• Volume 36, Issue 4

      December 2015,   pages  421-703

    • Editorial

      Milan S. Dimitrijević Luka Č. Popović

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    • Giant Broad Line Regions in Dwarf Seyferts

      Nick Devereux

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      High angular resolution spectroscopy obtained with the Hubble Space Telescope (HST) has revealed a remarkable population of galaxies hosting dwarf Seyfert nuclei with an unusually large broad-line region (BLR). These objects are remarkable for two reasons. Firstly, the size of the BLR can, in some cases, rival those seen in the most luminous quasars. Secondly, the size of the BLR is not correlated with the central continuum luminosity, an observation that distinguishes them from their reverberating counterparts. Collectively, these early results suggest that non-reverberating dwarf Seyferts are a heterogeneous group, and not simply scaled versions of each other. Careful inspection reveals broad H Balmer emission lines with single peaks, double peaks, and a combination of the two, suggesting that the broad emission lines are produced in kinematically distinct regions centered on the black hole (BH). Because the gravitational field strength is already known for these objects, by virtue of knowing their BH mass, the relationship between velocity and radius may be established, given a kinematic model for the BLR gas. In this way, one can determine the inner and outer radii of the BLRs by modeling the shape of their broad emission line profiles. In the present contribution, high quality spectra obtained with the Space Telescope Imaging Spectrograph (STIS) are used to constrain the size of the BLR in the dwarf Seyfert nuclei of M81, NGC 3998, NGC 4203, NGC 3227, NGC 4051 and NGC 3516.

    • Line Shape Variability in a Sample of AGN with Broad Lines

      D. Ilić L. Č. Popović A. I. Shapovalova A. N. Burenkov V. H. Chavushyan A. Kovačević

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      The spectral variability of active galactic nuclei (AGN) is one of the key features that enables us to study in more detail, the structure of AGN emitting regions. Especially, the broad line profiles that vary both in flux and shape, give us invaluable information about the kinematics and geometry of the broad line region (BLR) where these lines are originating from. We give here a comparative review of the line shape variability in a sample of five type 1 AGNs, those with broad emission lines in their spectra, of the data obtained from the international long-term optical monitoring campaign coordinated by the Special Astrophysical Observatory of the Russian Academy of Science. The main aim of this campaign is to study the physics and kinematics of the BLR on a uniform data set, focusing on the problems of the photoionization heating of the BLR and its geometry, where, in this paper, we give for a first time, a comparative analysis of the variabilty of five type 1 AGNs, discussing their complex BLR physics and geometry in the framework of the estimates of the supermassive black hole mass in AGN.

    • Optical Counterparts of Undetermined Type 𝛾-Ray Active Galactic Nuclei with Blazar-Like Spectral Energy Distributions

      Giovanni La Mura Graziano Chiaro Stefano Ciroi Piero Rafanelli David Salvetti Marco Berton Valentina Cracco Fermi-LAT collaboration

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      During its first four years of scientific observations, the Fermi Large Area Telescope (Fermi-LAT) detected 3033 𝛾-ray sources above a 4𝜎 significance level. Although most of the extra-galactic sources are active galactic nuclei (AGN) of the blazar class, other families of AGNs are observed too, while a still high fraction of detections (∼30%) remains with uncertain association or classification. According to the currently accepted interpretation, the AGN 𝛾-ray emission arises from inverse Compton (IC) scattering of low energy photons by relativistic particles confined in a jet, which, in the case of blazars, is oriented very close to our line-of-sight. Taking advantage of data from radio and X-ray wavelengths, which we expect to be produced together with 𝛾-rays, providing a much better source localization potential, we focused our attention on a sample of 𝛾-ray Blazar Candidates of Undetermined type (BCUs), starting a campaign of optical spectroscopic observations. The main aims of our investigation include a census of the AGN families that contribute to 𝛾-ray emission and a study of their redshift distribution, with the subsequent implications on the intrinsic source power. We furthermore analyze which 𝛾-ray properties can better constrain the nature of the source, thus helping in the study of objects not yet associated with a reliable low frequency counterpart. Here we report on the instruments and techniques used to identify the optical counterparts of 𝛾-ray sources, we give an overview on the status of our work, and we discuss the implications of a large scale study of 𝛾-ray emitting AGNs.

