The quiescent X-ray emission of X-ray novae and the coronal emission of late-type stars

The X-ray luminosities and spectra of F-M stars of luminosity classes IV–V are analyzed. In dwarfs with rotational velocities of about 100 km/s, such as the optical components of low-mass X-ray novae with black holes, hot plasma can be confined in coronal loops even in the presence of fairly weak magnetic fields. Thus, the soft X-ray emission of such systems in their quiescent state (to 10³¹ erg/s) could be associated with the coronal emission of the optical component/dwarf. Two systems studied with subgiants (V1033 Sco and V404 Cyg) have X-ray luminosities 2×10³²–2×10³³ erg/s. The X-ray emission of a solar-type corona cannot provide such luminosities. However, a transition to a non-solar corona is possible in rapidly rotating subgiants—a dynamical corona whose X-ray emission can be one to two orders of magnitude higher than observed for more slowly rotating late-type subgiants in the solar neighborhood. This suggests that the quiescent X-ray emission of these two systems is provided by emission from the corona of the subgiant optical component.     

Thorite versus huttonite: stability, electronic properties and X-ray emission spectra from first-principle calculations

The structural, electronic properties and stability of thorium orthosilicate ThSiO₄ polymorphs: thorite and huttonite are investigated by means of the full-potential linearized augmented-plane-wave method with the generalized gradient approximation for the exchange-correlation potential (FLAPW-GGA). The forbidden gaps of thorite and huttonite are estimated at about 7.8 and 7.6 eV, respectively. It is found that Th5f states in ThSiO₄ partially overlap with occupied O2p bands. The data obtained showed that thorite is more stable than huttonite; in turn both ThSiO₄ polymorphs are unstable with respect to their constituent binary oxides (thorianite ThO₂ and α-quartz SiO₂) in agreement with the experiments. The theoretical shapes of X-ray emission (XES) (Si,O)Kα,β spectra for thorite, huttonite as well as for SiO₂ and ThO₂ are calculated and discussed. We show that the XES spectroscopy near the (Si,O)K edge may be very useful technique not only for detailed investigation of the bulk-electronic structure of Th silicates but also for the phase analysis of complex mineral samples containing these species.     

The X-ray emission of magnetars

It is shown that cyclotron radiation by electrons near the surface of a neutron star with a magnetic field of ～10¹² G can easily provide the observed quiescent radiation of magnetars (Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters). Pulsed emission is generated by the synchrotron mechanism at the periphery of the magnetosphere. Short-time-scale cataclysms on the neutron star could lead to flares of gamma-ray radiation with powers exceeding the power of the X-ray emission by a factor of 2γ², where γ is the Lorentz factor of the radiating particles. It is shown that an electron cyclotron line with an energy of roughly 1 MeV should be generated in the magnetar model. The detection of this line would serve as confirmation of the correctness of this model.     

X-ray spectra of iron atoms in minerals

The results of X-ray fluorescence Ka _1,2 and Kβ₁,β′-spectra of iron in 54 samples, including 23 minerals, are presented. Spectrum characteristics are mainly dependent on the environment of iron in the sample, the most susceptible parameter being the chemical shift δβ₁. For the compounds investigated δβ₁ varies between ?1.58 eV and +1.56 eV when referring to metallic iron. Chemical shifts of compounds with predominantly covalent bonds exhibit more negative values, compounds with metallic bonds nearly vanishing values and compounds with predominantly ionic bonds more positive values. The FeKβ₁,β′-group of lines is actually a superposition of three lines: the long-wave line β′ and two overlapping lines β ₁ ⁵ and β ₁ ⁷ of which the superposition is conventionally denoted as β₁. The relative intensity of β ₁ ⁵ increases with ionicity of the bond, while that of β ₁ ⁷ decreases. The effective charge of iron has been determined for a group of iron-bearing minerals.     

Harmonic oscillations of the X-ray emission of a solar flare

An analysis is presented for the class-M9.3 solar flare of November 6, 2004, whose decay phase displayed weakly damped harmonic oscillations of the predominantly thermal X-ray flux detected by the RHESSI spacecraft (at energies ≲25 keV). The period of these oscillations was ≈78 s, and their characteristic decay time ≈100 min. Similar quasi-periodic pulsations were observed in the decimeter-centimeter radio flux (pulsations of a type-IV radio outburst), but were less pronounced in the non-thermal hard X-ray flux (≳25 keV). The area of the quasi-stationary X-ray source, which was located primarily at the apex of a set of flare loops (≲15 keV) that were cooled primarily via thermal conduction, was found to be in anti-phase with the oscillating X-ray flux it emitted. The observed oscillations are interpreted as harmonic modulations of the radiation flux emitted by the heated thermal flare-loop plasma, due to the global, standing, sausage mode of fast magnetoacoustic waves excited in the loop.     

