arXiv subjects

Anil K. Pradhan

Publications and source records attributed to Anil K. Pradhan.

At least 19 recordsLinked to original sources

arXiv2026

Emissivity line ratios for [Mn III] and spectral diagnostics of H II regions

We present the first theoretical emission-line study of Mn III as potential determinant of physical conditions and manganese abundance. We compute atomic data for a set of Mn III lines and line emissivity ratios as temperature-density diagnostics. Accurate relativistic Breit-Pauli R-matrix calculations have been carried out for electron impact excitation of Mn III transitions that have not been previously studied. Selected forbidden lines are shown to be sensitive to temperature and density under nebular conditions in H II regions such as SNRs. The coupled channel R-matrix calculations include a wavefunction expansion that includes 38 levels of Mn III dominated by the ground and first excited electronic configurations $3d^5$ and $3d^44s$. Collision strengths for all 703 forbidden transitions among those levels, and Maxwellian averaged rate coefficients are obtained. A collisional-radiative model yields emissivity line ratios using calculated collision strengths and radiative decay A-values. Based on the present analysis, certain [Mn III] lines are predicted to be sensitive to density and/or in the typical nebular range. The detection of those lines could also yield cosmic Mn abundance relative to other elements.

astro-ph.GA↗

arXiv2025

The Opacity Project: R-Matrix Calculations for Opacities of High-Energy-Density Astrophysical and Laboratory Plasmas

Accurate determination of opacity is critical for understanding radiation transport in both astrophysical and laboratory plasmas. We employ atomic data from R-Matrix calculations to investigate radiative properties in high-energy-density (HED) plasma sources. Specifically, we analyze environments such as the base of the convective zone (BCZ) of the Sun 2 x 10^6$ K, N_e = 10^{23}/cc and the inertial confinement fusion (ICF) device at the Sandia Z facility 2.11 x 10^6 K, N_e = 3.16 x 10^{22}/cc. We calculate Rosseland Mean Opacities (RMO) within a range of temperatures and densities and analyze how they vary under different plasma conditions. In this study, we specifically focus on electron collisional and Stark ion microfield broadening effects on autoionizing resonances in photoabsorption cross sections. Our results are relevant to astrophysical models, particularly in the context of the solar opacity problem, and provide insights into discrepancies between theoretical calculations and experimental measurements. In addition, we investigate the equation-of-state (EOS) and its impact on opacities. In addition, we examine the equation-of-state (EOS) and its impact on opacities of the "chemical picture" Mihalas-Hummer-Dappen EOS with respect to level populations of excited levels included in the R-matrix calculations. This study should contribute to improving opacity models of HED sources such as stellar interiors adn laboratory fusion plasma experiments.

astro-ph.HE↗

arXiv2023

Interface of Equation-of-State, Atomic Data and Opacities in the Solar Problem

Convergence of the Rosseland Mean Opacity (RMO) is investigated with respect to the equation-of-state (EOS) and the number of atomic levels of iron ions prevalent at the solar radiative/convection boundary. The "chemical picture" Mihalas-Hummer-Däppen MHD-EOS, and its variant QMHD-EOS, are studied at two representative temperature-density sets at the base of the convection zone (BCZ) and the Sandia Z experiment: $(2 \times 10^6K, \ 10^{23}/cc)$ and $(2.11 \times 10^6K, \ 3.16 \times 10^{22}/cc)$, respectively. It is found that whereas the new atomic datasets from accurate R-matrix calculations for opacities (RMOP) are vastly overcomplete, involving hundreds to over a thousand levels of each of the three Fe ions considered -- FeXVII, FeXVIII and FeXIX -- the EOS constrains contributions to RMOs by relatively fewer levels. The RMOP iron opacity spectrum is quite different from the Opacity Project distorted wave model and shows considerably more plasma broadening effects. This work points to possible improvements needed in the EOS for opacities in high-energy-density (HED) plasma sources.

astro-ph.SR↗

arXiv2023

Biosignature Line Ratios of [P II] in Exoplanetary and Nebular Environments

Being the backbone element of DNA, phosphorus is a key component in the search for life in the Universe. To aid in its detection, we present line emissivity ratios for the five lowest-lying forbidden [P~II] transitions, namely those among the levels $3s^23p^2(^3P_0,^3P_1, ^3P_2,^1D_2,^1S_0)$. The wavelengths range between 0.44-70 \mum, and several lie within the spectroscopic domain observable with the James Webb Space Telescope (JWST). These line ratios have been calculated using a new collisional-radiative-recombination (CRR) model combining calculated collision strengths and level-specific recombination rate coefficients; with both datasets computed using the accurate Breit-Pauli R-Matrix method. The CRR model includes a new scheme for \eion recombination to emission line formation. We compare its effect to models incorporating only electron impact excitation and spontaneous radiative decay. We find that electron-ion recombination has a significant impact on all line ratios, and represents a major improvement in physical accuracy of emission line models.