    • Observations of the Ca ${\rm\tiny II}$ IR Triplet in High Luminosity Quasars: Exploring the Sample

      Mary Loli Martínez-Aldama Paola Marziani Deborah Dultzin Jack W. Sulentic Alessandro Bressan Yang Chen Giovanna M. Stirpe

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      We present a new spectroscopic sample of 11 quasars at intermediate redshift observed with the Infrared Spectrometer and Array Camera (ISAAC) on the ESO Very Large Telescope (VLT), covering O ${\rm\tiny I}$ 𝜆8446 and the Ca ${\rm\tiny II}$ triplet 8498, 8542, 8662. The new observations – that supplement the sample presented by Martínez-Aldama et al. (2015) – allow us to confirm the constraints on physical conditions and location of the region emitting the low ionization lines, as well as the relation between Ca ${\rm\tiny II}$ and Fe ${\rm\tiny II}$.

    • Measures of the Soft X-ray Excess as an Eigenvector 1 Parameter for Active Galactic Nuclei

      K. Bensch A. del Olmo J. Sulentic J. Perea P. Marziani

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      We present a preliminary analysis of X-ray data of quasars in the context of the 4D eigenvector 1 parameter space (Sulentic et al. 2000a, b). 4DE1 serves as a surrogate H-R diagram for representing empirical diversity among quasars and identifying the physical drivers of the diversity. The soft X-ray spectral index (𝛤soft) was adopted as one of the key 4DE1 that correlates contrasting extremes in Type 1 properties. 4DE1 motivated the hypothesis of two quasar populations (A and B) divided by 𝐿/𝐿EDD ≈ 0.2. Pop. A is a largely radio-quiet population with FWHM 𝐻𝛽 < 4000 km/s and often showing a soft X-ray excess. Pop. B is a mix of radio-quiet and a majority of RL quasars shows only a hard X-ray power-law SED. The X-ray separation was based upon earlier ROSAT and ASCA data but we now confirm this dichotomy with large samples of X-ray spectra obtained with XMM-Newton and SWIFT. One popular idea connects the soft excess in Pop. A quasars as a signature of thermal emission from a hot accretion disk in sources radiating close to the Eddington limit.

    • Time Delay Evolution of Five Active Galactic Nuclei

      A. Kovačević L. Č. Popović A. I. Shapovalova D. Ilić A. N. Burenkov V. H. Chavushyan

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      Here we investigate light curves of the continuum and emission lines of five type 1 active galactic nuclei (AGN) from our monitoring campaign, to test time-evolution of their time delays. Using both modeled and observed AGN light curves, we apply Gaussian kernel-based estimator to capture variation of local patterns of their time evolving delays. The largest variations of time delays of all objects occur in the period when continuum or emission lines luminosity is the highest. However, Gaussian kernel-based method shows instability in the case of NGC 5548, 3C 390.3, E1821+643 and NGC 4051 possibly due to numerical discrepancies between damped random walk (DRW) time scale of light curves and sliding time windows of the method. The temporal variations of time lags of Arp 102B can correspond to the real nature of the time lag evolution.

    • Multicomponent Analysis of the UV Si IV and C IV Broad Absorption Troughs in BALQSO Spectra: The Examples of J01225+1339 and J02287+0002

      D. Stathopoulos E. Danezis E. Lyratzi A. Antoniou D. Tzimeas

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      Broad Absorption Line QSOs (BALQSOs) are a subtype of radio-quite QSOs that exhibit complex and unusually broad (FWHM ≥ 2,000 km/s) absorption lines. The existence of these lines in BALQSO spectra raises some questions with respect to the properties, the physical conditions and kinematics of the BAL material as well as the morphology of BAL troughs. In this study, taking into consideration the clumpy structure of the AGN outflow winds, we propose a physical model in order to explain the formation of BAL troughs and we give the mathematical description of this model. We also propose a method for analyzing spectroscopically the BAL profiles in the UV region of the electromagnetic spectrum. This method consists of the criteria we set during the fitting process of BAL troughs. The purpose of these criteria is to enable us to determine the exact number of components needed to simulate accurately the BAL troughs and guarantee the uniqueness of the fit. We give an application of the model and the method for Si IV and C IV resonance lines in the case of two BALQSOs. From the analysis, we conclude that the BAL material is in the form of clouds (density enhancements) that move radially and intercept the line-of-sight to the central continuum source. Using our method, we find the number of absorption components needed to simulate the BAL profiles, which means the number of clouds in the line-of-sight. We calculate the velocity shifts, the FWHM and the optical depths of the absorption components of BALs and we propose an internal structure for these clouds. Finally, we give some correlations between the properties of absorption components of Si IV and C IV.

