arXiv subjects

Luka Č. Popović

Publications and source records attributed to Luka Č. Popović.

At least 37 records · Page 2Linked to original sources

arXiv2021

The first supermassive black hole mass measurement in active galactic nuclei using the polarization of broad emission line Mg II

Spectropolarimetric efforts in the last few years have provided an efficient method that is based on the profiles of the polarization plane position angle of broad emission lines in active galactic nuclei (AGNs). Here we present black hole measurements of SBS 1419+538 using spectropolarimetric observations in the Mg II spectral band. The observations are performed by 6m telescope of SAO RAS using SCORPIO-2. We found a good agreement for the estimated supermassive black hole (SMBH) mass for this object using spectropolarimetry when compared with the mass obtained using other methods.

astro-ph.GA↗

arXiv2021

Linear Spectropolarimetric Analysis of Fairall 9 with VLT/FORS2

The quasar Main Sequence (MS) 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.

astro-ph.GA↗

arXiv2021

Detecting subparsec super-massive binary black holes: Long termmonitoring perspective

Here we consider the perspective to detect sub-pc super-massive binary black-hole (SMBBH) systems using long-term photometric and spectroscopic monitoring campaigns of active galactic nuclei. This work explores the nature of long-term spectral variability caused by the dynamical effects of SMBBH systems. We describe in great detail a model of SMBBH system which considers that both black holes have their accretion disc and additional line emitting region(s). We simulate the H$β$ spectral band (continuum+broad H$β$ line) for different mass ratios of components and different total masses of the SMBBH systems ($10^6-10^8\mathrm{M\odot}$). We analyze the set of continuum and broad line light curves for several full orbits of SMBBHs with different parameters, to test the possibility to extract the periodicity of the system. We consider different levels of the signal-to-noise ratio, which is added to the simulated spectra. Our analysis showed that the continuum and broad line profiles emitted from an SMBBH system are strongly dependent, not only on the mass ratio of the components but also on the total mass of the system. We found that the mean broad line profile and its rms could indicate the presence of an SMBBH. However, some effects caused by the dynamics of a binary system could be hidden due to a low signal-to-noise ratio. Finally, we can conclude that the long-term AGN monitoring campaigns could be beneficial for the detection of SMBBH candidates.

astro-ph.HE↗

arXiv2021

A ring accelerator? Unusual jet dynamics in the IceCube candidate PKS 1502+106

On 2019/07/30.86853 UT, IceCube detected a high-energy astrophysical neutrino candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 percent uncertainty region of the event. Our analysis of 15 GHz Very Long Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after the IceCube event, reveals evidence for a radio ring structure which develops with time. Several arc-structures evolve perpendicular to the jet ridge line. We find evidence for precession of a curved jet based on kinematic modelling and a periodicity analysis. An outflowing broad line region (BLR) based on the C IV line emission (Sloan Digital Sky Survey, SDSS) is found. We attribute the atypical ring to an interaction of the precessing jet with the outflowing material. We discuss our findings in the context of a spine-sheath scenario where the ring reveals the sheath and its interaction with the surroundings (narrow line region, NLR, clouds). We find that the radio emission is correlated with the $γ$-ray emission, with radio lagging the $γ$-rays. Based on the $γ$-ray variability timescale, we constrain the $γ$-ray emission zone to the BLR (30-200 $r_{\rm g}$) and within the jet launching region. We discuss that the outflowing BLR provides the external radiation field for $γ$-ray production via external Compton scattering. The neutrino is most likely produced by proton-proton interaction in the blazar zone (beyond the BLR), enabled by episodic encounters of the jet with dense clouds, i.e. some molecular cloud in the NLR.

astro-ph.HE↗

arXiv2020

Estimating supermassive black hole masses in AGNs using polarization of broad MgII, H$α$ and H$β$ lines

