X-ray Emission Lines Research Articles

OBSERVATIONS OF STAR FORMATION INDUCED BY GALAXY-GALAXY AND GALAXY-INTERGALACTIC MEDIUM INTERACTIONS WITH AKARI

Nearby spiral galaxies M101 and M81 are considered to have undergone a galaxy-galaxy interaction. M101 has experienced HI gas infall due to the interaction. With AKARI far-infrared (IR) photometric observations, we found regions with enhanced star forming activity, which are spatially close to regions affected by the interaction. In addition, the relation between the star formation rate (SFR) and the gas content for such regions shows a significant difference from typical spiral arm regions. We discuss possible explanations for star formation processes on a kiloparsec scale and the association with interaction-triggered star formation. We also observed the compact group of galaxies Stephan’s Quintet (SQ) with the AKARI Far-infrared Surveyor (FIS). The SQ shows diffuse intergalactic medium (IGM) due to multiple collisions between the member galaxies and the IGM. The intruder galaxy NGC 7318b is currently colliding with the IGM and causes a large-scale shock. The 160 micron image clearly shows the structure along the shock ridge as seen in warm molecular hydrogen line emission and X-ray emission. The far-IR emission from the shocked region comes from the luminous [CII]158 ㎛ line and cold dust (~20 K) that coexist with molecular hydrogen gas. Survival of dust grains is indispensable to form molecular hydrogen gas within the collision age (~5 Myr). At the stage of the dusty IGM environment, [CII] and H₂ lines rather than X-ray emission are powerful cooling channels to release the collision energy.

Quantitative measurement of hard x-ray spectra for high intensity laser produced plasma

X-ray line spectra ranging from 17 to 77 keV were quantitatively measured with a Laue spectrometer, composed of a cylindrically curved crystal and a detector. Either a visible CCD detector coupled with a CsI phosphor screen or an imaging plate can be chosen, depending on the signal intensities and exposure times. The absolute sensitivity of the spectrometer system was calibrated using pre-characterized laser-produced x-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an open code for x-ray diffraction. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer agency for specific x-ray line emissions, is derived as a consequence of this work.

A GENERALIZED MODEL OF NONLINEAR DIFFUSIVE SHOCK ACCELERATION COUPLED TO AN EVOLVING SUPERNOVA REMNANT

To better model the efficient production of cosmic rays (CRs) in supernova remnants (SNRs) with the associated coupling between CR production and SNR dynamics, we have generalized an existing cr-hydro-NEI code (i.e., Ellison et al. 2012) to include the following processes: (1) an explicit calculation of the upstream precursor structure including the position dependent flow speed, density, temperature, and magnetic field strength; (2) a momentum and space dependent CR diffusion coefficient; (3) an explicit calculation of magnetic field amplification (MFA); (4) calculation of the maximum CR momentum using the amplified magnetic field; (5) a finite Alfven speed for the particle scattering centers; and (6) the ability to accelerate a superthermal seed population of CRs as well as the ambient thermal plasma. While a great deal of work has been done modeling SNRs, most work has concentrated on either the continuum emission from relativistic electrons or ions, or the thermal emission from the shock heated plasma. Our generalized code combines these elements and describes the interplay between CR production and SNR evolution, including the nonlinear coupling of efficient diffusive shock acceleration (DSA), based mainly on the work of P. Blasi and co-workers, and a non-equilibrium ionization (NEI) calculation of thermal X-ray line emission. We believe our generalized model will provide a consistent modeling platform for SNRs, including those interacting with molecular clouds, and improve the interpretation of current and future observations, including the high-quality spectra expected from Astro-H. SNR RX J1713.7-3946 is modeled as an example.

