16 research outputs found
The WEBT BL Lacertae Campaign 2001 and its extension : Optical light curves and colour analysis 1994â2002
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 âŒ15 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 âŒ0.1 bluerwhen- brighter slope in the B â R versus R plane; a steeper slope of âŒ0.4 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 âŒ1.1. Doppler factor variations on a âconvexâ spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.Reig Torres, Pablo, [email protected]
The whole earth blazar telescope campaign on the intermediate BL Lac object 3C 66A in 2007-2008
Prompted by a high optical state in 2007 September, the Whole Earth Blazar Telescope 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. In this paper, we present data from 28 observatories in 12 countries, covering the observing season from late 2007 July through 2008 February. The source remained in a high optical state throughout the observing period and exhibited several bright flares on timescales of 10 days. This included an exceptional outburst around 2007 September 15-20, reaching a peak brightness at R 13.4. Our campaign revealed microvariability with flux changes up to |dR/dt| 0.02 mag hr-1. Our observations do not reveal evidence for systematic spectral variability in the overall high state covered by our campaign, in agreement with previous results. In particular, we do not find evidence for spectral hysteresis in 3C 66A for which hints were found in an earlier campaign in a somewhat lower flux state. We also did not find any evidence for spectral lags in the discrete correlation functions between different optical bands. We infer a value of the magnetic field in the emission region of B 19 e 2/7B Ï-6/7h D 13/71 G, where eB is the magnetic field equipartition fraction, Ïh is the shortest observed variability timescale in units of hours, and D 1 is the Doppler factor in units of 10. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D †28 Ï-1/8h e 3/16B. This is in perfect agreement with superluminal motion measurements with the VLBI/VLBA of ÎČapp †27 and argues against models with very high Lorentz factors of Î âł 50, required for a one-zone synchrotron-self-Compton interpretation of some high-frequency-peaked BL Lac objects detected at TeV Îł-ray energies. © 2009 The American Astronomical Society
Multifrequency variability of the blazar AO 0235+164 the WEBT campaign in 2004-2005 and long-term SED analysis
A huge multiwavelength campaign targeting the blazar AO 0235+164 was
organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the
variability properties of the source. Monitoring observations were carried out
at cm and mm wavelengths, and in the near-IR and optical bands, while three
pointings by the XMM-Newton satellite provided information on the X-ray and UV
emission. We present the data acquired during the second observing season,
2004-2005, by 27 radio-to-optical telescopes. They reveal an increased near-IR
and optical activity with respect to the previous season. Increased variability
is also found at the higher radio frequencies, down to 15 GHz, but not at the
lower ones. The radio (and optical) outburst predicted to peak around
February-March 2004 on the basis of the previously observed 5-6 yr
quasi-periodicity did not occur. The analysis of the optical light curves
reveals now a longer characteristic time scale of 8 yr, which is also present
in the radio data. The spectral energy distributions corresponding to the
XMM-Newton observations performed during the WEBT campaign are compared with
those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray
spectra can be described in terms of an extra component, which appears also
when the source is faint through a hard UV spectrum and a curvature of the
X-ray spectrum. Finally, there might be a correlation between the X-ray and
optical bright states with a long time delay of about 5 yr, which would require
a geometrical interpretation
The GASP-WEBT monitoring of 3C 454.3 during the 2008 optical-to-radio and Îł-ray outburst
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
The complex variability of blazars: time-scales and periodicity analysis in S4Â 0954+65
Among active galactic nuclei, blazars show extreme variability properties. We here investigate the case of the BL Lac object S4Â 0954+65 with data acquired in 2019â2020 by the Transiting Exoplanet Survey Satellite (TESS) and by the Whole Earth Blazar Telescope (WEBT) Collaboration. The 2-min cadence optical light curves provided by TESS during three observing sectors of nearly 1 month each allow us to study the fast variability in great detail. We identify several characteristic short-term time-scales, ranging from a few hours to a few days. However, these are not persistent, as they differ in the various TESS sectors. The long-term photometric and polarimetric optical and radio monitoring undertaken by the WEBT brings significant additional information, revealing that (i) in the optical, long-term flux changes are almost achromatic, while the short-term ones are strongly chromatic; (ii) the radio flux variations at 37Â GHz follow those in the optical with a delay of about 3 weeks; (iii) the range of variation of the polarization degree and angle is much larger in the optical than in the radio band, but the mean polarization angles are similar; (iv) the optical long-term variability is characterized by a quasi-periodicity of about 1 month. We explain the source behaviour in terms of a rotating inhomogeneous helical jet, whose pitch angle can change in time.Accepted manuscrip
The WEBT campaign on the blazar 3C 279 in 2006
The quasar 3C279 was the target of an extensive multiwavelength monitoring
campaign from January through April 2006, including an optical-IR-radio
monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration.
