arXiv subjects

M. S. Dimitrijevic

Publications and source records attributed to M. S. Dimitrijevic.

17 recordsLinked to original sources

arXiv2024

The extremes of AGN variability: outbursts, deep fades, changing looks, exceptional spectral states, and semi-periodicities

The extremes of Active Galactic Nuclei (AGN) variability offer valuable new insights into the drivers and physics of AGN. We discuss some of the most extreme cases of AGN variability; the highest amplitudes, deep minima states, extreme spectral states, Seyfert-type changes, and semi-periodic signals, including new X-ray observations. The properties of changing-look (CL) AGN are briefly reviewed and a classification scheme is proposed which encompasses the variety of CL phenomena; distinguishing slow and fast events, repeat events, and frozen-look AGN which do not show any emission-line response. Long-term light curves that are densely covered over multiple years, along with follow-up spectroscopy, are utilized to gain insight into the underlying variability mechanisms including accretion disk and broad-line region physics. Remarkable differences are seen, for instance, in the optical spectral response to extreme outbursts, implying distinct intrinsic variability mechanisms. Furthermore, we discuss methods for distinguishing between CL AGN and CL look-alike events (tidal disruption events or supernovae in dense media). Finally, semi-periodic light curve variability is addressed and the latest multiwavelength (MWL) light curve of the binary supermassive black hole (SMBH) candidate OJ 287 from the MOMO project is presented. Recent results from that project have clearly established the need for new binary SMBH modelling matching the tight new constraints from observations, including the measurement of a low (primary) SMBH mass of ~10^8 Msun which also implies that OJ 287 is no longer in the regime of near-future pulsar timing arrays.

astro-ph.HE↗

arXiv2020

The role of some collisional processes in AGNs: rate coefficients needed for modeling

The importance of some atom hydrogen collisions in AGN has been investigated. The results are useful for better estimate of the hydrogen Balmer lines uxes, which usage for effective temperature diagnostics in astrophysical plasma is limited by errors from the line formation models. The data could be also useful for modeling cooler and denser parts of AGN BLR clouds, as well as for the investigation of Rydberg states of hydrogen and for the study of their in uence during the cosmological recombination epoch. The results of the present work suggest that the investigated processes are of interest for the research and modelling of such media.

astro-ph.GA↗

arXiv2018

The collisional atomic processes of Rydberg alkali atoms in Geo-cosmical plasmas

The chemi-ionization (CI) processes in atom-Rydberg-atom collisions are investigated in this contribution. The rate coefficients for CI processes in $\textrm{Li}^{*}(n) + \textrm{Na}$, $\textrm{Li}^{*}(n) + \textrm{Li}$, $\textrm{Na}^{*}(n) + \textrm{Na}$ and $\textrm{H}^{*}(n)+\textrm{Li}$ collisions are calculated for wide region of temperatures $T \le 4000$ K and the principal quantum numbers $4 \le n\le 25$. These processes are investigated, with a particular accent on the possibility of involving alkali metals as the factors which may influence optical properties and modeling of weakly ionized layers of different stellar atmospheres. The obtained rate coefficients are applied to models of the Io atmosphere. Moreover, we present here further development of investigation of CI processes for other possible applications in spectroscopy such as in low temperature laboratory plasma created in gas discharges, for example in microwave-induced discharges at atmospheric pressure, where such plasma conditions may be favorable.

astro-ph.SR↗

arXiv2017

Astrophysical and laboratory plasmas: HF properties under extreme conditions

The values of electrical conductivity of plasma of stars with a magnetic field or moving in the magnetic field of the other component in a binary system could be of significant interest, since they are useful for the study of thermal evolution of such objects, cooling, nuclear burning of accreted matter, and the investigation of their magnetic fields. So, on the basis of numerically calculated values for the dense plasma conductivity in an external HF electric field, we determine the HF characteristics of astrophysical plasmas under extreme conditions. The examined range of frequencies covers the IR, visible and near UV regions and consider electronic number density and temperature are in the ranges of $10^{21} \textrm{cm}^{-3} \le Ne$ and 20 000 $\le T$, respectively. The method developed here represents a powerful tool for research into white dwarfs with different atmospheric compositions (DA, DC etc.), and for investigation of some other stars (M-type red dwarfs, Sun etc.).

