small shift of the frequency of the synchrotron peak that remains consistent with the values reported in past observations when the TeV activity was much weaker., Comment: 13 pages, 3 figures. ApJ Letters, accepted for publication

New data and results on the optical behaviour of the blazar PKS 0735+178 are presented. In addition the whole historical light curve, and a new photometric calibration of comparison stars are reported. Optical spectral indexes are calculated and studied on years scales, while several methods for time-series analysis are applied to the whole historical series and to each observing season of our data set. This allowed to search and identify optical variability modes, characteristic timescales and the signal power spectrum over 3 decades in time. In the last 10 years the optical flux of PKS 0735+178 exhibited a rather achromatic long-term behaviour and a variability mode resembling the shot-noise. The brightness level was in an intermediate/low state with a mild flaring activity and a superimposition/succession of rapid and slower flares, with no extraordinary/isolated outbursts but, at any rate, characterized by 1 major active phase in 2001. Several mid-term scales (days, weeks) were found, the more common falling into values of about 27-28 days, 50-56 days and 76-79 days. The rapid variability in the historical curve appear to be modulated by a general, slower and rather oscillating trend, where typical timescales of about 4.5, 8.5 and 11-13 years can be identified. This spectral and temporal analysis, accompanying our data publication, suggests the occurrence of distinctive variability signatures at days/weeks scales, that can likely be of transitory nature. On the other hand the possible pseudo-cyclical or multi-component modulations at long times could be more stable, recurrent and correlated to the bimodal radio flux behaviour and the twisted radio structure observed by many years in this blazar., Comment: 21 pages, 13 figures, 3 tables. Typeset with a LaTex2e-AMSLaTex code prepared by the author (using AA vers. 6.1, June 2006, class, and natbib, hyperref, graphicx, packages). Accepted for publication in Astronomy & Astrophysics

We performed optical spectroscopic monitoring of the BL Lac object AO 0235+164 (z=0.94) with the VLT and TNG telescopes from Aug. 2003 to Dec. 2004, during an extended WEBT campaign. The flux of this source is both contaminated and absorbed by a foreground galactic system at z=0.524, the stars of which can act as gravitational micro-lenses. In this period the object was in an optically faint, though variable state, and a broad Mg II emission line was visible at all epochs. The spectroscopic analysis reveals an overall variation in the Mg II line flux of a factor 1.9, while the corresponding continuum flux density changed by a factor 4.3. Most likely, the photoionising radiation can be identified with the emission component that was earlier recognised to be present as a UV-soft-X-ray bump in the source SED and that is visible in the optical domain only in very faint optical states. We estimate an upper limit to the BLR size of a few light months from the historical minimum brightness level; from this we infer the maximum amplification of the Mg II line predicted by the microlensing scenario. Unless we have strongly overestimated the size of the BLR, only very massive stars could significantly magnify the broad Mg II emission line, but the time scale of variations due to these (rare) events would be of several years. In contrast, the continuum flux, coming from much smaller emission regions in the jet, could be affected by microlensing from the more plausible MACHO deflectors, with variability time scales of some months., Comment: 10 pages, 6 figures (1 jpg, 5 eps included), in press for 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)

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 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. 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. We present the data acquired during the second observing season, 2004-2005, by 27 radio-to-optical telescopes. They reveal 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. 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 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., Comment: 14 pages, 10 figures (8 included in the text and 2 PNG files), in press for A&A

