Kα Emission Line Research Articles

Predicting the X-Ray Spectra of Stellar-mass Black Holes from Simulations

Abstract We describe results from a new technique for the prediction of complete, self-consistent X-ray spectra from three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of black hole accretion flows. Density and cooling rate data from a harm3d GRMHD simulation are post-processed by an improved version of the Monte Carlo radiation transport code pandurata (in the corona) and the Feautrier solver ptransx (in the disk), with xstar subroutines. The codes are run in a sequential, iterative fashion to achieve globally energy-conserving and self-consistent radiation fields, temperature maps, and photoionization equilibria. The output is the X-ray spectrum as seen by a distant observer, including features, such as the Fe Kα emission line and corresponding K-edge absorption trough, due to disk reprocessing of coronal power. For the example cases we consider—a non-rotating 10 M ⊙ black hole with solar abundances, accreting at 0.01, 0.03, 0.1, or 0.3 Eddington—we find spectra resembling actual observations of stellar-mass black holes in the soft or steep power-law state: broad thermal peaks (at 1–3 keV), steep power laws extending to high energy (Γ = 2.7–4.5), and prominent, asymmetric Fe Kα emission lines with equivalent widths in the range 40–400 eV (larger EW at lower accretion rates). By starting with simulation data, we obviate the need for parameterized descriptions of the accretion flow geometry—no a priori specification of the corona’s shape or flux, or the disk temperature or density, etc., is needed. Instead, we apply the relevant physical principles to simulation output using appropriate numerical techniques; this procedure allows us to calculate inclination-dependent spectra after choosing only a small number of physically meaningful parameters: black hole mass and spin, accretion rate, and elemental abundances.

Depth-graded Mo/Si multilayer coatings for hard x-rays.

This manuscript presents the first systematic study of non-periodic, broadband Mo/Si multilayer coatings with and without B4C interface barrier layers for hard x-ray applications with large field of view. The photon energy of operation in this work is 17.4 keV, the Mo Kα emission line. The coatings involve layers with varying thicknesses in the nanometer scale and the behavior at the layer interfaces plays a crucial role in their performance. Reflectivity measurements and modeling at 8.05 keV and 17.4 keV, Transmission Electron Microscopy (TEM), as well as thin film stress measurements, are employed to examine and optimize the reflective performance of these coatings and the physics of their constituent layers and interfaces. Mo/Si with B4C barrier layers on the Mo-on-Si interface is shown to produce the highest reflectivity among all design configurations considered in this work.

Study of an asynchronously rotating polar CD Ind

We present X-ray and ultaviolet (UV) analyses of the magnetic cataclysmic variable CD Ind. The X-ray light curves obtained from different observations show a very chaotic and spectacular pattern. X-ray and UV variations occur at orbital periods of (124.0±0.4) and (112.3±0.4) min and spin periods of (119.4±0.4) and (109.8±0.3) min, respectively. The degree of asynchronism of CD Ind is calculated as ~ 2.2% and ~ 3.7% from the UV and X-ray data, respectively. The variable shape of the X-ray and UV light curves indicate that the geometry of accretion changes in every epoch of observations. A two-pole accretion mode geometry is also visible in the X-ray and UV light curves of CD Ind. This two-pole accretion geometry is supported by the presence of double humps or dips in their X-ray and UV light curves. The rotational modulations of UV data are also found to be anti-correlated with respect to the X-rays, revealing that the UV pulsations are not originating in the region of magnetically confined accretion flow. Unlike other magnetic CVs, a strong fluorescent Ni Kα emission line is detected for the first time in the X-ray spectra of CD Ind. The X-ray and UV variations of CD Ind support the view that this system belongs to the class of asynchronous polars.

