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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 recent BeppoSAX observations of the two BL Lac objects OJ 287 and MS 1458+22 in bright optical states and of simultaneous photometric measurements in the bandpasses from I to U . OJ 287 was observed in November 2001 and MS 1458+22 in February of the same year. The X-ray flux of OJ 287 was rather low, F (2-10 keV$) = (1.35\pm0.15)\times 10^{-12}$ erg cm -2 s -1 and its spectrum is well described by a single power law with an energy index $0.45\pm0.08$, flatter than that measured in the optical, equal to $1.53\pm0.03$. The X-ray emission of MS 1458+22 was characterized by a low flux level of F (2-10 keV$) = (7.5\pm0.6)\times 10^{-13}$ erg cm -2 s -1 , with a steeper spectral index of $1.71\pm0.15$, while in the optical it was $0.99\pm0.06$. We show that the broad-band SEDs of these two sources can be well represented by log-parabolic spectral laws and evaluate their parameters. This law, already observed for the synchrotron components of other BL Lac sources, can be explained if the emitting particles are accelerated by some statistical mechanism having a probability of energy gain that is a decreasing function of the energy itself.

We present the results of the optical monitoring of the BL Lac object ON 231 from 1997 to the spring 2001, including the large outburst of spring 1998. After this event the mean source luminosity declined slowly and continued to show changes greater than one magnitude. A simple analysis of the variability time scales in the best-sampled segments of the light curve reveals that the luminosity changes were faster during the burst and slowed down in the subsequent years.

We present an optical intraday variability (IDV) study of S5 0716+714 carried out over 52 nights. The source showed detectable variations (>0.05 mag) in 80% of the nights. Typical variation rates of 0.02 mag/hour have been found to last on average 4 hours, with the average rising rate faster than the decreasing one. A maximum rising rate of 0.16 mag/hour has been observed. If the luminosity variation is due to jittering in the jet direction, a comoving dθ/dt rate of ∼100 arcsec/hour is required, assuming for the jet θ ∼ 7° and Γ ∼ 14.

We present intensive optical, infrared, and radio monitoring observations of the BL Lac object OJ 287, taken between the years 1993-1998. Two large optical outbursts were detected at the predicted times in November 1994 and December 1995. The detection of these outbursts supports the binary black hole model for OJ 287. Optical and radio polarisation observations show large variability in the degree of polarisation and position angle, very similar to those observed during the 1983/84 outburst in OJ 287. The polarisation position angles show very similar behaviour during these observations, indicating that, at least, the magnetic field orientations in radio and optical bands are related in OJ 287. Optical and infrared light curves show continuous variability in time scales ranging from tens of minutes to years. In the radio bands we have observed some of the lowest ever measured flux levels. During the first optical outburst in November 1994 the observed radio flux was very low, but during the second optical outburst radio bands also showed high flux levels. This is a puzzling observation, which can hopefully be used for discriminating between different outburst models. On top of the large outbursts OJ 287 has displayed flaring activity in time scales from days to weeks and shorter time scale flickering.

Understanding the nature of the instabilities of LBVs is important to understand the late evolutionary stages of very massive stars. We investigate the long term, S Dor-type variability of the luminous blue variable GR290 (Romano's star) in M33, and its 2006 minimum phase. New spectroscopic and photometric data taken in November and December 2006 were employed in conjunction with already published data on GR290 to derive the physical structure of GR290 in different phases and the time scale of the variability. We find that by the end of 2006, GR 290 had reached the deepest visual minimum so far recorded. Its present spectrum resembles closely that of the Of/WN9 stars, and is the hottest so far recorded in this star (and in any LBV as well), while its visual brightness decreased by about 1.4 mag. This first spectroscopic record of GR290 during a minimum phase confirms that, similarly to AG Car and other LBVs, the star is subject to ample S Dor-type variations, being hotter at minimum, suggesting that the variations take place at constant bolometric luminosity., 4 figures, 1 table, accepted for publication in A&A Letters