Spectroscopic evidence of charge exchange X‐ray emission from galaxies

AbstractWhat are the origins of the soft X‐ray line emission from non‐AGN galaxies? XMM‐Newton RGS spectra of nearby non‐AGN galaxies (including starforming ones: M82, NGC 253, M51, M83, M61, NGC 4631, M94, NGC 2903, and the Antennae galaxies, as well as the inner bulge of M31) have been analyzed. In particular, the Kα triplet of O VII shows that the resonance line is typically weaker than the forbidden and/or inter‐combination lines. This suggests that a substantial fraction of the emission may not arise directly from optically thin thermal plasma, as commonly assumed, and may instead originate at its interface with neutral gas via charge exchange. This latter origin naturally explains the observed spatial correlation of the emission with various tracers of cool gas in some of the galaxies. However, alternative scenarios, such as the resonance scattering by the plasma and the relic photo‐ionization by AGNs in the recent past, cannot be ruled out, at least in some cases, and are being examined. Such X‐ray spectroscopic studies are important to the understanding of the relationship of the emission to various high‐energy feedback processes in galaxies (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical shifts observed in Lα X-ray emission lines of elements in the range of 26 ≤ Z ≤ 30 in their halogen compounds

The chemical shifts and full widths at half maximum intensity of Lα X-ray emission lines of elements in the range 26 ≤ Z ≤ 30 were studied in their halogen compounds using a wavelength-dispersive X-ray fluorescence spectrometry. It was observed that the chemical shifts for F based compounds are higher than that of Cl and Br based compounds.

Excitation wavelength dependence of water-window line emissions from boron-nitride laser-produced plasmas

We investigated the effects of laser excitation wavelength on water-window emission lines of laser-produced boron-nitride plasmas. Plasmas are produced by focusing 1064 nm and harmonically generated 532 and 266 nm radiation from a Nd:YAG laser on BN target in vacuum. Soft x-ray emission lines in the water-window region are recorded using a grazing-incidence spectrograph. Filtered photodiodes are used to obtain complementary data for water-window emission intensity and angular dependence. Spectral emission intensity changes in nitrogen Ly-α and He-α are used to show how laser wavelength affects emission. Our results show that the relative intensity of spectral lines is laser wavelength dependent, with the ratio of Ly-α to He-α emission intensity decreasing as laser wavelength is shortened. Filtered photodiode measurements of angular dependence showed that 266 and 532 nm laser wavelengths produce uniform emission.

A comparative study of the ionic keV X-ray line emission from plasma produced by the femtosecond, picosecond and nanosecond duration laser pulses

We report here an experimental study of the ionic keV X-ray line emission from mag- nesium plasma produced by laser pulses of three widely different pulse durations (FWHM) of 45 fs, 25 ps and 3 ns, at a constant laser fluence of ∼1.5 × 10 4 Jc m −2 . It is observed that the X-ray yield of the resonance lines from the higher ionization states such as H- and He-like ions decreases on decreasing the laser pulse duration, even though the peak laser intensities of 3.5 × 10 17 Wc m −2 for the 45 fs pulses and 6.2 × 10 14 Wc m −2 for the 25 ps pulses are much higher than 5 × 10 12 Wc m −2 for the 3 ns laser pulse. The results were explained in terms of the ioniza- tion equilibrium time for different ionization states in the heated plasma. The study can be useful to make optimum choice of the laser pulse duration to produce short pulse intense X-ray line emission from the plasma and to get the knowledge of the degree of ionization in the plasma.

Charge-exchange X-ray emission of nearby star-forming galaxies

Properties of hot gas outflows from galaxies are generally measured from associated X-ray line emission assuming that it represents atomic transitions in thermally excited hot gas. X-ray line emission, however, can also arise from the charge exchange between highly ionised ions and neutral species. The K\alpha\ triplet of He-like ions can be used as a powerful diagnostic, because the charge exchange X-ray emission (CXE) favours the inter-combination and forbidden lines, while the thermal emission favours the resonance line. We analyse the OVII triplet of a sample of nine nearby star-forming galaxies observed by the XMM-Newton reflection grating spectrometers. For most galaxies, the forbidden lines are comparable to or stronger than the resonance lines, which is in contrast to the thermal prediction. For NGC 253, M51, M83, M61, NGC 4631, and the Antennae galaxy, the observed line ratios are consistent with the ratio of the CXE; for M94 and NGC 2903, the observed ratios indicate multiple origins; for M82, different regions show different line ratios, also indicating multiple origins. We discuss other possible mechanisms that can produce a relatively strong forbidden line, such as a collisional non-equilibrium-ionization recombining/ionizing plasma, which are not favoured. These results suggest that the CXE may be a common process and contribute a significant fraction of the soft X-ray line emission for galaxies with massive star formation.