The 6.7 keV Line Emission from the Stellar Flare of Algol

We present spectral analysis of Algol stellar flare data. The 2-day observation of Algol in our Galaxy was done by Suzaku Satellite. Algol binary system consists of two component stars that eclipse one another periodically. The stellar magnetic field activities and Roche Lobe overflow/mass transfer mechanisms during rapid rotation of the component stars generate an X-ray emission. The variation in brightness of the stellar flare from epoch to epoch provides useful information of stellar properties of the component stars. The atomic physics of the Kα line emission process in the vicinities of chromospherically active and X-ray flare stars, and binaries is well understood. The photo ionization/collisional excitation in the Algol’s corona produces 6.7 keV line emission. The accumulated spectra in X-ray Imaging Spectrometer (XIS) sensors fitted combined with bremsstrahlung model and Gaussian line show a strong fixed X-ray energy of 6.7 keV.

Chandra Detection of the Circumnuclear Molecular Torus of the Compton-thick Active Galactic Nucleus in NGC 5643

Abstract We report a clumpy elongated feature found with deep Chandra Advanced CCD Imaging Spectrometer high-resolution imaging of the Fe Kα line emission in the nuclear region of the Compton-thick active galactic nucleus galaxy NGC 5643. This feature extends for ∼65 pc north to south (N-S). No corresponding feature is seen in the 3.0–6.0 keV continuum. The Fe Kα feature is spatially consistent with the N-S elongation found in the CO(2–1) high-resolution imaging with the Atacama Large Millimeter/submillimeter Array, but slightly more extended than the rotating molecular disk of diameter 26 pc indicated by the kinematics of the CO(2–1) line. The Chandra detection of a corresponding north to south structure in the neutral Fe Kα line would argue for both CO and Fe Kα emission originating from the obscuring torus.

Characterization of hard X-ray sources produced via the interaction of relativistic femtosecond laser pulses with metallic targets

X-ray emission resulting from interactions of intense laser pulses with solid metal targets (Ni, Cu, Mo, Ag, and Sn) at 0.5 kHz repetition rate is measured using pulse energies of up to 12 mJ. A comparison of the conversion of laser pulse energy to total X-ray emission energy is made with respect to the previous measurements at lower energy (< 3 mJ). In the present experiments, the total bremsstrahlung conversion efficiency is found to increase by an order of magnitude for all targets as the energy in increased. The Kα line emission conversion efficiency also increases with incident pulse energy for all targets. In addition, the ratio between line and bremsstrahlung emission in the X-ray spectral region was significantly reduced at higher energies because of the large increase in bremsstrahlung. In general, the X-ray source size increases as the laser energy increased and the ellipticity of the X-ray source also increased in the laser polarization direction, with the effect becoming very pronounced at higher energies. Phase-contrast imaging of a nanospray emitter and a 3D printed plastic target was also performed using Cu and Mo targets.

Compton Shoulder Diagnostics in Active Galactic Nuclei for Probing the Metallicity of the Obscuring Compton-thick Tori

Abstract We analyzed the spectral shape of the Compton shoulder around the neutral Fe–K α line of the Compton-thick type II Seyfert nucleus of the Circinus galaxy. The characteristics of this Compton shoulder with respect to the reflected continuum and Fe–K α line core intensity are powerful diagnostics tools for analyzing the structure of the molecular tori, which obscures the central engine. We applied our Monte-Carlo-based X-ray reflection spectral model to the Chandra High Energy Transmission Grating data and successfully constrained the various spectral parameters independently, using only the spectral data only around the Fe–K α emission line. The obtained column density and inclination angle are consistent with previous observations and the Compton-thick type II Seyfert picture. In addition, we determined the metal abundance of the molecular torus for the case of the smooth and clumpy torus to be and 1.74 ± 0.16 solar abundance, respectively. Such slightly over-solar abundance can be useful information for discussing the star formation rate in the molecular tori of active galactic nuclei.