We study the synchrotron flaring behaviour of the blazar 3C279 based on an extensive dataset covering 10 years of monitoring at 19 different frequencies in the radio-to-optical range. The properties of a typical outburst are derived from the observations by decomposing the 19 lightcurves into a series of self-similar events. This analysis is achieved by fitting all data simultaneously to a succession of outbursts defined according to the shock-in-jet model of Marscher & Gear (1985). We compare the derived properties of the synchrotron outbursts in 3C279 to those obtained with a similar method for the quasar 3C273. It is argued that differences in the flaring behaviour of these two sources are intrinsic to the sources themselves rather than being due to orientation effects. We also compare the start times and flux densities of our modelled outbursts with those measured from radio components identified in Very Long Baseline Interferometry (VLBI) images. We find VLBI counterparts for most of our model outbursts, although some high-frequency peaking events are not seen in the radio maps. Finally, we study the link between the appearance of a new synchrotron component and the EGRET gamma-ray state of the source at 10 different epochs. We find that an early-stage shock component is always present during high gamma-ray states, while in low gamma-ray states the time since the onset of the last synchrotron outburst is significantly longer. This statistically significant correlation supports the idea that gamma-ray flares are associated with the early stages of shock components propagating in the jet. We note, however, that the shock wave is already beyond the broad line region during the gamma-ray flaring., Comment: 10 pages, 5 figures, accepted for publication in A&A

BL Lacertae has been the target of several observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration and is one of the best studied blazars at all accessible wavelengths. A recent analysis of the optical and radio variability indicates that part of the radio variability is correlated with the optical light curve. Here we present an analysis of a huge VLBI data set including 108 images at 15, 22, and 43 GHz obtained between 1995 and 2004. The aim of this study is to identify the different components contributing to the single-dish radio light curves. We obtain separate radio light curves for the VLBI core and jet and show that the radio spectral index of single-dish observations can be used to trace the core variability. Cross-correlation of the radio spectral index with the optical light curve indicates that the optical variations lead the radio by about 100 days at 15 GHz. By fitting the radio time lags vs. frequency, we find that the power law is steeper than expected for a freely expanding conical jet in equipartition with energy density decreasing as the square of the distance down the jet as in the K\"onigl model. There is a section of the compact radio jet where the emission is weak such that flares propagating down the jet are bright first in the core region with a secondary increase in flux about 1.0 mas from the core. This illustrates the importance of direct imaging to the interpretation of multi-wavelength light curves that can be affected by several distinct components at any given time. We discuss how the complex behaviour of the light curves and correlations can be understood within the framework of a precessing helical jet model., Comment: 13(+5) pages, 12 figures, accepted for publication in A&A, replaced because of layout problems

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, Comment: 7 pages, 4 figures, to be published in A&A

In this paper we analyse five observations of the BL Lac object AO 0235+16 performed with the Chandra and XMM-Newton satellites during the years 2000-2005. In the February 2002 observation the source is found in a bright state and presents a steep X-ray spectrum, while in all the other epochs it is faint and the spectrum is hard. The soft X-ray spectrum appears to be strongly absorbed, likely by the intervening system at z=0.524, which also absorbs the optical-UV radiation. We find that models that consider spectral curvature are superior to single power law ones in fitting the X-ray spectrum. In particular, we favour a double power law model, which agrees with the assumption of a superposition of two different components in the X-ray domain. Both in the Chandra and in one of the XMM-Newton observations, a tentative detection of the redshifted Fe Kalpha emission line may suggest its origin from the inner part of an accretion disc. Thermal emission from this accretion disc might explain the UV-soft-X-ray bump that appears in the spectral energy distributions, when the X-ray spectra are complemented with the optical-UV data from the Optical Monitor onboard XMM-Newton. More likely, the bump can be interpreted in terms of an additional synchrotron component emitted from an inner region of the jet with respect to that where the lower-energy emission comes from. An inspection of the X-ray light curves reveals that intraday variability occurs only when the source is in a bright state., Comment: In press for A&A