Interpretation of K X-ray emission spectra and chemical bonding in oxides of Mg,Al and Si using quantitative molecular orbital theory

Quantitative molecular orbital calculations are reported for Mg, Al and Si in tetrahedral and octahedral coordination with oxygen. These calculations are employed to assign and interpret the ME_α, ME_β and OK_α X-ray emission spectra of the corresponding oxides. The interpretation of the MK_β spectrum reproduces the observed trends in main peak and satellite energies with variation of metal, ligand and geometry. The splitting of the main K_β peak, observed in many oxides, is found to be a result of interaction between adjacent metal atoms. The calculations also reproduce the observed trends in OK_α spectra. The electronic structures of the various oxides are discussed briefly.     

A possible mechanism for the X-ray emission of Be/X stars

A new mechanism for the generation of X-ray emission in binary Be/X stars is proposed. It is shown that the mass-transfer rate through the point L₁ in a model in which the optical component of a Be/X star has an expanding envelope is sufficient to generate the observed X-ray luminosities of such stars. The results of computations indicate a dependence between the orbital periods and X-ray fluxes of these systems. The relationship between the orbital perod and the mass-transfer rate during flares obtained from modeling corresponds to the observed dependence of the maximum X-ray flux on the orbital period.     

Raman spectra and X-ray diffraction of tuite at various temperatures

High-temperature Raman spectra and thermal expansion of tuite, γ-Ca₃(PO₄)₂, have been investigated. The effect of temperature on the Raman spectra of synthetic tuite was studied in the range from 80 to 973 K at atmospheric pressure. The Raman frequencies of all observed bands for tuite continuously decrease with increasing temperature. The quantitative analysis of temperature dependence of Raman bands indicates that the changes in Raman frequencies for stretching modes (ν₃ and ν₁) are faster than those for bending modes (ν₄ and ν₂) of PO₄ in the present temperature range, which may be attributed to the structural evolution of PO₄ tetrahedron in tuite at high temperature. The thermal expansion of tuite was examined by means of in situ X-ray diffraction measurements in the temperature range from 298 to 923 K. Unit cell parameters and volume were analyzed, and the thermal expansion coefficients were obtained as 3.67 (3), 1.18 (1), and 1.32 (3) × 10^?5 K^?1 for V, a, and c, respectively. Thermal expansion of tuite shows an axial anisotropy with a larger expansion coefficient along the c-axis. The isothermal and isobaric mode Grüneisen parameters and intrinsic anharmonicity of tuite have been calculated by using present high-temperature Raman spectra and thermal expansion coefficient combined with previous results of the isothermal bulk modulus and high-pressure Raman spectra.     

The cyclotron spectrum of anisotropic ultrarelativistic electrons: Interpretation of X-ray pulsar spectra

The spectrum of cyclotron radiation produced by electrons with a strongly anisotropic velocity distribution is calculated taking into account higher harmonics. The motion of the electrons is assumed to be ultrarelativistic along the magnetic field and nonrelativistic across the field. One characteristic feature of the resulting spectrum is that harmonics of various orders are not equally spaced. The physical properties and observed spectra of four X-ray pulsars displaying higher cyclotron harmonics are analyzed. It was shown that at least in one of them, the cyclotron feature can apparently be only an emission line. Moreover, the observed harmonics are not equidistant, and display certain other properties characteristic of emission by strongly anisotropic ultrarelativistic electrons. In addition, there are indirect theoretical arguments that the electrons giving rise to cyclotron features in the spectra of X-ray pulsars are ultrarelativistic and characterized by strongly anisotropic distributions. As a result, estimates of the magnetic fields of X-ray pulsars (which are usually derived from the energies of cyclotron lines) and certain other physical parameters require substantial revision.     

X-ray K-edge absorption spectra of Fe minerals and model compounds: Near-edge structure

Synchrotron radiation has been used to collect high-resolution Fe K absorption near-edge spectra of a suite of Fe minerals and compounds having a range of Fe environments. These spectra, along with those of previous workers, indicate that the number, position, and intensity of near-edge features are characteristic of Fe valence and general site geometry. For example, the crest of the K-edge for Fe²⁺ in a six-coordinated site in the oxides studied is about 3 eV lower in energy than that for Fe³⁺ in a similar site. The K-edge crest for Fe³⁺ in a four-coordinated site is 1 to 2 eV lower than for Fe³⁺ in a regular site. The shape of the edge crest is sensitive to the details of first-neighbor bonding distances, tending to be broader in species with irregular Fe sites and varying in energy according to the average bond length. Comparison with Ca²⁺ and Zn²⁺ spectra from the literature is made and the applicability and utility of edge measurements discussed.     