astro-ph.EP↗

arXiv2018

Collision strengths for FIR and UV transtions in PIII and the phosphorus abundance

Phosphorus abundance is crucial for DNA-based extraterrestrial life in exoplanets. Atomic data for observed spectral lines of P-ions are needed for its accurate determination. We present the first calculations for collision strengths for the forbidden PIII fine structure transition $3s^23p (^2P^o_{1/2-3/2})$ within the ground state at 17.9 $μ$m, as well as allowed UV transitions in the $3s^23p (^2P^o_{1/2,3/2}) \rightarrow 3s3p^2 (^2D_{3/2,5/2}, ^2S_{1/2}, ^2P_{1/2,3/2})$ multiplets between 915-1345 $Å$. Collision strengths are computed using the Breit-Pauli R-Matrix method including the first 18 levels, and they exhibit extensive auto-ionizing resonance structures. In particular, the Maxwellian averaged effective collision strength for the FIR 17.9 $μ$m transition shows a factor 3 temperature variation broadly peaking at typical nebular temperatures. Its theoretical emissivity with solar phosphorus abundance is computed relative to H$β$ and found to be similar to observed intensties from planetary nebulae; the abundances derived in earlier works are 3-5 times sub-solar. The results pertain to the reported paucity of phosphorus from preferred production sites in supernovae, and abundances in planetary nebulae and supernova remnants.

astro-ph.SR↗

arXiv2018

Converged Close-Coupling R-Matrix calculations of Photoionization of Fe XVII in Astrophysical Plasmas: from Convergence to Completeness

Extensive resonance structures are manifest in R-Matrix (RM) calculations. However, there exist a large number of highly excited electronic configurations that may contribute to background non-resonant bound-free opacity in high-temperature plasmas. Since RM calculations are very complex, and not essential for background contributions, the Relativistic Distorted Wave (RDW) method is utilized to complement ("top-up") photoionization cross sections of Fe XVII obtained using Close-Coupling Breit-Pauli R-Matrix (CC-BPRM) method. There is good agreement between RDW and BPRM for background cross sections where resonances are not present, and individual fine structure levels can be correctly matched spectroscopically, though resonances are neglected in the RDW. To ensure completeness, a high energy range up to 500 Ry above the ionization threshold for each level is considered. Interestingly, the hydrogenic Kramer's approximation also shows the same energy behavior as the RDW. Grouping separately, the BPRM configurations consist of 454 bound levels with resonances corresponding to configurations $1s^22s^22p^4nln'l'$ (n $\leq$ 3, n' $\leq$ 10); including RDW configurations there are 51,558 levels in total. The topup contribution results in $\sim$20\% increment, in addition to the 35\% enhancement from BPRM calculations over the Opacity Project value for the Rosseland Mean Opacity at the Z-temperature of 2.11 $\times 10^6$K (Pradhan and Nahar 2017).

astro-ph.SR↗

arXiv2018

Recalculation of Astrophysical Opacities: Overview, Methodology and Atomic Calculations

A review of a renewed effort to recalculate astrophysical opacities using the R-Matrix method is presented. The computational methods and new extensions are described. Resulting enhancements found in test calculations under stellar interior conditions compared to the Opacity Project could potentially lead to the resolution of the solar abundances problem, as well as discrepancies between recent experimental measurements at the Sandia Z-pinch inertial confinement fusion device and theoretical opacity models. Outstanding issues also discussed are: (i) accuracy, convergence, and completeness of atomic calculations, (ii) improvements in the Equation-of-State of high-temperature-density plasmas, and (iii) redistribution of resonant oscillator strength in the bound-free continuum, and (iv) plasma broadening of auotionizing resonances.

astro-ph.SR↗

arXiv2016

Large enhancement in high-energy photoionization of Fe XVII and missing continuum plasma opacity

