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

W. N. Alston, A. C. Fabian, D. J. K. Buisson, E. Kara, M. L. Parker, A. M. Lohfink, P. Uttley, D. R. Wilkins, C. Pinto, B. De Marco, E. M. Cackett, M. J. Middleton, D. J. Walton, C. S. Reynolds, J. Jiang, L. C. Gallo, A. Zogbhi, G. Miniutti, M. Dovciak and A. J. Young, 2018, MNRAS 482, 2088

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∝flux2/3. 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 ∼10−3Hz. Using the high-frequency break we estimate the black hole mass MBH=[0.5−2]×106M⊙, and mass accretion rate in Eddington units, m˙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 0.7mHz, modelled with a Lorentzian the feature has Q∼8 and an rms∼3 %. We discuss the implication of these results for accretion of matter onto black holes.