Search

We use 78 reverberation-measured Mg II time-lag quasars (QSOs) in the redshift range $0.0033 \leq z \leq 1.89$ to constrain cosmological parameters in six different cosmological models. The basis of our method is the use of the radius-luminosity or $R-L$ relation to standardize these 78 Mg II QSOs. In each cosmological model we simultaneously determine $R-L$ relation and cosmological model parameters, thus avoiding the circularity problem. We find that the $R-L$ relation parameter values are independent of the cosmological model used in the analysis thus establishing that current Mg II QSOs are standardizable candles. Cosmological constraints obtained using these QSOs are significantly weaker than, but consistent with, those obtained from a joint analysis of baryon acoustic oscillation (BAO) observations and Hubble parameter [$H(z)$] measurements. So, we also analyse these QSOs in conjunction with the BAO + $H(z)$ data and find cosmological constraints consistent with the standard spatially-flat $\Lambda$CDM model as well as with mild dark energy dynamics and a little spatial curvature. A larger sample of higher-quality reverberation-measured QSOs should have a smaller intrinsic dispersion and so should provide tighter constraints on cosmological parameters., Comment: 16 pages, 11 figure, Mg II QSO data are given, MNRAS accepted version. arXiv admin note: text overlap with arXiv:2105.12692

As Setti & Woltjer noted back in 1973, quasars could be used to construct the Hubble diagram but the actualapplication was not that straightforward. It took years to implement the idea successfully. Most of the ways to use quasars for cosmology now require an advanced understanding of their structure, step by step. We briefly review this progress, with unavoidable personal bias, and concentrate on bright unobscured sources. We will mention the problem of the gas flow character close to the innermost stable circular orbit close to the black hole, discussed 50 years ago, which later led to the development of the slim disk scenario, but was recently revived in the context of Magnetically Arrested Disks (MAD) and Standard and Normal Evolution (SANE) disk models. We also discuss the hot/warm corona issue, which is still under discussion and complicates the analysis of the X-ray reflection. We present the scenario of the formation of the low ionization part of the Broad Line Region as a mostly failed wind powered by radiation pressure acting on dust (FRADO - Failed Radiatively Driven Dusty Outflow model). Next, we discuss the cosmological constraints that are currently achievable with quasars, mostly concentrating on light echo methods (continuum time delays and spectral line time delays with respect to the continuum) which are (or should be) incorporating the progress mentioned above. Finally, we briefly mention future prospects in this direction.

A black hole X-ray binary produces hard X-ray radiation from its corona and disk when the accreting matter heats up. During an outburst, the disk and corona co-evolves with each other. However, such an evolution is still unclear in both its geometry and dynamics. Here we report the unusual decrease of the reflection fraction in MAXI J1820+070, which is the ratio of the coronal intensity illuminating the disk to the coronal intensity reaching the observer, as the corona is observed to contrast during the decay phase. We postulate a jet-like corona model, in which the corona can be understood as a standing shock where the material flowing through. In this dynamical scenario, the decrease of the reflection fraction is a signature of the corona's bulk velocity. Our findings suggest that as the corona is observed to get closer to the black hole, the coronal material might be outflowing faster., 11 pages, 5 figures; published in Nature Communications; An artist's impression of Figure 5 is updated here, and the animation is available at http://youbei.work/research/

We study the multi-phase medium in the vicinity of the active galactic nucleus Centaurus A (Cen A). Combined high-resolution observations with the ALMA and Chandra observatories indicate that the hot X-ray emitting plasma coexists with the warm and cold media in Cen A. This complex environment is a source of CO lines with great impact for its diagnostics. We present the images from the two above-mentioned instruments covering the nuclear region (diameter of 10'' i.e., ~180 pc), and we study the conditions for plasma thermal equilibrium and possible coexistence of cool clouds embedded within the hot X-ray emitting gas. Further, we demonstrate that the multi-phase medium originates naturally by the thermal instability (TI) arising due to the interaction of the high-energy radiation field from the nucleus with the ambient gas and dust. We demonstrate that cold gas clouds can coexist in the mutual contact with hot plasma, but even colder dusty molecular clouds have to be distanced by several hundred pc from the hot region. Finally, we propose a 3-D model of the appearance of the hot plasma and the CO line-emitting regions consistent with the Chandra image and we derive the integrated emissivity in specific molecular lines observed by ALMA from this model. To reproduce the observed images and the CO line luminosity the dusty shell has to be ~420 pc thick and located at ~1000 pc from the centre., 16 pages, 16 figures, Accepted for publication in MNRAS

The nucleus of the active galaxy NGC 5548 was the target of two intensive spectroscopic monitoring campaigns at X-ray, ultraviolet (UV), and optical frequencies in 2013/14. These campaigns detected the presence of a massive obscuration event. In 2016/17, Landt et al. conducted a near-IR spectroscopic monitoring campaign on NGC 5548 and discovered He i 1.08 {\mu}m absorption. Here we decompose this absorption into its components and study its time variability. We attribute the narrow He i absorption lines to the warm absorber and, as for the newly appeared low-ionization warm absorber lines in the UV, their presence is most likely due to a reduction in ionization parameter caused by the obscurer. The observed variability of the narrow He i absorption is consistent with what is expected for the warm absorber. Most importantly, we also detect fast, broad He i absorption, which we attribute to the obscurer. This He i broad absorption, which is indicative of a high-column density gas, is unsaturated and variable on time-scales of a few months. The observed variability of the obscurer is mainly due to changes in ionization, although density changes also play a role. We test the physical cycle model of Dehghanian et al. which proposes that helium recombination can account for how the obscurer influences the physics of the warm absorber gas. Our results support their model, but also indicate that the reality might be more complex., Comment: 13 pages, 14 figures, accepted for publication in MNRAS

We perform non-hydrodynamical 2.5D simulations to study the dynamics of material above accretion disk based on the disk radiation pressure acting on dust. We assume a super-accreting underlying disk with the accretion rate of 10 times the Eddington rate with central black hole mass ranging from $10^7$ up to $10^9 M_{\odot}$. Such high accretion rates are characteristic for extreme sources. We show that for high accretors radiatively dust-driving mechanism based on FRADO model always leads to a massive outflow from the disk surface, and the failed wind develops only at larger radii. The outflow rate strongly depends on the black hole mass, and in optically-thick energy-driven solution can exceed the accretion rate for masses larger than $10^ 8 M_{\odot}$ but momentum-driven outflow does not exceed the accretion rate even for super-Eddington accretion, therefore not violating the adopted stationarity of the disk. However, even in this case the outflow from the disk implies a strong mechanical feedback., Comment: 13 pages, 3 figures, Accepted for publication in the journal of Dynamics (MDPI)