    • Line Shapes Emitted from Spiral Structures around Symmetric Orbits of Supermassive Binary Black Holes

      M. Smailagić E. Bon

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      Variability of active galactic nuclei is not well understood. One possible explanation is existence of supermassive binary black holes (SMBBH) in their centres. It is expected that major mergers are common in the Universe. It is expected that each supermassive black hole of every galaxy eventually finish as a SMBBH system in the core of newly formed galaxy. Here we model the emission line profiles of active galactic nuclei (AGN) assuming that the flux and emission line shape variations are induced by supermassive binary black hole systems (SMBBH). We assume that the accreting gas inside the circumbinary (CB) disk is photo ionized by mini accretion disk emission around each SMBBH. We calculate variations of emission line flux, shifts and shapes for different parameters of SMBBH orbits. We consider cases with different masses and inclinations for circular orbits and measure the effect to the shape of emission line profiles and flux variability.

    • Deterministic Chaos in the X-ray Sources

      M. Grzedzielski P. Sukova A. Janiuk

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      Hardly any of the observed black hole accretion disks in X-ray binaries and active galaxies shows constant flux. When the local stochastic variations of the disk occur at specific regions where a resonant behaviour takes place, there appear the quasi-periodic oscillations (QPOs). If the global structure of the flow and its non-linear hydrodynamics affects the fluctuations, the variability is chaotic in the sense of deterministic chaos. Our aim is to solve a problem of the stochastic versus deterministic nature of the black hole binary variabilities. We use both observational and analytic methods. We use the recurrence analysis and we study the occurence of long diagonal lines in the recurrence plot of observed data series and compare it to the surrogate series. We analyze here the data of two X-ray binaries – XTE J1550-564 and GX 339-4 observed by Rossi X-ray Timing Explorer. In these sources, the non-linear variability is expected because of the global conditions (such as the mean accretion rate) leading to the possible instability of an accretion disk. The thermal-viscous instability and fluctuations around the fixedpoint solution occurs at high accretion rate, when the radiation pressure gives dominant contribution to the stress tensor.

    • Possible Alternatives to the Supermassive Black Hole at the Galactic Center

      A. F. Zakharov

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      Now there are two basic observational techniques to investigate a gravitational potential at the Galactic Center, namely,

        bullet

      At the moment, one can use a small relativistic correction approach for stellar orbit analysis (however, in the future the approximation will not be precise enough due to enormous progress of observational facilities) while for smallest structure analysis in VLBI observations one really needs a strong gravitational field approximation. We discuss results of observations, their conventional interpretations, tensions between observations and models and possible hints for a new physics from the observational data and tensions between observations and interpretations. We discuss an opportunity to use a Schwarzschild metric for data interpretation or we have to use more exotic models such as Reissner–Nordstrom or Schwarzschild–de-Sitter metrics for better fits.

    • Radio Recombination Lines of Hydrogen

      G. Peach

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      The impact theory of spectral line broadening is used to obtain complete profiles for radio recombination lines perturbed by electron and proton impact. The collisions can be divided into two types: inelastic, where transitions take place between hydrogen levels with different principal quantum number 𝑛 and elastic, where the transitions are only between degenerate levels for a particular value of 𝑛. The widths of the radio lines are essentially determined by inelastic electron collisions and elastic proton collisions with the emitting hydrogen atom occupying either the upper or lower levels of the line.

      Here, earlier work is extended to examine the contribution from proton collisions to the line width in more detail, and it is shown that the trends in the behaviour of the widths again confirm previous results.

    • Comparative Analysis of VLF Signal Variation along Trajectory Induced by X-ray Solar Flares

      A. Kolarski D. Grubor

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      Comparative qualitative analysis of amplitude and phase delay variations was carried out along the trajectory of GQD/22.1 kHz and NAA/24.0 kHz VLF signal traces, propagating from Skelton (UK) and Maine (USA) toward Belgrade, induced by four isolated solar X-ray flare events occurred during the period from September 2005 to December 2006. For monitoring, recording and for storage of VLF data at the Institute of Physics in Belgrade, Serbia, the AbsPAL system was used. For modeling purposes of propagating conditions along GQD and NAA signal propagation paths, LWPCv21 program code was used. Occurred solar flare events induced lower ionosphere electron density height profile changes, causing perturbations in VLF wave propagation within Earth-ionosphere waveguides. As analyzed VLF signals characterize by different propagation parameters along trajectories from their transmitters to the Belgrade receiver site, their propagation is affected in different ways for different solar flare events and also for the same solar flare events.

