Low-resolution X-ray Spectra Research Articles

Analysis of X-Ray Emission from OB Stars. IV. About X-Ray Emission from Inhomogeneous Winds of OB Stars

We study the origin of X-ray emission from OB stars due to collisions of stellar winds and/or inhomogeneities in the winds. The low-resolution X-ray spectra of a big sample of OB stars were fitted by both the stationary APEC/MEKAL models and by this model with an additional PSHOCK component describing the nonstationary X-ray emission. These spectra were also described by two-temperature PSHOCK models. More than ∼50% of considered spectra can be described by the above-mentioned model combinations including the PSHOCK model and the quality of the fits appears to be better for O stars. The plasma temperature of the PSHOCK component is about 1–5 keV with the ionization timescale τ u ∼ 108–1013 s cm−3. The temperature of the PSHOCK component increases with the momentum and kinetic energy of the stellar wind by a power law with an index ∼0.12–0.14. Such dependencies were not revealed through modeling by the stationary APEC/MEKAL models only. At the same time the X-ray luminosity of OB stars depends on momentum and kinetic energy of their winds similarly either for stationary or for nonstationary models. We conclude that many O stars and some B stars can be sources of the nonstationary X-rays formed in their inhomogeneous stellar wind.

X-rays observations of a super-Chandrasekhar object reveal an ONe and a CO white dwarf merger product embedded in a putative SN Iax remnant

The merger of two white dwarfs (WDs) is a natural outcome of the evolution of many binary stars. Recently, a WD merger product, IRAS 00500+6713, was identified. IRAS 00500+6713 consists of a central star embedded in a circular nebula. The analysis of the optical spectrum of the central star revealed that it is hot, hydrogen, and helium free, and it drives an extremely fast wind with a record breaking speed. The nebula is visible in infrared and in the [O III] λ5007 Å line images. No nebula spectroscopy was obtained prior to our observations. Here we report the first deep X-ray imaging spectroscopic observations of IRAS 00500+6713. Both the central star and the nebula are detected in X-rays, heralding the WD merger products as a new distinct type of strong X-ray sources. Low-resolution X-ray spectra reveal large neon, magnesium, silicon, and sulfur enrichment of the central star and the nebula. We conclude that IRAS 00500+6713 resulted from a merger of an ONe and a CO WD, which supports earlier suggestion for a super-Chandrasekhar mass of this object. X-ray analysis indicates that the merger was associated with an episode of carbon burning and possibly accompanied by an SN Iax. In X-rays, we observe the point source associated with the merger product while the surrounding diffuse nebula is a supernova remnant. IRAS 00500+6713 will likely terminate its evolution with another peculiar Type I supernova, where the final core collapse to a neutron star might be induced by electron captures.

Analysis of the X-ray emission from OB stars III: low-resolution spectra of OB stars

This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars. In our two previous papers, we study the high-resolutionX-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field, mass loss rate and terminal wind velocity. Our previous analysis of high-resolution spectra showed that the correlations are weak or even absent. In the present paper, we analyzed low-resolution X-ray spectra, using model-independent X-ray hardness values for checking the above mentioned dependencies. We establish that X-ray luminosities LX weakly depend on the stellar magnetic field. At the same time, LX ∝ Ṁ0.5 and , where ṁ is the mass loss rate and Ekin is the kinetic energy of the wind. The X-ray luminosities decrease with growing magnetic confinement parameter η. We also argue that there is an additional (probably non-thermal) component contributed to the stellar X-ray emission.

XZ: Deriving redshifts from X-ray spectra of obscured AGN

Context. Redshifts are fundamental for our understanding of extragalactic X-ray sources. Ambiguous counterpart associations, expensive optical spectroscopy, and/or multimission multiwavelength coverage to resolve degeneracies often make estimation difficult in practice. Aims. We attempt to constrain redshifts of obscured active galactic nuclei (AGN) using only low-resolution X-ray spectra. Methods. Our method for determining redshifts from the X-ray spectrum (XZ) fits AGN X-ray spectra with a moderately complex spectral model incorporating a corona, a torus obscurer, and a warm mirror. Using the Bayesian X-ray Astronomy (BXA) package, we constrain redshift, column density, photon index, and luminosity simultaneously. The redshift information primarily comes from absorption edges in Compton-thin AGN, and from the Fe Kα fluorescent line in heavily obscured AGN. A new generic background fitting method allows us to extract more information from limited numbers of source counts. Results. We derive redshift constraints for 74/321 hard-band detected sources in the Chandra deep field South. Comparing with spectroscopic redshifts, we find an outlier fraction of 8%, indicating that our model assumptions are valid. For three Chandra deep fields, we release our XZ redshift estimates. Conclusions. The independent XZ estimate is easy to apply and effective for a large fraction of obscured AGN in today’s deep surveys without the need for any additional data. Compared to different redshift estimation methods, XZ can resolve degeneracies in photometric redshifts, help detect potential association problems, and confirm uncertain single-line spectroscopic redshifts. With high spectral resolution and a large collecting area, this technique will be highly effective for Athena/WFI observations.

