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The results of 8-year R-band photopolarimetric data of blazar Mrk 421 collected from February 2008 to May 2016 are presented, along with extensive multiwavelength observations covering from radio to TeV $\gamma$-rays around the flares observed in May 2008, March 2010, and April 2013. The most important results are found in 2013 when the source displayed in the R-band a very high brightness state of $11.29\pm 0.03$ mag ($93.60\pm 1.53$ mJy) on April 10th and a polarization degree of ($11.00\pm0.44$) on May 13th. The analysis of the optical data shows that the polarization variability is due to the superposition of two polarized components that might be produced in two distinct emitting regions. An intranight photopolarimetric variability study carried out over 7 nights after the 2013 April maximum found flux and polarization variations on the nights of April 14, 15, 16 and 19. In addition, the flux shows a minimum variability timescale of $\Delta\,t=$2.34$\pm$0.12 hours, and that the polarization degree presented variations of $\sim$ (1 - 2) in a timescale of $\Delta\,t\sim$ minutes. Also, a detailed analysis of the intranight data shows a coherence length of the large-scale magnetic field of $l_B\simeq$ 0.3 pc which is the same order of magnitude to the distance traveled by the relativistic shocks. This result suggests that there is a connection between the intranight polarimetric variations and spatial changes of the magnetic field. Analysis of the complete R-band data along with the historical optical light curve found for this object shows that Mrk 421 varies with a period of 16.26 $\pm$ 1.78 years., Comment: 30 pages and 16 figures. Accepted for publication in ApJ

An exhaustive analysis of 9-year optical R-band photopolarimetric data of the flat-spectrum radio quasar 3C279 from 2008 February 27 to 2017 May 25 is presented, alongside with multiwavelength observing campaigns performed during the flaring activity exhibited in 2009 February/March, 2011 June, 2014 March/April, 2015 June and 2017 February. In the R-band, this source showed the maximum brightness state of $13.68\pm 0.11$ mag ($1.36\pm0.20$ mJy) on 2017 March 02, and the lowest brightness state ever recorded of $18.20\pm 0.87$ mag ($0.16\pm0.03$ mJy) on 2010 June 17. During the entire period of observations, the polarization degree varied between $0.48\pm0.17$% and $31.65\pm0.77$% and the electric vector position angle exhibited large rotations between $82.98^\circ \pm0.92$ and $446.32^\circ \pm1.95$. Optical polarization data show that this source has a stable polarized component that varied from $\sim$6% (before the 2009 flare) to $\sim$13% after the flare. The overall behavior of our polarized variability data supports the scenario of jet precessions as responsible of the observed large rotations of the electric vector position angle. Discrete correlation function analysis show that the lags between gamma-rays and X-rays compared to the optical R-band fluxes are $\Delta t \sim$ 31 d and $1$ d in 2009. Lags were also found among gamma-rays compared with X-rays and radio of $\Delta t \sim$ 30 d and $43$ d in 2011, and among radio and optical-R band of $\Delta t \sim$ 10 d in 2014. A very intense flare in 2017 was observed in optical bands with a dramatic variation in the polarization degree (from $\sim$ 6% to 20%) in 90 days without exhibiting flaring activity in other wavelengths., Comment: 21 pages, 10 figures. Accepted for publication in ApJS