Chemical effects in the Kα and Kβ1,3 of X-ray emission spectra of Fe

Chemical shifts and full widths at half maximum intensity (FWHM) of Kα and Kβ1,3 X-ray emission lines were measured for the following Fe compounds: Fe, FeCl2, FeCl3·6H2O, FeF2, FeF2·4H2O, FeF3, FeF3·3H2O, FeO3, FeS2, FeSO4. The measurements were performed with a wavelength-dispersive X-ray fluorescence spectrometry (WDXRF). It was found that the calculated results for Fe compounds are strongly correlated with the oxidation state. At the same time chemical effects for halogens compounds are generally more than that other compounds.

TOWARD UNDERSTANDING THE COSMIC-RAY ACCELERATION AT YOUNG SUPERNOVA REMNANTS INTERACTING WITH INTERSTELLAR CLOUDS: POSSIBLE APPLICATIONS TO RX J1713.7–3946

Using three-dimensional magnetohydrodynamics simulations, we investigate general properties of a blast wave shock interacting with interstellar clouds. The shock-cloud interaction generates a turbulent shell through the vorticity generations. In the turbulent shell, the magnetic field is amplified as a result of turbulent dynamo action. In the case of a young supernova remnant, the corresponding strength of the magnetic field is approximately 1 mG. The propagation speed of the shock wave is significantly stalled in the clouds because of the high density, while the shock maintains a high velocity in the diffuse surrounding. In addition, when the shock wave hits the clouds, reflection shock waves are generated that propagate back into the shocked shell. From these simulation results, many observational characteristics of a young SNR RX J1713.7-3946 that is suggested to be interacting with molecular clouds, can be explained as follows: The reflection shocks can accelerate particles in the turbulent downstream region where the magnetic field strength reaches 1mG, which causes short-time variability of synchrotron X-rays. Since the shock velocity is stalled locally in the clouds, the temperature in the shocked cloud is suppressed below 1 keV. Thus, thermal X-ray line emissions would be faint even if the SNR is interacting with molecular clouds. We also find that the photon index of the $\pi^0$-decay gamma rays generated by cosmic-ray protons can be 1.5, because the penetration depth of high-energy particles into the clumpy clouds depends on their energy, which is consistent with the recent gamma-ray observation by Fermi space telescope.

Thermal instability in gravitationally stratified plasmas: implications for multiphase structure in clusters and galaxy haloes

We study the interplay among cooling, heating, conduction, and magnetic fields in gravitationally stratified plasmas using simplified, plane-parallel numerical simulations. Since the physical heating mechanism remains uncertain in massive halos such as groups or clusters, we adopt a simple, observationally-motivated prescription which enforces global thermal equilibrium when averaged over large scales. The plasma remains susceptible to local thermal instability, however, and cooling drives an inward flow of material. In contrast to previous results, we argue that the thermal stability of the plasma is independent of its convective stability. We find that the ratio of the cooling timescale to the dynamical timescale t_cool/t_ff controls the saturation of the thermal instability: when t_cool/t_ff < 1, the plasma develops extended multi-phase structure, whereas when t_cool / t_ff > 1 it does not. (In a companion paper, we show that the criterion for thermal instability in a spherical potential is somewhat less stringent, t_cool / t_ff < 10.) When thermal conduction is anisotropic with respect to the magnetic field, the criterion for multi-phase structure is essentially independent of the thermal conductivity of the plasma. Our criterion for local thermal instability to produce multi-phase structure is an extension of the cold vs. hot accretion modes in galaxy formation that applies at all radii in hot halos, not just to the virial shock. We show that this criterion is consistent with data on multi-phase gas in the ACCEPT sample of clusters; in addition, when t_cool / t_ff > 1, the net cooling rate to low temperatures and the mass flux to small radii are suppressed enough relative to models without heating to be qualitatively consistent with star formation rates and x-ray line emission in groups and clusters.