The Changing-look Quasar Mrk 590 Is Awakening

Abstract Mrk 590 was originally classified as a Seyfert 1 galaxy, but then it underwent dramatic changes: the nuclear luminosity dropped by over two orders of magnitude and the broad emission lines all but disappeared from the optical spectrum. Here we present follow-up observations to the original discovery and characterization of this “changing-look” active galactic nucleus (AGN). The new Chandra and Hubble Space Telescope observations from 2014 show that Mrk 590 is awakening, changing its appearance again. While the source continues to be in a low state, its soft excess has re-emerged, though not to the previous level. The UV continuum is brighter by more than a factor of two and the broad Mg ii emission line is present, indicating that the ionizing continuum is also brightening. These observations suggest that the soft excess is not due to reprocessed hard X-ray emission. Instead, it is connected to the UV continuum through warm Comptonization. Variability of the Fe Kα emission lines suggests that the reprocessing region is within ∼10 lt-yr or 3 pc of the central source. The change in AGN type is neither due to obscuration nor due to one-way evolution from Type 1 to Type 2, as suggested in the literature, but may be related to episodic accretion events.

NuSTAR view of the Seyfert galaxy HE 0436-4717

We present the multi-epoch spectral analysis of HE 0436-4717, a bright Seyfert 1 galaxy serendipitously observed by the high energy satellite NuSTAR four times between December 2014 and December 2015. The source flux shows modest variability within each pointing and among the four observations. Spectra are well modelled in terms of a weakly variable primary power law with constant photon index (Γ = 2.01 ± 0.08). A constant narrow Fe Kα emission line suggests that this feature has an origin far from the central black hole, while a broad relativistic component is not required by the data. The Compton reflection component is also constant in flux with a corresponding reflection fraction R = 0.7+0.2−0.3. The iron abundance is compatible with solar (AFe = 1.2+1.4−0.4), and a lower limit for the high energy cut-off Ec &gt; 280 keV is obtained. Adopting a self-consistent model accounting for a primary Comptonized continuum, we obtain a lower limit for the hot corona electron temperature kTe &gt; 65 keV and a corresponding upper limit for the coronal optical depth of τe &lt; 1.3. The results of the present analysis are consistent with the locus of local Seyfert galaxies in the kTe − τe and temperature-compactness diagrams.

Spectral evolution of the supergiant HMXB IGR J16320–4751 along its orbit using XMM-Newton

Context. The INTEGRAL satellite has revealed a previously hidden population of absorbed high-mass X-ray binaries (HMXBs) hosting supergiant stars. Among them, IGR J16320–4751 is a classical system intrinsically obscured by its environment, with a column density of ∼1023 cm−2, more than an order of magnitude higher than the interstellar absorption along the line of sight. It is composed of a neutron star rotating with a spin period of ∼1300 s, accreting matter from the stellar wind of an O8I supergiant star, with an orbital period of ∼9 days. Aims. We investigated the geometrical and physical parameters of both components of the binary system IGR J16320–4751. Since in systems of this type the compact object is usually embedded in the dense and powerful wind of an OB supergiant companion, our main goal here was to study the dependence of the X-ray emission and column density along the full orbit of the neutron star around the supergiant star. Methods. We analyzed all existing archival XMM-Newton and Swift/BAT observations collected between 2003 and 2008, performing a detailed temporal and spectral analysis of the X-ray emission of the source. We then fitted the parameters derived in our study, using a simple model of a neutron star orbiting a supergiant star. Results. The XMM-Newton light curves of IGR J16320–4751 display high-variability and flaring activity in X-rays on several timescales, with a clear spin period modulation of ∼1300 s. In one observation we detected two short and bright flares where the flux increased by a factor of ∼10 for ∼300 s, with similar behavior in the soft and hard X-ray bands. By inspecting the 4500-day light curves of the full Swift/BAT data, we derived a refined period of 8.99 ± 0.01 days, consistent with previous results. The XMM-Newton spectra are characterized by a highly absorbed continuum and an Fe absorption edge at ∼7 keV. We fitted the continuum with a thermally comptonized COMPTT model, and the emission lines with three narrow Gaussian functions using two TBABS absorption components, to take into account both the interstellar medium and the intrinsic absorption of the system. For the whole set of observations we derived the column density at different orbital phases, showing that there is a clear modulation of the column density with the orbital phase. In addition, we also show that the flux of the Fe Kα line is correlated with the N H column, suggesting a clear link between absorbing and fluorescent matter that, together with the orbital modulation, points towards the stellar wind being the main contributor to both continuum absorption and Fe Kα line emission. Conclusions. Assuming a simple model for the supergiant stellar wind we were able to explain the orbital modulation of the absorption column density, Fe Kα emission and the high-energy Swift/BAT flux, allowing us to constrain the geometrical parameters of the binary system. Similar studies applied to the analysis of the spectral evolution of other sources will be useful to better constrain the physical and geometrical properties of the sgHMXB class.