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. © 2021 Wiley-VCH GmbH., N. Bon and E. Bon acknowledge the support of Serbian Ministry of Education, Science and Technological Development, through the contract number 451-03-68/2020-14/200002., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

The spectra of active galactic nuclei exhibit broad-emission lines that presumably originate in the Broad-Line Region (BLR) with gaseous-dusty clouds in a predominantly Keplerian motion around the central black hole. Signatures of both inflow and outflow motion are frequently seen. The dynamical character of BLR is consistent with the scenario that has been branded as the Failed Radiatively Accelerated Dusty Outflow (FRADO; Czerny & Hryniewicz 2011). In this scheme, frequent high-velocity impacts of BLR clouds falling back onto the underlying accretion disk are predicted. The impact velocities depend mainly on the black-hole mass, accretion rate, and metallicity and they range from a few km s$^{-1}$ up to thousands of km s$^{-1}$. Formation of strong shocks due to the collisions can give rise to the production of relativistic particles and associated radiation signatures. In this work, the non-thermal radiation generated in this process is investigated, and the spectral energy distributions for different parameter sets are presented. We find that the non-thermal processes caused by the impacts of clouds can lead to emission in the X-ray and the gamma-ray bands, playing the cloud density and metallicity a key role., 21 pages, 14 figures, and 2 tables. Accepted for publication in ApJ; extended Appendix

The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will detect an unprecedentedly large sample of actively accreting supermassive black holes with typical accretion disk (AD) sizes of a few light days. This brings us to face challenges in the reverberation mapping (RM) measurement of AD sizes in active galactic nuclei (AGNs) using interband continuum delays. We examine the effect of LSST cadence strategies on AD RM using our metric AGNTimeLagMetric. It accounts for redshift, cadence, the magnitude limit, and magnitude corrections for dust extinction. Running our metric on different LSST cadence strategies, we produce an atlas of the performance estimations for LSST photometric RM measurements. We provide an upper limit on the estimated number of quasars for which the AD time lag can be computed within 01000 sources in each Deep Drilling field (DDF, 10 sq. deg) in any filter, with the redshift distribution of these sources peaking at z~1. We find the LSST observation strategies with a good cadence (~ 5 days) and a long cumulative season (~9 yr), as proposed for LSST DDF, are favored for the AD size measurement. We create synthetic LSST light curves for the most suitable DDF cadences and determine RM time lags to demonstrate the impact of the best cadences based on the proposed metric., Comment: accepted for publication in the Astrophysical Journal Focus Issue on Rubin LSST cadence and survey strategy

Context. Broad emission lines are the most characteristic features in the spectra of galaxies with an active galactic nucleus (AGN). They mostly show either single-peaked or double-peaked profiles and originate from a complex dynamics of the likely discrete clouds moving in a spatially extended region known as the broad line region (BLR). Aims. In this paper, we present a large grid of results, which is used to test the model based on calculations of the spectral line generic profiles. Methods. We followed a non-hydrodynamical single-cloud approach to BLR dynamics based on a radiatively dust-driven model. We previously showed in detail that the 2.5D version of the model could provide us with the 3D geometry of the BLR. Results. We show that the shape of profiles not only depends on the accretion rate of the source, the black hole mass, and the viewing angle, but it is most significantly affected by the adopted dust-to-gas mass ratio regulating the strength of the radiation pressure. We also show that the model can aptly explain the low ionized broad emission lines of the mean spectrum of quasars, such as MgII and Hβ. Conclusions. The radiatively dust-driving mechanism can appropriately account for the low-ionized part of BLR of AGNs.

Nuclear Activity in Galaxies Across Cosmic Time, Proceedings of the conference held 7-11 October 2019 in Addis Ababa, Ethiopia. Edited by Mirjana Pović et al. Proceedings of the International Astronomical Union, Volume 356, pp. 66-71, The main sequence offers a method for the systematization of quasar spectral properties. Extreme FeII emitters (or extreme Population A, xA) are believed to be sources accreting matter at very high rates. They are easily identifiable along the quasar main sequence, in large spectroscopic surveys over a broad redshift range. The very high accretion rate makes it possible that massive black holes hosted in xA quasars radiate at a stable, extreme luminosity-to-mass ratio. After reviewing the basic interpretation of the main sequence, we report on the possibility of identifying virial broadening estimators from low-ionization line widths, and provide evidence of the conceptual validity of redshift-independent luminosities based on virial broadening for a known luminosity-to-mass ratio., AdO acknowledges financial support from Spanish grants AYA2016-76682-C3-1-P and SEV-2017-0709.

The dynamics of the Broad Line Region (BLR) in Active galaxies is an open question, direct observational constraints suggest a predominantly Keplerian motion, with possible traces of inflow or outflow. In this paper we study in detail the physically motivated BLR model of (Czerny & Hryniewicz, 2011) based on the radiation pressure acting on dust at the surface layers of accretion disk (AD). We consider here a non-hydrodynamical approach to the dynamics of the dusty cloud under the influence of radiation coming from the entire AD. We use here the realistic description of the dust opacity, and we introduce two simple geometrical models of the local shielding of the dusty cloud. We show that the radiation pressure acting on dusty clouds is strong enough to lead to dynamical outflow from the AD surface, so the BLR has a dynamical character of (mostly failed) outflow. The dynamics strongly depend on the Eddington ratio of the source. Large Eddington ratio sources show a complex velocity field and large vertical velocities with respect to the AD surface, while for lower Eddington ratio sources vertical velocities are small and most of the emission originates close to the AD surface. Cloud dynamics thus determines the 3-D geometry of the BLR., 28 pages, 10 figures, Accepted in ApJ for publication