Aimed at solving the outstanding problem of solar opacity, and radiation transport plasma models in general, we report substantial photoabsorption in the high-energy regime due to atomic core photo-excitations not heretofore considered. In extensive R-Matrix calculations of unprecedented complexity for an important iron ion Fe xvii (Fe$^{16+}$), with a wave function expansion of 99 Fe xviii (Fe$^{17+}$) LS core states from $n \leq 4$ complexes (equivalent to 218 fine structure levels), we find: i) up to orders of magnitude enhancement in background photoionization cross sections, in addition to strongly peaked photo-excitation-of-core resonances not considered in current opacity models, and ii) demonstrate convergence with respect to successive core excitations. The resulting increase in the monochromatic continuum, and 35% in the Rosseland Mean Opacity, are compared with the "higher-than-predicted" iron opacity measured at the Sandia Z-pinch fusion device at solar interior conditions.

astro-ph.SR↗

arXiv2013

Fine structure collision strengths and line ratios for [Ne V] in infrared and optical sources

Improved collisions strengths for the mid-infrared and optical transitions in Ne V are presented. Breit-Pauli R-Matrix calculations for electron impact excitation are carried out with fully resolved near-threshold resonances at very low energies. In particular, the fine structure lines at 14 micron and 24 micron due to transitions among the ground state levels 1s^22s^22p^3 (^3P_{0,1,2}), and the optical/near-UV lines at 2973, 3346 and 3426 Angstrom transitions among the ^3P_{0,1,2}, ^1D_2, ^1S_0 levels are described. Maxwellian averaged collision strengths are tabulated for all forbidden transistion within the ground configuration. Significant differences are found in the low temperature range Te < 10000 K for both the FIR and the opitcal transitions compared to previous results. An analysis of the 14/24 line ratio in low-energy-density (LED) plasma conditions reveals considerable variation; the effective rate coefficient may be dominated by the very low-energy behaviour rather than the maxwellian averaged collision strengths. Computed values suggest a possible solution to the anomalous mid-IR ratios found to be lower than theoretical limits observed from planetary nebulae and Seyfert galaxies. While such LED conditions may be present in infrared sources, they might be inconsistent with photoionization equilibrium models.

physics.atom-ph↗

arXiv2012

Pumping K-Alpha Resonance Fluorescence by Monochromatic X-Ray Sources

We demonstrate the correspondence between theoretically calculated K-shell resonances lying below the K-edge in multiple ionization states of an element (Pradhan et al. 2009), and recently observed K-alpha resonances in high-intensity X-ray free-electron laser (XFEL) plasmas (Vinko et al. 2012). Resonant absorptions in aluminum ions are computed and found to reproduce experimentally observed features. Results are also presented for titanium for possible observation of K-alpha resonances in the 4.5-5.0 keV energy range. A possibly sustainable excitation mechanism for K-alpha resonance fluorescence might be implemented using two monochromatic X-ray beams tuned to the K-edge and the K-alpha resonant energies simultaneously. This targeted ionization/excitation would create inner-shell vacancies via Auger decay, as well as pump K-alpha resonances. The required X-ray fluence to achieve resonance fluorescence would evidently be much less than in the XFEL experiments, and might enable novel biomedical applications.

physics.atm-clus↗

arXiv2012

Improved collision strengths and line ratios for forbidden [O III] far-infrared and optical lines

Far-infrared and optical [O III] lines are useful temeprature-density diagnostics of nebular as well as dust obscured astrophysical sources. Fine structure transitions among the ground state levels 1s^22s^22p^3 \ ^3P_{0,1,2} give rise to the 52 and 88 micron lines, whereas transitions among the $^3P_{0,1,2}, ,^1D_2, ^1S_0$ levels yield the well-known optical lines 4363, 4959 and 5007 Angstroms. These lines are excited primarily by electron impact excitation. But despite their importance in nebular diagnostics collision strengths for the associated fine structure transitions have not been computed taking full account of relativistic effects. We present Breit-Pauli R-matrix calculations for the collision strengths with highly resolved resonance structures. We find significant differences of up to 20% in the Maxwellian averaged rate coefficients from previous works. We also tabulate these to lower temperatures down to 100 K to enable determination of physical conditions in cold dusty environments such photo-dissociation regions and ultra-luminous infrared galaxies observed with the Herschel space observatory. We also examine the effect of improved collision strengths on temperature and density sensitive line ratios.

astro-ph.CO↗

arXiv2011

Highly Excited Core Resonances in Photoionization of Fe XVII : Implications for Plasma Opacities