We present the most continuous data base of optical multiband data ever published on the BL Lacertae object ON 231 (W Com). The data have been collected during an intensive and coordinated BVRcIc monitoring campaign carried out in the period from March 1994 to March 1997. During our campaign, the source brightness was at the highest level ever observed. The light curve shows a complex structure, characterized by the pres- ence of three major outbursts having the observed max- ima in March 1995, February 1996, and January 1997, when ON 231 reached its historical maximum (B' 14:2). Variability on time scales from a few hours up to a month have frequently been observed and the light curve seems to be the superposition of many flares with dierent ampli- tudes and time scales. The broad-band optical spectral en- ergy distribution is characterized by a spectral slope which correlates with the flux level. In particular, the higher is the flux the flatter is the spectrum.

The results of optical photometry of BL Lac over a time interval of two years in the B, V, R, I l- ters are presented. The source is found for most of the time in a low state. The mean colour indexes are similar to those observed in higher luminosity states, however a flattening of the spectral slope was present during three small amplitude outbursts. A brief comparison with the observations available in the literature and some consid- erations on the physical conditions driving the appearance of emission lines are made.

We report on our optical monitoring of the BL Lac object S5 1803+78 from 1996 to 2011. The source showed no clear periodicity, but a time scale of about 1 300 days between major flares is possibly present. No systematic trend of the color index with flux variations is evident, at variance with other BL Lacs. In one flare, however, the source was bluer in the rising phase and redder in the falling one. Two ?-ray flares were detected by Fermi-GST during our monitoring: on the occasion of only one of them we found simultaneous optical brightening. A one-zone Synchrotron Self Compton (SSC) model appears too simple to explain the source behavior.

We present the relation between optical luminosity and spectral slope for eight BL Lac objects (3C66A, PKS 0422+00, S5 0716+71, OJ 287, ON 231, OQ 530, S5 1803+78 and BL Lacertae), derived from B, V, R, and I observations spanning a time interval of about 5 years. Four objects show a marked correlation between spectral slope and luminosity, being bluer when brighter, while for the other four the correlation is weaker or absent. Possible explanations are briefly discussed in the framework of current models of the BL Lac phenomenon.

Context. AG Draconis is the brightest symbiotic star in X-rays and one of the prototypes of the supersoft X-ray source class. Aims. Study of the X-ray spectrum of this peculiar binary system, covering both quiescence and activity periods, is necessary to investigate the physics of the high temperature spectral component, and to unveil the origin of the outbursts. Methods. X-ray and UV observations with XMM-Newton during 2003–2005 and coordinated optical spectrophotometric monitoring, together with archive data, are employed to derive the behaviour of the high energy source of the AG Dra system during different orbital and activity phases. Results. During quiescence the X-ray emission is very soft and is close in strength to the previous ROSAT observations, with an

Since 1980 the high velocity symbiotic system AG Dra has undergone three phases of activity (or “outbursts”) which were extensively observed in optical, ultraviolet and X-ray bands. We describe the historical light curve of the star, and summarize 17 years of UV monitoring with the IUE satellite.

We address the topic of the Intra-Night Optical Variability of the BL Lac object S5 0716+714. To this purpose a long term observational campaign was performed, from 1996 to 2003, which allowed the collection of a very large data set, containing 10,675 photometric measurements obtained in 102 nights. The source brightness varied in a range of about 2 mag, although the majority of observations were performed when it was in the range 13.0 < R < 13.75. Variability time scales were estimated from the rates of magnitude variation, which were found to have a distribution function well fitted by an exponential law with a mean value of 0.027 mag/h, corresponding to an e-folding time scale of the flux tau_F = 37.6 h. The highest rates of magnitude variation were around 0.10--0.12 mag/h and lasted less than 2 h. These rates were observed only when the source had an R magnitude < 13.4, but this finding cannot be considered significant because of the low statistical occurrence. The distribution of tau_F has a well defined modal value at 19 h. Assuming the recent estimate of the beaming factor delta about 20, we derived a typical size of the emitting region of about 5 times 10^{16}/(1 + z) cm. The possibility to search for a possible correlation between the mean magnitude variation rate and the long term changes of the velocity of superluminal components in the jet is discussed., 12 pages, 10 figures, accepted for Astronomy and Astrophysics