The quasar main sequence appears to be an incredibly powerful tool to organize the diversity in large samples of type-1 quasars but the most important physical parameters governing it are still unclear. Here, we investigate the origin of the broadening and of a defining feature of Population B sources: a strong redward asymmetry of the Balmer emission lines. We focus on a prototypical source, Fairall 9. Spectropolarimetric data of the Fairall 9 broad H β and H α profiles allowed for a view of the geometric and dynamical complexity of the line emitting regions. Measurements (1) provided evidence of rotational motion; (2) were helpful to test the presence of polar and equatorial scatterers, and their association with non-virial motions. However, we suggest that the polarization properties appear to be more consistent with a warped disc geometry induced by Lense–Thirring precession. © 2021 The Author(s)., BWJ and JMW acknowledge financial support from the National Natural Science Foundation of China (11833008 and 11991054), from the National Key Research and Development Program of China (2016YFA0400701), from the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (CAS; QYZDJ-SSW-SLH007), and from the CAS Key Research Program (KJZD-EW-M06). VLA and ES thank the grant of Russian Science Foundation project number 20-12-00030 ‘Investigation of geometry and kinematics of ionized gas in active galactic nuclei by polarimetry methods’, which supported the spectropolarimetric data analysis. ĐS and LČP acknowledge funding provided by the Astronomical Observatory Belgrade (the contract 451-03-68/2020-14/200002) through the grants by the Ministry of Education, Science, and Technological Development of the Republic of Serbia. This work was supported by the F.R.S.–FNRS under grant PDR T.0116.21. AdO acknowledges financial support from the Spanish grants MCI PID2019-106027GB-C41 and the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the IAA (SEV-2017-0709). Under the same award, PM acknowledges the Hypatia of Alexandria visiting grant.

OJ 287 is a well-studied binary black hole system, that occasionally exhibits bright X-ray and optical flares. Here we present a detailed spectral study of its second brightest X-ray flare observed during 2019-2020 using archival Swift and NuSTAR observations along with ToO observations from AstroSat. The entire flaring period is divided into three states, defined as low, intermediate, and high states. The variation of hardness ratio (HR) with 0.3-10.0 keV integrated flux suggests a \enquote{softer-when-brighter} behavior, as also previously reported based on flux-index variations. Simultaneous high state X-ray spectra obtained using Swift, NuSTAR and AstroSat are very steep with a power-law index $>$2. A significant spectral change is observed in AstroSat-SXT and LAXPC spectrum which is consistent with Swift-XRT and NuSTAR spectrum. Together, optical-UV and X-ray spectrum during the high flux state, suggesting the emergence of a new high BL Lacertae (HBL) component. We have modeled the synchrotron peak with publicly available code named GAMERA for low, intermediate, and high flux state. Our modeling suggests the need of high magnetic field to explain the high state under the leptonic scenarios. Increase in the magnetic field value inside the jet could be linked to the increase in accretion rate as expected in the BH-disk impact scenario. The color-magnitude diagram reveals a \enquote{bluer-when-brighter} spectral energy distribution chromatism during the flaring period. Different chromatism or no chromatism at various occasions suggests a complex origin of optical emission, which is believed to be produced by disc impact or through synchrotron emission in the jet.

Using the six years of the spectroscopic monitoring of the luminous quasar HE 0435-4312 ($z=1.2231$) with the Southern African Large Telescope (SALT), in combination with the photometric data (CATALINA, OGLE, SALTICAM, and BMT), we determined the rest-frame time-delay of $296^{+13}_{-14}$ days between the MgII broad-line emission and the ionizing continuum using seven different time-delay inference methods. Artefact time-delay peaks and aliases were mitigated using the bootstrap method, prior weighting probability function as well as by analyzing unevenly sampled mock light curves. The MgII emission is considerably variable with the fractional variability of $\sim 5.4\%$, which is comparable to the continuum variability ($\sim 4.8\%$). Because of its high luminosity ($L_{3000}=10^{46.4}\,{\rm erg\,s^{-1}}$), the source is beneficial for a further reduction of the scatter along the MgII-based radius-luminosity relation and its extended versions, especially when the high-accreting subsample that has an RMS scatter of $\sim 0.2$ dex is considered. This opens up a possibility to use the high-accretor MgII-based radius-luminosity relation for constraining cosmological parameters. With the current sample of 27 reverberation-mapped sources, the best-fit cosmological parameters $(\Omega_{\rm m}, \Omega_{\Lambda})=(0.19; 0.62)$ are consistent with the standard cosmological model within 1$\sigma$ confidence level., Comment: 34 pages, 23 figures, 6 tables; accepted by the Astrophysical Journal; Main body as well as the Appendix extended; added references

Recent measurements of the parameters of the Concordance Cosmology Model ($\Lambda$CDM) done in the low-redshift Universe with Supernovae Ia/Cepheids, and in the distant Universe done with Cosmic Microwave Background (CMB) imply different values for the Hubble constant (67.4 $\pm$ 0.5 km s$^{-1}$ Mpc$^{-1}$ from Planck vs 74.03 $\pm$ 1.42 km s$^{-1}$ Mpc$^{-1}$, Riess et al. 2019). This Hubble constant tension implies that either the systematic errors are underestimated, or the $\Lambda$CDM does not represent well the observed expansion of the Universe. Since quasars - active galactic nuclei - can be observed in the nearby Universe up to redshift z $\sim$ 7.5, they are suitable to estimate the cosmological properties in a large redshift range. Our group develops two methods based on the observations of quasars in the late Universe up to redshift z$\sim $4.5, with the objective to determine the expansion rate of the Universe. These methods do not yet provide an independent measurement of the Hubble constant since they do not have firm absolute calibration but they allow to test the $\Lambda$CDM model, and so far no departures from this model were found., Comment: 6 pages, 2 figures, contribution to the Proceedings of the 100 years of the Polish Physical Society Congress, 16-18 October 2020, Warsaw, Poland. http://info.ifpan.edu.pl/APP/

Broad Emission Lines are the most characteristic features of Active Galaxies, but the mechanism of creating a medium able to emit these intense lines is not quite clear. Observations clearly indicate that the motion of the material is predominantly Keplerian, with traces of inflow and clear signatures of outflow, but this still does not point out whether the lines partially come from the disk surface, or exclusively from the circumnuclear material, and whether this material originates from the disk as a wind, or comes, at least partially, from outside. I review the basic scenarios for the formation of the Broad Line Region (BLR), and the recent progress in modelling the physical conditions in the emitting medium. The current state is the the outer radius of the BLR is fixed by the dust sublimation temperature in the medium exposed to the irradiation from the central source, the inner radius is likely fixed by the dust sublimation temperature in the atmosphere of the non-illuminated accretion disk, and the local cloud density is a universal number fixed by the radiation pressure confinement. The time-dependent aspects of the BLR formation, however, still wait for serious modelling effort matching the quality of the observational data., Comment: Open Astronomy, in press. Proceedings of the conference 12th Serbian Conference on Spectral Line Shapes in Astrophysics, Vrdnik, Serbia, June 3-7, 2019