Kiehlmann, S. et. al., Context. Over the past few years, on several occasions, large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy gamma-ray flares and they have attracted considerable attention, since they could allow us to probe the magnetic field structure in the gamma-ray emitting region of the jet. The flat-spectrum radio quasar 3C 279 is one of the most prominent examples showing this behaviour. Aims. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. Methods. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C 279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of 2008-2012. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data. Results. 3C 279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth similar to 360 degrees rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. Conclusions. We conclude that, during the two different optical flux states, two different processes govern polarization variation, which is possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity., S.K. was supported for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Max Planck Institute for Radio Astronomy in cooperation with the Universities of Bonn and Cologne. T.S. was partly supported by the Academy of Finland project 274477. The research at Boston University was partly funded by NASA Fermi GI grant NNX11AQ03G. K.V.S. is partly supported by the Russian Foundation for Basic Research grants 13-02-12103 and 14-02-31789. N.G.B. was supported by the RFBR grant 12-02-01237a. E.B., M.S. and D.H. thank financial support from UNAM DGAPA-PAPIIT through grant IN116211-3. I. A. acknowledges support by a Ramon y Cajal grant of the Spanish Ministry of Economy and Competitiveness (MINECO). The research at the IAA-CSIC and the MAPCAT program are supported by the Spanish Ministry of Economy and Competitiveness and the Regional Government of Andalucia (Spain) through grants AYA2010-14844, AYA2013-40825-P, and P09-FQM-4784. The Calar Alto Observatory is jointly operated by the Max-Planck-Institut fur Astronomie and the Instituto de Astrofisica de Andalucia-CSIC. Data from the Steward Observatory spectropolarimetric monitoring project were used. This program is supported by Fermi Guest Investigator grants NNX08AW56G, NNX09AU10G, NNX12AO93G, and NNX14AQ58G. St. Petersburg University team acknowledges support from Russian RFBR grant 15-02-00949 and St. Petersburg University research grant 6.38.335.2015. The Abastumani team acknowledges financial support of the project FR/638/6-320/12 by the Shota Rustaveli National Science Foundation under contract 31/77.

We report the detection by the AGILE satellite of a rapid gamma-ray flare from the powerful gamma-ray quasar PKS 1510-089, during a pointing centered on the Galactic Center region from 1 March to 30 March 2008. This source has been continuosly monitored in the radio-to-optical bands by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT). Moreover, the gamma-ray flaring episode triggered three ToO observations by the Swift satellite in three consecutive days, starting from 20 March 2008. In the period 1-16 March 2008, AGILE detected gamma-ray emission from PKS 1510-089 at a significance level of 6.2-sigma with an average flux over the entire period of (84 +/- 17) x 10^{-8} photons cm^{-2} s^{-1} for photon energies above 100 MeV. After a predefined satellite re-pointing, between 17 and 21 March 2008, AGILE detected the source at a significance level of 7.3-sigma, with an average flux (E > 100 MeV) of (134 +/- 29) x 10^{-8} photons cm^{-2} s^{-1} and a peak level of (281 +/- 68) x 10^{-8} photons cm^{-2} s^{-1} with daily integration. During the observing period January-April 2008, the source also showed an intense and variable optical activity, with several flaring episodes and a significant increase of the flux was observed at millimetric frequencies. Moreover, in the X-ray band the Swift/XRT observations seem to show an harder-when-brighter behaviour of the source spectrum. The spectral energy distribution of mid-March 2008 is modelled with a homogeneous one-zone synchrotron self Compton emission plus contributions from inverse Compton scattering of external photons from both the accretion disc and the broad line region. Indeed, some features in the optical-UV spectrum seem to indicate the presence of Seyfert-like components, such as the little blue bump and the big blue bump., 10 pages, 4 figures and 2 tables, accepted for publication in A&A

In this work we present an analysis of the optical polarimetric variability of the TeV-blazar 1 ES 1959+650 from 2007 October 18 to 2011 May 5. The source showed a maximum variation of 1.2 mag in R - band, and a maximum polarization degree of 12.2%. The blazar presented a preferential position angle of the optical polarization of ~ 153°, with variations of 10° − 50°, which is in agreement with the projected position angle of the parsec-scale jet found at 43 GHz. We infer the existence of the two optically thin synchrotron components that contribute to the polarized flux, consistent with the spine-sheath model. One of them is stable with a constant polarization degree of 4%. Assuming a stationary shock, we estimated some parameters associated with the physics of the relativistic jet: the Doppler factor of δ D ∼ 23, the viewing angle of the jet of Φ ~ 2°.4, and the size of the emission region of r b ~ 5.6 × 10 17 cm.

Optical R-band polarimetric variability observations of the TeV Blazar W Comae are presented. Data obtained from 2008 February to 2013 May (∼5.2 years) are analyzed. The source presented a maximum flux variability of∼3 mJy. The minimum variability time scale displayed by the source during the monitored period ist∼ 3.3 d. A maximum linear polarisation degree value of P= (33.8± 1.6)% was observed in 2013 May 12. A rotation of the position angle from 78 ◦ (2008 March 10) to 229 ◦ (2008 July 11) was observed. This rotation corresponds to a large change of �� ∼ 150 ◦ in a period of 123 d or to a rotation of∼1.2 ◦ per day. After the high activity state observed in 2008, the position angle shows a preferential value of ∼ 56 ◦ , with variations of ∼60-120 ◦ . From the Stokes parameters we infer the existence of two optically-thin synchrotron components that contribute to the optical polarized flux. One of them is stable, with P∼11%.