Context. Understanding the origin of the instabilities of LBVs is important for shedding light on the late evolutionary stages of massive stars and on the chemical evolution of galaxies. Aims. To investigate the physical nature of variable stars in the upper H–R diagram, we performed a spectrophotometric study of the Romano’s star GR 290 and the Hubble-Sandage variables A, B, and C in the close galaxy M 33. Methods. New spectroscopic and photometric data were employed in conjunction with already published data of these stars in order to derive spectral types, energy distribution and bolometric luminosities. Results. The yellow hypergiant Var A is still at minimum, with a ∼G-type spectrum and strong Hα emission (Weq �− 35 A). Var B is in a low luminosity hot state (V = 17.5, B − V = −0.15) with very strong Hα emission (Weq = −310 A). Its absolute bolometric luminosity is 0.6 × 10 6 L� . Var C, at V = 16.4, is fainter than in the mid 1980s, but its spectrum shows the typical features of LBVs at maximum, a spectrum that is very rich in Fe ii emission lines. Its Lbol is about 0.7 × 10 6 L� . The Romano’s star GR 290 has a rich hot emission-line spectrum and is very bright with Lbol = 3 × 10 6 L� . During 2004 the star brightened by ∼half magnitude in each of the BVR filters. Conclusions. Our observations confirm that Var A probably is an intermediate type hypergiant star surrounded by an expanding envelope with a collisionally excited hydrogen emission, largely obscured by dusty disk and nebula. In recent years, Var B has undergone a blueward transition in the H–R diagram, probably at constant bolometric luminosity, while Var C is in a post-maximum phase with an η Car-type spectrum. GR 290 is notable for its spectrum and luminosity, and it is likely to develop ample spectral variations in the near future, similar to those observed in AG Car.

We present the historic light curve of the BL Lac object S5 0716+714, spanning the time interval from 1953 to 2003, built using Asiago archive plates and our recent CCD observations, together with literature data. The source shows an evident long term variability, over which well known short term variations are superposed. In particular, in the period from 1961 to 1983 the mean brightness of S5 0716+714 remained significantly fainter than that observed after 1994. Assuming a constant variation rate of the mean magnitude we can estimate a value of about 0.11 magnitude/year. The simultaneous occurrence of decreasing ejection velocities of superluminal moving components in the jet reported by Bach et al. (2005) suggests that both phenomena are related to the change of the direction of the jet to the line of sight from about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of about 12. A simple explanation is that of a precessing relativistic jet, which should presently be close to the smallest orientation angle. One can therefore expect in the next ten years a decrease of the mean brightness of about 1 magnitude., to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is given as a separated jpg file

Results of VLBI and optical observations are presented for the BL Lac object OQ 530 (B1418+546), whose long term optical variability is characterised by a decreasing mean luminosity trend. EVN images at 1.6 GHz and 5 GHz of OQ 530 show a very bright core with a much weaker jet extending to about 35–40 mas. The radio flux density from the core in June 2001 was much higher than that measured in February 1999. A similar increase was also found from nearly simultaneous optical photometry, while the total flux density of the jet remained unchanged. The structure of the jet shows several knots which move outward with an apparent superluminal motion with a $\beta_{\rm app}\geq3.5h^{-1}$. The region within about 3 mas from the core contains a component not well resolved in our images and emerging from it in the jet direction. That component was also detected in images taken from 1990 to 1997 and it seems to be a rather stable structural feature. Using nearly simultaneous optical and radio data, we can describe the SED of the synchrotron peak with a simple formula, approximating a power law at low frequencies and a log-parabola at the high ones, whose maximum lies in the range $3.5{-}5\times10^{13}$ Hz, and the extrapolation in the X-ray range gives a flux comparable to that observed with BeppoSAX.

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

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., 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for publication in A&A

The results of a photometric campaign in theB, V, R andI bands of the BL Lac object OJ 287 are presented. The source is found at the same level and spectral slope as in 1991, without any sign forecasting the predicted maximum in August 1994.

