arXiv subjects

Luka C. Popovic

Publications and source records attributed to Luka C. Popovic.

49 records · Page 3Linked to original sources

arXiv2015

Polarization in lines - a new method for measuring black hole masses in active galaxies

Measuring of the masses of galactic supermassive black holes (SMBHs) is an important task, since they correlate with the host galaxy properties and play an important role in evolution of galaxies. Here we present a new method for measuring of SMBH masses using the polarization of the broad lines emitted from active galactic nuclei (AGNs). We performed spectropolarometric observations of 9 AGNs and find that this method gives measured masses which are in a good agreement with reverberation measurements. An advantage of this method is that it can be used to measure the masses of SMBHs in a consistent way at different cosmological epochs.

astro-ph.GA↗

arXiv2013

A model for the Balmer pseudocontinuum in spectra of type 1 AGNs

Here we present a new method for subtracting the Balmer pseudocontinuum in the UV part of type 1 AGN spectra. We calculate the intensity of the Balmer pseudocontinuum using the prominent Balmer lines in AGN spectra. We apply the model on a sample of 293 type 1 AGNs from SDSS database, and found that our model of Balmer pseudocontinuum + power law continuum very well fits the majority of the AGN spectra from the sample, while in 15% of AGNs, the model fits reasonable the UV continuum, but a discrepancy between the observed and fitted spectra is noted. Some of the possible reasons for the discrepancy may be a different value for the optical depth in these spectra than used in our model or the influence of the intrinsic reddening.

astro-ph.CO↗

arXiv2013

Spectral properties of a sample of type 1 AGNs: influence of star formation

To find the spectral properties of AGNs in optical spectral band (around the Hb line) we constructed an Fe II template, that is covering Fe II emission from 4100 A to 5600 A. Using the new Fe II template we explored spectral properties of 302 type 1 AGNs. The most interesting results we found is the correlation of the Hb Full Width at Half Maximum (FWHM) and luminosity for a subsample of type 1 AGNs where the ratio of narrow lines indicates a significant starburst contribution.

astro-ph.CO↗

arXiv2013

The AGN dusty torus as a clumpy two-phase medium: radiative transfer modeling with SKIRT

We modeled the AGN dusty torus as a clumpy two-phase medium, with high-density clumps embedded in a low-density interclump dust. To obtain spectral energy distributions and images of the torus at different wavelengths, we employed the 3D Monte Carlo radiative transfer code SKIRT. Apart from the grid of two-phase models, we calculated the corresponding sets of clumps-only models and models with a smooth dust distribution for comparison. We found that the most striking feature of the two-phase model is that it might offer a natural solution to the common issue reported in a number of papers -- the observed excess of the near-infrared emission.

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arXiv2011

Influence of Microlensing on Spectral Anomaly of Lensed Objects

Here we consider the influence of the microlensing on the spectrum of a lensed object taking into account that composite emission is coming from different regions arranged subsequently around the central source. We assumed that the lensed object has three regions with the black body emission; first the innermost with the highest temperature of $10^4K$, second and third (located around the central) with slightly lower temperatures $7.5\cdot10^3$ and $5\cdot10^3$K, respectively. Than we explore the flux anomaly in lensed object due to microlensing. We compare U,V and B spectra of a such source. This results show that, due to microlensing, in a spectroscopically stratified object a flux anomaly is present.

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arXiv2011

AGN Dusty Tori as a Clumpy Two-Phase Medium: The 10 Micron Silicate Feature

We investigated the emission of active galactic nuclei dusty tori in the infrared domain, with a focus on the 10 micron silicate feature. We modeled the dusty torus as a clumpy two-phase medium with high-density clumps and a low-density medium filling the space between the clumps. We employed a three-dimensional radiative transfer code to obtain spectral energy distributions and images of tori at different wavelengths. We calculated a grid of models for different parameters and analyzed the influence of these parameters on the shape of the mid-infrared emission. A corresponding set of clumps-only models and models with a smooth dust distribution is calculated for comparison. We found that the dust distribution, the optical depth and a random arrangement of clumps in the innermost region, all have an impact on the shape and strength of the silicate feature. The 10 micron silicate feature can be suppressed for some parameters, but models with smooth dust distribution are also able to produce a wide range of the silicate feature strength.

astro-ph.CO↗

arXiv2011

Photo-centric variability of quasars caused by variations in their inner structure: Consequences on Gaia measurements