physics.plasm-ph↗

arXiv2016

Dynamics Resonances in Atomic States of Astrophysical Relevance

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 photons energy transfer in a cold atoms medium, should include elements of stochastic dynamics. Finally, the chaotic dynamics of a weakly bound Rydberg electron over a grid of the energy levels 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.

physics.atom-ph↗

arXiv2014

The ion-atom absorption processes as one of the factors of the influence on the sunspot opacity

As a continuation of the previous investigations of the symmetric and strongly non-symmetric ion-atom absorption processes in the far UV region within the models of the quiet Sun photosphere, these processes are studied here within a model of the sunspot. Here we mean the absorption processes in the H$(1s)$+H$^{+}$ and H$(1s)+X^{+}$ collisions and the processes of the photo-dissociation of the H$_{2}^{+}$ and H$X^{+}$ molecular ions, where $X$ is one of the metal atoms: $X=$Na, Ca, Mg, Si and Al. Obtained results show that the influence of the considered ion-atom absorption processes on the opacity of sunspots in the considered spectral region (110 nm $\lesssim λ\lesssim$ 230 nm) is not less and in some parts even larger than the influence of the referent electron-atom processes. In such a way, it is shown that the considered ion-atom absorption processes should be included \emph{ab initio} in the corresponding models of sunspots of solar-type and near solar-type stars. Apart of that, the spectral characteristics of the considered non-symmetric ion-atom absorption processes (including here the case $X$ = Li), which can be used in some further applications, have been determined and presented within this work.

astro-ph.SR↗

arXiv2014

Absorption non-symmetric ion-atom processes in helium-rich white dwarf atmospheres

In this work the processes of absorption charge-exchange and photo-association in He+H$^{+}$ collisions together with the process of ion HeH$^{+}$ photo-dissociation are considered as factors of influence on the opacity of the atmospheres of helium-rich white dwarfs in the far UV and EUV region. It is shown that they should be taken into account even in the cases of the atmospheres of white dwarfs with H:He =$10^{-5}$. Than, it is established that in the cases of white dwarfs with H:He $\gtrsim 10^{-4}$, particulary when H:He $\approx 10^{-3}$, these processes have to be included \emph{ab initio} in the corresponding models of their atmospheres, since in the far UV and EUV region they become dominant with respect to the known symmetric ion-atom absorption processes.

astro-ph.SR↗

arXiv2013

Ion-atom radiative processes in the solar atmosphere: quiet Sun and sunspots

In the previous works the significance of the symmetric and non-symmetric ion-atom absorption processes in far UV and EUV regions within a model of the quiet Sun atmosphere, has been studied. The considered processes were the processes of the photo-dissociation of the molecular ions H$_{2}^{+}$ and H$X^{+}$ and absorption processes in H+H$^{+}$ and H$+X^{+}$ collisions, where $X$ denotes the metal atom. As the continuation of the previous investigation, these processes are considered also within the corresponding sunspot model. In this work the non-symmetric ion-atom absorption processes are considered with $X=$ Mg, Si, etc. It was analyzed the significance of such processes in far UV and EUV regions in comparison with the concurrent absorption processes, especially with the processes of the photo-ionization of the metal atoms (Na, Mg, Ca, etc.) which were not included in the consideration in the case of the quiet Sun atmosphere. From our analysis it follows that the non-symmetric ion-atom absorption processes considered here, are significant not only for quiet Sun modeling but also for sunspots and should be included {\it ab initio} in both cases.

astro-ph.SR↗

arXiv2013

The influence of the radiative non-symmetric ion-atom collisions on the stellar atmospheres in VUV region

