Variability of the soft X-ray excess in IRAS 13224-3809

We study the soft excess variability of the narrow line Seyfert 1 galaxy IRAS 13224-3809. We considered all five archival XMM-Newton observations, and we applied the flux-flux plot (FFP) method. We found that the FFPs were highly affected by the choice of the light curves' time bin size, most probably because of the fast and large amplitude variations, and the intrinsic non-linear flux-flux relations in this source. Therefore, we recommend that the smallest bin-size should be used in such cases. Hence, We constructed FFPs in 11 energy bands below 1.7 keV, and we considered the 1.7-3 keV band, as being representative of the primary emission. The FFPs are reasonably well fitted by a "power-law plus a constant" model. We detected significant positive constants in three out of five observations. The best-fit slopes are flatter than unity at energies below ~0.9 keV, where the soft excess is strongest. This suggests the presence of intrinsic spectral variability. A power-law-like primary component, which is variable in flux and spectral slope (as Γ∝ N_PL^0.1) and a soft-excess component, which varies with the primary continuum (as F_{excess∝ F_primary^0.46}), can broadly explain the FFPs. In fact, this can create positive "constants", even when a stable spectral component does not exist. Nevertheless, the possibility of a stable, soft-band constant component cannot be ruled out, but its contribution to the observed 0.2-1 keV band flux should be less than ~15%. The model constants in the FFPs were consistent with zero in one observation, and negative at energies below 1 keV in another. It is hard to explain these results in the context of any spectral variability scenario, but they may signify the presence of a variable, warm absorber in the source.