We report the main results of our six year long intensive optical monitoring on blazars ON 231 (W Com), BL Lac, and 3C 273. Intensive optical monitoring is an indispensable tool to obtain well sampled light curves and thus to understand the correlation with the variability in other bands and to discriminate among the proposed emission models. The curves based on our work show different variability modes: intermittent mode, quasi-regular mode and mixed mode. It is not clear if different variability modes can be present, at various times, in the life of an object suggesting that their occurrence can be related to the evolution of blazars. An optimized and nearly continuous optical monitoring is the only way to know the activity status of the sources and to retrieve useful information on their physical dynamics. Also, the rapid availability of information about the luminosity of a source is very important to trigger space based observations and to activate large multifrequency collaborations. We stress that a significant sample of blazars can be observed with small size telescopes (0.35-0.80 cm) equipped with CCD cameras. A world-wide network of several instruments in different countries, is very useful to increase the time coverage and to reduce the number of nights lost for bad meteorological conditions., Comment: Paper submitted to the AIP Conference Proceedings "High Energy Gamma-Ray Astronomy" - Conf. Proc. of the gamma-2000 symposium held in Heidelberg. 4 pages, doc format source (AIP Proc.), 1 gif figure

Like many radio-selected blazars, 0716+714 shows a high level of variability on different time scales, as short as a few days (e.g. Wagner et al 1996, Ghisellini et al. 1997). The mechanism of the emission in the optical band, in the general scheme of a relativistic plasma jet highy collimated toward the observer, is generally believed to be synchrotron radiation from electrons in a strong magnetic field. Our monitoring of S5 0716+714 is aimed to clarify whether the flux variations are chromatic or achromatic. In the first case, one could guess that variations in the spectrum of the injected electrons are responsible for the flux variations; in the second case, geometrical effects like small changes in the angle between the jet and the line of sight could be more likely (Wagner et al. 1993).

The latest observations of BL Lacertae with the EGRET instrument on CGRO show that the gamma-ray flux was four times higher than the previous detection. Within the 1997 July 15-22 observation there was a dramatic factor of 2.5 increase in the gamma-ray flux, with a sharp peak of about 8 hours, and apparently preceding a brief optical flare by several hours. The gamma-ray photon spectral index was significantly harder than that for the previous detection., Comment: 11 pages, 3 figures. To be published in the Astrophysical Journal Letters

We monitored the γ-ray loud BL Lac object S5 0716+714 between November 1994 and April 1995 (J.D. 2449671–2449838), from the Observatories of Torino, Perugia and Roma. The source was always active in the R and B bands, showing a particularly high optical state during the period of the CGRO pointing (February 14–28, 1995). The lightcurve in the R band is presented in Fig. 1. The complete set of observations is discussed in Ghisellini et al. (1995).

Like many radio-selected blazars, 0716+714 shows a high level of variability on different time scales, as short as a few days (e.g. Wagner et al 1996, Ghisellini et al. 1997). The mechanism of the emission in the optical band, in the general scheme of a relativistic plasma jet highy collimated toward the observer, is generally believed to be synchrotron radiation from electrons in a strong magnetic field. Our monitoring of S5 0716+714 is aimed to clarify whether the flux variations are chromatic or achromatic. In the first case, one could guess that variations in the spectrum of the injected electrons are responsible for the flux variations; in the second case, geometrical effects like small changes in the angle between the jet and the line of sight could be more likely (Wagner et al. 1993).

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., to be published in A&A

We present the results of photometric observations of BL Lacertae at 1 mm performed during the strong outburst that occurred in the summer of 1997. Measurements were carried out using the F99 filter of the Arizona system. Simultaneous observations in the B and V bands were made with other telescopes. Large and fast variations were observed on the night of August 2 whenthebrightnessofBLLacertaechangedby,0.5maginabout2h:thevariationamplitude is greater at higher frequency. Some implications of these results for the origin of the fast variations are discussed. We also report the first F99 calibration for three reference stars in the BL Lac field.

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., 19 pages, 6 figures, accepted by Astronomy & Astrophysics