(abridged) We performed a homogeneous and systematic analysis of simultaneous X-ray and optical/UV properties of a group of 15 gamma-ray loud AGN, using observations performed with XMM-Newton. The sample is composed of 13 blazars (6 BL Lac and 7 Flat-Spectrum Radio Quasar) and 2 radio galaxies, that are associated with detections at energies >100 MeV. The data for 7 of them are analyzed here for the first time, including the first X-ray observation of PKS 1406-706. Then, the spectral characteristics of the sources in the present sample have been compared with those in previous catalogs of blazars and other AGN, in order to search for any difference or long term change. All the selected sources appear to follow the classic "blazar sequence" and the spectral energy distributions (SED) built with the present X-ray and optical/UV data and completed with historical data, confirm the findings of previous studies on this type of sources. Some sources display interesting features worth noting: four of them, namely AO 0235+164, PKS 1127-145, S5 0836+710 and PKS 1830-211 show the presence of an intervening absorption system along the line of sight, but only the latter is known to be surely gravitationally lensed. AO 0235+164 was detected during an outburst and its SED shows a clear shift of the synchrotron peak. 3C 273 shows a change in the state with respect to the previous BeppoSAX observations, that can be interpreted as an increase of the Seyfert-like component and a corresponding decline of the jet emission. This is consistent with the monitoring at radio wavelengths performed during the same years. PKS 1406-706 is detected with a flux higher than in the past, but with a corresponding low optical flux. Moreover, although it is classified as FSRQ, the SED can be modelled with a simple synchrotron self-Compton model., Comment: 13 pages + 12 pages of on-line material; 6 figures, 12 Tables. Accepted for publication on A&A Main Journal. Revision of English grammar

Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 10^{12} K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, possible under non-stationary conditions, would lead to IC avalanches in the soft-gamma-ray energy band during transient periods. For the first time, broadband signatures of possible IC catastrophes were searched for in S5 0716+71. A multifrequency observing campaign targetting S5 0716+71 was carried out in November 2003 under the framework of the European Network for the Investigation of Galactic nuclei through Multifrequency Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope (WEBT), involving a pointing by the soft-gamma-ray satellite INTEGRAL, optical, near-infrared, sub-millimeter, millimeter, radio, and Very Long Baseline Array (VLBA) monitoring. S5 0716+71 was very bright at radio frequencies and in a rather faint optical state during the INTEGRAL pointing; significant inter-day and low intra-day variability was recorded in the radio regime, while typical fast variability features were observed in the optical band. No correlation was found between the radio and optical emission. The source was not detected by INTEGRAL, neither by the X-ray monitor JEM-X nor by the gamma-ray imager ISGRI, but upper limits to the source emission in the 3-200 keV energy band were estimated. A brightness temperature Tb>2.1x10^{14} K was inferred from the radio variability, but no corresponding signatures of IC avalanches were recorded at higher energies. The absence of IC-catastrophe signatures provides either a lower limit delta>8 to the Doppler factor affecting the radio emission or strong constraints for modelling of the Compton catastrophes in S5 0716+71., Comment: 15 pages, 3 EPS figures, 3 tables, to appear in A&A

Notwithstanding the big efforts devoted to the investigation of the mechanisms responsible for the high-energy (E>100 MeV) gamma-ray emission in active galactic nuclei (AGN), the definite answer is still missing. The X-ray energy band (0.4-10 keV) is crucial for this type of study, since both synchrotron and inverse Compton emission can contribute to the formation of the continuum. Within an ongoing project aimed at the investigation of the gamma-ray emission mechanism acting in the AGN detected by the EGRET telescope onboard CGRO, we firstly focused on the sources for which X-ray and optical/UV data are available in the XMM-Newton public archive. The preliminary results are outlined here., Comment: 2 pages, no figures. Proceedings of the Conference "The X-ray Universe", San Lorenzo de El Escorial (Madrid, Spain), 26-30 September 2005

A multiwavelength campaign on the BL Lac object AO 0235+16 has been organized by the WEBT collaboration during the observing seasons 2003-2004 and 2004-2005. Here we report on the results of the first season, which saw 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. In the optical and near-IR the source has been very active in the last 3 years, though rather faint most of the time, with noticeable variations of more than a mag in a few days. In contrast, in the radio bands it appears "quiescent" since early 2000. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at z=0.524. No significant variation of the X-ray flux occurred during the pointing. In contrast, contemporaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ~2-3% flux decrease in 6-7 hours, which, if intrinsic, would imply a brightness temperature well above the Compton limit. 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, optical-UV data from the Optical Monitor onboard XMM-Newton, and X-ray data by the EPIC instruments. Particular care was 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., Comment: 15 pages, 12 figures (7 included, 5 separate PNG files), in press for Astronomy and Astrophysics