Quasar 3C 279 is known to exhibit episodes of optical polarization angle rotation. We present new, well-sampled optical polarization data for 3C 279 and introduce a method to distinguish between random and deterministic electric vector position angle (EVPA) variations. We observe EVPA rotations in both directions with different amplitudes and find that the EVPA variation shows characteristics of both random and deterministic cases. Our analysis indicates that the EVPA variation is likely dominated by a random process in the low brightness state of the jet and by a deterministic process in the flaring state., Comment: 6 pages, 2 figures, 4 tables, proceeding to appear on EPJ Web of Conferences "The Innermost Regions of Relativistic Jets and their Magnetic Fields (Granada, Spain,10-14 June 2013)"

A physical description of the formation and propagation of the working surface inside the relativistic jet of the Blazar PKS 1510-089 are used to model its gamma-ray variability light curve using FERMI-LAT data from 2008 to 2012. The physical model is based on conservation laws of mass and momentum at the working surface as explained by Mendoza et al. (2009). The hydrodynamical description of the working surface is parametrised by the initial velocity and mass injection rate at the base of the jet. We show that periodic variations on the injected velocity profiles are able to account for the observed luminosity. With this, we are able to obtain mass ejection rates of the central engine which are injected at the base of the jet, and oscillation frequencies of the flow, amongst other physical parameters., 5 pages, 2 figures, 1 table, accepted for publication in MNRAS

In 2007-2008 we carried out a new multiwavelength campaign of the Whole Earth Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the XMM-Newton satellite, to study its emission properties. The source was monitored in the optical-to-radio bands by 37 telescopes. The brightness level was relatively low. Some episodes of very fast variability were detected in the optical bands. The X-ray spectra are well fitted by a power law with photon index of about 2 and photoelectric absorption exceeding the Galactic value. However, when taking into account the presence of a molecular cloud on the line of sight, the data are best fitted by a double power law, implying a concave X-ray spectrum. The spectral energy distributions (SEDs) built with simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations suggest that the peak of the synchrotron emission lies in the near-IR band, and show a prominent UV excess, besides a slight soft-X-ray excess. A comparison with the SEDs corresponding to previous observations with X-ray satellites shows that the X-ray spectrum is extremely variable. We ascribe the UV excess to thermal emission from the accretion disc, and the other broad-band spectral features to the presence of two synchrotron components, with their related SSC emission. We fit the thermal emission with a black body law and the non-thermal components by means of a helical jet model. The fit indicates a disc temperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s., 11 pages, 7 figures, accepted for publication in A&A

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

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

An $R$--band photopolarimetric variability analysis of the TeV bright blazar W Comae, between 2008 February 28 and 2013 May 17, is presented. The source showed a gradual tendency to decrease its mean flux level with a total change of ~3~mJy. A maximum and minimum brightness states in the $R$-band of 14.25$\pm$0.04 and 16.52$\pm$0.1~mag respectively were observed, corresponding to a maximum variation of $\Delta$F = 5.40 mJy. We estimated a minimum variability timescale of $\Delta$t=3.3 days. A maximum polarization degree $P$=33.8\%$\pm$1.6\%, with a maximum variation of $\Delta$P = 33.2\%, was found. One of our main results is the detection of a large rotation of the polarization angle from 78\degr to 315\degr ($\Delta\theta\sim$237\degr) that coincides in time with the $\gamma$-ray flare observed in 2008 June. This result indicates that both optical and $\gamma$-ray emission regions could be co-spatial. During this flare, a correlation between the $R$-band flux and polarization degree was found with a correlation coefficient of $r_{F-p}=0.93\pm$0.11. From the Stokes parameters we infer the existence of two optically thin synchrotron components that contribute to the polarized flux. One of them is stable with a constant polarization degree of 11\%. Assuming a shock-in jet model during the 2008 flare, we estimated a maximum Doppler factor $\delta_D\sim 27$ and a minimum of $\delta_D\sim 16$; a minimum viewing angle of the jet $\sim$2\degr.0; and a magnetic field $B \sim$ 0.12~G., Comment: Accepted in ApJ,34 pages,6 figures,4 tables. arXiv admin note: text overlap with arXiv:1304.2819

