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As Setti and Woltjer noted back in 1973, one can use quasars to construct the Hubble diagram; however, the actual application of the idea was not that straightforward. It took years to implement the proposition successfully. Most ways to employ quasars for cosmology now require an advanced understanding of their structure, step by step. We briefly review this progress, with unavoidable personal biases, and concentrate on bright unobscured sources. We will mention the problem of the gas flow character close to the innermost stable circular orbit near the black hole, as discussed five decades ago. This problem later led to the development of the slim disk scenario and is recently revived in the context of Magnetically Arrested Disks (MAD) and Standard and Normal Evolution (SANE) models. We also discuss the hot or warm corona issue, which is still under debate and complicates the analysis of X-ray reflection. We present the scenario of the formation of the low ionization part of the Broad Line Region as a failed wind powered by radiation pressure acting on dust (Failed Radiatively Driven Dusty Outflow – FRADO). Next, we examine the cosmological constraints currently achievable with quasars, primarily concentrating on light echo methods (continuum time delays and spectral-line time delays to the continuum) that are (or should be) incorporating the progress mentioned above. Finally, we briefly discuss prospects in this lively subject area. [ABSTRACT FROM AUTHOR]

Using bona-fide black hole (BH) mass estimates from reverberation mapping and the line ratio [Si  vi ] 1.963 |$\rm{\mu m}$| /Brγbroad as tracer of the AGN ionizing continuum, a novel BH-mass scaling relation of the form log(M BH) = (6.40 ± 0.17) − (1.99 ± 0.37) × log ([Si  vi ]/Brγbroad), dispersion 0.47 dex, over the BH mass interval, 106–108 M⊙ is found. Following on the geometrically thin accretion disc approximation and after surveying a basic parameter space for coronal lines production, we believe one of main drivers of the relation is the effective temperature of the disc, which is effectively sampled by the [Si  vi ] 1.963 |$\rm{\mu m}$| coronal line for the range of BH masses considered. By means of cloudy photoionization models, the observed anticorrelation appears to be formally in line with the thin disc prediction T disc ∝  M BH−1/4. [ABSTRACT FROM AUTHOR]

We report preliminary results on the analysis of the continuum and Hβ light curves of the type‐1 active galactic nucleus (AGN) NGC 5548. We notice a clear signature of shallowing in the trend between the Hβ and the continuum luminosities. We attempt the recovery of this observed Hβ emission trend as a response to large continuum flux increase using CLOUDY photoionization simulations. We explore a wide range in the physical parameters space for modeling the Hβ emission from the broad‐line region (BLR) appropriate for this source. We employ a constant density, single‐cloud model approach in this study and successfully recover the observed shallowing of the Hβ emission with respect to rising AGN continuum. With our modeling, we are able to provide the constraints on the local BLR cloud density and recover the BLR distances (from the continuum source) consistent with the Hβ reverberation mapping estimates. We further discuss the implications of the BLR covering factor and their sizes on recovering the observed trend. [ABSTRACT FROM AUTHOR]

The last few years have seen the confirmation of several trends associated with the quasar main sequence. The idea of a main sequence for quasars is relatively recent, and its full potential for the observational classification and contextualization of quasars' properties has yet to be fully exploited. The main sequence drivers are discussed in terms of the properties of extreme objects. We briefly summarize developments that constrain the viewing angle of the accretion disk in a particular class of quasars (extreme Population B, radiating at low Eddington ratio), as well as inferences on the chemical composition of the broad line emitting gas, and on the nature of radio emission along the quasar main sequence. [ABSTRACT FROM AUTHOR]

Active galaxies form a clear pattern in the optical plane showing the correlation between the FWHM of the Hβ line and the ratio of the equivalent width (EW) of the optical Fe ii emission and the broad EW(Hβ). This pattern is frequently referred to as the quasar main sequence. In this paper, we study the UV plane showing the FWHM of Mg ii line against the ratio of the EW of UV Fe ii emission to the broad EW(Mg ii). We show that the UV plane trends are different, with the underlying strong correlation between the FWHM(Mg ii) and the EW(Mg ii). This correlation is entirely driven by the choice of the continuum used to measure the EW(Mg ii). If instead of the observationally determined continuum, we use a theoretically motivated power law extrapolated from the wide wavelength range, the behavior of the FWHM versus EW for Mg ii becomes similar to the behavior for Hβ. Such a similarity is expected since both the lines belong to the low-ionization group of emission lines and come from a similar region. We discuss the behavior of the lines in the context of the broad line region model based on the presence of dust in the accretion disk atmosphere. [ABSTRACT FROM AUTHOR]

The interpretation of the main sequence of quasars has become a frontier subject in the last years. This considers the effect of a highly flattened, axially symmetric geometry for the broad line region (BLR) on the parameters related to the distribution of quasars along their main sequence. We utilize the photoionization code CLOUDY to model the BLR, assuming 'un-constant' virial factor with a strong dependence on the viewing angle. We show the preliminary results of the analysis to highlight the co-dependence of the Eigenvector 1 parameter, RFeii on the broad Hβ FWHM (i.e. the line dispersion) and the inclination angle (θ), assuming fixed values for the Eddington ratio (Lbo1/LEdd), black hole mass (MBH) and spectral energy distribution (SED) shape. We consider four cases with changing cloud density (nH) and composition. Understanding the emitting region is crucial as this knowledge can be extended to the use of quasars as distance indicators for Cosmology.‡ [ABSTRACT FROM AUTHOR]

We address the effect of orientation of the accretion disk plane and the geometry of the broad line region (BLR) in the context of understanding the distribution of quasars along their Main Sequence. We utilize the photoionization code CLOUDY to model the BLR, incorporating the 'un-constant' virial factor. We show the preliminary results of the analysis to highlight the co-dependence of the Eigenvector 1 parameter, RFeII on the broad HβFWHM (i.e. the line dispersion) and the inclination angle (θ), assuming fixed values for the Eddington ratio (Lbol/ LEdd), black hole mass (MBH), spectral energy distribution (SED) shape, cloud density (nH) and composition.† [ABSTRACT FROM AUTHOR]

In Panda et al.2018a, we constructed a refined sample from the original Shen et al.(2011) QSO catalog. Based on our hypothesis — the main driver of the Quasar Main Sequence is the maximum of the accretion disk temperature (TBBB) defined by the Big Blue Bump on the Spectral Energy Distribution (Panda et al.2017; Panda et al.2018b). We select the four extreme sources that have RFeII ⩾ 4.0 and use {CIGALE (Boquien et al.2018) to fit their multi—band photometric data. We also perform detailed spectral fitting including the Fe II pseudo—continuum (based on Śniegowska et al.2018)) to estimate and compare the value of RFEII. We show the dependence of FeII strength on changing metallicity. [ABSTRACT FROM AUTHOR]

In Panda et al.2018a, we constructed a refined sample from the original Shen et al.(2011) QSO catalog. Based on our hypothesis — the main driver of the Quasar Main Sequence is the maximum of the accretion disk temperature (TBBB) defined by the Big Blue Bump on the Spectral Energy Distribution (Panda et al.2017; Panda et al.2018b). We select the four extreme sources that have RFeII ⩾ 4.0 and use {CIGALE (Boquien et al.2018) to fit their multi—band photometric data. We also perform detailed spectral fitting including the Fe II pseudo—continuum (based on Śniegowska et al.2018)) to estimate and compare the value of RFEII. We show the dependence of FeII strength on changing metallicity. [ABSTRACT FROM AUTHOR]