Emission Lines In X-ray Spectra Research Articles

Supercritically charged objects and electron-positron pair creation

We investigate the stability and $e^+e^-$ pair creation of supercritically charged superheavy nuclei, $ud$QM nuggets, strangelets, and strangeon nuggets based on Thomas-Fermi approximation. The model parameters are fixed by reproducing their masses and charge properties reported in earlier publications. It is found that $ud$QM nuggets, strangelets, and strangeon nuggets may be more stable than ${}^{56}$Fe at $A\gtrsim 315$, $5\times10^4$, and $1.2\times10^8$, respectively. For those stable against neutron emission, the most massive superheavy element has a baryon number $\sim$965, while $ud$QM nuggets, strangelets, and strangeon nuggets need to have baryon numbers larger than $39$, 433, and $2.7\times10^5$. The $e^+e^-$ pair creation will inevitably start for superheavy nuclei with charge numbers $Z\geq177$, $ud$QM nuggets with $Z\geq163$, strangelets with $Z\geq 192$, and strangeon nuggets with $Z\geq 212$. A universal relation $Q/R_e = \left(m_e - \bar{\mu}_e\right)/\alpha$ is obtained at a given electron chemical potential $\bar{\mu}_e$, where $Q$ is the total charge and $R_e$ the radius of electron cloud. This predicts the maximum charge number by taking $\bar{\mu}_e=-m_e$. For supercritically charged objects with $\bar{\mu}_e<-m_e$, the decay rate for $e^+e^-$ pair production is estimated based on the JWKB approximation. It is found that most positrons are emitted at $t\lesssim 10^{-15}$ s, while a long lasting positron emission is observed for large objects with $R\gtrsim 1000$ fm. The emission and annihilation of positrons from supercritically charged objects may be partially responsible for the short $\gamma$-ray burst during the merger of binary compact stars, the 511 keV continuum emission, as well as the narrow faint emission lines in X-ray spectra from galaxies and galaxy clusters.

Signatures of the non-Maxwellian κ-distributions in optically thin line spectra

Aims. We investigated the possibility of diagnosing the degree of departure from the Maxwellian distribution using the Fe XVII–Fe XVIII spectra originating in plasmas in collisional ionization equilibrium, such as in the cores of solar active regions or microflares. Methods. The original collision strengths for excitation are integrated over the non-Maxwellian electron κ-distributions characterized by a high-energy tail. Synthetic X-ray emission line spectra were calculated for a range of temperatures and κ. We focus on the 6–24 Å spectral range to be observed by the upcoming Marshall Grazing-Incidence X-ray Spectrometer MaGIXS. Results. We find that many line intensity ratios are sensitive to both T and κ. Best diagnostic options are provided if a ratio involving both Fe XVII and Fe XVIII is combined with another ratio involving lines formed within a single ion. The sensitivity of such diagnostics to κ is typically a few tens of per cent. Much larger sensitivity, of about a factor of two to three, can be obtained if the Fe XVIII 93.93 Å line observed by SDO/AIA is used in conjuction with the X-ray lines. Conclusions. We conclude that the MaGIXS instrument is well-suited for detection of departures from the Maxwellian distribution, especially in active region cores.

The independent pulsations of Jupiter’s northern and southern X-ray auroras

Auroral hot spots are observed across the Universe at different scales 1 and mark the coupling between a surrounding plasma environment and an atmosphere. Within our own Solar System, Jupiter possesses the only resolvable example of this large-scale energy transfer. Jupiter’s northern X-ray aurora is concentrated into a hot spot, which is located at the most poleward regions of the planet’s aurora and pulses either periodically 2,3 or irregularly 4,5 . X-ray emission line spectra demonstrate that Jupiter’s northern hot spot is produced by high charge-state oxygen, sulfur and/or carbon ions with an energy of tens of MeV (refs 4–6 ) that are undergoing charge exchange. Observations instead failed to reveal a similar feature in the south 2,3,7,8 . Here, we report the existence of a persistent southern X-ray hot spot. Surprisingly, this large-scale southern auroral structure behaves independently of its northern counterpart. Using XMM-Newton and Chandra X-ray campaigns, performed in May–June 2016 and March 2007, we show that Jupiter’s northern and southern spots each exhibit different characteristics, such as different periodic pulsations and uncorrelated changes in brightness. These observations imply that highly energetic, non-conjugate magnetospheric processes sometimes drive the polar regions of Jupiter’s dayside magnetosphere. This is in contrast to current models of X-ray generation for Jupiter 9,10 . Understanding the behaviour and drivers of Jupiter’s pair of hot spots is critical to the use of X-rays as diagnostics of the wide range of rapidly rotating celestial bodies that exhibit these auroral phenomena. The authors discover that Jupiter's southern X-ray aurora is concentrated into a hot spot (until now only the north pole was known to have one), which behaves completely differently in brightness and timing pulsation from its northern counterpart.