Optical Fe II emission is a strong feature in quasar spectra originating in the broad-line region (BLR). The difficulty in understanding the complex Fe II pseudo-continuum has led us to search for other reliable, simpler ionic species such as Ca II. In this first part of the series, we confirm the strong correlation between the strengths of two emission features, the optical Fe II and the NIR Ca II, both from observations and photoionization modelling. With the inclusion of an up-to-date compilation of observations with both optical Fe II and NIR Ca II measurements, we span a wider and more extended parameter space and confirm the common origin of these two spectral features with our photoionization models using CLOUDY. Taking into account the effect of dust into our modelling, we constrain the BLR parameter space (primarily, in terms of the ionization parameter and local cloud density) as a function of the strengths of Fe II and Ca II emission., 21 pages, 12 figures, 1 table; accepted for publication in ApJ

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, $\mathrm{R_{FeII}}$ on the broad H$\beta$ FWHM (i.e. the line dispersion) and the inclination angle ($\theta$), assuming fixed values for the Eddington ratio ($\mathrm{L_{bol}}/\mathrm{L_{Edd}}$), black hole mass ($\mathrm{M_{BH}}$), spectral energy distribution (SED) shape, cloud density (n$\rm{_{H}}$) and composition., Comment: 3 pages, 1 figure, Proceedings of the IAU356: Nuclear Activity in Galaxies across Cosmic Time, Addis Ababa, Ethiopia (7-11 October, 2019)

Context. The blazar OJ 287 has been proposed as a binary black hole system based on its periodic optical outburst. Among blazars with parsec scale jets, the black hole binary systems are very rare and hence this source is very interesting to study. Aims. The BL Lac OJ 287 is an interesting object for multi-wavelength study due to its periodic outbursts. We have analyzed the optical, X-ray, and gamma-ray data of OJ 287 for the period of 2017-2020. There are several high states in optical-UV and X-ray frequencies during this period. Based on the observed variability in optical and X-ray frequencies the entire period 2017-2020 is divided into five segments, referred to as A, B, C, D, & E in this paper. A detailed temporal and spectral analysis is performed to understand the nature of its flaring activities. Methods. To understand the temporal variability in this source we have studied the intra-day, and fractional variability for all the various states, and along with that fast variability time was also estimated to understand the nature of variability. Further, the multi-wavelength SED modeling is performed to know more about the physical processes responsible for the simultaneous broadband emission and the fast variability. Results. The Fermi-LAT observations show a moderate flux level of this source in gamma-ray frequency throughout this period, though flux variability has been observed. The source has shown a strong flux variability in X-ray, optical, and UV during early 2017 and mid-2020 when the source was in a very high state. A single zone SSC emission model is considered to model the spectral energy distributions and this helps us to explore the nature of this BL Lac with binary super-massive black holes., 14 pages, 5 figures, 6 tables, Accepted for publication in A&A

A soft X-ray excess above the 2-10 keV power law extrapolation is generally observed in AGN X-ray spectra. Presently there are two competitive models to explain it: blurred ionized reflection and warm Comptonisation. In the latter case, observations suggest a corona temperature $\sim$ 1 keV and a corona optical depth $\sim$ 10. Moreover, radiative constraints from spectral fits with Comptonisation models suggest that most of the accretion power should be released in the warm corona. The disk below is basically non-dissipative, radiating only the reprocessed emission from the corona. The true radiative properties of such a warm and optically thick plasma are not well-known, however. For instance, the importance of the Comptonisation process, the potential presence of strong absorption/emission features or the spectral shape of the output spectrum have been studied only very recently. We present in this paper simulations of warm and optically thick coronae using the TITAN radiative transfer code coupled with the NOAR Monte-Carlo code, the latter fully accounting for Compton scattering of continuum and lines. Illumination from above by a hard X-ray emission and from below by an optically thick accretion disk is taken into account as well as (uniform) internal heating. Our simulations show that for a large part of the parameter space, the warm corona with sufficient internal mechanical heating is dominated by Compton cooling and neither strong absorption nor emission lines are present in the outgoing spectra. In a smaller part of the parameter space, the calculated emission agrees with the spectral shape of the observed soft X-ray excess. Remarkably, this also corresponds to the conditions of radiative equilibrium of an extended warm corona covering almost entirely a non-dissipative accretion disk. These results confirm the warm Comptonisation as a valuable model for the soft X-ray excess., Accepted for publication in A&A. 13 pages, 8 Figures

We address the effect of the viewing angle of the accretion disk plane and the geometry of the broad-line region (BLR) with the goal of interpreting the distribution of quasars along the main sequence (MS). We utilize photoionization code CLOUDY to model the BLR FeII emission, incorporating the grossly underestimated role of the form factor (f). We recover the dependence of the strength of the FeII emission in the optical (R$_{\rm{FeII}}$) on L$_{\rm{bol}}$/L$_{\rm{Edd}}$ ratio and related observational trends - as a function of the spectral energy distribution (SED) shape, cloud density, composition and intra-cloud dynamics, assumed following prior observational constraints. With this approach, we are now able to explain the diversity of quasars and the change of the quasar properties along the Main Sequence (MS). Our approach also explains the rarity of the highest FeII emitters known as the extreme xA sources and can be used as a predictive tool in future reverberation mapping studies of Type-1 AGNs. This approach further justifies the use of quasars as `cosmological probes'., Comment: 15 pages, 5 figures; Contributions of the Astronomical Observatory Skalnat\'e Pleso (CAOSP) as a Special Issue "Spectral Line Shapes in Astrophysics and Related Topics"