The aim of this work is to draw attention to the processes of radiative charge exchange in non-symmetric ion-atom collisions as a factor of influence on the opacity of stellar atmospheres in VUV region. For that purpose calculations of the spectral absorption coefficients for several ion-atom systems, namely: He + H$^{+}$ and H + X$^{+}$, where X = Na and Li have been performed. On chosen examples it has been established that the examined processes generate rather wide molecular absorption bands in the VUV region, which should be taken into account for the interpretation of data obtained from laboratory measurements or astrophysical observations. In this paper the potential significance is discussed of the considered radiative processes for DB white dwarfs and solar atmospheres, as well as for the atmospheres of the so-called lithium stars.

astro-ph.SR↗

arXiv2013

Excitation and deexcitation processes in atom-Rydberg atom collisions in helium-rich white dwarf atmospheres

We aim to show the importance of non-elastic excitation and deexcitation processes in $\textrm{He}^{*}(n)+ \textrm{He}(1s^{2})$ collisions with the principal quantum number $n \ge 3$ for helium-rich white dwarf atmospheres. We compare the efficiencies of these processes with those of the known non-elastic electron-$\textrm{He}^{*}(n)$ atom processes in the atmospheres of some DB white dwarfs. We show that in significant parts of the considered atmospheres, which contain weakly ionized layers (the ionization degree $\lesssim 10^{-3}$), the influence of the studied atom-Rydberg atom processes on excited helium atom populations is dominant or at least comparable to the influence of the concurrent electron-$\textrm{He}^{*}(n)$-atom processes.

astro-ph.SR↗

arXiv2013

The non-symmetric ion-atom radiative processes in the stellar atmospheres

The aim of this research is to show that the processes of absorption charge-exchange and photo-association in $A+B^{+}$ collisions together with the processes of $AB^{+}$ photo-dissociation in the case of strongly non-symmetric ion-atom systems, significantly influence the opacity of stellar atmospheres in ultraviolet (UV) and extreme UV (EUV) region. In this work, the significance of such processes for solar atmosphere is studied. In the case of the solar atmosphere the absorption processes with $A=$ H and $B=$ Mg and Si are treated as dominant ones, but the cases $A=$ H and $B=$ Al and $A=$ He and $B=$ H are also taken into consideration. The choice of just these species is caused by the fact that, of the species relevant for the used solar-atmosphere model, it was only for them that we could determine the necessary characteristics of the corresponding molecular ions, i.e. the molecular potential curves and dipole matrix elements. It is shown that the efficiency of the examined non-symmetric processes within the rather wide corresponding quasi-molecular absorption bands in the far-UV and EUV regions is comparable and sometimes even greater than the intensity of the known symmetric ion-atom absorption processes, which are included now in the models of the solar atmosphere. Consequently, the presented results suggest that the non-symmetric ion-atom absorption processes also have to be included \emph{ab initio} in the corresponding models of the stellar atmospheres.

astro-ph.SR↗

arXiv2012

Electrical conductivity of plasmas of DB white dwarf atmospheres

The static electrical conductivity of non-ideal, dense, partially ionized helium plasma was calculated over a wide range of plasma parameters: temperatures $1\cdot 10^{4}\textrm{K} \lesssim T \lesssim 1\cdot 10^{5}\textrm{K}$ and mass density $1 \times 10^{-6} \textrm{g}/\textrm{cm}^{3} \lesssim ρ\lesssim 2 \textrm{g}/\textrm{cm}^{3}$. Calculations of electrical conductivity of plasma for the considered range of plasma parameters are of interest for DB white dwarf atmospheres with effective temperatures $1\cdot 10^{4}\textrm{K} \lesssim T_{eff} \lesssim 3\cdot 10^{4}\textrm{K}$. Electrical conductivity of plasma was calculated by using the modified random phase approximation and semiclassical method, adapted for the case of dense, partially ionized plasma. The results were compared with the unique existing experimental data, including the results related to the region of dense plasmas. In spite of low accuracy of the experimental data, the existing agreement with them indicates that results obtained in this paper are correct.

