Breaking the rules at z ≃ 0.45: The rebel case of RBS 1055

Abstract

Context. Very luminous quasars are unique sources for studying the circumnuclear environment around supermassive black holes. Several components contribute to the overall X-ray spectral shape of active galactic nuclei (AGN). The hot (kTe = 50 − 100 keV) and warm (kTe = 0.1 − 1 keV) coronae are responsible for the hard and soft power-law continua, while the circumnuclear toroidal reflector accounts for the Fe Kα emission line and the associated Compton hump. However, all these spectral features are simultaneously observed only in a handful of sources above z ≃ 0.1. Aims. An ideal astrophysical laboratory for this investigation is the quasar RBS 1055, at z ≃ 0.45. With a luminosity L2 − 10 keV = 2 × 1045 erg s−1, it is the brightest radio-quiet quasar from the ROSAT Bright Survey. Despite the known anti-correlation between the equivalent width (EW) of the narrow neutral Fe Kα line and L2 − 10 keV, an intense Fe Kα was previously detected for this source. Methods. We report findings based on a long (250 ks) NuSTAR observation performed in March 2021 and archival XMM-Newton pointings (185 ks) taken in July 2014. We also analyzed an optical spectrum of the source taken with the Double Spectrograph at the Palomar Observatory quasi-simultaneously to the NuSTAR observations. Results. We find that the two-corona model, in which a warm and hot corona coexist, well reproduces the broad band spectrum of RBS 1055, with temperatures kTe = 0.12−0.03+0.08 keV, kTe = 30−10+40 keV and Thomson optical depths τ = 30−10+15 and τ = 3.0−1.4+1.0 for the former and the latter component, respectively. We confirm the presence of an intense Fe Kα emission line (EW = 55 ± 6 eV) and find, when a toroidal model is considered for reproducing the Compton reflection, a Compton-thin solution with NH = (3.2−0.8+0.9) × 1023 cm−2 for the circumnuclear reflector. A detailed analysis of the optical spectrum reveals a likely peculiar configuration of our line of sight with respect to the nucleus, and the presence of a broad [O III] component tracing outflows in the Narrow Line Region, with a velocity shift v = 1500 ± 100 km s−1, leading to a mass outflow rate Ṁout = 25.4 ±1.5 M⊙ yr−1 and outflow kinetic power normalized by the bolometric luminosity Ēkin/LBol ∼ 0.33%. We estimate the BH mass to be in the range 2.8 × 108–1.2 × 109 M⊙, according to different broad line region emission lines, with an average value of ⟨MBH⟩ = 6.5 × 108 M⊙. Conclusions. With an Fe Kα that is 3σ above the value predicted from the EW–L2 − 10 keV relation and an extreme source brightness at 2 keV (a factor 10−15 higher than the one expected from the optical/UV), we can confirm that RBS 1055 is an outlier in the X-rays compared to other objects in the same luminosity and redshift range.