A comprehensive study of high-accuracy photoionization cross sections is carried out using the relativistic Breit-Pauli R-matrix (BPRM) method for (hnu + Fe XVII --> Fe XVIII + e). Owing to its importance in high-temperature plasmas the calculations cover a large energy range, particularly the myriad photoexciation-of-core (PEC) resonances including the n = 3 levels not heretofore considered. The calculations employ a close coupling wave function expansion of 60 levels of the core ion Fe XVIII ranging over a wide energy range of nearly 900 eV between the n = 2 and n = 3 levels. Strong coupling effects due to dipole transition arrays 2p^5 --> 2p^4 (3s,3d) manifest themselves as large PEC resonances throughout this range, and enhance the effective photoionization cross sections orders of magnitude above the background. Comparisons with the erstwhile Opacity Project (OP) and other previous calculations shows that the currently available cross sections considerably underestimate the bound-free cross sections. A level-identification scheme is used for spectroscopic designation of the 454 bound fine structure levels of Fe XVII. Level-specific photoionization cross sections are computed for all levels. In addition, partial cross sections for leaving the core ion Fe XVII in the ground state are also obtained. These results should be relevant to modeling of astrophysical and laboratory plasma sources requiring (i) photoionization rates, (ii) extensive non-local-thermodynamic-equilibrium models, (iii) total unified electron-ion recombination rates including radiative and dielectronic recombination, and (iv) plasma opacities. We particularly examine PEC and non-PEC resonance strengths and emphasize their expanded role to incorporate inner-shell excitations for improved opacities, as shown by the computed monochromatic opacity of Fe XVII.

astro-ph.SR↗

arXiv2008

XMM-Newton observations of the Narrow-Line Seyfert 1 galaxy Mrk 335 in an historical low X-ray flux state

We report the discovery of strong soft X-ray emission lines and a hard continuum above 2 keV in the Narrow-Line Seyfert 1 galaxy Mrk 335 during an extremely low X-ray flux state. Mrk 335 was observed for 22 ks by XMM-Newton in July 2007 as a Target of Opportunity to examine it in its X-ray low-flux state, which was discovered with Swift. Long-term light curves suggest that this is the lowest flux state this AGN has ever been seen in. However, Mrk 335 is still sufficiently bright that its X-ray properties can be studied in detail. The X-ray continuum spectrum is very complex and requires several components to model. Statistically, partial covering and blurred reflection models work well. We confirm the presence of a strong narrow Fe line at 6.4 keV. High-resolution spectroscopy with the XMM-RGS reveals strong, soft X-ray emission lines not detected in previous, higher signal-to-noise, XMM-Newton observations, such as: highly ionized Fe lines, O VII, Ne IX and Mg XI lines. The optical/UV fluxes are similar to those previously measured with Swift. Optical spectroscopy taken in 2007 September do not show any changes to optical spectra obtained 8 years earlier.

astro-ph↗

arXiv2007

A study of the breakdown of the quasi-static approximation at high densities and its effect on the helium-like K ALPHA complex of nickel, iron, and calcium

The General Spectral Modeling (GSM) code employs the quasi-static approximation, a standard, low-density methodology that assumes the ionization balance is separable from a determination of the excited-state populations that give rise to the spectra. GSM also allows for some states to be treated only as contributions to effective rates. While these two approximations are known to be valid at low densities, this work investigates using such methods to model high-density, non-LTE emission spectra and determines at what point the approximations break down by comparing to spectra produced by the LANL code ATOMIC which makes no such approximations. As both approximations are used by other astrophysical and low-density modeling codes, the results should be of broad interest. He-like K$α$ emission spectra are presented for Ni, Fe, and Ca, in order to gauge the effect of both approximations employed in GSM. This work confirms that at and above the temperature of maximum abundance of the He-like ionization stage, the range of validity for both approximations is sufficient for modeling the low- and moderate-density regimes one typically finds in astrophysical and magnetically confined fusion plasmas. However, a breakdown does occur for high densities; we obtain quantitative limits that are significantly higher than previous works. This work demonstrates that, while the range of validity for both approximations is sufficient to predict the density-dependent quenching of the z line, the approximations break down at higher densities. Thus these approximations should be used with greater care when modeling high-density plasmas such as those found in inertial confinement fusion and electromagnetic pinch devices.

physics.atom-ph↗

arXiv2006

The Fe-Line Feature In The X-Ray Spectrum of Solar Flares: First Results From The SOXS Mission