Searching for Narrow Emission Lines in X‐Ray Spectra: Computation and Methods

The detection and quantification of narrow emission lines in X-ray spectra is a challenging statistical task. The Poisson nature of the photon counts leads to local random fluctuations in the observed spectrum that often result in excess emission in a narrow band of energy resembling a weak narrow line. From a formal statistical perspective, this leads to a (sometimes highly) multimodal likelihood. Many standard statistical procedures are based on (asymptotic) Gaussian approximations to the likelihood and simply cannot be used in such settings. Bayesian methods offer a more direct paradigm for accounting for such complicated likelihood functions, but even here multimodal likelihoods pose significant computational challenges. The new Markov chain Monte Carlo (MCMC) methods developed in 2008 by van Dyk and Park, however, are able to fully explore the complex posterior distribution of the location of a narrow line, and thus provide valid statistical inference. Even with these computational tools, standard statistical quantities such as means and standard deviations cannot adequately summarize inference and standard testing procedures cannot be used to test for emission lines. In this paper, we use new efficient MCMC algorithms to fit the location of narrow emission lines, we develop new statistical strategies for summarizing highly multimodal distributions and quantifying valid statistical inference, and we extend the method of posterior predictive p-values proposed by Protassov and coworkers to test for the presence of narrow emission lines in X-ray spectra. We illustrate and validate our methods using simulation studies and apply them to the Chandra observations of the high-redshift quasar PG 1634+706.

Ion‐by‐Ion Differential Emission Measure Determination of Collisionally Ionized Plasma. II. Application to Hot Stars

In a previous paper we have described a technique to derive constraints on the differential emission measure (DEM) distribution, a measure of the temperature distribution, of collisionally ionized hot plasmas from their X-ray emission line spectra. We apply this technique to the Chandra HETGS spectra of all of the nine hot stars available to us at the time that this project was initiated. We find that DEM distributions of six of the seven O stars in our sample are very similar, but that θ1 Ori C has an X-ray spectrum characterized by higher temperatures. The DEM distributions of both of the B stars in our sample have lower magnitudes than those of the O stars, and one, τ Sco, is characterized by higher temperatures than the other, β Cru. These results confirm previous work in which high temperatures have been found for θ1 Ori C and τ Sco and taken as evidence for channeling of the wind in magnetic fields, the existence of which is related to the stars' youth. Our results demonstrate the utility of our method for deriving temperature information for large samples of X-ray emission-line spectra.

Ion‐by‐Ion Differential Emission Measure Determination of Collisionally Ionized Plasma. I. Method

We describe a technique to derive constraints on the differential emission measure (DEM) distribution, a measure of the temperature distribution, of collisionally ionized hot plasmas from their X-ray emission-line spectra. This technique involves fitting spectra using a number of components, each of which is the entire X-ray emission-line spectrum for a single ion. It is applicable to high-resolution X-ray spectra of any collisionally ionized plasma, and it is particularly useful for spectra in which the emission lines are broadened and blended, such as those of the winds of hot stars. This method does not require that any explicit assumptions about the form of the DEM distribution be made and is easily automated.

The X‐Ray Line Emission from the Supernova Remnant W49B

The Galactic supernova remnant W49B has one of the most impressive X-ray emission-line spectra obtained with ASCA. We use both plasma line diagnostics and broadband model fits to show that the Si and S emission lines require multiple spectral components. The spectral data do not necessarily require individual elements to be spatially stratified, as suggested by earlier work, although when ASCA line images are considered, it is possible that Fe is stratified with respect to Si and S. Most of the X-ray-emitting gas is from ejecta, based on the element abundances required, but is surprisingly close to being in collisional ionization equilibrium. A high ionization age implies a high internal density in a young remnant. The fitted emission measure for W49B indicates a minimum density of 2 cm-3, with the true density likely to be significantly higher. W49B probably had a Type Ia progenitor, based on the relative element abundances, although a low-mass Type II progenitor is still possible. We find persuasive evidence for Cr and possibly Mn emission in the ASCA spectrum—the first detection of these elements in X-rays from a cosmic source.

Ne IX and Ne X spectra on the JET tokamak

Observations of H-like Ne x and He-like Ne IX soft X-ray emission line spectra have been made on the JET (Joint European Torus) tokamak. Recently calculated electron impact excitation rates for He-like neon are used in the derivation of the electron temperature sensitive line ratios G=((x+y+z)/w), R1=(1s3p1P1 to 1s2 1S0/w Lyalpha /w, and the electron density sensitive ratio R=(z/(x+y)), where w, x, y and z are the resonance line (1s2p 1P1 to 1s21S0), intercombination (1s2p3P2,1 to 1s2 1S0) and forbidden lines (1s2s3S1 to 1s21S0) respectively and Lyalpha is the H-like Lymanalpha , transition. Extensive modelling is carried out to allow for non-coronal conditions due to diffusion effects in these calculations, which are compared with experimental ratios from JET, where independent temperature and density measurements are available. Departures from equilibrium are found for both ohmic and additionally heated plasmas, resulting in significant changes to the G and Lyalpha /w ratios. The R ratio is approaching its upper density limit and is relatively unaffected by non-coronal conditions. Good agreement between the experimental and calculated ratios is found, giving an indication of the accuracy of the combination of the atomic data and transport employed in the calculations.