    • Modeling of Stark–Zeeman Lines in Magnetized Hydrogen Plasmas

      J. Rosato H. Bufferand H. Capes M. Koubiti L. Godbert-Mouret Y. Marandet R. Stamm

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      The action of electric and magnetic fields on atomic species results in a perturbation of the energy level structure, which alters the shape of spectral lines. In this work, we present the Zeeman–Stark line shape simulation method and perform new calculations of hydrogen Lyman and Balmer lines, in the framework of magnetic fusion research. The role of the Zeeman effect, fine structure and the plasma's non-homogeneity along the line-of-sight are investigated. Under specific conditions, our results are applicable to DA white dwarf atmospheres.

    • Spectroscopic Data of W I, Mo I and Cr I Spectral Lines: Selection and Analysis

      A. N. Veklich A. V. Lebid T. A. Tmenova

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      Plasma of electric arc discharges between composite Cu–W, Cu–Mo and Cu–Cr electrodes in argon flow and their spectra were studied by optical emission spectroscopy. Since values of oscillator strengths for W I, Mo I and Cr I presented in various sources are significantly different, selection of spectroscopic data for these elements (particularly oscillator strength) was expected to be useful for plasma diagnostics. The Boltzmann plot method was used as a tool for the selection of appropriate spectral lines and their spectroscopic data. The main result of the paper is W I, Mo I and Cr I spectral lines and spectroscopic data recommended for diagnostics of plasma with such metal impurities.

    • Radiative Transfer Reconsidered as a Quantum Kinetic Theory Problem

      J. Rosato

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      We revisit the radiative transfer theory from first principles approach, inspired from quantum kinetic theory. The radiation field is described within the second quantization formalism. A master equation for the radiation density operator is derived and transformed into a balance relation in the phase space, which involves nonlocal terms owing to radiation coherence. In a perturbative framework, we focus on the lowest order term in $\hbar$-expansion and show that the radiation coherence results in an alteration of the photon group velocity. An application to the formation of hydrogen lines in stellar atmospheres is performed as an illustration.

    • Dynamics Resonances in Atomic States of Astrophysical Relevance

      K. N. Arefieff K. Miculis N. N. Bezuglov M. S. Dimitrijević A. N. Klyucharev A. A. Mihajlov V. A. Srećković

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      Ionized geocosmic media parameters in a thermal and a subthermal range of energy have a number of unique features. The photoresonance plasma that is formed by optical excitation of the lowest excited (resonance) atomic states is one example of conversion of radiation energy into electrical one. Since spontaneous fluorescence of excited atoms is probabilistic, the description of the radiating quantized system evolution along with photon energy transfer in a cold atom medium, should include elements of stochastic dynamics. Finally, the chaotic dynamics of a weakly bound Rydberg electron over a grid of the energy level diagram of a quasi-molecular Rydberg complex provides an excitation migration of the electron forward to the ionization continuum. This work aims at discussing the specific features of the dynamic resonances formalism in the description of processes involving Rydberg states of an excited atom, including features in the fluorescence spectrum partially caused by the quantum defect control due to the presence of statistic electromagnetic fields.

    • Non-Elastic Processes in Atom Rydberg-Atom Collisions: Review of State of Art and Problems

      A. A. Mihajlov V. A. Srećković Lj. M. Ignjatović A. N. Klyucharev M. S. Dimitrijević N. M. Sakan

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      Inour previous research, it has been demonstrated that inelastic processes in atom Rydberg-atom collisions, such as chemi-ionization and ($n-n'$) mixing, should be considered together. Here we will review the present state-of-the-art and the actual problems. In this context, we will consider the influence of the ($n-n'$)-mixing during a symmetric atom Rydberg-atom collision processes on the intensity of chemi-ionization process. It will be taken into account H(1s) + H*(𝑛) collisional systems, where the principal quantum number is $n \gg 1$. It will be demonstrated that the inclusion of ($n-n'$) mixing in the calculation, influences significantly on the values of chemi-ionization rate coefficients, particularly in the lower part of the block of the Rydberg states. Different possible channels of the ($n-n'$)-mixing influence on chemi-ionization rate coefficients will be demonstrated. The possibility of interpretation of the ($n-n'$)-mixing influence will be considered on the basis of two existing methods for describing the inelastic processes in symmetrical atom Rydberg-atom collisions.

    • Inverse Bremsstrahlung in Astrophysical Plasmas: The Absorption Coefficients and Gaunt Factors

      A. A. Mihajlov V. A. Srećković N. M. Sakan

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      The electron–ion inverse Bremsstrahlung is considered here as a factor of the influence on the opacity of the different stellar atmospheres and other astrophysical plasmas. It is shown that this process can be successfully described in the frames of cut-off Coulomb potential model within the regions of the electron densities and temperatures. The relevant quantum mechanical method of the calculation of the corresponding spectral coefficient processes is described and discussed. The results obtained for the plasmas with the electron densities from 1014 cm$^{-3}$ to 2 · 1019 cm$^{−3}$ and temperatures from 5 · 103 K to 3 · 104 K in the wavelength region 100 nm < 𝜆 < 3000 nm are presented. Also, these results can be of interest for different laboratory plasmas.

