D. Hiriart

Multifrequency monitoring of the blazar 0716+714 during the GASP-WEBT-AGILE campaign of 2007

Aims. Since the CGRO operation in 1991–2000, one of the primary unresolved questions about the blazar γ -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 γ -ray detections by the AGILE and GLAST satellites. This new WEBT project started in early September 2007, just before a strong γ -ray detection of 0716+714 by AGILE. Methods. 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. Results. 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 γ -ray events.

The GASP-WEBT monitoring of 3C 454.3 during the 2008 optical-to-radio and γ-ray outburst

Context. 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. Aims. The GLAST-AGILE Support Program (GASP) was born from the WEBT to provide long-term continuous optical-to-radio monitoring of a sample of γ -loud blazars, during the operation of the AGILE and GLAST (now known as Fermi GST) γ -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 γ -ray emission from the source. Methods. 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. Results. An optical outburst was observed to peak in mid July 2008, when Fermi detected the brightest γ -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.

WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007-2008 Unveiling different emission components

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.

The long-lasting activity of 3C 454.3 - GASP-WEBT and satellite observations in 2008–2010

Context. The blazar 3C 454.3 is one of the most active sources from the radio to the γ-ray frequencies observed in the past few years. Aims. We present multiwavelength observations of this source from April 2008 to March 2010. The radio to optical data are mostly from the GASP-WEBT, UV and X-ray data from Swift, and γ-ray data from the AGILE and Fermi satellites. The aim is to understand the connection among emissions at different frequencies and to derive information on the emitting jet. Methods. Light curves in 18 bands were carefully assembled to study flux variability correlations. We improved the calibration of optical-UV data from the UVOT and OM instruments and estimated the Lyα flux to disentangle the contributions from different components in this spectral region. Results. The observations reveal prominent variability above 8 GHz. In the optical-UV band, the variability amplitude decreases with increasing frequency due to a steadier radiation from both a broad line region and an accretion disc. The optical flux reaches nearly the same levels in the 2008–2009 and 2009–2010 observing seasons; the mm one shows similar behaviour, whereas the γ and X-ray flux levels rise in the second period. Two prominent γ-ray flares in mid 2008 and late 2009 show a double-peaked structure, with a variable γ/optical flux ratio. The X-ray flux variations seem to follow the γ-ray and optical ones by about 0.5 and 1 d, respectively. Conclusions. We interpret the multifrequency behaviour in terms of an inhomogeneous curved jet, where synchrotron radiation of increasing wavelength is produced in progressively outer and wider jet regions, which can change their orientation in time. In particular, we assume that the long-term variability is due to this geometrical effect. By combining the optical and mm light curves to fit the γ and X-ray ones, we find that the γ (X-ray) emission may be explained by inverse-Comptonisation of synchrotron optical (IR) photons by their parent relativistic electrons (SSC process). A slight, variable misalignment between the synchrotron and Comptonisation zones would explain the increased γ and X-ray fl ux levels in 2009–2010, as well as the change in the γ/optical flux ratio during the outbursts peaks. The time delays of the X-ray flux changes after the γ, and optical ones are consistent with the proposed scenario.

The awakening of BL Lacertae: observations by Fermi, Swift and the GASP-WEBT

Since the launch of the Fermi satellite, BL Lacertae has been moderately active at ?-rays and optical frequencies until 2011 May, when the source started a series of strong flares. The exceptional optical sampling achieved by the GLAST–AGILE Support Program of the Whole Earth Blazar Telescope in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily ?-ray observations by Fermi. Discrete correlation analysis between the optical and ?-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 ?-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 ?-ray-emitting zone in the jet. The mean optical degree of polarization slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarization 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 polarization. A helical magnetic field model predicts an evolution of the mean polarization 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.

Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies

Context. After years of modest optical activity, the quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum, renewing interest in this source. Aims. We present the results of low-energy multifrequency monitoring by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. This combined study aims to provide insights into the source broad-band emission and variability properties. Methods. We assemble optical, near-infrared, millimetre, and radio light curves and investigate their features and correlations. In the optical, we also analyse the spectroscopic and polarimetric properties of the source. We then compare the low-energy emission behaviour with that at high energies. Results. In the optical-UV band, several results indicate that there is a contribution from a quasi-stellar-object (QSO) like emission component, in addition to both variable and polarised jet emission. In the optical, the source is redder-when-brighter, at least for R ≳ 16. The optical spectra display broad emission lines, whose flux is constant in time. The observed degree of polarisation increases with flux and is higher in the red than the blue. The spectral energy distribution reveals a bump peaking around the U band. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R QSO ∼ 17.85-18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and γ-ray flux apparently fades in time, likely because of an increasing optical to γ-ray flux ratio. Conclusions. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor δ. Under the hypothesis that in the period 2008-2011 all the γ-ray and optical variability on a one-week timescale were due to changes in δ, this would range between ∼7 and ∼21. If the variability were caused by changes in the viewing angle θ only, then θ would go from ∼2.6° to ∼5°. Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model.