FAR-ULTRAVIOLET SPECTRAL IMAGES OF THE ORION-ERIDANUS SUPERBUBBLE REGION

The far-ultraviolet (FUV) continuum and spectral images of C IV and H{sub 2} emission lines for the region of the Orion-Eridanus Superbubble are hereby presented and compared with maps obtained in other wavelengths. While the region shows complex structures, consisting of hot gases and cold dust, a close examination reveals that the FUV emission in this region can be understood reasonably as the result of their interactions. We confirm the origin of most diffuse FUV continuum to be starlight scattered by dust, but we also find that ionized gas contributes 50%-70% of the total FUV intensity in the regions of H{alpha} arcs. We note the bright diffuse FUV continuum in the eastern part of the northern region, which has a lot of dust, and attribute it to a greater abundance of bright, early-type stars in this region than in the west, as the amount of dust itself does not seem to be much different across 'arc A', which separates the two regions. In addition, two P Cygni-type stars are identified in this eastern region and their peculiar spectral profiles around the C IV emission line are manifested in the scattered diffuse spectrum. Besides this, the C IV emission is generallymore » enhanced at the boundaries of the hot X-ray cavities where thin dust regions are located, confirming the thermal interface nature of the origin of this cooling emission line. The morphology of the H{sub 2} emission shows a general correlation with dust extinction features but its intensity peaks are located in thin dust areas, rather than the peak dust regions. Furthermore, H{sub 2} emission is weak in the arc A region even though the arc passes through the center of the dust-rich area. Hence, the H{sub 2} emission and dust features, together with those of X-ray and ion line emissions, show the stratified structure of arc A quite well, again confirming its thermal interface nature.« less

Charge-exchange X-ray emission of M82: Ka triplets of OVII, Ne IX and Mg XI

Abstract Starburst galaxies are primary feedback sources of mechanical energy and metals, which are generally measured from associated X-ray emission lines, assuming that they are from the thermal emission of the outflowing hot gas. Such line emission, however, can also arise from the charge-exchange X-ray emission (CXE) between highly ionized ions and neutral species. To understand the feedback of energy and metals, it is crucial to determine the origin of the X-ray emission lines and to distinguish the contributions from the CXE and the thermal emission. The origin of the lines can be diagnosed by the Kα triplets of He-like ions, because the CXE favours the intercombination and forbidden lines, while the thermal emission favours the resonance line. We analyse the triplets of O vii, Ne ix and Mg xi observed in the XMM—Newton reflection grating spectra of the starburst galaxy M82. The flux contribution of the CXE is 90, 50 and 30 per cent to the O vii, Ne ix and Mg xi triplets, respectively. Averaged over all the three triplets, the contribution of the CXE is ∼50 per cent of the total observed triplet flux. To correctly understand the hot outflow of starburst galaxies, it is necessary to include the CXE. Based on the measured CXE fluxes of the O vii, Ne ix and Mg xi triplets, we estimate the relative abundances of O, Ne and Mg of the outflow and find they are similar to the solar ratios.

On the driver of relativistic effect strength in Seyfert galaxies

Spectroscopy of X-ray emission lines emitted in accretion discs around supermassive black holes is one of the most powerful probes of the accretion flow physics and geometry, while also providing in principle observational constraints on the black hole spin.[...] We aim at determining the ultimate physical driver of the strength of this relativistic reprocessing feature. We first extend the hard X-ray flux-limited sample of Seyfert galaxies studied so far (FERO, de la Calle Perez et al. 2010) to obscured objects up to a column density N_H=6x10^23 atoms/cm/cm. We verify that none of the line properties depends on the AGN optical classification, as expected from the Seyfert unification scenarios. There is also no correlation between the accretion disc inclination, as derived from formal fits of the line profiles, and the optical type or host galaxy aspect angle, suggesting that the innermost regions of the accretion disc and the host galaxy plane are not aligned. [...]. Data are not sensitive enough to the detailed ionisation state of the line-emitting disc. However, the lack of dependency of the line EW on either the luminosity or the rest-frame centroid energy rules out that disc ionisation plays an important role on the EW dynamical range in Seyferts. The dynamical range of the relativistically broadened K-alpha iron line EW in nearby Seyferts appears to be mainly determined by the properties of the innermost accretion flow. We discuss several mechanisms (disc ionisation, disc truncation, aberration due to a mildly relativistic outflowing corona) which can explain this. [...] Observational data are still not in contradiction with scenarios invoking different mechanisms for the spectral complexity around the iron line, most notably the "partial covering" absorption scenario. (abridged).