X-Ray Structure between the Innermost Disk and Optical Broad-line Region in NGC 4151

Abstract We present an analysis of the narrow Fe Kα line in Chandra/HETGS observations of the Seyfert active galactic nucleus (AGN) NGC 4151. The sensitivity and resolution afforded by the gratings reveal asymmetry in this line. Models including weak Doppler boosting, gravitational redshifts, and scattering are generally preferred over Gaussians at the 5σ level of confidence, and generally measure radii consistent with . Separate fits to “high/unobscured” and “low/obscured” phases reveal that the line originates at smaller radii in high-flux states; model-independent tests indicate that this effect is significant at the 4–5σ level. Some models and Δt ≃ 2 × 104 s variations in line flux suggest that the narrow Fe Kα line may originate at radii as small as in high-flux states. These results indicate that the narrow Fe Kα line in NGC 4151 is primarily excited in the innermost part of the optical broad line region (BLR), or X-ray BLR. Alternatively, a warp could provide the solid angle needed to enhance Fe Kα line emission from intermediate radii, and might resolve an apparent discrepancy in the inclination of the innermost and outer disk in NGC 4151. Both warps and the BLR may originate through radiation pressure, so these explanations may be linked. We discuss our results in detail, and consider the potential for future observations with Chandra, XARM, and ATHENA to measure black hole masses and to study the intermediate disk in AGNs using narrow Fe Kα emission lines.

Hard X-ray Generation from ZnO Nanowire Targets in a Non-Relativistic Regime of Laser-Solid Interactions

We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5 × 1016 W/cm2 intensity laser pulses at a 0.8 µm wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150–800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic Kα line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in highenergy density (HED) physics.

Multi-wavelength campaign on NCG 7469

We conducted a multi-wavelength 6-month campaign to observe the Seyfert Galaxy NGC 7469, using the space-based observatories HST, Swift, XMM-Newton and NuSTAR. We report the results of the spectral analysis of the seven simultaneous XMM-Newton and NuSTAR observations. The source shows significant flux variability within each observation, but the average flux is less variable among the different pointings of our campaign. Our spectral analysis reveals a prominent narrow neutral Fe Kα emission line in all the spectra and weaker contributions from Fe Kβ, neutral Ni Kα, and ionized iron. We find no evidence for variability or relativistic effects acting on the emission lines, which indicates that they originate from distant material. In the joint analysis of XMM-Newton and NuSTAR data, a constant photon index is found (Γ = 1.78 ± 0.02) together with a high energy cut-off Ecut = 170−40+60 keV. Adopting a self-consistent Comptonization model, these values correspond to an average coronal electron temperature of kT = 45−12+15 keV and, assuming a spherical geometry, an optical depth τ = 2.6 ± 0.9. The reflection component is consistent with being constant and the reflection fraction is in the range R = 0.3−0.6. A prominent soft excess dominates the spectra below 4 keV. This is best fit with a second Comptonization component, arising from a warm corona with an average kT = 0.67 ± 0.03 keV and a corresponding optical depth τ = 9.2 ± 0.2.

