Constraining Intrinsic Power Spectra Of Active Galactic Nuclei And Revealing Ultrafast X-Ray Variability Of TeV Blazars

Active galactic nuclei (AGNs) are powered by accretion of matter onto supermas-sive black holes at the centers of galaxies. A common character of black hole accreiton systems is the persistent flux variability over a wide range of wavelengths, occurring on timescales that span many orders of magnitude. Timing studies of AGNs not only pro-vide us powerful tools to investigate the structure around the central black hole, but also reveal important properties of the accretion process. One approach is to calculate the Fourier Transform based power spectral density (PSD). People use periodogram to es-timate the underlying PSD of the physical process. Caused both by the intrinsic PSD of AGN and limitations of astronomical observations, periodogram usually suffers from systematic biases, among which the so called red-noise leakage seriously distorts the high-frequency PSD that describes short term variability. Based on the fact that red-noise leakage problem has not been well solved, we propose a novel way of describing the distribution of red-noise leakage at different frequencies, which is called normalized leakage spectrum (NLS). Utilizing Monte Carlo simulations, we prove that NLS can be used to constrain the intrinsic PSD of AGN.According to the unification model of AGNs, blazars are radio galaxies with one of their radio jets pointing nearly directly toward us. The subgroup of blazars that can be detected at very high energy (VHE;>100GeV) γ-rays are called TeV blazars. Their spectral energy distributions usually manifest a broad peak at X-ray band, which is due to the synchrotron emission of the most energetic electron population in the relativistic j et, thus the X-ray flux usually varies faster than other bands. The most rapid variability, which usually manifests in sub-hour flares, can constrain the physical properties of the emission region, e.g., the magnetic field, Doppler factor and physical scale of emission zone. We search in the complete RXTE archival observations of TeV blazars for such flares. We find 3 new events, of which the fastest one varies in sub-minute timescale. This timescale is by far the shortest of all the AGN classes, and reveals an emission region even smaller than the black hole event horizon.