Kepler light-curve analysis of the blazar W2R 1926+42

Abstract

We study the long term Kepler light curve of the blazar W2R 1926+42 (~1.6 yr) which indicates a variety of variability properties during different intervals of observation. The normalized excess variance, Fvar ranges from 1.8 per cent in the quiescent phase and 43.3 per cent in the outburst phase. We find no significant deviation from linearity in the Fvar-flux relation. Time series analysis is conducted using the Fourier power spectrum and the wavelet analysis methods to study the power spectral density (PSD) shape, infer characteristic time-scales and statistically significant quasi-periodic oscillations (QPOs). A bending power law with an associated time-scale of TB = 6.2+6.4-3.1 hours is inferred in the PSD analysis. We obtain a black hole mass of M• = (1.5-5.9) × 107M⊙ for the first time using Fvar and the bend time-scale for this source. From a mean outburst lifetime of days, we infer a distance from the jet base r ≤ 1.75 pc indicating that the outburst originates due to a shock. A possible QPO peaked at 9.1 d and lasting 3.4 cycles is inferred from the wavelet analysis. Assuming that the QPO is a true feature, r = (152-378)GM•/c2 and supported by the other timing analysis products such as a weighted mean PSD slope of -1.5 ± 0.2 from the PSD analysis, we argue that the observed variability and the weak and short duration QPO could be due to jet based processes including orbital features in a relativistic helical jet and others such as shocks and turbulence.