Large-amplitude rapid X-ray variability in the narrow line Seyfert 1 galaxy PG 1404+226: the accretion disk/corona connection [HEAP]

We present results from a detailed analysis of our 2016 \xmm{} observation of the narrow-line Seyfert~1 galaxy PG~1404+226 which showed a large-amplitude, rapid X-ray variability by a factor of $\sim8$ in $\sim11$\ks{}. We use this variability event to investigate the origin of the soft X-ray excess emission and the connection between the disk, hot corona and the soft excess emitting region through UV/X-ray cross-correlation, time-resolved spectroscopy and root mean square (rms) spectral modelling. The weakly variable UV emission ($F_{\rm var,UV}$=3.9$\pm0.2\%$) appears to lead the strongly variable X-ray emission ($F_{\rm var, X}$=89.0$\pm0.7\%$) by $\sim33$\ks{}. Such a UV lead is consistent with the crossing time ($\sim23$\ks{}) of the seed photons from the disk to a compact ($\sim 10r_s$) hot corona and the time required for their thermal Comptonization ($\sim9$\ks{}) giving rise to the X-ray power-law emission. The strong soft X-ray excess below 1\keV{} seen in the mean X-ray spectrum as well as in the time-resolved spectra is well described by both the intrinsic disk Comptonization and the blurred reflection models. The soft excess emission is found to vary together with the power-law component as $F_{{\rm primary}}\propto F_{{\rm excess}}^{2.01}$. The X-ray fractional rms spectrum shows an increase in variability with energy which can be described only in the framework of blurred reflection model in which both the intrinsic continuum and the reflected emission are highly variable in normalization only and are perfectly coupled with each other. Our results suggest that accretion disk provides the seed photons for thermal Comptonization giving rise to the X-ray power-law component which in turn illuminates the innermost accretion disk and gives rise to the soft X-ray excess emission.

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L. Mallick and G. Dewangan
Tue, 28 Feb 17

Comments: 16 pages, 12 figures, 4 tables, Submitted to ApJ