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 the WEBT 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 $\sim 15600$ 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 few-day 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 $\sim 0.1$ bluer-when-brighter slope in the $B-R$ versus $R$ plane; a steeper slope of $\sim 0.4$ 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 $\sim 1.1$. Doppler factor variations on a "convex" spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism., Comment: 11 pages, 9 figures (4 included in the text, 5 in jpg format), to be published in Astronomy & Astrophysics

The long-term variability of the multiwavelength blazar emission can be interpreted in terms of orientation variations of a helical, inhomogeneous, non-thermally emitting jet, possibly caused by the orbital motion of the parent black hole in a binary system (Villata & Raiteri 1999). The helical-jet model is here applied to explain the quasi-periodic radio-optical light curves and the broad-band spectral energy distributions (SEDs) of the BL Lac object AO 0235+16. Through a suitable choice of the model parameters, the helix rotation can well account for the periodicity of the main radio and optical outbursts and for the corresponding SED variability, while the interspersed minor radio events could be interpreted as due either to some local distortions of the helical structure or to other phenomena contributing to the source emission. In particular, the probable existence of flow instabilities provides a viable interpretation for the non-periodic features., Comment: 14 pages, 7 eps figures, 1 jpg figure, A&A, in press

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one., Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for publication in A&A

We present the most continuous data base of optical $BVR_{c}I_{c}$ observations ever published on the BL Lacertae object GC 0109+224, collected mainly by the robotic telescope of the Perugia University Observatory in the period November 1994-February 2002. These observations have been complemented by data from the Torino Observatory, collected in the period July 1995-January 1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid optical variations and six major outbursts were observed at the beginning and end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the beginning of 2002. Fast and large-amplitude drops characterized its flux behaviour. The $R_c$ magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy), with a mean value of 14.9 (3.38 mJy). In the periods where we collected multi-filter observations, we analyzed colour and spectral indexes, and the variability patterns during some flares. The long-term behaviour seems approximatively achromatic, but during some isolated outbursts we found evidence of the typical loop-like hysteresis behaviour, suggesting that rapid optical variability is dominated by non-thermal cooling of a single emitting particle population. We performed also a statistical analysis of the data, through the discrete correlation function (DCF), the structure function (SF), and the Lomb-Scargle periodogram, to identify characteristic times scales, from days to months, in the light curves, and to quantify the mode of variability. We also include the reconstruction of the historical light curve and a photometric calibration of comparison stars, to favour further extensive optical monitoring of this interesting blazar., Comment: 13 pages, 11 PS figures, 1 EPS figure, 3 tables, accepted by Astronomy and Astrophysics. Uses A&A documentclass aa.cls, and the package graphicx.sty

We present two BeppoSAX observations of BL Lacertae as part of a multiwavelength radio-to-TeV campaign. During the first observation we observe a faint Compton spectrum, while during the second, we detect a synchrotron spectrum with the highest [2-10] keV flux ever measured; above 10 keV an inverse Compton component begin to dominate. The synchrotron flux is very variable with time scales of 1 hr. We describe four different SED shifting the synchrotron peak both in frequency and flux intensity and we sketch a scenario in which a blob moves along a jet and can be located in or outside the BLR. This implies different radiative mechanism at work, SSC or external Compton, producing different high energy spectra., Comment: 5 pages, 2 figures; to appear in the proccedings of the V Italian Conference on AGNs

We have organized an extensive multiwavelength campaign on BL Lacertae in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio frequencies, in the optical - carried out by the WEBT collaboration -, in X-rays - using BeppoSAX and RXTE -, and at VHE gamma-rays with the CAT and HEGRA Cherenkov telescope facilities. In this paper, we are presenting first results from this campaign. The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10 - 20 hour segments, and revealed intraday variability on time scales of ~ 1.5 hours. The X-ray observations of Nov. 1 - 2, 2000, revealed significant variability on similar time scales, and provided evidence for the synchrotron spectrum extending out to ~ 10 keV during that time. At higher energies, the onset of the hard power-law commonly observed in radio-loud quasars and radio-selected BL Lac objects is seen. From the energy-resolved X-ray variability patterns, we find evidence that electron cooling might not be dominated by one single radiation mechanism; rather, a combination of synchrotron and Compton cooling seems to be required., Comment: To appear in "Blazar Astrophysics with BeppoSAX and other Observatories", Frascati, Italy, Dec. 10 - 11, 2001. 6 pages, including 5 figures. Uses BSAXwork.sty