For type-1 active galactic nuclei (AGNs) for which the equatorial scattering is the dominant broad line polarization mechanism, it is possible to measure the supermassive black hole mass by tracing the Keplerian motion across the polarization plane positionangle. So far this method has been used for 30 objects but only for H$α$ emission line. We explore the possibilities this method for determining SMBH masses using polarization in broad emission lines by applying it for the first time to Mg II$λ$2798AA spectral line. We use 3D Monte Carlo radiative transfer code stokes for simultaneous modeling of equatorial scattering of H$α$, H$β$ and Mg II lines. We included vertical inflows and outflows in the MgII broad line region (BLR). We find that polarization states of H$α$ and H$β$ lines are almost identical and SMBH mass estimates differ by 7%. For Mg II line, we find that polarization angle $φ$ exhibits an additional "plateau" with a constant $φ$, which deviates than the profiles expected for pure Keplerian motion. SMBH mass estimates using Mg II line are higher by up to 35% than those obtained from H$α$ and H$β$ lines. Our model shows that for vertical inflows and outflows in the BLR that is higher or comparable to Keplerian velocity, this method can be applied as a first approximation for obtaining SMBH mass.

astro-ph.GA↗

arXiv2020

Variability and the size-luminosity relation of the intermediate mass AGN in NGC 4395

We present the variability study of the lowest-luminosity Seyfert 1 galaxy NGC 4395 based on the photometric monitoring campaigns in 2017 and 2018. Using 22 ground-based and space telescopes, we monitored NGC 4395 with a $\sim$5 minute cadence during a period of 10 days and obtained light curves in the UV, V, J, H, and K/Ks bands as well as the H$α$ narrow-band. The RMS variability is $\sim$0.13 mag on \emph{Swift}-UVM2 and V filter light curves, decreasing down to $\sim$0.01 mag on K filter. After correcting for continuum contribution to the H$α$ narrow-band, we measured the time lag of the H$α$ emission line with respect to the V-band continuum as ${55}^{+27}_{-31}$ to ${122}^{+33}_{-67}$ min. in 2017 and ${49}^{+15}_{-14}$ to ${83}^{+13}_{-14}$ min. in 2018, depending on the assumption on the continuum variability amplitude in the H$α$ narrow-band. We obtained no reliable measurements for the continuum-to-continuum lag between UV and V bands and among near-IR bands, due to the large flux uncertainty of UV observations and the limited time baseline. We determined the AGN monochromatic luminosity at 5100Å $λL_λ= \left(5.75\pm0.40\right)\times 10^{39}\,\mathrm{erg\,s^{-1}}$, after subtracting the contribution of the nuclear star cluster. While the optical luminosity of NGC 4395 is two orders of magnitude lower than that of other reverberation-mapped AGNs, NGC 4395 follows the size-luminosity relation, albeit with an offset of 0.48 dex ($\geq$2.5$σ$) from the previous best-fit relation of Bentz et al. (2013).

astro-ph.GA↗

arXiv2020

Broad spectral lines in AGNs and supermassive black hole mass measurements

The mass measurement of supermassive black holes (SMBHs) is a very complex task. Between several methods for SMBH mass measurements, some of them use the spectral lines, which indicate the motion of the emitting/absorbing material around an SMBH. Mostly there is assumption of virialization of line emitting gas in the region close to SMBHs that is used for their mass measurements. In this paper we will give an overview of methods for the SMBH mass measurements using broad emission spectral lines observed in Type 1 AGNs. First we give the basic idea to use the parameters of broad lines to SMBH mass measurements. After that we give an overview of broad lines from X-ray (Fe kα) to the IR (Pashen and Brecket lines) which have been used for SMBH mass estimates. Additionally, we describe and discuss a new method for SMBH mass measurements using the polarization in the broad lines emitted from Type 1 AGNs.

astro-ph.GA↗

arXiv2019

Mapping the Inner Structure of Quasars with Time-Domain Spectroscopy

The ubiquitous variability of quasars across a wide range of wavelengths and timescales encodes critical information about the structure and dynamics of the circumnuclear emitting regions that are too small to be directly resolved, as well as detailed underlying physics of accretion and feedback processes in these active supermassive black holes. We emphasize the importance of studying quasar variability with time-domain spectroscopy, focusing on two science cases: (1) reverberation mapping (RM) to measure the broad-line region sizes and black hole masses in distant quasars; (2) spectroscopic follow-up of extreme variability quasars that dramatically change their continuum and broad-line flux within several years. We highlight the need for dedicated optical-infrared spectroscopic survey facilities in the coming decades to accompany wide-area time-domain imaging surveys, including: (1) the next phase of the Sloan Digital Sky Survey (SDSS-V; ~2020-2025), an all-sky, time-domain multi-object spectroscopic survey with 2.5m-class telescopes; (2) the planned Maunakea Spectroscopic Explorer, a dedicated 10m-class spectroscopic survey telescope with a 1.5 sq. deg field-of-view and multiplex of thousands of fibers in both optical and near-IR (J+H) to begin operations in 2029; (3) the Time-domain Spectroscopic Observatory (TSO), a proposed Probe-class ~1.3m telescope at L2, with imaging and spectroscopy (R=200, 1800) in 4 bands (0.3 - 5 micron) and rapid slew capability to 90% of sky, which will extend the coverage of Hbeta to z=8.