A detailed analysis of the optical polarimetric variability of the TeV blazar 1ES 1959+650 from 2007 October 18 to 2011 May 5 is presented. The source showed a maximum and minimum brightness states in the R-band of 14.08$\pm$0.03 mag and 15.20$\pm$0.03 mag, respectively, with a maximum variation of 1.12 mag, and also a maximum polarization degree of $P=$(12.2$\pm$0.7)%, with a maximum variation of 10.7%. From August to November 2009, a correlation between the optical $R$-band flux and the degree of linear polarization was found, with a correlation coefficient $r_{pol}$=0.984$\pm$0.025. The source presented a preferential position angle of optical polarization of $\sim153^{\circ}$, with variations of $10\degr$-$50\degr$, that is in agreement with the projected position angle of the parsec scale jet found at 43 GHz. From the Stokes parameters we infer the existence of two optically-thin synchrotron components that contribute to the polarized flux. One of them is stable, with a constant polarization degree of 4%. Assuming a stationary shock for the variable component, we estimated some parameters associated with the physics of the relativistic jet: the magnetic field, $B\sim$0.06 G, the Doppler factor, $\delta_{0}\sim$23, the viewing angle, $\Phi\sim2.4\degr$, and the size of the emission region $r_b\sim5.6\times10^{17}$ cm. Our study is consistent with the spine-sheath model to explain the polarimetric variability displayed by this source during our monitoring., Comment: 58 pages, 8 figures, 6 tables. Accepted for publication in ApJ Supplement

A physical description of the formation and propagation of working surfaces inside the relativistic jet of the blazar PKS 1510-089 are used to model its γ -ray variability light curve using Fermi-LAT data from 2008 to 2012. The physical model is based on conservation laws of mass and momentum at the working surface as explained by Mendoza et al. (2009). The hydrodynamical description of a working surface is parametrized by the initial velocity and mass injection rate at the base of the jet. We show that periodic variations on the injected velocity profiles are able to account for the observed luminosity, fixing model parameters such as mass ejection rates of the central engine injected at the base of the jet, oscillation frequencies of the flow and maximum Lorentz factors of the bulk flow during a particular burst.

Observing Delta Scuti stars is most important as their multi-frequency spectrum of radial pulsations provide strong constraints on the physics of the stars interior; so any new detection and observation of these stars is a valuable contribution to asteroseismology. While performing uvby-beta photoelectric photometry of some RR Lyrae stars acquired in 2005 at the Observatorio Astronomico Nacional, Mexico, we also observed several standard stars, HD115520 among them. After the reduction this star showed indications of variability. In view of this, a new observing run was carried out in 2006 during which we were able to demonstrate its variability and its nature as a Delta Scuti star. New observations in 2007 permitted us to determine its periodic content with more accuracy. This, along with the uvby-beta photoelectric photometry allowed us to deduce its physical characteristics and pulsational modes.

We report results from an intensive multiwavelength campaign on the intermediate-frequency-peaked BL Lacertae object W Com (z=0.102) during a strong outburst of very high energy gamma-ray emission in June 2008. The very high energy gamma-ray signal was detected by VERITAS on 2008 June 7-8 with a flux F(>200 GeV) = (5.7+-0.6)x10^-11 cm-2s-1, about three times brighter than during the discovery of gamma-ray emission from W Com by VERITAS in 2008 March. The initial detection of this flare by VERITAS at energies above 200 GeV was followed by observations in high energy gamma-rays (AGILE, E>100 MeV), and X-rays (Swift and XMM-Newton), and at UV, and ground-based optical and radio monitoring through the GASP-WEBT consortium and other observatories. Here we describe the multiwavelength data and derive the spectral energy distribution (SED) of the source from contemporaneous data taken throughout the flare., 26 pages, 5 figures; Accepted for publication by The Astrophysical Journal