    • Stark Widths of Spectral Lines of Neutral Neon

      Milan S. Dimitrijević Zoran Simić Andjelka Kovačević Aleksandar Valjarević Sylvie Sahal-Bréchot

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      In order to complete Stark broadening data for Ne I spectral lines which are needed for analysis of stellar atmospheres, collisional widths and shifts (the so-called Stark broadening parameters) of 29 isolated spectral lines of neutral neon have been determined within the impact semiclassical perturbation method. Calculations have been performed for the broadening by collisions with electrons, protons and ionized helium for astrophysical applications, and for collisions with ionized neon and argon for laboratory plasma diagnostics. The shifts have been compared with existing experimental values. The obtained data will be included in the STARK-B database, which is a part of the Virtual Atomic and Molecular Data Center – VAMDC.

    • Stark Broadening Parameters for Neutral Oxygen Spectral Lines

      N. Alonizan R. Qindeel N. Ben Nessib S. Sahal-Bréchot Milan S. Dimitrijević

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      Stark broadening parameters for nine neutral oxygen (O I) lines have been determined within the impact approximation and the semiclassical perturbation method. The atomic data have been taken from the TOPbase and NIST atomic databases. The electron and proton Stark widths and shifts and ion broadening parameter values for these O I lines have been calculated for electron density of 1016 cm$^{−3}$ and for 4 different electron temperatures in the range of 5000 K to 40000 K. These Stark broadening parameters are compared with our previous results (Ben Nessib, N. et al. 1996, Physica Scripta, 54, 603–613), where we calculated Stark broadening parameters for only four O I spectral lines and where Stark widths and shifts were compared with experimental and theoretical data available in the literature. In the present paper, we have also compared our results with the Griem's book (Griem, H. R. 1974, Spectral line broadening by plasmas) and VALD (Ryabchikova, T. et al. 2015, Physica Scripta, 90, 054005) values.

    • On the Stark Broadening of Lu III Spectral Lines

      Zlatko Majlinger Zoran Simić Milan S. Dimitrijević

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      The electron-impact widths for 27 Lu III spectral lines have been calculated by using the modified semiempirical method. Calculations have been also performed with the published relativistic Hartree-Fock oscillator strengths and additionally, with the approximate formula of Cowley.With the obtained results, the influence of Stark broadening on Lu III lines was investigated in the spectra of A-type stars. The obtained data will be included in the STARK-B database, which is part of the Virtual Atomic and Molecular Data Center – VAMDC.

    • Stark Broadening in Compact Stars: Xe VI Lines

      Milan S. Dimitrijević Zoran Simić Andjelka Kovačević Aleksandar Valjarević Sylvie Sahal-Bréchot

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      We will consider Stark broadening of non hydrogenic spectral lines in the impact approximation in compact stars: pre-white dwarf and white dwarf atmospheres. In order to show an example, Stark broadening parameters have been calculated, using the impact semiclassical perturbation approach for four Xe VI spectral lines. Obtained results have been used to demonstrate the influence of Stark broadening in DA and DB white dwarf atmospheres.

    • MOL-D: A Collisional Database and Web Service within the Virtual Atomic and Molecular Data Center

      V. Vujčić D. Jevremović A. A. Mihajlov Lj. M. Ignjatović V. A. Srećković M. S. Dimitrijević M. Malović

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      MOL-D database is a collection of cross-sections and rate coefficients for specific collisional processes and a web service within the Serbian Virtual Observatory (SerVO) and the Virtual Atomic and Molecular Data Center (VAMDC). This database contains photo-dissociation cross-sections for the individual ro-vibrational states of the diatomic molecular ions and rate coefficients for the atom-Rydberg atom chemiionization and inverse electron–ion–atom chemi-recombination processes. At the moment it contains data for photodissociation crosssections of hydrogen H+2 and helium H+2 molecular ions and the corresponding averaged thermal photodissociation cross-sections. The ro-vibrational energy states and the corresponding dipole matrix elements are provided as well. Hydrogen and helium molecular ion data are important for calculation of solar and stellar atmosphere models and for radiative transport, as well as for kinetics of other astrophysical and laboratory plasma (i.e. early Universe).

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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