Analyses On Asymmetric Variability Of AGNs

The high quality light curves of Kepler space telescope make it possible to ana-lyze the optical variability of AGNs with an unprecedented time resolution. Studying the asymmetry in variations could give independent constraints on the physical models for AGN variability. In this paper, we use Kepler observations of 19 sources to per-form analyses on the variability asymmetry of AGNs. We apply smoothing-correction to light curves to deduct the bias to high frequency variability asymmetry, caused by long term variations which are poorly sampled due to the limited length of light curves. A parameter β based on structure functions is introduced to quantitively describe the asymmetry and its uncertainty is measured using extensive Monte-Carlo simulations. Individual sources show no evidence of asymmetry at timescales of 1-20 days and there is not a general trend toward positive or negative asymmetry over the whole sam-ple. Stacking data of all 19 AGNs, we derive averaged β of 0.00±0.03 and -0.02±0.04 over timescales of 1-5 days and 5-20 days, respectively, statistically consistent with zero. Quasars and Seyfert galaxies show similar asymmetry parameters. Our results indicate that short term optical variations in AGNs are highly symmetric.We also analyze the asymmetry of color-variations of 8944 quasars from SDSS Stripe 82. Results calculated directly from observations show that the’redder when darker’trend is stronger than’bluer when brighter’trend on dozens of days, however, this difference is not physical, but due to systematic color residuals in photometric data. After reducing these residuals, we find that there are no difference between ’bluer when brighter’trend and’redder when darker’trend, i.e., the color-variations of quasars are symmetric. In addition, with observations from XMM-Newton and GALEX satellite, we also calculate the variability asymmetry of AGNs in shorter bands. After smoothing-correction and reduction of random noise, the X-ray data of 17 Seyfert galaxies yield an averaged asymmetry of -0.01±0.02 at timescales of 103-104 seconds. As for 5434 quasars from GALEX sample, the averaged asymmetry in NUV and FUV band is -0.06±0.08 and -0.01±0.06, respectively, at timescales of 1-100 days. These results show that variations of AGNs in X-ray and UV bands are also symmetric, while improvement of photometric accuracy is required for better constraints.