We report on two BeppoSAX observations of BL Lac (2200+420) performed respectively in June and December 1999, as part of a ToO program to monitor blazars in high states of activity. During both runs the source has been detected up to 100 keV, but it showed quite different spectra: in June it was concave with a very hard component above 5-6 keV (alpha_1 ~ 1.6; alpha_2 ~ 0.15); in December it was well fitted by a single power law (alpha ~ 0.6). During the first BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3 - 2 keV flux doubled in ~ 20 minutes, while the flux above 4 keV was almost constant. This frequency-dependent event is one of the shortest ever recordered for BL Lac objects and places lower limits on the dimension and the magnetic field of the emitting region and on the energy of the synchrotron radiating electrons. A similar but less extreme behaviour is detected also in optical light curves, that display non-simultaneous, smaller fluctuations of ~ 20% in 20 min. We fit the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but effective SSC + external Compton scenario, highlightning the importance of the location of the emitting region with respect to the Broad Line Region and the relative spectral shape dependence. We compare our data with historical radio to gamma-ray Spectral Energy Distributions., Comment: 19 pages, 6 figures, accepted by Astronomy & Astrophysics

We present the BeppoSAX observations of 3C 279 performed in 1997 January 13-23 simultaneously with gamma-ray pointings with EGRET on board CGRO and optical observations at the Torino Observatory (1997 January 11-18). ISO data close to this epoch are also available. We compare the derived Spectral Energy Distribution with those obtained at all other epochs with adequate multiwavelength coverage. Simple spectral models fitted to the multiepoch SEDs suggest that changes in intensity can be ascribed mainly to the variation of the bulk Lorentz factor of the plasma in the jet., Comment: 34 pages (draft format), 6 figures, Accepted for publication in ApJ

New optical and radio data on the BL Lacertae object AO 0235+16 have been collected in the last four years by a wide international collaboration, which confirm the intense activity of this source. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) first-light campaign organized in November 1997. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection, Discrete Correlation Function analysis, and Discrete Fourier Transform technique. The major radio outbursts are found to repeat quasi-regularly with a periodicity of about 5.7 years; this period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them., Comment: to be published in A&A

Light curves of 3C 279 are presented in optical (R-band), X-rays (RXTE/PCA), and gamma rays (CGRO/EGRET) for 1999 Jan-Feb and 2000 Jan-Mar. During both of those epochs the gamma-ray levels were high, and all three observed bands demonstrated substantial variation, on time scales as short as one day. Correlation analyses provided no consistent pattern, although a rather significant optical/gamma-ray correlation was seen in 1999, with a gamma-ray lag of ~2.5 days, and there are other suggestions of correlations in the light curves. For comparison, correlation analysis is also presented for the gamma-ray and X-ray light curves during the large gamma ray flare in 1996 Feb and the two gamma-bright weeks leading up to it; the correlation at that time was strong, with a gamma-ray/X-ray offset of no more than 1 day., Comment: 20 pages, including 7 figures; accepted by The Astrophysical Journal

We present the preliminary results of a long BeppoSAX observation of the BL Lac object Mkn501 carried out in June 1999. The source was fainter than found during the BeppoSAX pointings of 1997 and 1998, but is still detected with a good signal-to-noise ratio up to ~40 keV. The X-ray spectrum in the energy range 0.1-40 keV, produced through synchrotron radiation, is steeper than in the previous years, it is clearly curved, and peaks (in nu*F_nu) at ~0.5 keV. This energy is much lower than those at which the synchrotron component was found to peak in 1997 and 1998. Some intraday variability suggests that activity of the source on small time scales accompanies the large long time scale changes of brightness and spectrum., Comment: 4 pages, Latex, 2 PostScript figures, to appear in the Proceedings of the Conference "X-ray Astronomy '999: Stellar Endpoints, AGNs, and the Diffuse X-ray Background" (Bologna, 6-10 September 1999)

