Letter to the Editor

Fe Kα line from the broad-line region of PDS456 with XRISM/Resolve

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio, Italy
² INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Caveliere 100 00133 Roma, Italy
³ Dipartimento di Matematica e Fisica, Universitá degli Studi Roma Tre, Via della Vasca Navale 84 I-00146 Roma, Italy
⁴ Cahill Center for Astronomy & Astrophysics, California Institute of Technology, 1216 East California Boulevard Pasadena CA 91125, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 June 2026
Accepted: 11 July 2026

Abstract

The high-luminosity, high-mass and likely super-Eddington quasar PDS 456 is known for its extremely dynamic environment. A wealth of observational features from the millimetric up to the X-ray band shows energetic outflows at all scales, from accretion disc up to galaxy scales. Broad emission lines in the optical and UV show significant widths and line-of-sight velocities. Moreover, NIR interferometry revealed a partially outflowing Broad Line Region. Thanks to the unprecedented energy resolution of the X-ray microcalorimeter Resolve onboard XRISM, we detect a neutral Fe Kα line, the main tracer of cold matter around AGNs. The line blueshift (v_out = 2700 km s⁻¹) and width (σ ≤ 700 km s⁻¹) are on the lower bound of the range of values of the optical-UV emission lines, possibly suggesting a stratified medium. The derived column density, N_H ≈ 10²² cm⁻², is significantly lower than what expected for Broad Line Regions, again in agreement with a composite emission. The very small Equivalent Width (9 eV) marks one of the smallest measurements in the literature and probes the high-luminosity end of the so-called Iwasawa-Taniguchi (or X-ray Baldwin) effect.