Comparison of the X-ray and optical variabilities in the Seyfert galaxy 3C 120

• Published in: Astronomy letters 2009-06, Vol.35 (6), p.361-374
• Main Authors: Doroshenko, V. T., Sergeev, S. G., Efimov, Yu. S., Klimanov, S. A., Nazarov, S. V.
• Format: Article
• Language: English
• Subjects: Astronomy; Astrophysics and Astroparticles; Earth, ocean, space; Exact sciences and technology; Observations and Techniques; Optics; Physics; Physics and Astronomy; Stars & galaxies; Studies; X-rays
• ISSN: 1063-7737; 1562-6873
• DOI: 10.1134/S1063773709060012

Based on our UBV RI observations and X-ray data from the RXTE satellite, we have investigated the variability of the galaxy 3C 120 over the period 1996–2008. The relative variability amplitude in the U and B bands without any subtraction of the contribution from the underlying galaxy is 23 and 22%, respectively, against 21% in the X-ray band. The autocorrelation function based on the B -band data is considerably wider than that based on the X-ray data. The structure functions on a time scale from 1 to ∼100–300 days in the X-ray and optical spectral ranges have the form of a power law ( SF ∼ τ b ). However, their indices differ significantly: b = 0.42 in the X-ray band and b = 1.36 in the B band. Considering the X-ray and optical variabilities as a superposition of independent flares in a wide range of durations, we may conclude that the amplitudes of short flares in the X-ray band are higher than those in the optical one and, conversely, the relative amplitudes of long flares in the X-ray band are slightly lower than those in the optical one, i.e., short events dominate in the X-ray band. The optical flux variations in the R c and I c bands lag significantly behind those in the B band, by 3.9 −0.7 +1.0 and 6.2 −0.6 +1.1 days, respectively, if the lag is estimated from the centroid of the cross-correlation function. The X-ray variability on a time scale of about 1800 days (∼5 yr) lags behind the B -band variations by 5.3 −3.3 +2.7 days, but the confidence level of this estimate is only 87%. A more detailed analysis of the correlation between the X-ray and optical emissions has revealed a fairly complex picture: different degrees of correlation between the optical and X-ray fluxes are observed at different times.