Observations in the near-infrared domain showed the presence of the flat core of bright late-type stars inside $\sim 0.5\,{\rm pc}$ from the Galactic center supermassive black hole (Sgr A*), while young massive OB/Wolf-Rayet stars form a cusp. Several dynamical processes were proposed to explain this apparent paradox of the distribution of the Galactic center stellar populations. Given the mounting evidence about a significantly increased activity of Sgr A* during the past million years, we propose a scenario based on the interaction between the late-type giants and a nuclear jet, whose past existence and energetics can be inferred from the presence of $\gamma$-ray Fermi bubbles and bipolar radio bubbles. Extended, loose envelopes of red giant stars can be ablated by the jet with kinetic luminosity in the range of $L_{\rm j}\approx 10^{41}$-$10^{44}\,{\rm erg\,s^{-1}}$ within the inner $\sim 0.04\,{\rm pc}$ of Sgr A* (S cluster region), which would lead to their infrared luminosity decrease after several thousand jet-star interactions. The ablation of the atmospheres of red giants is complemented by the process of tidal stripping that operates at distances of $\lesssim 1\,{\rm mpc}$, and by the direct mechanical interaction of stars with a clumpy disc at $\gtrsim 0.04\,{\rm pc}$, which can explain the flat density profile of bright late-type stars inside the inner half parsec from Sgr A*., Comment: 24 pages, 10 figures, 3 tables; accepted for publication by the Astrophysical Journal

We present an analysis of UV spectra of 13 quasars believed to belong to extreme Population A (xA) quasars, aimed at the estimation of the chemical abundances of the broad-line-emitting gas. Metallicity estimates for the broad-line-emitting gas of quasars are subject to a number of caveats; xA sources with the strongest Fe ii emission offer several advantages with respect to the quasar general population, as their optical and UV emission lines can be interpreted as the sum of a low-ionization component roughly at quasar rest frame (from virialized gas), plus a blueshifted excess (a disk wind), in different physical conditions. Capitalizing on these results, we analyze the component at rest frame and the blueshifted one, exploiting the dependence of several intensity line ratios on metallicity Z. We find that the validity of intensity line ratios as metallicity indicators depends on the physical conditions. We apply the measured diagnostic ratios to estimate the physical properties of sources such as density, ionization, and metallicity of the gas. Our results confirm that the two regions (the low-ionization component and the blueshifted excess) of different dynamical conditions also show different physical conditions and suggest metallicity values that are high, and probably the highest along the quasar main sequence, with Z ∼ 20-50 Z o˙, if the solar abundance ratios can be assumed constant. We found some evidence of an overabundance of aluminum with respect to carbon, possibly due to selective enrichment of the broad-line-emitting gas by supernova ejecta. © 2021. The American Astronomical Society. All rights reserved., M.S. acknowledges the support of the Erasmus+ program of the European Union and would like to express very great appreciation to Istituto Nazionale di Astrofisica (INAF) Osservatorio Astronomico di Padova, University of Padova, and Astronomical Observatory of the University of Warsaw for enabling her to complete an internship. The project was partially supported by the Polish Funding Agency National Science Centre project 2017/26/A/ST9/00756 (MAESTRO 9) and MNiSW grant DIR/WK/2018/12. P.M. acknowledges the Hypatia of Alexandria visiting grant SO-IAA (SEV-2017-0709) through the Center of Excellence Severo Ochoa and is deeply indebted to Drs. J. Perea and A. del Olmo for the generous allocation of computing resources and for a stay at IAA. A.d.O. acknowledges financial support from the Spanish grants MCI PID2019-106027GB-C41 and the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). Funding for the Sloan Digital Sky Survey (SDSS) has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are the University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington.

We present the monitoring of the AGN continuum and MgII broad line emission for the quasar HE 0413-4031 ($z=1.38$) based on the six-year monitoring by the South African Large Telescope (SALT). We managed to estimate a time-delay of $302.6^{+28.7}_{-33.1}$ days in the rest frame of the source using seven different methods: interpolated cross-correlation function (ICCF), discrete correlation function (DCF), $z$-transformed DCF, JAVELIN, two estimators of data regularity (Von Neumann, Bartels), and $\chi^2$ method. This time-delay is below the value expected from the standard radius-luminosity relation. However, based on the monochromatic luminosity of the source and the SED modelling, we interpret this departure as the shortening of the time-delay due to the higher accretion rate of the source, with the inferred Eddington ratio of $\sim 0.4$. The MgII line luminosity of HE 0413-4031 responds to the continuum variability as $L_{\rm line}\propto L_{\rm cont}^{0.43\pm 0.10}$, which is consistent with the light-travel distance of the location of MgII emission at $R_{\rm out} \sim 10^{18}\,{\rm cm}$. Using the data of 10 other quasars, we confirm the radius-luminosity relation for broad MgII line, which was previously determined for broad H$\beta$ line for lower-redshift sources. In addition, we detect a general departure of higher-accreting quasars from this relation in analogy to H$\beta$ sample. After the accretion-rate correction of the light-travel distance, the MgII-based radius-luminosity relation has a small scatter of only $0.10$ dex., Comment: 39 pages (23 pages - Main text, 16 pages - Appendix), 21 figures, 14 tables; accepted for publication in the Astrophysical Journal

Cygnus X-1 is one of the brightest X-ray sources observed and shows the X-ray intensity variations on time scales from milliseconds to months in both the soft and hard X-rays. The accretion onto the black hole is believed to be wind fed due to focused stellar wind from the binary companion HDE-226868. We aim to understand the physical mechanism responsible for the short timescale X-ray variability ($, Comment: 16 pages, 10 figures, 2 tables, accepted for publication in ApJ

Changing-look phenomenon observed now in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole. We propose a simple explanation for the sources where multiple semi-periodic outbursts are observed, and the sources are operating at a few per cent of the Eddington limit. The outburst are caused by the radiation pressure instability operating in the narrow ring between the standard gas-dominated outer disk and the hot optically thin inner Advection-Dominated Accretion Flow. The corresponding limit cycle is responsible for periodic outbursts, and the timescales are much shorter than the standard viscous timescale due to the narrowness of the unstable radial zone. Our toy model gives quantitative predictions and works well for multiple outbursts like those observed in NGC 1566, NGC 4151, NGC 5548 and GSN 069, although the shapes of the outbursts are not yet well modeled, and further development of the model is necessary., Comment: 10 pages, 10 figures. Accepted for publication in A&A. arXiv admin note: text overlap with arXiv:1904.06767