Doppler effects on 3-D non-LTE radiation transport and emission spectra

Spatially and temporally resolved X-ray emission lines contain information about temperatures, densities, velocities, and the gradients in a plasma. Extracting this information from optically thick lines emitted from complex ions in dynamic, three-dimensional, non-LTE plasmas requires self-consistent accounting for both non-LTE atomic physics and non-local radiative transfer. We present a brief description of a hybrid-structure spectroscopic atomic model coupled to an iterative tabular on-the-spot treatment of radiative transfer that can be applied to plasmas of arbitrary material composition, conditions, and geometries. The effects of Doppler line shifts on the self-consistent radiative transfer within the plasma and the emergent emission and absorption spectra are included in the model. Sample calculations for a two-level atom in a uniform cylindrical plasma are given, showing reasonable agreement with more sophisticated transport models and illustrating the potential complexity – or richness – of radially resolved emission lines from an imploding cylindrical plasma. Also presented is a comparison of modeled L- and K-shell spectra to temporally and radially resolved emission data from a Cu:Ni plasma. Finally, some shortcomings of the model and possible paths for improvement are discussed.

Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate

We present analysis of both the resolved X-ray emission line profiles and the broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the Chandra HETGS. This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10% of the X-ray flux. We find results consistent with the predictions of numerical simulations of the line-driving instability, including line broadening indicating an onset radius of X-ray emission of several tenths Rstar. Helium-like forbidden-to-intercombination line ratios are consistent with this onset radius, and inconsistent with being formed in a wind-collision interface with the star's closest visual companion at a distance of ~100 AU. The broadband X-ray spectrum is fit with a dominant emission temperature of just kT = 0.6 keV along with significant wind absorption. The broadband wind absorption and the line profiles provide two independent measurements of the wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} \times 10^{-6} Msun/yr and Mdot = 6.8_{-2.2}^{+2.8} \times 10^{-6} Msun/yr, respectively. This is the first consistent modeling of the X-ray line profile shapes and broadband X-ray spectral energy distribution in a massive star, and represents a reduction of a factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a smooth wind.

Configuration interaction in statistically complete hybrid-structure atomic models1This article is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

Configuration interaction can have significant effects on the transition energies and strengths of diagnostically important X-ray emission lines and features. However, including full configuration interaction (CI) effects by computing extensive collections of fine structure (LS term) levels may be computationally prohibitive for complex ions. In this paper, we show that CI effects in a simple ion vary little with the configuration of spectator electrons, and that CI effects in complex ions are fairly consistent from one ionization stage to the next, particularly for highly charged ions. Therefore, we argue that the CI effects within an ion can be approximated by extending the CI effects computed from a small subset of configurations in that ion to all transitions of the type (nlj)–(nlj)′ in that ion, regardless of the spectator electron. This approach to CI enforces consistency between the fine structure and averaged states in hybrid-structure atomic models, which are designed to provide a computationally efficient balance of spectroscopic accuracy and statistical completeness.

