Origin of the characteristic X-ray spectral variations of IRAS 13224−3809

Abstract

The Narrow-line Seyfert 1 galaxy (NLS1) IRAS 13224−3809 is known to exhibit significant Xray spectral variation, a sharp spectral drop at ∼ 7 keV, strong soft excess emission, and a hint of iron L-edge feature, which is very similar to the NLS1 1H 0707−495. We have proposed the "Variable Double Partial Covering (VDPC) model" to explain the energy spectra and spectral variability of 1H 0707−495 (Mizumoto, Ebisawa and Sameshima 2014, PASJ, 66, 122). In this model, the observed flux/spectral variations below 10 keV within a ∼day are primarily caused by change of the partial covering fraction of patchy clouds composed by double absorption layers in the line of sight. In this paper, we apply the VDPC model to IRAS 13224−3809. Consequently, we have found that the VDPC model can explain the observed spectral variations of IRAS 13224-3809 in the 0.5-10 keV band. In particular, we can explain the observed Root Mean Square (RMS) spectra (energy dependence of the fractional flux variation) in the entire 0.5 -10 keV band. In addition to the well-known significant drop in the iron K-band, we have found intriguing iron L-peaks in the RMS spectra when the iron L-edge is particularly deep. This feature, which is also found in 1H 0707-495, is naturally explained with the VDPC model, such that the RMS variations increase at the energies where optical depths of the partial absorbers are large. The absorbers have a larger optical depth at the iron L-edge than in the adjacent energy bands, thus a characteristic iron L-peak appears. On the other hand, just below the iron K-edge, the optical depth is the lowest and the RMS spectrum has a broad dip.

Nature of the warm absorber outflow in NGC 4051

“…We have shown that 'double partial covering' with the same partial covering fractions can explain the observed spectral/flux variability of NGC 4051 in the 0.4-10 keV. Indeed, equation 13is equivalent to the 'variable double partial covering (VDPC) model', which was originally proposed for MCG-6-30-15 by Miyakawa, Ebisawa & Inoue (2012), refined by Mizumoto et al (2014), and confirmed for various NLS1s by Iso et al (2016) and Yamasaki et al (2016). In the VDPC model, the commonality of the two partial covering fractions is explained by double-layer absorbers, and the spectral variations of NLS1s are explained by the partial covering fraction of the double-layer absorbers, as well as independent variation of the continuum normalization.…”

“…Equations (5) and (7) are mathematically equivalent; both equations monotonically increase with σ (E), which means that rms peaks appear at the energy band where the absorption features are deeper than the adjacent bands (see fig. 10 of Yamasaki et al 2016). We calculated simulated rms spectra in order to illustrate effects of the warm absorber variations in the rms spectra.…”

X-ray reverberation lags of the Fe–K line due to AGN disc winds

“…Such variable partial covering models can fit the observed spectral variability (e.g. Miyakawa et al 2012;Mizumoto et al 2014;Iso et al 2016;Yamasaki et al 2016). The occultation timescale is t occ ∼ 2000(M BH /2 × 10 6 M ⊙ )(R/500 R g ) 1/2 (D/10 R g ) s (Mizumoto & Ebisawa 2017), where R and D are the location and size of the cold clumps, respectively.…”

Modelling X-ray RMS spectra II: the ultrafast outflow of PDS 456

“…This restricts the results that can be obtained, and makes comparison between different spectra difficult. The typical analysis of such a spectrum is a qualitative examination by eye, and a more sophisticated analysis will include comparison with a simulated spectrum based on the model fit to the count spectrum (e.g., Vaughan & Fabian 2004;Matzeu et al 2016;Yamasaki et al 2016;Mallick et al 2017;Alston et al 2020).…”