同步辐射软X射线吸收谱与发射谱测定天然针铁矿能带结构

天然半导体矿物由于成分、缺陷复杂,传统测试方法如紫外可见漫反射等难以准确测定其禁带宽度.本文以针铁矿为例,通过第一性原理计算得到纯针铁矿及掺Al针铁矿的电子结构.计算结果显示,纯针铁矿导带底与价带顶均由Fe3d与O2p轨道组成,而当含杂质Al时,Al2p与O2p发生杂化参与了价带组成.在此基础上,利用同步辐射X射线氧的K边吸收谱与发射谱对纯针铁矿及天然针铁矿的能带结构进行了测定.结果表明,天然含Al的针铁矿禁带宽度为2.30eV,小于纯针铁矿(2.57eV).本研究提供了一种测定天然氧化物矿物禁带宽度的新方法,为深入研究天然半导体可见光催化活性产生机制提供了理论依据.     

实验条件对X射线衍射物相定量分析结果的影响

通过实验验证了多物相试样X射线衍射定量分析的基体效应,着重研究了参比强度K值、衍射强度、混合物各物相粒度、X射线管功率等实验条件对物相定量分析结果的影响。结果表明,物相定性分析是定量分析的前提,采用积分强度、使各物相颗粒尽可能细而且均匀、采用较高的X射线管功率以及适合的扫描速率均有利于提高物相X射线衍射定量分析的准确度。     

Aluminum X-ray absorption near-edge spectra of some oxide minerals: Calculation versus experimental data

Al X-ray absorption K-edge spectra were calculated for atom clusters which simulate Al surrounded by oxygens in tetrahedral and octahedral environments in five minerals, and are compared with experimental Al K-edge spectra from the same minerals. The experimental data show simple edges, for Al tetrahedra, that are shifted to lower energy by 2 eV compared with more complicated edges for Al octahedra. The calculated edge itself is due to a strong 1s to 3p resonance in the μ₀ for Al. The shifting of the calculated edges is due to different molecular potentials used and different types of atomic scattering. The calculated edge features for tetrahedral Al are all similar, and are due primarily to single scattering from the tetrahedral oxygens. Calculated edge features for octahedral Al are more complicated and are dominated by multiple scattering. The calculated edge features resemble the experimental data, but are much larger in amplitude than their experimental counterparts.     

Polarized X-ray absorption spectra and electronic structure of molybdenite (2H-MoS2)

Polarized S K- and L-edge, Mo L₃- and L₂-edge x-ray absorption near-edge structure (XANES) of natural molybdenite (2H-MoS₂) have been measured with synchrotron radiation. These results are qualitatively interpreted using the energy band model of molybdenite and provide important information on the unoccupied states of molybdenite. The valence band (VB) maximum of molybdenite is characterized by fully occupied Mo 4d_z ², and the conduction band (CB) minimum of molybdenite is characterized by unoccupied Mo 4d states. The unoccupied Mo 4d band is split into two sub-bands, designated as t _2g ^– /t _2g ⁺ and e _g ^– /e _g ⁺ sets. Although the relative energy of these two sets are difficult to be evaluated, probably the former has the lower energy than the latter, both two sets have the combination wave functions of the other unoccupied Mo 4d components, rather than the simple 4d_x ² — y₂ and 4d_xy states. The unoccupied Mo 4d sub-bands contain significant DOS of both S 3 p- and 3 s-like states, indicating strong hybridization with S 3s and 3 p states. In the lower energy sub-band, the DOS of the S p_z- and p_x,y-like states are very similar. However, in the higher energy sub-band, the DOS of the S 3 p_x,y-like state is lower than that of the S 3p_z state. Polarized S K-edge XANES also reveal the features of antibonding S p_z- and p_x,y-like states in molybdenite. The feature assigned to the S 3 p_z-like states is stronger and sharper, and shifts to lower energy by about 2 eV relative to that for the S 3 p_x,y-like states.     