In this second paper in the series, we carefully analyze the observational properties of the optical FeII and NIR CaII triplet in Active Galactic Nuclei, as well as the luminosity, black hole mass, and Eddington ratio in order to define the driving mechanism behind the properties of our sample. The CaII shows an inverse Baldwin effect, bringing out the particular behavior of this ion with respect to the other low-ionization lines such as H$\beta$. We performed a Principal Component Analysis, where 81.2% of the variance can be explained by the first three principal components drawn from the FWHMs, luminosity, and equivalent widths. The first principal component (PC1) is primarily driven by the combination of black hole mass and luminosity with a significance over 99.9%, which in turn is reflected in the strong correlation of the PC1 with the Eddington ratio. The observational correlations are better represented by the Eddington ratio, thus it could be the primary mechanism behind the strong correlations observed in the CaII-FeII sample. Since calcium belongs to the $\alpha$-elements, the FeII/CaII flux ratio can be used as a chemical clock for determining the metal content in AGN and trace the evolution of the host galaxies. We confirm the de-enhancement of the ratio FeII/CaII by the Eddington ratio, suggesting a metal enrichment of the BLR in intermediate-$z$ with respect to low-$z$ objects. A larger sample, particularly at $z$>2, is needed to confirm the present results., Comment: 35 pages, 16 figures, 4 tables. Accepted for publication in ApJ

We study the hydrogen ionization instability mechanism in the context of low-mass X-ray binaries with a black hole as a central object. We make numerical calculations of the predicted outbursts’ light curves and compare them to the data observed by X-ray satellites. The comparison to the data is done for five sources observed by RXTE/ASM (XTE J1550−564, 4U 1630−472, XTE J1859+226, GX 339-4, XTE J1818−245) and one source observed by MAXI (MAXI J1659−152). The aim of this paper is to show that the hydrogen ionization instability operating in an accretion disk is responsible for the shape of outbursts observed in low-mass X-ray binaries. From the data fitting process, we put tight constraints on global source parameters such as black hole mass and disk accretion rate. The influence of chemical composition on the overall analysis is also shown. In the case of each outburst, we found the overall bolometric light curve shape that qualitatively matches the data. We were able to model the main outburst and secondary reflare often seen in the data, the latter one caused by the presence of metals in disk gas. In the case of 4U 1630−472, we analyzed two outbursts, which allowed us to put tight constraints on the black hole mass of 4 ± 0.5M ⊙ and on the accretion rate of yr−1.

The symmetry axes of active galactic nuclei (AGN) are randomly distributed in space but highly inclined sources are heavily obscured and are not seen as quasars with broad emission lines. The obscuring torus geometry determines the average viewing angle, and if the torus geometry changes with the redshift, this average viewing angle will also change. Thus the ratio between the isotropic luminosity and observed luminosity may change systematically with redshift. Therefore, if we use quasars to measure the luminosity distance by evaluating the isotropic absolute luminosity and measuring the observed flux, we can have a redshift-dependent bias which can propagate to cosmological parameters. We propose a toy model for testing the effect of viewing angle uncertainty on measurement of the luminosity distance. The model is based on analytical description of the obscuring torus applied to one-parameter observational data. It illustrates the possible change of the torus covering factor between the two chosen redshift ranges. We have estimated the possible error on specific cosmological parameters (H0,Omega_m) for the flat Lambda-CDM cosmology if a method is calibrated at low redshift and applied to the higher redshift. The errors on cosmological parameters due to potential dependence of viewing angle on redshift are found to be potentially significant, and the effect will have to be accommodated in the future in all quasar-based cosmological methods. A careful systematic study of AGN mean viewing angle across redshift is necessary, with the use of appropriate samples and models which uniquely determine the inclination of each source., 15 pages, 10 figures, Accepted for publication in ApJ

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited., Highly accreting quasars show fairly distinctive properties in their optical, UV, and X spectra, and are easy to recognize because of their specific location in the quasar main sequence: they are the strongest optical FeII emitters. They show a surprisingly high rate of radio detections and, at variance with the classical radio-loud (jetted) sources, the origin of their radio emission is probably "thermal." The chemical composition of the broad line emitting gas implies high metallicity values, above 10 times solar. A fraction of highly accreting quasars at intermediate and high redshift might therefore be in a particular evolutionary stage that is unobscured albeit still involving a contribution of nuclear and circum-nuclear star formation in their multifrequency properties. © 2021. The Author(s). Published by the American Astronomical Society., A.d.O. acknowledges financial support from the Spanish MCIU grant PID2019-106027GB-C41 and from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofsica de An- daluca (SEV-2017-0709).

The usage of the radius-luminosity (R-L) relation for the determination of black hole masses across the cosmic history as well as its application for cosmological studies motivates us to analyze its scatter, which has recently increased significantly both for the optical (H$\beta$) and UV (Mg II) lines. To this purpose, we determined the scatter along the R-L relation for an up-to-date reverberation-mapped Mg II sample. Studying linear combinations of the luminosity at 3000 A with independent parameters such as the Full Width at Half Maximum (FWHM), Fe II strength (R$_\mathrm{FeII}$), and the fractional variability ($F\mathrm{_{var}}$) for the whole sample, we get only a small decrease in the scatter ($\sigma{_{\rm rms}}=0.29-0.30$ dex). Linear combinations with the dimensionless accretion rate ($\dot{\mathcal{M}}$) and the Eddington ratio ($\alpha_\mathrm{Edd}$) lead to significant reductions of the scatter ($\sigma_{\rm rms}\sim 0.1$ dex), albeit both suffering from the inter-dependency on the observed time-delay. After the division into two sub-samples considering the median value of the $\dot{\mathcal{M}}$ in the full sample, we find that the scatter decreases significantly for the highly accreting sub-sample. In particular, the smallest scatter of $\sigma{_{\rm rms}}=0.17$ dex is associated with the independent parameter R$_\mathrm{FeII}$, followed by the combination with $F\mathrm{_{var}}$, with $\sigma{_{\rm rms}}=0.19$ dex. Both of these independent observationally-inferred parameters are in turn correlated with $\dot{\mathcal{M}}$ and $\alpha_\mathrm{Edd}$. These results suggest that the large scatter along the R-L relation is driven $\textit{mainly}$ by the accretion rate intensity., Comment: 20 pages, 9 figures, 2 tables. Accepted for publication in ApJ