Hard X-ray fluorescence spectroscopy of high pressure cavitating fluids in aluminum nozzles

X-rays are frequently used to study the internal geometry of dense objects, and to measure the density of multiphase flows. However, quantitatively measuring fluid density inside a metallic object such as a high pressure spray nozzle is difficult. X-rays of sufficiently high energy to penetrate a metal object are not appreciably absorbed by the fluid inside. This requires the use of plastic or beryllium test sections, which are not suited to high pressure conditions. We present a high-energy X-ray fluorescence technique which can overcome this problem. The experiments were conducted at the 7-BM beamline of the Advanced Photon Source at Argonne National Laboratory. A hydrocarbon fluid was seeded with cerium nanoparticles. The fluid was pumped at high pressure through aluminum nozzles with inner diameters of 0.36–0.90 mm and wall thicknesses of 2–3 mm. A collimated, monochromatic 42.5 keV X-ray beam excited K-edge fluorescence from the cerium. The Kα emission lines at 34–35 keV were recorded by a cryogenic germanium detector. Changes in fluid density due to cavitation of the liquid inside the nozzle were measured by raster scanning the nozzle across the beam. A spatial resolution of 20 × 20 µm2 was achieved with a slitted beam, which was improved to 5 × 10 µm2 with X-ray focusing mirrors. The uncertainty in the path-integrated vapor fraction was 30–40 µm at 95% confidence. A limitation of this approach is that for low vapor pressure fluids, the nanoparticles increase the vapor pressure of the fluid and act as additional nucleation sites. These experiments demonstrate a path forward for measurements of multiphase flows inside metal components under conditions that are not feasible in optically accessible materials.

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-energy Transmission Grating X-Ray Spectroscopy

Abstract The narrow, neutral Fe Kα fluorescence emission line in X-ray binaries (XRBs) is a powerful probe of the geometry, kinematics, and Fe abundance of matter around the accretion flow. In a recent study it has been claimed, using Chandra High-Energy Transmission Grating (HETG) spectra for a sample of XRBs, that the circumnuclear material is consistent with a solar-abundance, uniform, spherical distribution. It was also claimed that the Fe Kα line was unresolved in all cases by the HETG. However, these conclusions were based on ad hoc models that did not attempt to relate the global column density to the Fe Kα line emission. We revisit the sample and test a self-consistent model of a uniform, spherical X-ray reprocessor against HETG spectra from 56 observations of 14 Galactic XRBs. We find that the model is ruled out in 13/14 sources because a variable Fe abundance is required. In two sources a spherical distribution is viable, but with nonsolar Fe abundance. We also applied a solar-abundance Compton-thick reflection model, which can account for the spectra that are inconsistent with a spherical model, but spectra with a broader bandpass are required to better constrain model parameters. We also robustly measured the velocity width of the Fe Kα line and found FWHM values of up to ∼5000 km s−1. Only in some spectra was the Fe Kα line unresolved by the HETG.

Evidence for a Neutral Iron Line Generated by MeV Protons from Supernova Remnants Interacting with Molecular Clouds

Abstract Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray accelerators. When low-energy cosmic-ray protons (LECRp) collide with interstellar gas, they ionize neutral iron atoms and emit the neutral iron line (Fe i Kα) at 6.40 keV. We search for the iron K-shell line in seven SNRs from the Suzaku archive data of the Galactic plane in the region. All of these SNRs interact with molecular clouds. We discover Fe i Kα line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe i Kα line is produced by interactions between LECRp and the adjacent cold gas. The proton energy density is estimated to be ≳10–100 eV cm−3, which is more than 10 times higher than that in the ambient interstellar medium.

