XMM/HST monitoring of the ultra-soft highly accreting narrow-line Seyfert 1 RBS 1332

¹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, (RM), Italy
² Space Science Data Center, Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
³ Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁶ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti, 93/3, 40129 Bologna, Italy
⁷ Departament de Física, EEBE, Universitat Politécnica de Catalunya, Av. Eduard Maristany 16, 08019 Barcelona, Spain
⁸ INAF, Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere, 100, I-00133 Rome, Italy
⁹ Quasar Science Resources SL for ESA, European Space Astronomy Centre (ESAC), Science Operations Department, 28692 Villanueva de la Cañada, Madrid, Spain

^⋆ Corresponding author; riccardo.middei@ssdc.asi.it

Received: 16 September 2024
Accepted: 14 January 2025

Abstract

Ultra-soft narrow-line Seyfert 1s (US-NLSy 1s) are a poorly observed class of active galactic nuclei characterised by significant flux changes and an extreme soft X-ray excess. This peculiar spectral shape represents a golden opportunity to test whether the standard framework commonly adopted for modelling local AGNs is still valid. We thus present the results of the joint XMM-Newton and HST monitoring campaign of the highly accreting US-NLSy RBS 1332. The optical-to-UV spectrum of RBS 1332 exhibits evidence for both a stratified narrow-line region and an ionised outflow that produces absorption troughs over a wide range of velocities (from ∼–1500 km s⁻¹ to ∼1700 km s⁻¹) in several high-ionisation transitions (Lyα, N V, C IV). From a spectroscopic point of view, the optical/UV/FUV/X-ray emission of this source is due to the superposition of three distinct components that are best modelled in the context of the two-coronae framework in which the radiation of RBS 1332 can be ascribed to a standard outer disc, a warm Comptonisation region, and a soft coronal continuum. The present dataset is not compatible with a pure relativistic reflection scenario. Finally, the adoption of the novel model REXCOR allowed us to determine that the soft X-ray excess in RBS 1332 is dominated by the emission of the optically thick and warm Comptonising medium, and only a marginal contribution is expected from relativistic reflection from a lamppost-like corona.