RADIO PHYSICS AND RADIO ASTRONOMY

PACS number: 98.54.Cm

Purpose: Interrelation and difference in the appearance of quasiperiodic activity of BL-Lac objects OJ 287 and BL Lac is investigated according to optical and radio observations. The aim of the work is to determine and compare the basic quasi-periods of these BL-Lac objects in different light filters of optical range and in radio frequency range (at 15 and 14.5 GHz), as well as brief overview of the results obtained by other authors. Also, the method of comparing optical and radio data in separate bands of close periods was tested. This method will make it possible to better determine the delays between optical and radio data only in the bands of the main quasiperiodic oscillations, which form light curves and screen out noises and irregular variations in the source magnitude and flux.

Design/methodology/approach: The authors used the data of optical observations of OJ 287 in 1978–2018 and of BL Lac in 1970–2018 from the AAVSO (American Association of Variable Star Observers) catalog and from the catalog of a long-term (2008–2018) radio source monitoring at the 40-meter radio telescope, OVRO observatory (Owens Valley Radio Observatory, USA) at 15 GHz, as well as the data from UMRAO observatory (Radio observatory of Michigan University) obtained at 14.5 GHz within 1974–2011. To calculate periodograms and wavelet spectra, a “fast” modification of the Lomb-Scargle method was used, as well as a “fast” method of calculating wavelet spectra via fast Fourier transform with the Morlet analyzing function. Data interpolation has been made by using smoothing cubic splines. To isolate the bands of individual quasi-periods in optical and radio data, Fourier filtering with a Hamming spectral window is used providing the edge effects of about 1 % of time series length.

Findings: Radio source OJ 287 shows good accordance between quasi-periods in optics and radio within 1.1 to 2 years. However, long-term periods in the optical range, close to 12 and 6 years, mentioned in many works, are practically imperceptible in the radio range, against the background of 25-year trend wave. The BL Lac radio source has more differences. In the optical range, a quasi-period of 9 years (about 8 years in the radio one) is observed in the visual light curve. A long wave with the possible period of about 12–13 years, in the optical and radio data is unnoticeable, and the greatest similarity between rapid variability in optical and radio ranges is observed within the periods of 0.6–4 years. Comparison of individual oscillations in close periods for optical and radio data allocated by the Fourier filtering showed their good similarity and perspective in further use of this method in analyzing time delays between these frequency ranges.

Conclusions: Study of variability properties of OJ 287 and BL Lac according to the data of optical and radio observations showed similarities and differences in quasi-periods of their activity, which can be due to the difference of emitting regions in optical and radio ranges. In the optical range, in addition to the jet radiation due to the inverse Compton effect, there exists a contribution from the accretion radiation disk, whose wave processes can give different set of quasi-periods than those observed in the radio range. Therefore, qualitative observations of these radio sources (especially optical) are very important for further construction of models capable of taking into account differences in the processes which form radiation variability in optical and radio ranges.

Key words: BL-Lac objects, bandpass filtering, photometry, periodogram, wavelet analysis

Manuscript submitted  10.05.2019

Radio phys. radio astron. 2019, 24(4): 254-271

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