X-ray emission-line spectra of photoionized plasmas - Density sensitivity of the Fe L-shell series

The circumsource environments of accretion-powered X-ray sources are likely to support relatively dense (greater than 10 exp 11/cu cm) photoionized X-ray emission-line regions. The Fe L-shell ions provide a versatile class of plasma diagnostics in this regime, their multielectron structures resulting in diverse spectral phenomena. Attention is given to the spectral response of Fe L-shell ions to variations in electron density over the range 10 exp 11 to 10 exp 16/cu cm. It is found that density-sensitive line ratios exist in the wavelength interval 12-17 A for the ions Fe XVII-XXI. The prominent role of radiative recombination in the population kinetics distinguishes the density-sensitive Fe lines in photoionized plasmas from those which operate in coronal equilibrium plasmas. The results of detailed atomic modeling of these ions are presented and applications to spectroscopic observations of accretion-driven X-ray sources are discussed.

Accuracy in quantitative electron-probe microanalysis

Electron probe microanalysis (EPMA) is based on the interpretation of x-ray spectra emitted by specimens which were exposed to a focused, accelerated electron beam. One of the attractions of this technique is the simplicity of the x-ray emission line spectra which is particularly striking if we do not pay attention to the fine details of x-ray spectrometry such as line position and shape changes with chemical composition or the extended structure of edges. In close analogy, the inventor of EPMA, R. Castaing, found in early investigations that quite simple approximations to quantitation with this technique could yield results of remarkable analytical accuracy, considering the state of art of instrumentation at that time, and the small volumes from which the compositional analysis was elicited [1]. While Castaing favored a simple approach based on physical principles, Ziebold and Ogilvie [2] chose an empirical technique based on the use of composite standard materials; again, a pleasing simplicity was observed, and the empirical method is still widely used, particularly in the analysis of minerals. As experience and areas of applications widened, it became obvious that to increase the accuracy of the procedure, some of this apparent simplicity had to be abandoned. Fortunately, the availability of small computers permitted the on-line execution of more involved data reduction schemes; larger computers could be used in Monte Carlo simulations of the events in the target [3], and the quality of estimates of pertinent parameters, such as the x-ray coefficients, was also improved [4]. The x-ray line spectra observed in EPMA are caused by electrons which penetrate the specimen surface with energies which typically range between 5 and 30 keV. In the typical flat, thick specimen, most of the electron paths are contained within a homogeneous specimen matrix. The penetrating electrons lose energy due to inelastic interactions which usually are treated as a continuous process and described by Bethe's law of electron deceleration or by expressions related to this law [4]. The direction of the penetrating electron is altered mainly due to inelastic collisions; the scattering of electrons causes a significant number of electrons to be re-emitted from the specimen (backscattering), and the fraction of electrons which are backscattered depends strongly on the mean number of the target. The backscattered electrons conserve most of the energy they had when entering the specimen; therefore, a significant fraction of the energy which would otherwise cause x-ray emission is lost through backscattering, and the loss varies strongly with specimen composition. A small fraction of electron-target interactions, described by ionization cross-sections produces xrays, and the x-ray emission directed toward the specimen surface suffers attenuation which is determined by the distribution in depth of the loci of excitation and by the x-ray coefficients of the specimen. Besides this primary x-ray generation, processes such as fluorescent x-ray generation by continuous as well as characteristic x-rays affect the relation between x-ray emission intensity and specimen composition. It is common, though not justifiable by present standards of computation, to group the data processing steps into three multiplicative factors (Z,A,F), which, respectively, represent the atomic number correction (i.e., consideration of electron deceleration and backscatter), the absorption correction (which only considers the attenuation of primary x-rays), and the correction (which describes, usually in an oversimplified way, the generation and attenuation of x-rays excited by other characteristic x-ray lines). The fluorescence due to the continuum is ignored in most data reduction schemes. In addition to the uncertainties in parameters involved in the aforementioned processes, the accuracy of the procedure is affected by lack of flatness and homogeneity of the standards whose emission is compared with that of the specimen, by errors in the determination of the composition of nonele-

The effects of a multidensity plasma on ultraviolet spectroscopic electron density diagnostics

Spectroscopic electron density diagnostics have been developed for interpretation of UV, EUV, and X-ray emission line spectra of solar and other astrophysical plasmas, and tokamak plasmas. In principle, accurate electron densities can be determined. However, in practice, a number of difficulties arise with respect to the determination of very accurate electron densities in the 1100-3000 A region. The present study has the objective to investigate one of these difficulties, taking into account the effect on line ratios produced by a source composed of several regions of substantially different densities, all at the same temperature. The study is in particular concerned with a source in which small high density knots are embedded in low-density plasma. Attention is given to line ratios involving the O IV multiplet near 1400 A, obtained from the spectrum of a surge observed outside the solar limb.