We study the photocenter position variability due to variations in the quasar inner structure. We consider variability in the accretion disk emissivity and torus structure variability due to different illumination by the central source. We discuss possible detection of these effects by Gaia. Observations of the photocenter variability in two AGNs, SDSS J121855+020002 and SDSS J162011+1724327 have been reported and discussed. With investigation of the variations in the quasar inner structure we explore how much this effect can affect the position determination and whether it can be (or not) detected with Gaia mission. We used (a) a model of a relativistic disk, including the perturbation that can increase brightness of a part of the disk, and consequently offset the photocenter position, and (b) a model of a dusty torus which absorbs and re-emits the incoming radiation from accretion disk. We estimated the value of the photocenter offset due to these two effects. We found that perturbations in the inner structure can significantly offset the photocenter. It depends on the characteristics of perturbation and accretion disk and structure of the torus. In the case of two considered QSOs the observed photocenter offsets cannot be explained by variations in the accretion disk and other effects should be considered. We discussed possibility of exploding stars very close to the AGN source, and also possibility that there are two variable sources in the center of these two AGNs that may indicate a binary super-massive black hole system on a kpc (pc) scale. The Gaia mission seems to be very perspective, not only for astrometry, but also for exploring the inner structure of AGNs. We conclude that variations in the quasar inner structure can affect the observed photocenter (up to several mas). There is a chance to observe such effect in the case of bright and low-redshifted QSOs.

astro-ph.GA↗

arXiv2011

Three-dimensional radiative transfer modeling of AGN dusty tori as a clumpy two-phase medium

We investigate the emission of active galactic nuclei (AGN) dusty tori in the infrared domain. Following theoretical predictions coming from hydrodynamical simulations, we model the dusty torus as a 3D two-phase medium with high-density clumps and low-density medium filling the space between the clumps. Spectral energy distributions (SED) and images of the torus at different wavelengths are obtained using 3D Monte Carlo radiative transfer code SKIRT. Our approach of generating clumpy structure allows us to model tori with single clumps, complex structures of merged clumps or interconnected sponge-like structure. A corresponding set of clumps-only models and models with smooth dust distribution is calculated for comparison. We found that dust distribution, optical depth, clump size and their actual arrangement in the innermost region, all have an impact on the shape of near- and mid-infrared SED. The 10 micron silicate feature can be suppressed for some parameters, but models with smooth dust distribution are also able to produce a wide range of the silicate feature strength. Finally, we find that having the dust distributed in a two-phase medium, might offer a natural solution to the lack of emission in the near-infrared, compared to observed data, which affects clumpy models currently available in the literature.

astro-ph.CO↗

arXiv2011

Super-massive binary black holes and emission lines in active galactic nuclei

The broad emission spectral lines emitted from AGNs are our main probe of the geometry and physics of the broad line region (BLR) close to the SMBH. There is a group of AGNs that emits very broad and complex line profiles, showing two displaced peaks, one blueshifted and one redshifted from the systemic velocity defined by the narrow lines, or a single such peak. It has been proposed that such line shapes could indicate a supermassive binary black hole (SMB) system. We discuss here how the presence of an SMB will affect the BLRs of AGNs and what the observational consequences might be. We review previous claims of SMBs based on broad line profiles and find that they may have non-SMB explanations as a consequence of a complex BLR structure. Because of these effects it is very hard to put limits on the number of SMBs from broad line profiles. It is still possible, however, that unusual broad line profiles in combination with other observational effects (line ratios, quasi-periodical oscillations, spectropolarimetry, etc.) could be used for SMBs detection. Some narrow lines (e.g., [O\,III]) in some AGNs show a double-peaked profile. Such profiles can be caused by streams in the Narrow Line Region (NLR), but may also indicate the presence of a kilo-parsec scale mergers. A few objects indicated as double-peaked narrow line emitters are confirmed as kpc-scale margers, but double-peaked narrow line profiles are mostly caused by the complex NLR geometry. We briefly discuss the expected line profile of broad Fe K$α$ that probably originated in the accretion disk(s) around SMBs. Finally we consider rare configurations where a SMB system might be gravitationally lensed by a foreground galaxy, and discuss the expected line profiles in these systems.

astro-ph.CO↗

arXiv2011

Optical emission line properties of a sample of the broad-line AGNs: the Baldwin effect and eigenvector 1