astro-ph.SR↗

arXiv2012

Stark Broadening of in III Lines in Astrophysical and Laboratory Plasma

Besides the need of Stark broadening parameters for a number of problems in physics, and plasma technology, in hot star atmospheres the conditions exist where Stark widths are comparable and even larger than the thermal Doppler widths. Using the semiclassical perturbation method we investigated here the influence of collisions with charged particles for In III spectral lines. We determined a number of Stark broadening parameters important for the investigation of plasmas in the atmospheres of A-type stars and white dwarfs. Also, we have compared the obtained results with existing experimental data. The results will be included in the STARK-B database, the Virtual Atomic and Molecular Data Center and the Serbian Virtual Observatory.

astro-ph.SR↗

arXiv2006

The flux ratio of the [OIII] 5007,4959 lines in AGN: Comparison with theoretical calculations

By taking into account relativistic corrections to the magnetic dipole operator, the theoretical [OIII] 5006.843/4958.511 line intensity ratio of 2.98 is obtained. In order to check this new value using AGN spectra we present the measurements of the flux ratio of the [OIII] 4959,5007 emission lines for a sample of 62 AGN, obtained from the Sloan Digital Sky Survey (SDSS) Database and from published observations. We select only high signal-to-noise ratio spectra for which the line shapes of the [OIII] 4959,5007 lines are the same. We obtained an averaged flux ratio of 2.993 +/- 0.014, which is in a good agreement with the theoretical one.

astro-ph↗

arXiv2005

On the influence of Stark broadening on Cr I lines in stellar atmospheres

Using the semiclassical perturbation method, electron-, proton-, and ionized helium-impact line widths and shifts for the nine Cr I spectral lines from the $4p^7P^0-4d^7D$ multiplet, have been calculated for a perturbers density of 10$^{14}$ cm$^{-3}$ and temperatures T =2,500 - 50,000 K. The obtained results have been used to investigate the influence of Stark broadening effect in the Cr-rich Ap star $β$ CrB atmosphere on line shapes of these lines. It has been found that the contribution of the proton and He II collisions to the line width and shift is significant, and it is comparable and sometimes (depending of the electron temperature) even larger than electron-impact contribution. Moreover, not only the Stark line width, but, depending on the electron-, proton-, and He\ii density in stellar atmosphere also the Stark shift may contribute to the blue as well as to the red asymmetry of the same line. The obtained results have been used to investigate the influence of Stark broadening effect on line shapes of Crılines in the atmosphere of Cr-rich Ap star $β$ CrB.

astro-ph↗

arXiv2003

On the influence of Stark broadening on Si I lines in stellar atmospheres

We study the influence of Stark broadening and stratification effects on Siılines in the rapidly oscillating (roAp) star 10 Aql, where the Siı6142.48 Åand 6155.13 Ålines are asymmetrical and shifted. First we have calculated Stark broadening parameters using the semiclassical perturbation method for three Siılines: 5950.2 Å, 6142.48 Åand 6155.13 Å. We revised the synthetic sp$ calculation code taking into account both Stark width and shift for these lines. From the comparison of our calculations with the observations we found that Stark broadening + the stratification effect can explain asymmetry of the Siı6142.48 Åand 6155.13 Ålines in the atmospere of roAp star 10 Aql.

astro-ph↗

arXiv1998

A programme for electron-impact broadening parameter calculations of ionized rare-earth element lines

In order to provide atomic data needed for astrophysical investigations, a set of electron-impact broadening parameters for ionized rare-earth element lines should be calculated. We are going to calculate the electron-impact broadening parameters for more than 50 transitions of ionized rare-earth elements. Taking into account that the spectra of these elements are very complex, for calculation we can use the modified semiempirical approach - MSE or simplified MSE. Also, we can estimate these parameters on the basis of regularities and systematic trends.

astro-ph↗