In this paper we focus on the results of the WEBT campaign. The source
exhibited substantial variability of optical flux and spectral shape, with a
characteristic time scale of a few days. The variability patterns throughout
the optical BVRI bands were very closely correlated with each other. In
intriguing contrast to other (in particular, BL Lac type) blazars, we find a
lag of shorter- behind longer-wavelength variability throughout the RVB ranges,
with a time delay increasing with increasing frequency. Spectral hardening
during flares appears delayed with respect to a rising optical flux. This, in
combination with the very steep IR-optical continuum spectral index of ~ 1.5 -
2.0, may indicate a highly oblique magnetic field configuration near the base
of the jet. An alternative explanation through a slow (time scale of several
days) acceleration mechanism would require an unusually low magnetic field of <
0.2 G, about an order of magnitude lower than inferred from previous analyses
of simultaneous SEDs of 3C279 and other FSRQs with similar properties
Multiwavelength behaviour of the blazar 3CÂ 279: decade-long study from Îł-ray to radio
We report the results of decade-long (2008â2018) Îł-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 Îł-ray light curves correlate well, with no delay âł 3 h, implying general cospatiality of the emission regions. The Îł-rayâoptical fluxâflux 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 Very Long Baseline Array 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 Îł-ray variability on very short time-scales. The Mgâii emission line flux in the âblueâ and âredâ 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 Ï = 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 âŒ5 GHz. These tendencies suggest different Doppler boosting of stratified radio-emitting zones in the jet.First author draf
Multiwavelength observations of 3C 454.3. III. Eighteen months of agile monitoring of the "crazy diamond"
We report on 18 months of multiwavelength observations of the blazar 3C 454.3 (Crazy Diamond) carried out in the period 2007 July-2009 January. In particular, we show the results of the AGILE campaigns which took place on 2008 May-June, 2008 July-August, and 2008 October-2009 January. During the 2008 May-2009 January period, the source average flux was highly variable, with a clear fading trend toward the end of the period, from an average Îł-ray flux F E>100 MeV âł 200 Ă 10-8photonscm -2s-1 in 2008 May-June, to F E>100 MeV 80 Ă 10-8photonscm-2s-1 in 2008 October-2009 January. The average Îł-ray spectrum between 100 MeV and 1 GeV can be fit by a simple power law, showing a moderate softening (from ÎGRID ⌠2.0 to ÎGRID ⌠2.2) toward the end of the observing campaign. Only 3Ï upper limits can be derived in the 20-60 keV energy band with Super-AGILE, because the source was considerably off-axis during the whole time period. In 2007 July-August and 2008 May-June, 3C 454.3 was monitored by Rossi X-ray Timing Explorer (RXTE). The RXTE/Proportional Counter Array (PCA) light curve in the 3-20 keV energy band shows variability correlated with the Îł-ray one. The RXTE/PCA average flux during the two time periods is F 3-20 keV = 8.4 Ă 10-11ergcm-2s -1, and F 3-20 keV = 4.5 Ă 10 -11ergcm-2s-1, respectively, while the spectrum (a power law with photon index ÎPCA = 1.65 0.02) does not show any significant variability. Consistent results are obtained with the analysis of the RXTE/High-Energy X-Ray Timing Experiment quasi-simultaneous data. We also carried out simultaneous Swift observations during all AGILE campaigns. Swift/XRT detected 3C 454.3 with an observed flux in the 2-10 keV energy band in the range (0.9-7.5) Ă 10-11ergcm-2s-1 and a photon index in the range ÎXRT = 1.33-2.04. In the 15-150 keV energy band, when detected, the source has an average flux of about 5mCrab. GASP-WEBT monitored 3C 454.3 during the whole 2007-2008 period in the radio, millimeter, near-IR, and optical bands. The observations show an extremely variable behavior at all frequencies, with flux peaks almost simultaneous with those at higher energies. A correlation analysis between the optical and the Îł-ray fluxes shows that the Îł-optical correlation occurs with a time lag of Ï = -0.4+0.6-0.8 days, consistent with previous findings for this source. An analysis of 15 GHz and 43 GHz VLBI core radio flux observations in the period 2007 July-2009 February shows an increasing trend of the core radio flux, anti-correlated with the higher frequency data, allowing us to derive the value of the source magnetic field. Finally, the modeling of the broadband spectral energy distributions for the still unpublished data, and the behavior of the long-term light curves in different energy bands, allow us to compare the jet properties during different emission states, and to study the geometrical properties of the jet on a time-span longer than one year. © 2010. The American Astronomical Society. All rights reserved
Optical variability of the blazar 3C 454.3 during 2007-2010
The optical variability of the blazar 3C 454.3 during 2007-2010 has been studied using the results of multicolor (BVRIJHK) observations. The existence of two variable synchrotron sources was found. The first is responsible for the flux variability of small amplitude, the second â for the flares. In each flare the relative SED of variable source was found to be constant but spectral indexes were different in different flares. This fact points to impossibility to explain the global variability only by geometrical reasons. The observed color variability is due to a superposition of the variable and constant components with different SEDs