New multiwavelength observations of the Of?p star CPD -28 2561

A rather strong mean longitudinal magnetic field of the order of a few hundred Gauss was detected a few years ago in the Of?p star CPD -28 2561 using FORS2 low-resolution spectropolarimetric observations. In this work we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS2 (FOcal Reducer low dispersion Spectrograph) mounted at the 8-m Antu telescope of the VLT. These observations cover a little less than half of the stellar rotation period of 73.41d mentioned in the literature. The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle, indicating a dominant dipolar contribution to the magnetic field topology. The estimated polar strength of the surface dipole B_d is larger than 1.15kG. Further, we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths. The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum. The comparison of the spectral behaviour of CPD -28 2561 with that of another Of?p star, HD148937 of similar spectral type, reveals remarkable differences in the degree of variability between both stars. Finally, we present new X-ray observations obtained with the Suzaku X-ray Observatory. We report that the star is X-ray bright with log L_X/L_bol ~= -5.7. The low resolution X-ray spectra reveal the presence of a plasma heated up to 24MK. We associate the 24MK plasma in CPD -28 2561 with the presence of a kG strong magnetic field capable to confine stellar wind.

Coronal Abundances in Orion Nebula Cluster Stars

Following the Chandra Orion Ultradeep Project (COUP) observation, we have studied the chemical composition of the hot plasma in a sample of 146 X-ray bright pre-main sequence stars in the Orion Nebula Cluster. We report measurements of individual element abundances for a subsample of 86 slightly-absorbed and bright X-ray sources, using low resolution X-ray spectra obtained from the Chandra ACIS instrument. The X-ray emission originates from a plasma with temperatures and elemental abundances very similar to those of active coronae in older stars. A clear pattern of abundances vs. First Ionization Potential (FIP) is evident if solar photospheric abundances are assumed as reference. The results are validated by extensive simulations. The observed abundance distributions are compatible with a single pattern of abundances for all stars, although a weak dependence on flare loop size may be present. The abundance of calcium is the only one which appears to vary substantially between stars, but this quantity is affected by relatively large uncertainties. The ensemble properties of the X-ray bright COUP sources confirm that the iron in the emitting plasma is underabundant with respect to both the solar composition and to the average stellar photospheric values. Comparison of the present plasma abundances with those of the stellar photospheres and those of the gaseous component of the nebula, indicates a good agreement for all the other elements with available measurements, and in particular for the high-FIP elements (Ne, Ar, O, and S) and for the low-FIP element Si. We conclude that there is evidence of a significant chemical fractionation effect only for iron, which appears to be depleted by a factor 1.5--3 with respect to the stellar composition.

ChandraObservations of Associates of η Carinae. II. Spectra

The low-resolution X-ray spectra around η Car covering Trumpler 16 and part of Trumpler 14 have been extracted from a Chandra CCD ACIS image. Various analysis techniques have been applied to the spectra based on their count rates. The spectra with the greatest number of counts (HD 93162 = WR 25, HD 93129 AB, and HD 93250) have been fitted with a wind model, which uses several components with different temperatures and depths in the wind. Weaker spectra have been fitted with Raymond-Smith models. The weakest spectra are simply intercompared with strong spectra. In general, fits produce reasonable parameters based on knowledge of the extinction from optical studies and on the range of temperatures for high- and low-mass stars. Direct comparisons of spectra confirm the consistency of the fitting results and also hardness ratios for cases of unusually large extinction in the clusters. The spectra of the low-mass stars are harder than the more massive stars. Stars in the sequence evolving from the main sequence (HD 93250) through the system containing the O supergiant (HD 93129 AB) and then through the Wolf-Rayet stage (HD 93162), presumably ending in the extreme example of η Car, share the property of being unusually luminous and hard in X-rays. For these X-ray-luminous stars, their high mass and evolutionary status (from the very last stages of the main sequence and beyond) is the common feature. Their binary status is mixed, and their magnetic status is still uncertain.

