The remarkable X-ray variability of IRAS 13224-3809 I: The variability process

We present a detailed X-ray timing analysis of the highly variable NLS1 galaxy, IRAS 13224-3809. The source was recently monitored for 1.5 Ms with XMM-Newton which, combined with 500 ks archival data, makes this the best studied NLS1 galaxy in X-rays to date. We apply standard time- and Fourier-domain in order to understand the underlying variability process. The source flux is not distributed lognormally, as would be expected for accreting sources. The first non-linear rms-flux relation for any accreting source in any waveband is found, with rms∝flux^2/3. The light curves exhibit significant strong non-stationarity, in addition to that caused by the rms-flux relation, and are fractionally more variable at lower source flux. The power spectrum is estimated down to ∼ 10^-7 Hz and consists of multiple peaked components: a low-frequency break at ∼ 10^-5 Hz, with slope α < 1 down to low frequencies; an additional component breaking at ∼ 10^-3 Hz. Using the high-frequency break we estimate the black hole mass M_BH = [0.5-2] × 10^6 M_⊙, and mass accretion rate in Eddington units, ṁ_ Edd≳ 1. The non-stationarity is manifest in the PSD with the normalisation of the peaked components increasing with decreasing source flux, as well as the low-frequency peak moving to higher frequencies. We also detect a narrow coherent feature in the soft band PSD at 0.7 mHz, modelled with a Lorentzian the feature has Q ∼ 8 and an rms∼ 3 accretion of matter onto black holes.