Correlated radio and optical variations in a sample of AGN

Hanski, M.; Takalo, L. O.; Valtaoja, E.

doi:10.1051/0004-6361:20020988

Cited by 32 publications

(28 citation statements)

References 41 publications

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“…Other works on this subject always found radio variations noticeably delayed with respect to the optical ones (Bregman et al 1990;Hufnagel & Bregman 1992;Tornikoski et al 1994b;Clements et al 1995;Hanski et al 2002). On the contrary, the correlation between flux variations at different radio wavelengths has been found very strong, with no or short time delay.…”

Section: Introductionmentioning

confidence: 87%

The WEBT campaigns on BL Lacertae

Villata

Raiteri

Aller

et al. 2004

A&A

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Abstract. The Whole Earth Blazar Telescope (WEBT) collaboration has collected a large amount of optical and radio data on BL Lacertae in the last years, which, when added to literature data, allow to construct well-sampled light curves of the source from 1968 to the end of 2003. These optical and radio data are here analysed with three statistical methods designed for unevenly-sampled data trains in order to search for possible periodicities. While the main radio outbursts repeat every ∼8 years, with a possible progressive stretching of the period, the evidence of an optical periodicity is much less clear. Radio light curves from 4.8 to 37 GHz are well correlated, with variations at the higher frequencies leading the lower-frequency ones by a few weeks for contiguous bands, up to a few months when considering the largest frequency separations. The radio behaviour reveals the presence of two different components, the softer-spectrum one constituting the bulk of the radio emission. On the other hand, the harder component shows itself as radio events which appear enhanced at the higher frequencies and seem to have optical counterparts. Cross-correlation between the optical light curve and radio hardness ratios indicates a radio time delay of more than 3 months. Thus, our analysis suggests a scenario where flux variations propagate towards less and less opaque regions, giving rise to related optical and hard radio events and, in more extended zones, to soft events apparently uncorrelated with the former ones.

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Section: Introductionmentioning

confidence: 87%

The WEBT campaigns on BL Lacertae

Villata

Raiteri

Aller

et al. 2004

A&A

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“…Some of these found correlations with radio time delays of several hundred days (Pomphrey et al 1976;Hufnagel & Bregman 1992;Tornikoski et al 1994b;Clements et al 1995;Hanski et al 2002). Sometimes simultaneous events were reported (Balonek 1982;Tornikoski et al 1994a).…”

Section: Optical-radio Time Delaysmentioning

confidence: 98%

Structure and flux variability in the VLBI jet of BL Lacertae during the WEBT campaigns (1995–2004)

Bach¹,

Villata²,

Raiteri³

et al. 2006

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BL Lacertae has been the target of several observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration and is one of the best studied blazars at all accessible wavelengths. A recent analysis of the optical and radio variability indicates that part of the radio variability is correlated with the optical light curve. Here we present an analysis of a huge VLBI data set including 108 images at 15, 22, and 43 GHz obtained between 1995 and2004. The aim of this study is to identify the different components contributing to the single-dish radio light curves. We obtain separate radio light curves for the VLBI core and jet and show that the radio spectral index of single-dish observations can be used to trace the core variability. Cross-correlation of the radio spectral index with the optical light curve indicates that the optical variations lead the radio by about 100 days at 15 GHz. By fitting the radio time lags vs. frequency, we find that the power law is steeper than expected for a freely expanding conical jet in equipartition with energy density decreasing as the square of the distance down the jet as in the Königl model. The analysis of the historical data back to 1968 reveals that during a time range of 16 years the optical variability was reduced and its correlation with the radio emission was suppressed. There is a section of the compact radio jet where the emission is weak such that flares propagating down the jet are bright first in the core region with a secondary increase in flux about 1.0 mas from the core. This illustrates the importance of direct imaging to the interpretation of multi-wavelength light curves that can be affected by several distinct components at any given time. We discuss how the complex behaviour of the light curves and correlations can be understood within the framework of a precessing helical jet model.

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“…PKS 0735+178 has been extensively studied in the radio regime. The radio flux appear to vary quite slowly with some outbursts (Bååth & Zhang 1991;Teräsranta et al 1992;Aller et al 1999;Teräsranta et al 2004), but there is not any evidence of a radio-optical correlation (Clements et al 1995;Hanski, Takalo, & Valtaoja 2002) or periodicity (Ciaramella et al 2004). Early radio observations of PKS 0735+178 showed a peculiar spectrum soon interpreted as the superposition of incoherent synchrotron radiation emitted by distinct and homogeneous radio components, conspiring to add up to an overall very flat shape (the source was indeed nicknamed as the "Cosmic Conspiracy" Marscher 1977Marscher , 1980Cotton et al 1980).…”

Section: Optical Properties Of Pks 0735+178mentioning

confidence: 99%

Ten-year optical monitoring of PKS 0735+178: historical comparison, multiband behavior, and variability timescales

Ciprini

Takalo

Tosti

et al. 2007

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Aims. New data and results on the optical behaviour of the prominent blazar PKS 0735+178 (also know as OI 158, S3 0735+17, DA 237, 1ES 0735+178, 3EG J0737+1721) are presented, through the most continuous BV RI data available in the period 1994-2004 (about 500 nights of observations). In addition, the whole historical light curve, and a new photometric calibration of comparison stars in the field of this source are reported. Methods. Several methods for timeseries analysis of sparse data sets are developed, adapted and applied to the reconstructed historical light curve and to each observing season of our unpublished optical database on PKS 0735+178. Optical spectral indexes are calculated from the multi-band observations and studied on long-term (years) durations as well. For the first time in this source, variability modes, characteristic timescales and the signal power spectrum are explored and identified over 3 decades in time with a sufficient statistics. The novel investigation of mid-term optical scales (days, weeks), could be also applied and compared to blazar gamma-ray light curves that will be provided, on the same timescales, by the forthcoming GLAST observatory. Results. In the last 10 years the optical emission of PKS 0735+178 exhibited a rather achromatic behaviour and a variability mode resembling the shot-noise. The source was in a intermediate or low brightness level, showing a mild flaring activity and a superimposition/succession of rapid and slower flares, without extraordinary and isolated outbursts but, at any rate, characterized by one major active phase in 2001. Several mid-term scales of variability were found, the more common falling into duration intervals of about 27-28 days, 50-56 days and 76-79 days. Rapid variability in the historical light curve appears to be modulated by a general, slower and rather oscillating temporal trend, where typical amplitudes of about 4.5, 8.5 and 11-13 years can be identified. This spectral and temporal analysis, accompanying our data publication, suggests the occurrence of distinctive signatures at mid-term durations that can likely be of transitory nature. On the other hand the possible pseudo-cyclical or multi-component modulations at long times could be more stable, recurrent and correlated to the bimodal radio flux behaviour and the twisted radio structure observed by several years in this blazar.

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Correlated radio and optical variations in a sample of AGN

Cited by 32 publications

References 41 publications

The WEBT campaigns on BL Lacertae

The WEBT campaigns on BL Lacertae

Structure and flux variability in the VLBI jet of BL Lacertae during the WEBT campaigns (1995–2004)

Ten-year optical monitoring of PKS 0735+178: historical comparison, multiband behavior, and variability timescales

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