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

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 (kT(e) = 50-100 keV) and warm (kT(e) = 0.1-1 keV) coronae are responsible for the hard and soft power-law continua, while the circumnuclear toroidal reflector accounts for the Fe K emission line and the associated Compton hump. However, all these spectral features are simultaneously observed only in a handful of sources above z similar or equal to 0.1.Aims. An ideal astrophysical laboratory for this investigation is the quasar RBS 1055, at z similar or equal to 0.45. With a luminosity L-2-10(keV) = 2 x 10(45)( )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 K alpha line and L2-10keV, an intense Fe K alpha 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 kT(e) = 0.12(-0.03)(+0.)(08) keV, kT(e) = 30(-10)(+40) keV and Thomson optical depths tau = 30(-1.4)(+1.0) and T = 3.01:4 for the former and the latter component, respectively. We confirm the presence of an intense Fe K alpha emission line (EW = 55 +/- 6 eV) and find, when a toroidal model is considered for reproducing the Compton reflection, a Compton-thin solution with N-H = (3.2(-0.8)(+0.9)) x 10(23) 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 (M) over dot(out) = 25.4 +/- 1.5 M-circle dot yr(-1) and outflow kinetic power normalized by the bolometric luminosity (E)over dot(kin)/L-Bol similar to 0.33%. We estimate the BH mass to be in the range 2.8 x 10(8) -1.2 x 10(9) M-circle dot, according to different broad line region emission lines, with an average value of < M-BH > = 6.5 x 10(8) M-circle dot.Conclusions. With an Fe K alpha that is 3 sigma above the value predicted from the EW-L-2-(10keV) 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.