Bright, point X-ray source based on a commercial portable 40 ps Nd:YAG laser system

We present some experimental results on X-ray spectra obtained from plasmas produced using a compact Nd:YAG laser system. The beam was focused on different targets (Cu, Al, Ge,…) and both high resolution and low resolution X-ray spectra were recorded.

Multiple ionization effects in low-resolution X-ray spectra induced by energetic heavy ions

The method of analysis of the multiple ionization effects in low-resolution X-ray spectra induced by heavy ions is described here. It is shown that, by fitting the X-ray spectra measured by a semiconductor detector with the proposed model, which accounts for the multiple ionization effects, the ionization probabilities can be determined as free fitting parameters. This approach is based on the assumption that the intensity distribution of emitted X-ray satellites is approximately described by the binomial distribution parameterized by the ionization probability at the moment of X-ray emission. In particular, we demonstrate that the excited X-ray satellites, when measured by a low-resolution semiconductor detector, appear as the Gaussian profile, which is shifted and broadened with respect to the diagram line. Moreover, we find that both X-ray line shift and width are expressed in terms of the multiple ionization probabilities, as well as the X-ray energy shifts per vacancy. These observations allowed to develop a novel method of X-ray spectra fitting. Detailed discussion of the approximations used in the present approach is given. The method developed, using the calculated Dirac–Fock X-ray energy shifts per vacancy, was applied to determine the multiple ionization probabilities in M- and N-shell by fitting the measured X-ray spectra for L γ(L→N,O) transitions induced by heavy ions in selected high-Z atoms.

Coronal and Transition Region Structure in the RS Canum Venaticorum Binaries V711 Tauri, AR Lacertae, and II Pegasi. I. Data Analysis and Emission Measure Distributions

We present a study of the transition regions and coronae of the RS CVn binaries V711 Tau, AR Lac, and II Peg. Archival data from the Extreme Ultraviolet Explorer (EUVE), International Ultraviolet Explorer (IUE), Goddard High-Resolution Spectrograph (GHRS), and a variety of X-ray telescopes have been analyzed and modeled. Emission measure distributions have been determined for the transition region of each system. The upper transition region (6.1 < log Te < 7.4) has been studied using line fluxes from the EUVE. Emission measure distributions for the lower transition region (3.8 < log Te < 5.3) have been found using emission-line fluxes obtained with IUE and the GHRS. Electron densities of ~1.6 × 1011 and ~5 × 1011 cm-3 have been determined at log Te = 7.0 and log Te = 4.7 respectively, using the density-sensitive Fe XXI λ128.7 and Si III] λ1892 lines. The total pressure (gas + turbulent) is smaller at the lower temperature, indicating that the high- and low-temperature material originates in physically unconnected regions. The emission measure distributions derived from the EUVE line spectra are compared with emission measures found from spectral fits to a variety of low-resolution X-ray spectra, including our reanalysis of archival EXOSAT spectra. Although the emission measure distribution is continuous, both types of analysis lead to two distinct peaks in the high-temperature emission measure distribution, suggesting that there may be two classes of structure in the upper transition region and corona, e.g., two families of loops.

An Objective Multicolor Method for the Characterization of Low-Resolution X-Ray Spectra

view Abstract Citations (5) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Objective Multicolor Method for the Characterization of Low-Resolution X-Ray Spectra Collura, A. ; Micela, G. ; Sciortino, S. ; Harnden, F. R., Jr. ; Rosner, R. Abstract We describe a new technique for the characterization of low-resolution X-ray spectra, based on generalized colors defined by means of a principal component analysis. Application of the method to a sample of optically and radio-selected Einstein Observatory X-ray sources yields groups that are dominated either by stars or by extragalactic sources. The method thereby provides an immediate interpretation in terms of differences in the physics of the emission processes dominating the objects studied. The principal advantages of the method are that it provides an objective means of classifying source spectra and that it provides an objective means of obtaining the minimal set of spectral classification parameters. Though the method does not significantly improve results obtained with previously employed techniques when applied to Einstein data, its objective operation should prove powerful in aiding source classification in the more recent medium spectral resolution X-ray surveys based on the ROSAT and ASCA mission. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175772 Bibcode: 1995ApJ...446..108C Keywords: X-RAYS: GALAXIES; X-RAYS: STARS full text sources ADS |