Active galaxies form a clear pattern in the optical plane showing the correlation between the Full Width Half Maximum (FWHM) of the H$\beta$ line and the ratio of the Equivalent Width (EW) of the optical FeII emission and the EW(H$\beta$). This pattern is frequently refereed to as the quasar main sequence. In this paper we study the UV plane showing FWHM of the MgII line against the ratio of EW of UV FeII emission to EW(MgII). We show that the UV plane trends are different, with the underlying strong correlation between FWHM(MgII) and EW(MgII). This correlation is entirely driven by the choice of the continuum used to measure EW(MgII). If we correct for the strong curvature in the UV continuum related to the character of emission expected from accretion disk, the behavior of the FWHM vs. EW for MgII becomes similar to the behaviour of H$\beta$. Such a similarity is expected since both lines belong to the Low Ionization group of emission lines and come from a similar region. Such a similarity and weak coupling between FWHM and EW of the corresponding line is then also consistent with the Broad Line Region model based on the presence of dust in the accretion disk atmosphere., Comment: 15 pages, 11 figures

Context: Giant radio galaxies (with projected linear size of radio structure larger than 0.7 Mpc) are very rare and unusual objects. Only $\sim$5% of extended radio sources reach such sizes. Understanding of the processes responsible for their large sizes is crucial to further our knowledge about the radio source's evolution. Aims: We check the hypothesis that giants become extremely large due to the specific history of their host galaxy formation, as well as in the context of the cluster or group of galaxies where they evolve. Therefore we study the star formation histories in their host galaxies and in galaxies located in their neighbourhood. Methods: We studied 41 giant-size radio galaxies as well as galaxies located within a radius of 5 Mpc around giants to verify whether the external conditions of the intergalactic medium somehow influence the internal evolution of galaxies in the group/cluster. We compared the results with a control sample of smaller-sized Fanaroff--Riley type II radio galaxies and their neighbouring galaxies. We fit stellar continua in all galaxy spectra using the spectral synthesis code STARLIGHT and provide statistical analysis of the results. Results: We find that hosts of giant radio galaxies have a larger amount of intermediate age stellar populations compared with smaller-sized FRII radio sources. The same result is also visible when we compare neighbouring galaxies located up to 1.5 Mpc around giants and FRIIs. This may be evidence that star formation in groups with giants was triggered due to global processes occurring in the ambient intergalactic medium. These processes may also contribute to mechanisms responsible for the extremely large sizes of giants., 20 pages, 9 figures, to be publish in A&A

We review the current status of the understanding how the accretion onto the central black hole proceeds in Active Galaxies. Standard accretion disk is a key element in all relatively bright active galaxies like Seyferts and quasars, although it is not present in very low luminosity sources, like Sgr A*. However, the standard disk does not explain the broad band spectrum, so the disk has to be supplemented with a number of additional components, and our deeper understanding of these components is still far from being complete. These additional elements are: compact hard X-ray corona, inner hot flow, warm corona, disk wind and the Broad Line Region, and finally dusty/molecular torus. All these elements seem to be needed in various proportions, depending predominantly on the Eddington ratio of a given source. These elements also interact with each other which is not yet fully taken into account., 17 pages, 7 figures, Accepted for publication on PoS Proceedings, Contribution "PoS(APCS2018)006" to the Conference on "Accretion Processes in Cosmic Sources", Saint Petersburg, Russia

Quasars have been proposed as a new class of standard candles analogous to Supernovae, as their large redshift range and high luminosities make them excellent candidates. Reverberation mapping (RM) method enables to estimate the distance to the source from the time delay measurement of the emission lines with respect to the continuum, since the time delay depends on the absolute luminosity of the source. This radius-luminosity (RL) relation exhibits a low scatter and it was thus proposed for cosmological purposes. However, in recent years the increase in the studied sample, and in particular the inclusion of highly accreting QSO has increased the dispersion in the RL relation, with many objects showing time delays shorter than the expected. Using H$\beta$ RM measurements for 117 sources with $0.002, Comment: 26 pages, 9 Figures, 4 tables; accepted for publication in ApJ

Bright active galaxies show a range of properties but many of these properties are correlated which has led to the concept of the Quasar Main Sequence. We test whether our current understanding of the quasar structure allows to reproduce the pattern observed in the optical plane formed by the kinematic line width of H$\beta$ and the relative importance of the Fe II optical emission. We performed simulations of the H$\beta$ and Fe II production using the code CLOUDY and well justified assumptions about the broad band spectra, distance of the emission line region, and the cloud properties. We show that the presence of the warm corona is an important element of the broad band spectrum which decreases the dependence of the relative Fe II emissivity on the Eddington ratio, and allows to reproduce the rare cases of the particularly strong Fe II emitters. Results are sensitive to the adopted cloud distance, and strong Fe II emission can be obtain either by adopting strongly super-solar metallicity, or much shorter distance than traditionally obtained from reverberation mapping. We modeled in a similar way the UV plane defined by the Mg II line and Fe II UV pseudo-continuum, but here our approach is less successful, in general overproducing the Fe II strength. We found that the Fe II optical and UV emissivity depend in a different way on the turbulent velocity and metallicity, and the best extension of the model in order to cover both planes is to allow very large turbulent velocities in the Broad Line Region clouds., Comment: 15 pages, 12 figures, accepted for publication in ApJ

The interpretation of the main sequence of quasars has become a frontier subject in the last years. The consider 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, R$_{\mathrm{FeII}}$ on the broad H$\beta$ FWHM (i.e. the line dispersion) and the inclination angle ($\theta$), assuming fixed values for the Eddington ratio ($\mathrm{L_{bol}}/\mathrm{L_{Edd}}$), black hole mass ($\mathrm{M_{BH}}$) and spectral energy distribution (SED) shape. We consider four cases with changing cloud density (n$\rm{_{H}}$) and composition. Understanding the FeII emitting region is crucial as this knowledge can be extended to the use of quasars as distance indicators for Cosmology., Comment: 5 pages, 2 figures, Proceedings of the IAU356: Nuclear Activity in Galaxies across Cosmic Time, Addis Ababa, Ethiopia (7-11 October, 2019)

We address the effect of orientation of the accretion disk plane and the geometry of the broad-line region (BLR) as part of an effort to understand the distribution of quasars in the optical plane of the quasar main sequence. We utilize the photoionization code CLOUDY to model the BLR incorporating the grossly underestimated form factor ($f$). Treating the aspect of viewing angle appropriately, we confirm the dependence of the $R_{\mathrm{FeII}}$ sequence on Eddington ratio and on the related observational trends - as a function of the SED shape, cloud density and composition, verified from prior observations. Sources with $R_{\mathrm{FeII}}$ in the range 1 -- 2 (about 10\% of all quasars, the so-called extreme Population A [xA] quasars) are explained as sources of high, and possibly extreme Eddington ratio along the $R_{\mathrm{FeII}}$ sequence. This result has important implication for the exploitation of xA sources as distance indicators for Cosmology. $\mathrm{FeII}$ emitters with $R_{\mathrm{FeII}} > 2$ are very rare (, 9 pages, 4 figures, 1 table; accepted for publication in ApJ