astro-ph.GA↗

arXiv2018

The shape of Fe K$α$ line emitted from relativistic accretion disc around AGN black holes

The relativistically broadened Fe K$α$ line, originating from the accretion disc in a vicinity of a super massive black hole, is observed in only less than 50\% of type 1 Active Galactic Nuclei (AGN). In this study we investigate could this lack of detections be explained by the effects of certain parameters of the accretion disc and black hole, such as the inclination, the inner and outer radius of disc and emissivity index. In order to determine how these parameters affect the Fe K $α$ line shape, we simulated about 60,000 Fe K $α$ line profiles emitted from the relativistic disc. Based on simulated line profiles, we conclude that the lack of the Fe K$α$ line detection in type 1 AGN could, be caused by the specific emitting disc parameters, but also by the limits in the spectral resolution and sensitivity of the X-ray detectors.

astro-ph.GA↗

arXiv2018

The intrinsic Baldwin effect in NLSy 1 galaxies

The intrinsic Baldwin effect is an anti-correlation between the line equivalent width and the flux of the underlying continuum detected in a single variable active galactic nucleus (AGN). This effect, in spite of the extensive research, is still not well understood, and might give us more information about the physical properties of the line and continuum emission regions in AGNs. Here we present preliminary results of our investigation of the intrinsic Baldwin effect of the broad H$β$ line in several Narrow-line Seyfert 1 (NLSy 1) galaxies, for which data were taken from the long-term monitoring campaigns and from the Sloan Digital Sky Survey Reverberation Mapping project.

astro-ph.GA↗

arXiv2018

Narrow vs. Broad line Seyfert 1 galaxies: X-ray, optical and mid-infrared AGN characteristics

We investigated narrow line Seyfert 1 galaxies (NLS1s) at optical, mid-infrared (MIR) and X-ray wavelengths, comparing them to the broad line active galactic nuclei (BLAGNs). We found that black hole mass, coronal line luminosities, X-ray hardness ratio and X-ray, optical and MIR luminosities are higher for the BLAGNs than for NLS1s, while policyclic aromatic hydrocarbon (PAH) contribution and the accretion rates are higher for the NLS1s. Furthermore, we found some trends among spectral parameters that NLS1s have and BLAGNs do not have. The evolution of FWHM(H$β$) with the luminosities of MIR and coronal lines, continuum luminosities, PAH contribution, H$β$ broad line luminosity, FWHM[O\,III] and EW(H$β$NLR), are important trends found for NLS1s. That may contribute to the insight that NLS1s are developing AGNs, growing their black holes, while their luminosities and FWHM(H$β$) consequently grow, and that BLAGNs are mature, larger objects of slower and/or different evolution. Black hole mass is related to PAH contribution only for NLS1s, which may suggest that PAHs are more efficiently destroyed in NLS1s.

astro-ph.GA↗

arXiv2018

Detection of short term response of the low ionosphere on gamma ray bursts

In this paper, we study the possibility of detection of short term terrestrial lower ionospheric response to gamma ray bursts (GRBs) using a statistical analysis of perturbations of six very low or low frequency (VLF/LF) radio signals emitted by transmitters located worldwide and recorded by VLF/LF receiver located in Belgrade (Serbia). We consider a sample of 54 short lasting GRBs (shorter than 1 min) detected by the SWIFT satellite during the period 2009-2012. We find that a statistically significant perturbations can be present in the low ionosphere, and reactions on GRBs may be observed immediately after the beginning of the GRB event or with a time delay of 60 s - 90 s.

physics.space-ph↗

arXiv2017

The optical vs. mid-infrared spectral properties of 82 Type 1 AGNs: coevolution of AGN and starburst

