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, A. J. Young. 2019. The remarkable X-ray variability of IRAS 13224-3809-I. The variability process. Monthly Notices of the Royal Astronomical Society 482, 2, 2088-2106 DOI: 10.1093/mnras/sty2527

We present a detailed X-ray timing analysis of the highly variable narrow-line Seyfert 1 ( NLS1) galaxy IRAS 13224-3809. The source was recently monitored for 1.5Ms 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 techniques in order to understand the underlying variability process. The source flux is not distributed lognormally, as expected for all types of 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 similar to 10(-7) Hz and consists of multiple peaked components: a low-frequency break at similar to 10(-5) Hz, with slope alpha < 1 down to low frequencies, and an additional component breaking at similar to 10(-3) Hz. Using the high-frequency break, we estimate the black hole mass MBH = [0.5-2] x 10(6)M(circle dot) and mass accretion rate in Eddington units,. m Edd greater than or similar to 1. The broad-band power spectral density (PSD) and accretion rate make IRAS 13224-3809 a likely analogue of very high/intermediate-state black hole X-ray binaries. The non-stationarity is manifest in the PSD with the normalization 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 7 x 10(-4) Hz; modelled with a Lorentzian the feature has Q similar to 8 and an rms similar to 3 per cent. We discuss the implication of these results for accretion of matter on to black holes.

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