We present the first results from the "Low Energy Detector" payload of the "Solar X-ray Spectrometer (SOXS)" mission, which was launched onboard the GSAT-2 Indian spacecraft on 08 May 2003 by the GSLV-D2 rocket to study solar flares. The SOXS Low Energy Detector (SLD) payload was designed, developed, and fabricated by the Physical Research Laboratory (PRL) in collaboration with the Space Application Centre (SAC), Ahmedabad and the ISRO Satellite Centre (ISAC), Bangalore of Indian Space Research Organization (ISRO). The energy ranges of the Si PIN and CZT detectors are 4 - 25 keV and 4 - 56 keV respectively. The Si PIN provides sub-keV energy resolution while the CZT reveals ~1.7 keV energy resolution throughout the energy range. The high sensitivity and sub-keV energy resolution of the Si PIN detector allows measuring the intensity, peak energy, and the equivalent width of the Fe-line complex at approximately 6.7 keV as a function of time in all ten M-class flares studied in this investigation. The peak energy (Ep) of the Fe-line feature varies between 6.4 and 6.7 keV with increasing in temperature from 9 to 58 MK. We found that the equivalent width (w) of the Fe-line feature increases exponentially with temperature up to 30 MK and then increases very slowly up to 40 Mk. It remains between 3.5 and 4 keV in the temperature range of 30 - 45 MK. We compare our measurements of w with calculations made earlier by various investigators and propose that these measurements may improve theoretical models. We interpret the variation of both Ep and w with temperature as due to the changes in the ionization and recombination conditions in the plasma during the flare interval and as a consequence the contribution from different ionic emission lines also varies.

astro-ph↗

arXiv2006

Electron impact excitation of Helium-like ions up to n=4 levels including radiation damping

Helium-like ions provide the most important X-ray spectral diagnostics in high temperature fusion and astrophysical plasmas. We previously presented computed collision strengths for O~VII including relativistic fine structure, levels up to the $n=4$ complex and radiation damping of autoionizing resonances. We have extended this work to other He-like ions (N, Ne, Mg, Al, Si, S, Ca). The calculations are carried out using the Breit-Pauli R-matrix (BPRM) method with a 31-level eigenfunction expansion. Collision strengths for the principal lines important in X-ray plasma diagnostics, w, x, y and z, corresponding to the 4 transitions to the ground level 1s^2(^1S_0) <- 1s2p(^1P^o_1), 1s2p(^3P^o_2), 1s2p(^3P^o_1), 1s2s(^3S_1), are explicitly shown. We find the effect of radiation damping to be significant for the forbidden transitions in heavier He-like ions, which should affect the diagnostic line ratios. We extrapolated the collision strengths to their values at infinite energy using the Burgess-Tully extrapolation technique. This is required to calculate the Maxwellian average collision strengths at high temperature. We show that the coupling between dipole allowed and inter-combination transitions affects increasingly the effective collision strengths for the n ^1S_0 - n' ^3P_1 transition as the charge of the ion increases. This clearly affects the treatment of the extrapolation toward the infinite energy point of the collision strength. This work is carried out as part of the Iron Project-RmaX Network.

astro-ph↗

arXiv2005

Electron distributions in X-Ray plasmas: spectral diagnostics with the 3s/3d line ratio in Fe XVII

Efforts to benchmark astrophysical observations with X-ray laboratory measurements have been stymied by observed and measured differences of up to a factor of two in the ratio '3s/3d' of Fe XVII lines at ~17 Åand ~15 Årespectively. Using the electron distribution function (EDF) as a new physical parameter, we compute the Fe XVII line ratios and account for these differences. Based on large-scale relativistic close coupling calculations using the Breit-Pauli R-matrix method, revealing the precise effect of resonances in collisional excitation, we employ collisional-radiative models using cross sections convolved with both the Gaussian and the Maxwellian EDF. Comparison with astrophysical observations and laboratory measurements demonstrates that (a) the 3s/3d line ratio depends not only on the EDF but also on the electron temperature/energy of the source, (b) plasma conditions in experimental measurements and astrophysical observations may be quite different, and (c) departure from a Maxwellian should manifest itself in, and be used as a diagnostics of, particle distributions in plasmas.

astro-ph↗

arXiv2005

[O II] line ratios

Based on new calculations we reconfirm the low and high density limits on the forbidden fine structure line ratio [O II] I(3729)/I(3726): lim_{N_ e} --> 0} = 1.5 and lim_{N_ e} --> \infty} = 0.35. Employing [O II] collision strengths calculated using the Breit-Pauli R-matrix method we rule out any significant deviation due to relativistic effects from these canonical values. The present results are in substantial agreement with older calculations by Pradhan (1976) and validate the extensive observational analysis of gaseous nebulae by Copetti and Writzel (2002) and Wang et al (2004) that reach the same conclusions. The present theoretical results and the recent observational analyses differ significantly from the calculations by MacLaughlin and Bell (1998) and Keenan et al (1999). The new maxwellian averaged effective collision strengths are presented for the 10 transitions among the first 5 levels to enable computations of [O II] line ratios.

astro-ph↗