In the near future, new surveys promise a significant increase in the number of quasars (QSO) at large redshifts. This will help to constrain the dark energy models using quasars. The Large Synoptic Survey Telescope (LSST) will cover over 10 million QSO in six photometric bands during its 10-year run. QSO will be monitored and subsequently analyzed using the photometric reverberation mapping (RM) technique. In low-redshift quasars, the combination of reverberation-mapped and spectroscopic results have provided important progress. However, there are still some facts which have to be taken into account for future results. It has been found that super-Eddington sources show time delays shorter than the expected from the well-known Radius-Luminosity ($R\mathrm{_{H\beta}}-L_{5100}$) relation. Using a sample of 117 H$\beta$ reverberation-mapped AGN with $0.02, Comment: 12 pages, 5 figures. Proceedings of the Frascati Workshop 2019: Multifrequency Behaviour of High Energy Cosmic Sources XIII, 3 - 8 June 2019, Palermo. Italy. To be published in the proceedings of in electronic form by the Proceedings of Science (PoS-SISSA)

By applying different statistically robust methods, we analyze the time-lag between the continuum and ionized line-emission (Mg II line) light curves for the distant bright quasar CTS C$30.10$ (redshift $z \sim 0.9$). The data were obtained by the 10-meter SALT telescope in South Africa. In detail, we demonstrate the application of several methods using interpolated cross-correlation function (ICCF), discrete correlation function (DCF), z-transformed discrete correlation function (zDCF), von Neumann estimator, and the JAVELIN code package. In particular, we discuss the uncertainties of these methods. In conclusion, we find that the quasar lies on the broad-line region (BLR) size -- monochromatic luminosity power-law scaling, $R_{\rm BLR}\propto L_{5100}^{1/2}$, which was already confirmed for low-redshift sources. In case the BLR size-luminosity relation holds for other distant sources, quasars could be used for probing cosmological models as ``standard candles'' complementary to supernovae Ia., Comment: 9 pages, 5 figures, 2 tables; to appear in Astronomical Notes as a contribution presented at the IBWS 2019 conference (Karlovy Vary, Czechia, 20 - 24 May 2019)

Recently, near-infrared GRAVITY@ESO observations at $2.2\,\mu{\rm m}$ have announced the detection of three bright "flares" in the vicinity of the Galactic center supermassive black hole (SMBH) that exhibited orbital motion at a distance of about $6 - 11$ gravitational radii from an $\sim 4\times 10^6\, M_{\odot}$ black hole. There are indications of the presence of a large-scale, organized component of the magnetic field at the Galactic center. Electromagnetic effects on the flare dynamics were previously not taken into account despite the relativistic motion of a plasma in magnetic field leading to the charge separation and nonnegligible net charge density in the plasma. Applying various approaches, we find the net charge number density of the flare components of the order of $10^{-3} - 10^{-4}$ cm$^{-3}$, while the particles' total number density is of the order of $10^{6} - 10^{8}$ cm$^{-3}$. However, even such a tiny excess of charged particles in the quasi-neutral plasma can significantly affect the dynamics of flare components, which can then lead to the degeneracy in the measurements of spin of the SMBH. Analyzing the dynamics of recent flares in the case of the rapidly rotating black hole, we also constrain the inclination angle between the magnetic field and spin axis to $\alpha < 50^{\circ}$, as for larger angles, the motion of the hot spot is strongly chaotic., Comment: 28 pages, 7 figures; to appear in the Astrophysical Journal; accepted version

The Absorption Measure Distribution (AMD) in the X-ray outflows of Seyfert active galactic nuclei (AGN) describes the distribution of absorbing column density as a function of ionization parameter. Up to now, AMD has been measured only for seven objects with high-resolution X-ray data that contain absorption lines from ionized heavy elements. Even though the number of measured AMDs is not large, they display a universal broad shape containing a prominent dip, for which the absorbing column drops by around two orders of magnitude. In this paper, we tested a range of photoionization models against the overall shape of the AMD as observed in Seyferts. In particular, we demonstrate that the shape of the AMD depends both on the spectral energy distribution (SED) of radiation which enters the outflow, and the density of the warm absorber (WA). The model that best reproduces the observed shape of the AMD is one wherein the gas density of the WA is of the order of $10^{12}$ cm$^{-3}$, irradiated by an SED whose optical/UV luminosity is 100 times higher than the X-ray luminosity. When the cloud density is higher than $\sim 10^{11}$ cm$^{-3}$, free-free heating dominates the entire absorber, and only one instability zone occurs, which is in agreement with observations., 13 pages, 17 figures, submitted to ApJ

In this contribution, we summarize our results concerning the observational constraints on the electric charge associated with the Galactic centre black hole - Sgr A*. According to the no-hair theorem, every astrophysical black hole, including supermassive black holes, is characterized by at most three classical, externally observable parameters - mass, spin, and the electric charge. While the mass and the spin have routinely been measured by several methods, the electric charge has usually been neglected, based on the arguments of efficient discharge in astrophysical plasmas. From a theoretical point of view, the black hole can attain charge due to the mass imbalance between protons and electrons in fully ionized plasmas, which yields about $\sim 10^8\,{\rm C}$ for Sgr A*. The second, induction mechanism concerns rotating Kerr black holes embedded in an external magnetic field, which leads to electric field generation due to the twisting of magnetic field lines. This electric field can be associated with the induced Wald charge, for which we calculate the upper limit of $\sim 10^{15}\,{\rm C}$ for Sgr A*. Although the maximum theoretical limit of $\sim 10^{15}\,{\rm C}$ is still 12 orders of magnitude smaller than the extremal charge of Sgr A*, we analyse a few astrophysical consequences of having a black hole with a small charge in the Galactic centre. Two most prominent ones are the effect on the X-ray bremsstrahlung profile and the effect on the position of the innermost stable circular orbit., 16 pages, 4 figures, a short review on the charge of the Galactic centre black hole (Sgr A*) presented at FISICPAC-2018 conference (University of Sharjah, UAE), submitted to the Journal of Physics: Conference Series; minor revision after the referee round; additional references added