Revealing the burning and soft heart of the bright bare active galactic nucleus ESO 141-G55: X-ray broadband and SED analysis

Abstract

ESO 141-G55 is a nearby X-ray bright broad-line Seyfert\,1 (BLS1) that has been classified as a bare active galactic nucleus (AGN) due to a lack of warm absorption along its line of sight, providing an unhampered view into its disc-corona system. We aim to probe its disc-corona system thanks to the first simultaneous XMM-Newton and NuSTAR observation obtained October 1--2, 2022. We carried out an X-ray broadband spectral analysis to determine the dominant process(es) at work as well as a spectral energy distribution (SED) analysis to determine the disc-corona properties. The simultaneous broadband X-ray spectrum of ESO\,141-G55 is characterised by the presence of a prominent smooth soft X-ray excess, a broad Fe\,Kalpha emission line, and a significant Compton hump. The high-resolution reflection grating spectrometer spectra confirmed the lack of intrinsic warm-absorbing gas along our line of sight in the AGN rest frame, verifying that it is still in a bare state. However, soft X-ray emission lines were observed, indicating substantial warm gas out of our line of sight. The intermediate inclination of the disc-corona system (sim circ $) may offer us a favourable configuration to observe ultra-fast outflows from the disc, but none were found in this 2022 observation, contrary to a previous 2007 XMM-Newton one. We ruled out relativistic reflection alone on a standard disc based on the X-ray broadband analysis, while a combination of soft and hard Comptonisation by a warm and hot corona ( relagn ) plus relativistic reflection ( reflkerrd ) reproduces the ESO\,141-G55 SED quite well. The hot corona temperature is very hot, sim 140\,keV, and much higher than about 80 of AGNs, whereas the warm corona temperature, sim 0.3\,keV, is similar to the values found in other sub-Eddington AGNs. ESO\,141-G55 is accreting at a moderate Eddington accretion rate (sim 10--20). Our analysis points to a significant contribution of an optically thick warm corona to both the soft X-ray and UV emission in ESO\,141-G55, adding to the growing evidence that the accretion of AGNs (even at a moderate accretion rate) appears to deviate from standard disc theory.