Since the launch of the Fermi satellite, BL Lacertae has been moderately active at γrays and optical frequencies until May 2011, when the source started a series of strong flares. The exceptional optical sampling achieved by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily γ-ray observations by Fermi. Discrete correlation analysis between the optical and γ-ray emission reveals correlation with a time lag of 0 ± 1 d, which suggests cospatiality of the corresponding jet emitting regions. A better definition of the time lag is hindered by the daily gaps in the sampling of the extremely fast flux variations. In general, optical flares present more structure and develop on longer time scales than corresponding γ-ray flares. Observations at X-rays and at millimetre wavelengths reveal a common trend, which suggests that the region producing the mm and X-ray radiation is located downstream from the optical and γ-ray-emitting zone in the jet. The mean optical degree of polarisation slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarisation angle (EVPA) shows a preferred orientation of about 15 • , nearly aligned with the radio core EVPA and mean jet direction. Oscillations around it increase during the 2011-2012 outburst. We investigate the effects of a geometrical interpretation of the long-term flux variability on the polarisation. A helical magnetic field model predicts an evolution of the mean polarisation that is in reasonable agreement with the observations. These can be fully explained by introducing slight variations in the compression factor in a transverse shock waves model.

Context. The investigations of the photometric and spectral variability of PMS stars are essential to a better understanding of the early phases of stellar evolution. We are carrying out a photometric monitoring program of some fields of active star formation. One of our targets is the dark cloud region between the bright nebulae NGC 7000 and IC 5070. Aims. We report the discovery of a large amplitude outburst from the young star HBC 722 (LkHα 188 G4) located in the region of NGC 7000/IC 5070. On the basis of photometric and spectroscopic observations, we argue that this outburst is of the FU Orionis type. Methods. We gathered photometric and spectroscopic observations of the object both in the pre-outburst state and during a phase of increase in its brightness. The photometric BVRI data (Johnson-Cousins system) that we present were collected from April 2009 to September 2010. To facilitate transformation from instrumental measurements to the standard system, fifteen comparison stars in the field of HBC 722 were calibrated in the BVRI bands. Optical spectra of HBC 722 were obtained with the 1.3-m telescope of Skinakas Observatory (Crete, Greece) and the 0.6-m telescope of Schiaparelli Observatory in Varese (Italy). Results. The pre-outburst photometric and spectroscopic observations of HBC 722 show both low amplitude photometric variations and an emission-line spectrum typical of T Tau stars. The observed outburst started before May 2010 and reached its maximum brightness in September 2010, with a recorded ΔV ∼ 4. m 7 amplitude. Simultaneously with the increase in brightness the color indices changed significantly and the star became appreciably bluer. The light curve of HBC 722 during the period of rise in brightness is similar to the light curves of the classical FUors -FU Ori and V1057 Cyg. The spectral observations during the time of increase in brightness showed significant changes in both the profiles and intensity of the spectral lines. Only Hα remained in emission, while the Hβ, Na I 5890/5896, Mg I triplet 5174, and Ba II 5854/6497 lines were in strong absorption.

We present the results of extensive multi-band intra-night optical monitoring of BL Lacertae during 2010-2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them observations were done quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with an average sampling interval of around 8 minutes. BL Lacertae showed significant variations on hour-like timescales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states deviates from the linear trend suggesting different jet components contributing to the emission at different times.

We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacertae objects (BL Lacs) and one flat spectrum radio quasar (FSRQ). We have measured the multiband optical flux and colour variations in these blazars on intraday and short-term time-scales of months and have limited data for two more blazars. These photometric observations were made during 2009-2011, using six optical telescopes, four in Bulgaria, one in Greece and one in India. On short-term time-scales we found significant flux variations in nine of the sources and colour variations in three of them. Intraday variability was detected on six nights for two sources out of the 18 nights and four sources for which we collected such data. These new optical observations of these blazars plus data from our previous published papers (for three more blazars) were used to analyse their spectral flux distributions in the optical frequency range. Our full sample for this purpose includes six high-synchrotron-frequency-peaked BL Lacs (HSPs), three intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and six lowsynchrotron-frequency-peaked BL Lacs (LSPs; including both BL Lacs and FSRQs). We also investigated the spectral slope variability and found that the average spectral slopes of LSPs show a good accordance with the synchrotron self-Compton loss dominated model. Our analysis supports previous studies that found that the spectra of the HSPs and FSRQs have significant additional emission components. The spectra of all these HSPs and LSPs get flatter when they become brighter, while for FSRQs the opposite appears to hold. This supports the hypothesis that there is a significant thermal contribution to the optical spectrum for FSRQs.

Aims. We present results from optical photometric and spectroscopic observations of the eruptive pre-main sequence star V582 Aur. Variability of the star was reported a few years ago when it was suspected as a possible FU Orionis object. Due to the small number of currently known FUors, a new object of this type is ideal target for follow-up photometric and spectroscopic observations. Methods. We carried out BVRI CCD photometric observations in the field of V582 Aur from 2009 August to 2013 February. We acquired high-, medium-, and low-resolution spectroscopy of V582 Aur during this period. To study the pre-outburst variability of the target and construct its historical light curve, we searched for archival observations in photographic plate collections. Both CCD and photographic observations were analyzed using a sequence of 14 stars in the field of V582 Aur calibrated in BVRI. Results. The pre-outburst photographic observations of V582 Aur show low-amplitude light variations typical of T Tauri stars. Archival photographic observations indicate that the increase in brightness began in late 1984 or early 1985 and the star reached the maximum level of brightness at 1986 January. The spectral type of V582 Aur can be defined as G0I with strong P Cyg profiles of Hα and Na I D lines, which are typical of FU Orionis objects. Our BVRI photometric observations show large amplitude variations (ΔV ∼ 2. m 8) during the 3.5 year period of observations. Most of the time, however, the star remains in a state close to the maximum brightness. The deepest drop in brightness was observed in the spring of 2012, when the brightness of the star fell to a level close to the pre-outburst. The multicolor photometric data show a color reversal during the minimum in brightness, which is typical of UX Ori variables. The corresponding spectral observations show strong variability in the profiles and intensities of the spectral lines (especially Hα), which indicate significant changes in the accretion rate. On the basis of photometric monitoring performed over the past three years, the spectral properties of the maximal light, and the shape of the long-term light curve, we confirm the affiliation of V582 Aur to the group of FU Orionis objects.