Detection of an Unidentified Soft X-ray Emission Feature in NGC 5548

Abstract

Context. NGC 5548 is an X-ray bright Seyfert 1 active galaxy. It exhibits a variety of spectroscopic features in the soft X-ray band, including in particular the absorption by the AGN outflows of a broad range of ionization states, with column densities up to 10 27 m −2 , and having speeds up to several thousand kilometers per second. The known emission features are in broad agreement with photoionized X-ray narrow and broad emission line models. Aims. We report on an X-ray spectroscopic study using 1.1 Ms XMM-Newton and 0.9 Ms Chandra grating observations of NGC 5548 spanning two decades. The aim is to search and characterize any potential spectroscopic features in addition to the known primary spectral components that are already modeled in high precision. Methods. For each observation, we model the data using a global fit including an intrinsic spectral energy distribution of the AGN and the known distant X-ray absorbers and emitters. We utilize as much knowledge from previous studies. The fit residuals are stacked and scanned for possible secondary features. Results. We detect a weak unidentified excess emission feature at ∼ 18.4 Å (18.1 Å in the restframe). The feature is seen at > 5σ statistical significance taking into account the look elsewhere effect. No known instrumental issues, atomic transitions, and astrophysical effects can explain this excess. The observed intensity of the possible feature seems to anti-correlate in time with the hardness ratio of the source. However, the variability might not be intrinsic, it might be caused by the time-variable obscuration by the outflows. An intriguing possibility is the line emission from charge exchange between a partially ionized outflow and a neutral layer in the same outflow, or in the close environment. Other possibilities, such as emission from a highly-ionized component with high outflowing speed, cannot be fully ruled out.