We present the results of two ToO BeppoSAX observations of BL Lac. During the first observation we detected both the synchrotron and the Compton components. Fast time variability was present, but only for the synchrotron component. During the second observation the spectrum was flatter and only the Compton component was present. Four different SED are presented and can be described by a blob moving along a jet and responsible for the SSC emission. The seed photons for the Compton scattering are the synchrotron photons themself plus possibly external photons coming from the broad line region., Comment: 3 pages, 2 figures, proc. of AGN2000, MemSAiT, in press

We present BeppoSAX observations of three gamma-ray emitting quasars, namely 0836+710, 1510-089 and 2230+114. The three objects have been detected up to 100 keV showing extremely flat power-law spectra above 2 keV (energy index alpha_{2-10}=0.3-0.5). The soft X-ray spectrum of 0836+710 implies either an absorption column density higher than the galactic one or an intrinsically very hard slope (alpha_{0.1-1}=-0.2) below 1 keV. 1510-089 shows a soft excess, with the low energy spectrum steeper (alpha_{0.1-1}=1.6) than the high energy power-law. The results are discussed in the framework of current Inverse Compton models for the high energy emission of Flat Spectrum Radio Quasars and are used to estimate the physical quantities in the jet emitting region and to shed light on the energy transport mechanisms in jets. Finally we discuss the estimates of the jet luminosity in the context of the Blandford & Znajek mechanism for jet production., Comment: 9 figures, accepted for publication in ApJ

We report first observational results of multifrequency campaigns on the prominent Virgo blazars 3C 273 and 3C 279 which were carried out in January and February 1999. Both blazars are detected from radio to gamma-ray energies. We present the measured X- to gamma-ray spectra of both sources, and for 3C 279 we compare the 1999 broad-band (radio to gamma-ray) spectrum to measured previous ones., Comment: 5 pages including 3 figures, latex2e, to appear in: 'Proc. of the 5th Compton Symposium', AIP, in press

During the observation period 1997, the nearby Blazar Mkn 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using radio, optical, soft and hard X-ray and TeV data. We concentrate on the medium-timescale variability of the broadband spectra, averaged over weekly intervals. We confirm the previously found correlation between soft and hard X-ray emission and the emission at TeV energies, while the source shows only minor variability at radio and optical wavelengths. The non-linear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet, and flux variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with a Synchrotron Self-Compton (SSC) dominated leptonic jet model, using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet, accounting for gamma-gamma absorption and pair production within the source as well as due to the intergalactic infrared background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found to be negligible at TeV energies. We find that high levels of the X-ray and TeV fluxes can be explained by a hardening of the energy spectra of electrons injected at the base of the jet, in remarkable contrast to the trend found for gamma-ray flares of the flat-spectrum radio quasar PKS 0528+134., Comment: accepted for publication in ApJ, 31 pages, 11 figures

ON 231 was observed with BeppoSAX in May and June 1998, following an exceptional optical outburst which occured in April--May. We measured the X-ray spectrum from 0.1 up to 100 keV. In both occasions the spectrum had a concave shape, with a break detected at about 4 and 2.5 keV, respectively. We interpret the steeper component at energies below the break as due to synchrotron emission and the extremely flat component at energies above the break as due to inverse Compton emission. This is so far the best example in which both the synchrotron and the Inverse Compton component are detected simultaneously and with the same instruments in the X-ray spectrum of a blazar. We detect a decrease of the X-ray flux of about 30% below the break between the first and the second observation, and smaller variability amplitude between 4 and 10 keV. During the May observation we also detected a fast variability event with the flux below 4 keV increasing by about a factor of three in 5 hours. Above 4 keV no variability was detected. We discuss these results in the framework of synchrotron self-Compton models., Comment: Accepted for publication in A&A, main journal

