Anatomy of the AGN in NGC 5548. VIII. XMM-Newton's EPIC detailed view of an unexpected variable multilayer absorber
Kaastra, J. S.; Harrison, F. A.; Kriss, G. A.; Arav, N.; Bianchi, S.; Cappi, M.; Costantini, E.; Ebrero, J.; Mehdipour, M.; Paltani, S.; Ponti, G.; Behar, E.; Branduardi-Raymont, G.; Petrucci, P. -O.; Steenbrugge, K. C.; Peterson, B. M.; De Marco, B.; Matt, G.; Ursini, F.; Di Gesu, L.; Kaspi, S.; Walton, D. J.; Whewell, M.; Boissay, R.
Italy, Germany, France, Netherlands, United States, United Kingdom, Israel, Switzerland, Spain, Chile
Abstract
In 2013, we conducted a large multi-wavelength campaign on the archetypical Seyfert 1 galaxy NGC 5548. Unexpectedly, this usually unobscured source appeared strongly absorbed in the soft X-rays during the entire campaign, and signatures of new and strong outflows were present in the almost simultaneous UV HST/COS data. Here we carry out a comprehensive spectral analysis of all available XMM-Newton observations of NGC 5548 (precisely 14 observations from our campaign plus three from the archive, for a total of ~763 ks) in combination with three simultaneous NuSTAR observations. We obtain a best-fit underlying continuum model composed by I) a weakly varying flat (Γ ~ 1.5-1.7) power-law component; II) a constant, cold reflection (FeK + continuum) component; III) a soft excess, possibly owing to thermal Comptonization; and iv) a constant, ionized scattered emission-line dominated component. Our main findings are that, during the 2013 campaign, the first three of these components appear to be partially covered by a heavy and variable obscurer that is located along the line of sight (LOS), which is consistent with a multilayer of cold and mildly ionized gas. We characterize in detail the short timescale (mostly ~ks-to-days) spectral variability of this new obscurer, and find it is mostly due to a combination of column density and covering factor variations, on top of intrinsic power-law (flux and slope) variations. In addition, our best-fit spectrum is left with several (but marginal) absorption features at rest-frame energies ~6.7-6.9 keV and ~8 keV, aswell as a weak broad emission line feature redwards of the 6.4 keV emission line. These could indicate a more complex underlying model, e.g. a P-Cygni-type emission profile if we allow for a large velocity and wide-angle outflow. These findings are consistent with a picture where the obscurer represents the manifestation along the LOS of a multilayer of gas, which is also in multiphase, and which is likely outflowing at high speed, and simultaneously producing heavy obscuration and scattering in the X-rays, as well as broad absorption features in the UV.