X-ray K-edge absorption spectra of Fe minerals and model compounds: II. EXAFS

K-edge extended X-ray absorption fine structure (EXAFS) spectra of Fe in varying environments in a suite of well-characterized silicate and oxide minerals were collected using synchrotron radiation and analyzed using single scattering approximation theory to yield nearest neighbor Fe-O distances and coordination numbers. The partial inverse character of synthetic hercynite spinal was verified in this way. Comparison of the results from all samples with structural data from X-ray diffraction crystal structure refinements indicates that EXAFS-derived first neighbor distances are generally accurate to ±0.02 Å using only theoretically generated phase information, and may be improved over this if similar model compounds are used to determine EXAFS phase functions. Coordination numbers are accurate to ±20 percent and can be similarly improved using model compound EXAFS amplitude information. However, in particular cases the EXAFS-derived distances may be shortened, and the coordination number reduced, by the effects of static and thermal disorder or by partial overlap of the longer Fe-O first neighbor distances with second neighbor distances in the EXAFS structure function. In the former case the total information available in the EXAFS is limited by the disorder, while in the latter case more accurate results can in principle be obtained by multiple neighbor EXAFS analysis. The EXAFS and XANES spectra of Fe in Nain, Labrador osumulite and Lakeview, Oregon plagioclase are also analyzed as an example of the application of X-ray absorption spectroscopy to metal ion site occupation determination in minerals.     

Effect of coulomb losses on the spectra of heavy particles accelerated in prolonged solar events

This paper examines the possibility of using the energy spectra of accelerated solar cosmic-ray ions and features formed by Coulomb losses to study the solar plasma (the power-law index S for the scattering turbulence, particle number density N, and temperature T of the background medium). For an individual solar flare, Coulomb losses can be manifest to different degrees in the spectra of different ions, providing a means to determine S. A comparison of theoretical spectra for H, He, C, O, and Fe ions with observed spectra for the prolonged solar flare of October 20, 1995 yields S≈3, N≈5×10⁹ cm^?3, and T≈10⁶ K, assuming that the characteristic time scale over which these particles gain energy is about a second.     

Site occupancies of minor elements in synthetic olivines as determined by channeling-enhanced X-ray emission

Major- and minor-element site occupancies have been determined for Ni- and Mn-rich synthetic forsteritic olivines that were equilibrated to 300, 600, 900, or 1,050° C, by analyzing electron-channeling-enhanced x-ray emissions. Site preferences for minor elements (Ni>Cr for M1; Ca>Mn for M2) are consistent with those reported in previous studies. Degrees of ordering at different elevated temperatures differ less than analytical errors, although there may be significant differences in ordering between olivines that have been rapidly quenched from 300° C or higher and those that have undergone normal lava-flow cooling histories. An x-ray structure refinement of San Carlos olivine (R=0.031) gives major-element occupancies similar to those determined by electron-channeling on the same material. Thus, the channeling technique gives results that are consistent with other methods, and it may be possible to use the technique to identify olivines that have undergone very rapid quenching.     

Search for differences in the velocities and directions of the kiloparsec-scale jets of quasars with and without X-ray emission

The X-ray emission of the kiloparsec-scale jets of core-dominant quasars is usually interpreted as inverse Compton scattering on the cosmic microwave background (CMB) emission (Sample I). By analogy with the situation on parsec scales, ultrarelativistic motion along a jet oriented at a small angle to the line of sight is usually invoked to explain the X-ray emission while also satisfying the condition of equipartition between the energies associated with the relativistic particles and the magnetic field on kiloparsec scales. This leads to an increase in the energy flux of the CMB radiation in the rest frame of the kiloparsec-scale jets. Consequently, the intensity of the CMB radiation is enhanced to the level required for detectable X-ray emission. This suggests that kiloparsec jets of quasars with similar extents and radio flux densities that are not detected in the X-ray (Sample II) could have subrelativistic speeds and larger angles to the line of sight, due to deceleration and bending of the jet between parsec and kiloparsec scales. This suggests the possible presence of differences in the distributions of the difference between the position angle for the parsec-scale and kiloparsec-scale jets for these two groups of quasars; this is not confirmed by a statistical analysis of the data for Samples I and II. It is deduced that most of the sources considered exhibit bending of their jets by less than about 1.5 times the angle of the parsec-scale jet to the line of sight. This suggests that the X-ray emission is generated by other mechanisms that there is no equipartition.     

Modeling of emission spectra of the flaring red dwarf EV Lac: Active regions,flares, and microflares

Echelle spectrograms of the flaring red dwarf EV Lac obtained on the Nordic Optical Telescope are used to study the chromosphere of the star during the impulsive flare of 23:19 UT on August 30, 1994, and in two nearby time intervals when UBV RI photometry showed the star to be in a quiescent state. The high spectral resolution of the observations enabled the detection of broad Balmer emission wings in the nonflaring state, which are interpreted as evidence for microflares. Semiempirical models for active regions in the chromosphere, flares, and microflares are constructed and used to compare the main physical characteristics of these structures. The place of the detected microflares in the overall energetics of flare events in EV Lac is discussed.