Polarization angle swings in blazars: The case of 3C 279

International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Max Planck Institute for Radio Astronomy; Universities of Bonn and Cologne; Academy of Finland project [274477]; NASA Fermi GI grant [NNX11AQ03G]; Russian Foundation for Basic Research [13-02-12103, 14-02-31789]; RFBR [12-02-01237a]; UNAM DGAPA-PAPIIT [IN116211-3]; Ramon y Cajal grant of the Spanish Ministry of Economy and Competitiveness (MINECO); Spanish Ministry of Economy and Competitiveness (Spain); Regional Government of Andalucia (Spain) [AYA2010-14844, AYA2013-40825-P, P09-FQM-4784]; Fermi Guest Investigator [NNX08AW56G, NNX09AU10G, NNX12AO93G, NNX14AQ58G]; Russian RFBR [15-02-00949]; St. Petersburg University research [6.38.335.2015]; Shota Rustaveli National Science Foundation [FR/638/6-320/12, 31/77]

The Whole Earth Blazar Telescope Campaign on the Intermediate BL Lac Object 3C 66A in 2007-2008

Prompted by a high optical state in September 2007, the Whole Earth Blazar Telescope (WEBT) consortium organized an intensive optical, near-IR (JHK) and radio observing campaign on the intermediate BL Lac object 3C 66A throughout the fall and winter of 2007 -- 2008. The source remained in a high optical state throughout the observing period and exhibited several bright flares on time scales of ~ 10 days. This included an exceptional outburst around September 15 - 20, 2007, reaching a peak brightness at R ~ 13.4. Our campaign revealed microvariability with flux changes up to |dR/dt| ~ 0.02 mag/hr. Our observations do not reveal evidence for systematic spectral variability or spectral lags. We infer a value of the magnetic field in the emission region of B ~ 19 e_B^{2/7} \tau_h^{-6/7} D_1^{13/7} G. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D 50, required for a one-zone SSC interpretation of some high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.

The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emission

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 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 becomesharderwhentheX-rayfluxincreases.ThelongXMM‐Newtonexposurerevealsacurved X-ray spectrum. In the SED, the XMM‐Newton data show a hard near-UV spectrum, while SwiftdatadisplayasoftershapethatisconfirmedbypreviousHubbleSpaceTelescope/Cosmic Origins Spectrograph and International Ultraviolet Explorer 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.

Multiwavelength behaviour of the blazar OJ 248 from radio to γ-rays

We present an analysis of the multiwavelength behaviour of the blazar OJ 248 at z = 0.939 in the period 2006-2013. We use low-energy data (optical, near-infrared, and radio) obtained by 21 observatories participating in the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT), as well as data from the Swift (optical-UV and X-rays) and Fermi (gamma-rays) satellites, to study flux and spectral variability and correlations among emissions in different bands. We take into account the effect of absorption by the Damped Lyman Alpha intervening system at z = 0.525. Two major outbursts were observed in 2006-2007 and in 2012-2013 at optical and near-IR wavelengths, while in the high-frequency radio light curves prominent radio outbursts are visible peaking at the end of 2010 and beginning of 2013, revealing a complex radio-optical correlation. Cross-correlation analysis suggests a delay of the optical variations after the gamma-ray ones of about a month, which is a peculiar behaviour in blazars. We also analyse optical polarimetric and spectroscopic data. The average polarization percentage P is less than 3 per cent, but it reaches about 19 per cent during the early stage of the 2012-2013 outburst. A vague correlation of P with brightness is observed. There is no preferred electric vector polarisation angle and during the outburst the linear polarization vector shows wide rotations in both directions, suggesting a complex behaviour or structure of the jet and possible turbulence. The analysis of 140 optical spectra acquired at the Steward Observatory reveals a strong Mg II broad emission line with an essentially stable flux of 6.2 e-15 erg cm-2 s-1 and a full width at half-maximum of 2053 km s-1.