Periodicities in blazar light curves may be related to helical trajectories in extragalactic radio jets by differential Doppler boosting effects. We consider ballistic and nonballistic (i.e., radial) trajectories and discuss three possible periodic driving mechanisms for the origin of helical jet paths, namely, orbital motion in a binary black hole system, jet precession, and intrinsic jet rotation. It is shown that precessional-driven ballistic motion is unlikely to result in observable periods of less than several tens of years. We demonstrate that for nonballistic helical motion the observed period is generally strongly shortened relative to the real physical driving period because of light-travel time effects. Internal jet rotation may thus account for observed periods Pobs 10 days. Periodicity due to orbital-driven (nonballistic) helical motion, on the other hand, is usually constrained to periods of Pobs 10 days, while Newtonian-driven precession is unlikely to be responsible for periodicity on a timescale Pobs 100 days but may well be associated with periods of Pobs 1 yr.

On the Geometrical Origin of Periodicity in Blazar-type Sources

Aims. 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. Methods. 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. Results. We present the data acquired during the second observing season, 2004-2005, by 27 radio-to-optical telescopes. The ∼2600 data points collected allow us to trace the low-energy behaviour of the source in detail, revealing 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. While the X-ray (and optical) light curves obtained during the XMM-Newton pointings reveal no significant short-term variability, the simultaneous intraday radio observations with the 100 m telescope at Effelsberg show flux-density changes at 10.5 GHz, which are more likely due to a combination of intrinsic and extrinsic processes. Conclusions. 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 variability 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.

/pdf/multifrequency-variability-of-the-blazar-ao-0235-164-the-24juq6ejp4.pdf

Multifrequency variability of the blazar AO 0235+164 . The WEBT campaign in 2004-2005 and long-term SED analysis

Aims. 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. Methods. 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. Results. We present the data acquired during the second observing season, 2004–2005, by 27 radio-to-optical telescopes. The ∼2600 data points collected allow us to trace the low-energy behaviour of the source in detail, revealing 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. While the X-ray (and optical) light curves obtained during the XMM-Newton pointings reveal no significant short-term variability, the simultaneous intraday radio observations with the 100 m telescope at Effelsberg show flux-density changes at 10.5 GHz, which are more likely due to a combination of intrinsic and extrinsic processes. Conclusions. 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 variability of ∼ 8y r, 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 WEBT campaign in 2004-2005 and long-term SED analysis

We present optical photopolarimetric observations with high temporal resolution of the blazar AO 0235+164. Our data, the first to test the photopolarimetric behaviour of this object at very short time-scales, show significant micro-variability in total flux, colour index, linear polarization degree and position angle. Strong inter-night variations are also detected for these parameters. Although no correlation between colour index and total flux was found, our data seem to support the general bluer-when-brighter trend already known for this object. The polarization degree, in turn, shows no correlation with total flux, but a clear trend in the sense that colour index is redder (the spectrum is softer) when the measured polarization is higher.

/pdf/extreme-photopolarimetric-behaviour-of-the-blazar-ao-0235-1n2d8etf7h.pdf

Extreme photopolarimetric behaviour of the blazar AO 0235+164★

Supermassive binary black holes (SBBHs) are a natural outcome of galaxy mergers. Here we show that low-frequency (f≤10−6 Hz) quasi-periodic variability observed from cosmic blazar sources can provide substantial inductive support for the presence of close (d≲0.1 pc) SBBHs at their centers. It is argued on physical grounds that such close binary systems are likely to give rise to different (although not independent) periodicities in the radio, optical and X-ray/TeV regime, and, hence that detection of appropriate period ratios significantly corroborates the SBBH interpretation. This is illustrated for a binary model where optical longterm periodicity is related to accretion disk interactions, radio periodicity to Newtonian jet precession, and periodicities in the high energy bands to the orbital motion of the jet. We use the observed periodicities to constrain the properties for a sample of SBBH candidates including OJ 287 and AO 0235+16, and discuss the results within the context of jet activity and binary evolution.

Supermassive binary black holes among cosmic gamma-ray sources

Recent analysis of the long term radio light curve of the extremely variable BL Lacertae object AO 0235+16 by Raiteri et al. have revealed the presence of recurrent outbursts with a period of 5.7 ± 0.5yr. Periodicity analysis of the optical light curve also shows evidence for a shorter period. Here we discuss whether such a behavior can be explained by a binary black hole model where the accretion disk of one of the supermassive black holes is precessing due to the tidal eects of the companion. We estimate the mass of the accreting hole and analyze constraints on the secondary mass and the orbital parameters of the system. It is possible to provide a viable interpretation of the available multiwavelength data.

G. E. Romero

Papers

The Binary Black Hole Scenario for the BL Lacertae Object AO 0235+16