We investigated the connection between the mid-infrared (MIR) and optical spectral characteristics in a sample of 82 Type 1 active galactic nuclei (AGNs), observed with Infrared Spectrometer on Spitzer (IRS) and Sloan Digital Sky Survey (SDSS, DR12). We found several interesting correlations between optical and MIR spectral properties: i) as starburst significators in MIR increase, the EWs of optical lines H$β$NLR and FeII, increase as well; ii) as MIR spectral index increases, EW([OIII]) decreases, while fractional contribution of AGN (RAGN) is not connected with EW([OIII]); iii) The log([OIII]5007/$\rm Hβ$NLR) ratio is weakly related to the fractional contribution of polycyclic aromatic hydrocarbons (RPAHs). We compare the two different MIR and optical diagnostics for starburst contribution to the overall radiation (RPAH and BPT diagram, respectively). The significant differences between optical and MIR starburst diagnostics were found. The starburst influence to observed correlations between optical and MIR parameters is discussed.

astro-ph.GA↗

arXiv2017

Black Hole Mass Estimation in Type 1 AGN: H$β$ vs. Mg II lines and the role of Balmer continuum

Here we investigate the H$β$ and Mg II spectral line parameters used for the black hole mass (M$_{\rm BH}$) estimation for a sample of Type 1 Active Galactic Nuclei (AGN) spectra selected from the Sloan Digital Sky Survey (SDSS) database. We have analyzed and compared the virialization of the H$β$ and Mg II emission lines, and found that the H$β$ line is more confident virial estimator than Mg II. We have investigated the influence of the Balmer continuum emission to the M$_{\rm BH}$ estimation from the UV parameters, and found that the Balmer continuum emission can contribute to the overestimation of the M$_{\rm BH}$ on average for ~ 5% (up to 10%).

astro-ph.GA↗

arXiv2017

Low ionospheric reactions on tropical depressions prior hurricanes

We study the reactions of the low ionosphere during tropical depressions (TDs) which have been detected before the hurricane appearances in the Atlantic Ocean. We explore 41 TD events using very low frequency (VLF) radio signals emitted by NAA transmitter located in the USA and recorded by VLF receiver located in Belgrade (Serbia). We found VLF signal deviations (caused ionospheric turbulence) in the case of 36 out of 41 TD events (88%). Additionally, we explore 27 TDs which have not been developed in hurricanes and found similar low ionospheric reactions. However, in the sample of 41 TDs which are followed by hurricanes the typical low ionosphere perturbations seem to be more frequent than other TDs.

astro-ph.EP↗

arXiv2017

Diagnostics of plasma in the ionospheric D-region: detection and study of different ionospheric disturbance types

Here we discuss our recent investigations of the ionospheric plasma by using very low and low frequency (VLF/LF) radio waves. We give a review of how to detect different low ionospheric reactions (sudden ionospheric disturbances) to various terrestrial and extra-terrestrial events, show their classification according to intensity and time duration, and present some methods for their detections in time and frequency domains. Investigations of detection in time domain are carried out for intensive long-lasting perturbations induced by solar X-ray flares and for short-lasting perturbations caused by gamma ray bursts. We also analyze time variations of signals used in the low ionospheric monitoring after earthquake events. In addition, we describe a procedure for the detection of acoustic and gravity waves from the VLF/LF signal analysis in frequency domain. The research of the low ionospheric plasma is based on data collected by the VLF/LF receivers located in Belgrade, Serbia.

astro-ph.EP↗

arXiv2013

Broad spectral line and continuum variabilities in QSO spectra induced by microlensing of diffusive massive substructure

We investigate the variability of the continuum and broad lines in QSO spectra (particularly in the H$β$ line and continuum at $λ$ 5100 Å) caused by microlensing of a diffuse massive structure (like an open star cluster). We modeled the continuum and line emitting region and simulate a lensing event by a star cluster located in an intervening galaxy. Such a type of microlensing event can have a significant influence on magnification and centroid shift of the broad lines and continuum source. We explore relationships between the continuum and broad line flux variability during the microlensing event.

astro-ph.CO↗

arXiv2013

Stark broadening data for spectral lines of rare-earth elements: Nb III

The electron-impact widths for 15 doubly charged Nb ion lines have been theoretically determined by using the modified semiempirical method. Using the obtained results, we considered the influence of the electron-impact mechanism on line shapes in spectra of chemically peculiar stars and white dwarfs.

astro-ph.SR↗