New Frontiers in Black Hole Astrophysics Proceedings IAU Symposium No. 324, 2016 Andreja Gomboc, ed. c  International Astronomical Union 2017 doi:10.1017/S1743921317003490 X-ray fluctuation timescale and Black Hole mass relation in AGN Amri Wandel 1 and Mathew Malkan 2 1 Racah Institute, The Hebrew University of Jerusalem, Jerusalem 91904, Israel email: amri@huji.ac.il 2 Dept. of Astronomy, University of California, Los Angeles, California , U.S.A. email: malkan@astro.ucla.edu Abstract. We analyze the fluctuations in the X-ray flux of 20 AGN (mainly Seyfert 1 galaxies) monitored by RXTE and XMM-Newton with a sampling frequency ranging from hours to years, using structure function (SF) analysis. We derive SFs over four orders of magnitude in the time domain (0.03-300 days). Most objects show a characteristic time scale, where the SF flattens or changes slope. For 10 objects with published power-spectral density (PSD) the break time scales in the SF and PSD are similar and show a good correlation . We also find a significant correlation between the SF timescale and the mass of the central black hole, determined for most objects by reverberation mapping. Keywords. AGN, X-ray, Fluctuations, Structure Functions, Power Spectral Density, Black Holes 1. Introduction The strong, irregular X-ray variability observed in many Seyfert galaxies on timescales as short as hours indicates that these X-rays are emitted from a region close to the central black hole. Doubling time scales were used as the first quantification of variability; characteristic time scales of X-ray variability seemed to correlate with luminosity and black hole mass Wandel & Mushotzky (1986). For quasi-periodic and stochastic light curves the variability is often characterized through the power spectral density (PSD). The PSD is the variability amplitude per frequency, so it describes the variability power contained within a frequency interval. A similar measure that is sometimes used is the structure function, which describes the variability amplitude as a function of time scale. The variability of quasi-periodic and stochastic light curves may then be characterized by representing the power spectrum in a parametric form. However, irregular sampling or sequences of regular sampling separated by gaps often cause problems in measuring variability features from a light curve, which is the case in all ground-based and often also space-based data. Ideally the variability must be quantified over a broad a range of time scales, to maximize the probability of detecting a characteristic variability time scale and to avoid spurious features caused by an insufficiently long observing time. The Rossi X-ray Timing Explorer (XTE) has been the first mission to provide sustained monitoring on given objects for time scales up to months and years. High dynamic- range X-ray PSDs were published for approximately 10 AGN, (e.g., Edelson & Nandra (1999), Uttley, McHardy & Papadakis (2002), Markowitz et al. (2003), Chatterjee et al. (2009;2011), Rothshild et al. (2011)). Typically they are parameterized as a broken power law; on long timescales the fluctuation power is flat, scaling as a power of f 0-1 , and on short timescales it is descending more steeply, with slopes of 2-3. The break between 172 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921317003490 Downloaded from https://www.cambridge.org/core. IP address: 3.239.7.17, on 15 Dec 2021 at 16:07:40, subject to the Cambridge Core terms of use, available at