We report intensity variations of different spectral components in the BL Lac object S5 0716+714 detected during coordinated BeppoSAX and optical observations in 1996 and 1998. The transition between synchrotron and inverse Compton emission has been clearly detected as sharp X-ray spectral breaks at around 2-3 keV on both occasions. Correlated optical and soft X-ray variability was found during the second BeppoSAX pointing when intensive optical monitoring could be arranged. The hard (Compton) component changed by a factor of 2 between the two observations, but remained stable within each exposure. During events of rapid variability S5 0716+714 showed spectral steepening with intensity, a behaviour rarely observed in BL Lacs. We interpret these findings as the probable consequence of a shift of the synchrotron peak emission from the IR/optical band to higher energies, causing the synchrotron tail to push into the soft X-ray band more and more as the source brightens., Comment: LaTex, 6 pages, Accepted by A&A

We present X-ray spectra obtained with BeppoSAX (Satellite per Astronomia X) of 10 BL Lac objects, selected from the Einstein Medium Sensitivity and Slew Surveys. We find that in about half of the objects a fit in the 0.1-10 keV range with a single power law and free absorption yields values of NH larger than the Galactic ones. In most of these cases, however, broken power law fits with NH fixed at the Galactic values yield an alternative, better description of the data and indicate a steepening of the spectrum with increasing energy. One object (1ES1101-232) is detected up to ~100 keV. Its spectral energy distribution (SED) peaks in the medium energy X-ray band. For each object we compute the peak frequency of the SED from multifrequency data. The spectral indices alpha_x in the 2-10 keV band (Fnu prop to nu^{-alpha_x}) are smaller (i.e. flatter spectrum) for objects with higher peak frequencies. We therefore confirm and extend to higher energies the behavior already known for X-ray selected BL Lac objects in the ROSAT band. We do not find spectral indices smaller than 1; however, the flat distribution of alpha_x and the correlation between alpha_x and peak frequency found from our data suggest that a number of objects may exist, which in the quiescent status have flatter spectrum and peak frequency in the hard X-ray range., Comment: 14 pages Latex, 6 figures, accepted for publication in A&A (uses psfig.tex, l-aa.sty)

We report on the BeppoSAX observations of the gamma-ray blazar PKS 0528+134 performed in Feb and Mar 1997, during a multiwavelength campaign involving EGRET and ground based telescopes. The source was in a faint and hard state, with energy spectral index alpha=0.48+-0.04 between 0.1 and 10 keV, and [2-10] keV flux of 2.7E-12 erg/cm2/s. No significant variability was observed. The source was detected in the 20-120 keV band by the PDS, with a flux lying slightly above the extrapolation from lower X-ray energies. Comparing this low state with previous higher states of the source, there is an indication that the X-ray spectrum hardens and the gamma-ray spectrum steepens when the source is fainter., Comment: 4 pages, LateX, 5 figures (included). Uses espcrc2.sty and epsf.sty (included). To appear in The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE, Rome, Italy, 21-24 October, 1997. Eds.: L. Scarsi, H. Bradt, P. Giommi and F. Fiore

We monitored the BL Lac object S5 0716+714 in the optical band during the period November 1994-April 1995, which includes the time of a gamma-ray observation by the Energetic Gamma Ray Experiment Telescope (EGRET) on February 14--28, 1995. The light curves in the R and B bands show fast fluctuations superimposed on longer timescale variations. The color index correlates with intensity during the rapid flares (the spectrum is flatter when the flux is higher), but it is rather insensitive to the long term trends. Over the 5 month observational period the light curve shows an overall brightening of about 1 mag followed by a fast decline. The EGRET pointing covers part of the very bright phase (V about 13.2) and the initial decline. An ultraviolet spectrum was also obtained with the International Ultraviolet Explorer (IUE) (1200-3000 A) during the EGRET observations. The variability of the optical emission of S5 0716+714 by itself sets important constraints on the magnetic field strength and on the physical processes responsible for it. Interpreting the whole electromagnetic spectrum with synchrotron self Compton models leads to the prediction of a bright gamma-ray state during the EGRET pointing. We discuss how the gamma-ray data could be used as a diagnostic of the proposed models., Comment: 13 pages, LateX, 10 figures (included). Uses l-aa.sty and epsf.sty. To appear in A&A