Effect of chemical environment on K shell emission lines of transition and post transition compounds

The chemical shift in Kα and Kβ1,3 X-ray emission lines of 47Ag, 48Cd and 50Sn compounds have been studied by using highly equipped advanced S8 TIGER WDXRF spectrometer. From the results, it is found that halogen compounds are more sensitive for chemical shifts than other compounds. With change of transition probabilities and outermost shell energy levels, chemical shift measured in Kβ1,3 X-ray emission lines are observed more than that of Kα emission lines. It is also found that chemical shift (ΔE) and effective charge (q) are increased with increasing number of ligands attached to the central metal atom. To show the dependency behaviour of chemical shift and effective charge with each other, we establish a linear relation between them in different 48Cd and 50Sn compounds.

On the Calculation of the Fe K-alpha Line Emissivity of Black Hole Accretion Disks

Abstract Observations of the fluorescent Fe Kα emission line from the inner accretion flows of stellar mass black holes in X-ray binaries and supermassive black holes in active galactic nuclei have become an important tool to study the magnitude and inclination of the black hole spin, and the structure of the accretion flow close to the event horizon of the black hole. Modeling spectral, timing, and soon also X-ray polarimetric observations of the Fe Kα emission requires the calculation of the specific intensity in the rest frame of the emitting plasma. We revisit the derivation of the equation used for calculating the illumination of the accretion disk by the corona. We present an alternative derivation leading to a simpler equation, and discuss the relation to previously published results.

Simulations of the Fe Kα Energy Spectra from Gravitationally Microlensed Quasars

Abstract The analysis of the Chandra X-ray observations of the gravitationally lensed quasar RX J1131−1231 revealed the detection of multiple and energy-variable spectral peaks. The spectral variability is thought to result from the microlensing of the Fe Kα emission, selectively amplifying the emission from certain regions of the accretion disk with certain effective frequency shifts of the Fe Kα line emission. In this paper, we combine detailed simulations of the emission of Fe Kα photons from the accretion disk of a Kerr black hole with calculations of the effect of gravitational microlensing on the observed energy spectra. The simulations show that microlensing can indeed produce multiply peaked energy spectra. We explore the dependence of the spectral characteristics on black hole spin, accretion disk inclination, corona height, and microlensing amplification factor and show that the measurements can be used to constrain these parameters. We find that the range of observed spectral peak energies of QSO RX J1131−1231 can only be reproduced for black hole inclinations exceeding 70° and for lamppost corona heights of less than 30 gravitational radii above the black hole. We conclude by emphasizing the scientific potential of studies of the microlensed Fe Kα quasar emission and the need for more detailed modeling that explores how the results change for more realistic accretion disk and corona geometries and microlensing magnification patterns. A full analysis should furthermore model the signal-to-noise ratio of the observations and the resulting detection biases.

Excess Galactic Molecular Absorption Toward the Radio Galaxy 3C 111

Abstract We show the combined spectral analysis of Chandra high-energy transmission grating and XMM-Newton reflection-grating spectrometer observations of the broad-line radio galaxy 3C 111. This source is known to show excess neutral absorption with respect to the one estimated from 21 cm radio surveys of atomic H i in the Galaxy. However, previous works were not able to constrain the origin of such an absorber as local to our Milky Way or intrinsic to the source (z = 0.0485). The high signal-to-noise grating spectra allow us to constrain the excess absorption as being due to intervening gas in the Milky Way, and we estimate a time-averaged total column density of cm−2, a factor of two higher than the tabulated H i value. We recommend using the total average Galactic column density estimated here when studying 3C 111. The origin of the extra Galactic absorption of cm−2 is likely due to molecular gas associated with the Taurus molecular cloud complex toward 3C 111, which is our nearest star-forming region. We also detect a weak (EW = 16 ± 10 eV) and narrow (FWMH &lt; 5500 km s−1, consistent with optical Hα) Fe Kα emission line at E = 6.4 keV, likely from the torus in the central regions of 3C 111, and we place an upper limit on the column density of a possible intrinsic warm absorber of N H &lt; 2.5 × 1020 cm−2. These complexities make 3C 111 a very promising object for studying both the intrinsic properties of this active radio galaxy and the Galactic interstellar medium, if used as a background source.