Evidence for a circumnuclear and ionized absorber in the X-ray obscured broad-line radio galaxy 3C 445

Here we present the results of a Suzaku observation of the broad-line radio galaxy 3C 445. We confirm the results obtained with the previous X-ray observations which unveiled the presence of several soft X-ray emission lines and an overall X-ray emission which strongly resembles a typical type 2 Seyfert galaxy despite of the optical classification as an unobscured active galactic nucleus. The broad-band spectrum allowed us to measure for the first time the amount of reflection (R∼ 0.9) which together with the relatively strong neutral Fe Kα emission line (EW ∼ 100 eV) strongly supports a scenario where a Compton-thick mirror is present. The primary X-ray continuum is strongly obscured by an absorber with a column density of NH= 2–3 × 1023 cm−2. Two possible scenarios are proposed for the absorber: a neutral partial covering or a mildly ionized absorber with an ionization parameter logξ∼ 1.0 erg cm s−1. A comparison with the past and more recent X-ray observations of 3C 445 performed with XMM–Newton and Chandra is presented, which provided tentative evidence that the ionized and outflowing absorbers varied. We argue that the absorber is probably associated with an equatorial disc wind located within the parsec scale molecular torus.

THE INTEGRATED DIFFUSE X-RAY EMISSION OF THE CARINA NEBULA COMPARED TO OTHER MASSIVE STAR-FORMING REGIONS

The Chandra Carina Complex Project (CCCP) has shown that the Carina Nebula displays bright, spatially-complex soft diffuse X-ray emission. Here we `sum up' the CCCP diffuse emission work by comparing the global morphology and spectrum of Carina's diffuse X-ray emission to other famous sites of massive star formation with pronounced diffuse X-ray emission: M17, NGC 3576, NGC 3603, and 30 Doradus. All spectral models require at least two diffuse thermal plasma components to achieve adequate spectral fits, a softer component with kT = 0.2--0.6 keV and a harder component with kT = 0.5--0.9 keV. In several cases these hot plasmas appear to be in a state of non-equilibrium ionization that may indicate recent and current strong shocks. A cavity north of the embedded giant HII region NGC 3576 is the only region studied here that exhibits hard diffuse X-ray emission; this emission appears to be nonthermal and is likely due to a recent cavity supernova, as evidenced by a previously-known pulsar and newly-discovered pulsar wind nebula also seen in this cavity. All of these targets exhibit X-ray emission lines that are not well-modeled by variable-abundance thermal plasmas and that might be attributed to charge exchange at the shock between the hot, tenuous, X-ray-emitting plasma and cold, dense molecular material; this is likely evidence for dust destruction at the many hot/cold interfaces that characterize massive star-forming regions.

USING THE X-RAY MORPHOLOGY OF YOUNG SUPERNOVA REMNANTS TO CONSTRAIN EXPLOSION TYPE, EJECTA DISTRIBUTION, AND CHEMICAL MIXING

Supernova remnants (SNRs) are a complex class of sources, and their heterogeneous nature has hindered the characterization of their general observational properties. To overcome this challenge, we use statistical tools to analyze the Chandra X-ray images of Galactic and Large Magellanic Cloud SNRs. We apply two techniques, a power-ratio method (a multipole expansion) and wavelet-transform analysis, to measure the global and local morphological properties of the X-ray line and thermal emission in twenty-four SNRs. We find that Type Ia SNRs have statistically more spherical and mirror symmetric thermal X-ray emission than core-collapse (CC) SNRs. The ability to type SNRs based on thermal emission morphology alone enables, for the first time, the typing of SNRs with weak X-ray lines or with low-resolution spectra. We identify one source, SNR G344.7-0.1, as originating from a CC explosion that was previously unknown, and we confirm the tentative Type Ia classifications of G337.2-0.7 and G272.2-3.2. Although the global morphology is indicative of the explosion type, the relative morphology of the X-ray line emission within SNRs is not: all sources in our sample have well-mixed ejecta, irrespective of stellar origin. In particular, we find that 90% of the bright metal-line emitting substructures are spatially coincident and have similar scales, even if the metals arise from different burning processes. Moreover, the overall X-ray line morphologies within each SNR are the same, with <6% differences. These findings reinforce that hydrodynamical instabilities can efficiently mix ejecta in Type Ia and CC SNRs. The only exception is W49B, which is likely from its jet-driven/bipolar SN explosion. Finally, we describe observational constraints that can be used to test hydrodynamical models of SNR evolution; notably, the filling factor of X-ray emission decreases with SNR age.