Blazars are active galactic nuclei, which are powerful sources of radiation whose central engine is located in the core of the host galaxy. Blazar emission is dominated by non-thermal radiation from a jet that moves relativistically towards us, and therefore undergoes Doppler beaming. This beaming causes flux enhancement and contraction of the variability timescales, so that most blazars appear as luminous sources characterized by noticeable and fast changes in brightness at all frequencies. The mechanism that produces this unpredictable variability is under debate, but proposed mechanisms include injection, acceleration and cooling of particles, with possible intervention of shock waves or turbulence. Changes in the viewing angle of the observed emitting knots or jet regions have also been suggested as an explanation of flaring events and can also explain specific properties of blazar emission, such as intra-day variability, quasi-periodicity and the delay of radio flux variations relative to optical changes. Such a geometric interpretation, however, is not universally accepted because alternative explanations based on changes in physical conditions-such as the size and speed of the emitting zone, the magnetic field, the number of emitting particles and their energy distribution-can explain snapshots of the spectral behaviour of blazars in many cases. Here we report the results of optical-to-radio-wavelength monitoring of the blazar CTA 102 and show that the observed long-term trends of the flux and spectral variability are best explained by an inhomogeneous, curved jet that undergoes changes in orientation over time. We propose that magnetohydrodynamic instabilities or rotation of the twisted jet cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the extreme optical outburst of 2016-2017 (brightness increase of six magnitudes) occurred when the corresponding emitting region had a small viewing angle. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory.

Abstract. New visual light curves and infrared multi-epoch photometry are presented for a sample of AGBstars spectroscopically observed with ISO. While the ISO work is or will be presented elsewhere, the aim of this paper is to give an overview of the properties of the light change of the objects. This information is crucial for the interpretation of the valuable ISO material. Using the University of Vienna Twin Automatic Photoelectric Telescope (APT) we monitored the stars of our sample in the photometric bands V and IC. We present the light curve of each object and derive the parameters of the current light change, such as period(s) and amplitude. Furthermore we give V −IC colours and colour variations for these objects. Our results allow us to derive some general results on semiregular and irregular variables. Only in less than 50% of the cases could we confirm the GCVS period. Moreover, we did not find any pronounced difference between SRb and Lb variables in the regularity of the light curve. The existence of rapid oscillations indicated by Hipparcos data could not be confirmed. In addition to the visual light changes we present new near infrared photometry data. Although typically only few data points are available, they can be viewed relative to the better-monitored visual light curves providing information on possible phase shifts and differences in amplitude in different parts of the spectrum. Furthermore, multi-epoch photometry allows us to derive mean colours for these objects.

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After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from γ -ray to near-infrared (NIR) frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive Glast-Agile support programme of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in [2004][2005]. We find remarkable similarity between the optical and γ -ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ≈1 h, indicative of cospatiality of the optical and γ -ray emission regions. The relation between the γ -ray and optical fluxes is consistent with the synchrotron self-Compton (SSC) mechanism, with a quadratic dependence of the SSC γ -ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the γ -ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in γ -ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.

We report the results of decade-long (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) γ-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, Fermi and Swift data, as well as polarimetric and spectroscopic data. The X-ray and γ-ray light curves correlate well, with no delay > ∼ 3 hours, implying general co-spatiality of the emission regions. The γ-ray-optical flux-flux relation changes with activity state, ranging from a linear to a more complex dependence. The behaviour of the Stokes parameters at optical and radio wavelengths, including 43 GHz VLBA images, supports either a predominantly helical magnetic field or motion of the radiating plasma along a spiral path. Apparent speeds of emission knots range from 10 to 37c, with the highest values requiring bulk Lorentz factors close to those needed to explain γ-ray variability on very short time scales. The Mg II emission line flux in the 'blue' and 'red' wings correlates with the optical synchrotron continuum flux density, possibly providing a variable source of seed photons for inverse Compton scattering. In the radio bands we find progressive delays of the most prominent light curve maxima with decreasing frequency, as expected from the frequency dependence of the τ = 1 surface of synchrotron self-absorption. The global maximum in the 86 GHz light curve becomes less prominent at lower frequencies, while a local maximum, appearing in 2014, strengthens toward decreasing frequencies, becoming pronounced at ∼ 5 GHz. These tendencies suggest different Doppler boosting of stratified radio-emitting zones in the jet.

We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSSJ133941.11+484727.5 (SDSS1339) which has been discovered in the Sloan Digital Sky Survey Data Release 4. The orbital period determined from radial velocity studies is 82.524(24)min, close to the observed period minimum. The optical spectrum of SDSS1339 is dominated to 90% by emission from the white dwarf. The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with Twd=12500K for a fixed log g=8.0, d=170pc, and a spectral type of the secondary later than M8. The mass transfer rate corresponding to the optical luminosity of the accretion disc is very low,~1.7x10^-13Msun/yr. Optical photometry reveals a coherent variability at 641s with an amplitude of 0.025mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320min or 344min and an amplitude of 0.025mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSSJ123813.73-033933.0, SDSSJ204817.85-061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available.Comment: MNRAS, in press, 8 pages, 10 figures, some figures downgraded to meet the file size constraint of arxiv.or

We report the results of CCD V and R photometry of the RR Lyrae stars in M2. The periodicities of most variables are revised and new ephemerides are calculated. Light‐curve decomposition of the RR Lyrae stars was carried out and the corresponding mean physical parameters [Fe/H]=−1.47, Teff= 6276 K, log L= 1.63 L⊙ and MV= 0.71 from nine RRab and [Fe/H]=−1.61, M= 0.54 M⊙, Teff= 7215 K, log L= 1.74 L⊙ and MV= 0.71 from two RRc stars were calculated. A comparison of the radii obtained from the above luminosity and temperature with predicted radii from non‐linear convective models is discussed. The estimated mean distance to the cluster is 10.49 ± 0.15 kpc. These results place M2 correctly in the general globular cluster sequences for Oosterhoff type, mass, luminosity and temperature, all as a function of the metallicity. Mean relationships for M, log L/L⊙, Teff and MV as a function of [Fe/H] for a family of globular clusters are offered. These trends are consistent with evolutionary and structural notions on the horizontal branch. Eight new variables are reported.

A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in April-July. We also analyse the UV and X-ray data acquired by the Swift and XMM-Newton satellites in the same period. The WEBT and satellite observations allow us to detail the synchrotron emission bump in the source spectral energy distribution (SED). In the optical we found a general bluer-when-brighter trend. The X-ray spectrum remained stable during 2013, but a comparison with previous observations suggests that it becomes harder when the X-ray flux increases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In the SED, the XMM-Newton data show a hard near-UV spectrum, while Swift data display a softer shape that is confirmed by previous HST-COS and IUE observations. Polynomial fits to the optical-X-ray SED show that the synchrotron peak likely lies in the 4-30 eV energy range, with a general shift towards higher frequencies for increasing X-ray brightness. However, the UV and X-ray spectra do not connect smoothly. Possible interpretations include: i) orientation effects, ii) additional absorption, iii) multiple emission components, and iv) a peculiar energy distribution of relativistic electrons. We discuss the first possibility in terms of an inhomogeneous helical jet model.