We divide a sample of 302 type-1 AGNs into two subsamples based on the narrow line [OIII]/Hbeta_{NLR} ratio, since we expect that there will be a stronger starburst (HII region) contribution to the narrow line emission for R=log([OIII]/Hbeta_{NLR})<0.5. For the two samples we {find significant differences in correlations between} spectral properties of objects with $R<0.5$ and R>0.5. {We find similar differences when we divided the sample based on the FWHM ratios of [OIII] and broad Hbeta lines (R_1=log(FWHM[OIII]/FWHM Hbeta_broad)^>_<-0.8), i.e. similar correlations between R>0.5 and R_1<-0.8 subsamples from one side and R<0.5 and R_1>-0.8 subsamples from the other side.} The most interesting difference is in the correlation between the broad Hbeta FWHM and luminosity in the R<0.5 (R_1>-0.8) sample that indicates a connection between the BLR kinematics and photoionization source. We discuss possible effects which can cause such differences in spectral properties of two subsamples.

astro-ph.CO↗

arXiv2010

Analysis of optical Fe II emission in a sample of AGN spectra

We present a study of optical Fe II emission in 302 AGNs selected from the SDSS. We group the strongest Fe II multiplets into three groups according to the lower term of the transition (b $^4$F, a $^6$S and a $^4$G terms). These correspond approximately to the blue, central, and red part respectively of the "iron shelf" around Hb. We calculate an Fe II template which takes into account transitions into these three terms and an additional group of lines, based on a reconstruction of the spectrum of I Zw 1. This Fe II template gives a more precise fit of the Fe II lines in broad-line AGNs than other templates. We extract Fe II, Ha, Hb, [O III] and [N II] emission parameters and investigate correlations between them. We find that Fe II lines probably originate in an Intermediate Line Region. We notice that the blue, red, and central parts of the iron shelf have different relative intensities in different objects. Their ratios depend on continuum luminosity, FWHM Hb, the velocity shift of Fe II, and the Ha/Hb flux ratio. We examine the dependence of the well-known anti-correlation between the equivalent widths of Fe II and [O III] on continuum luminosity. We find that there is a Baldwin effect for [O III] but an inverse Baldwin effect for the Fe II emission. The [O III]/Fe II ratio thus decreases with L\lambda5100. Since the ratio is a major component of the Boroson and Green eigenvector 1, this implies a connection between the Baldwin effect and eigenvector 1, and could be connected with AGN evolution. We find that spectra are different for Hb FWHMs greater and less than ~3000 km/s, and that there are different correlation coefficients between the parameters.

astro-ph.CO↗

arXiv2004

Contribution of microlensing to X-ray variability of distant QSOs

We consider a contribution of microlensing to the X-ray variability of high-redshifted QSOs. Cosmologically distributed gravitational microlenses could be localized in galaxies (or even in bulge or halo of gravitational macrolenses) or could be distributed in a uniform way. We have analyzed both cases of such distributions. We found that the optical depth for gravitational microlensing caused by cosmologically distributed deflectors could be significant and could reach $10^{-2} - 0.1$ at $z\sim 2$. This means that cosmologically distributed deflectors may contribute significantlly to the X-ray variability of high-redshifted QSOs ($z>2$). Considering that the upper limit of the optical depth ($τ\sim 0.1$) corresponds to the case where dark matter forms cosmologically distributed deflectors, observations of the X-ray variations of unlensed QSOs can be used for the estimation of the dark matter fraction of microlenses.

astro-ph↗

arXiv2003

Diagnostics of Plasma Properties in Broad Line Region of AGNs

The Boltzmann-plot (BPT) method for laboratory plasma diagnostic was used for a quick estimate of physical conditions in the Broad Line Region (BLR) of 14 Active Galactic Nuclei (AGNs). For the BLR of nine AGNs, where PLTE exist, the estimated electron temperature are in the range T $\sim$ 130000K - 37000K are in good agreement with previous estimates. The estimated electron densities depend on the opacity of the emitting plasma in the BLR, and they are in range from $10^9\rm cm^{-3}$ (for optically thick plasma) to $10^{14}\rm cm^{-3}$ (for optically thin plasma). Although an alternative of PLTE in some AGNs may be very high intrinsic reddening effect, this method may be used for fast insight into physical processes in the BLR prior to applying more sophisticated physical models.

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