Strong X-ray Absorption Research Articles

Evidence for X-ray obscuration in Type II quasar candidates from the Sloan Digital Sky Survey

Recently, Zakamska et al. selected 291 high-ionization narrow-emission-line active galactic nuclei (AGN) in the redshift range 0.3-0.83 from the Sloan Digital Sky Survey (SDSS) spectroscopic data. The sample includes both type 2 Seyfert galaxies and their higher-luminosity 'cousins', Type II quasar candidates. Here we present the results on the X-ray properties of 17 of these objects for which archival X-ray data (ROSAT and XMM-Newton) are available. Three sources have been significantly (≥6a) detected, one being the most radio-loud source of the sample; its X-ray emission, possibly enhanced by jet emission, is consistent with the absence of absorption. Another source has 6σ detection in the ROSAT All-Sky Survey (RASS), possibly complex radio structure, and no evidence for strong X-ray absorption. For the third X-ray detection, the XMM-Newton spectrum indicates a column density of N H = 1.26 +0.75 -0.51 x 10 22 cm -2 ; this result, coupled with the 2-10 keV luminosity of 4 × 10 44 erg s -1 , makes this source a genuine Type II quasar. Using the [O III] λ5007 line luminosities, we estimated the intrinsic X-ray power of the AGN and found that ≥47 per cent of the observed sample shows indications of X-ray absorption, with column densities ≥10 22 cm -2 . This provides further evidence that a considerable fraction are obscured quasars. Support for our conclusions also comes from the very recent analysis of RASS data performed by Zakamska et al., who found five additional lower-significance (2.1σ-3.6σ) X-ray matches.

The molecular clouds in the environs of the supernova remnants G349.7+0.2 and G18.8+0.3

We present the results of a new high-resolution study of the molecular gas associated with the supernova remnants (SNRs) G349.7+0.2 and G18.8+0.3. The observations were performed with the SEST telescope in the 12 CO J = 1-0, 2-1 and 3-2 lines (beams of 45, 23 and 15, respectively). The present observations have provided, for the two SNRs, new evidence in support of the existence of physical interaction between the SN shocks and the adjoining molecular clouds. In the case of G349.7+0.2, the new observations revealed for the first time the internal structure of the shocked cloud, as well as the kinematical consequences of the impact of the SNR shock on the molecular cloud. From these observations we were able to constrain the conditions of the pre-shocked gas. The molecular cloud associated with G349.7+0.2, centered near ν LSR = +16.2 km s -1 , has a linear size of about 7 pc, a mass of ∼10 4 M ○. and a volume density of ∼10 3 cm -3 . The high line ratios derived are indicative of the existence of shocks in the cloud. From the asymmetries observed in the line shapes we propose that the SN shock cloud is running into the denser part of the cloud and has probably begun to disrupt it, pushing the eastern component clumps away from us, and the western fragments toward us. After comparing our estimates of the column density of the intervening gas with similar calculations based on ASCA X-rays spectral fitting we conclude that the best way to make these results compatible is by assuming that the associated cloud is placed behind G349.7+0.2 along the line of sight, and the SNR/molecular cloud encounter is taking place on the far side of the SNR. This model also provides a natural explanation for the lack of strong X-ray absorption in the central region of G349.7+0.2. Evaporation of part of the associated cloud must be responsible for the central X-ray emission. The comparison with IRAS infrared data provides additional support for the hypothesis of SNR/cloud physical interaction. From the study of the molecular gas in the neighborhood of the five OH (1720 MHz) masers detected in G349.7+0.2 we find that in three cases the maser peak velocity coincides with the local CO peak velocity, while in the remaining two cases the maser peak velocity agrees with a secondary, blended CO component. We conclude that the masers are excited at the sites where a non-dissociative C-type shock, locally transverse to the line of sight (or forming a large angle with it), hits a denser molecular clump. For the SNR G18.8+0.3, the new higher resolution observations have revealed excellent morphological agreement between one of the cloud components and the SNR shock front towards the eastern limb. The associated molecular mass is estimated to be ∼4.4 x 10 4 M ○. and the cloud volume density ∼1200 cm -3 . The analysis of the line ratios in this case revealed a maximum of R 2-1/1-0 = 1.25 at a position that exactly matches an indentation in the radio continuum emission in the remnant's shell, providing additional evidence of SNR/molecular cloud interaction.

XMM-Newton observation of the Chandra Deep Field-South: statistical treatment of faint source spectra

Abstract We present first results of the X-ray spectral analysis of 500 ksec deep survey images obtained with XMM-Newton on the Chandra Deep Field South (CDFS). Statistical distributions of spectral index and intrinsic absorption are derived for a sample containing 70 sources with a count limit of 100 (flux limit in the 2-10 keV band of 8.9×10 −16 erg cm −2 s −1 ), of which 44 sources have had redshifts identified. We observe a spatial separation between the type-1 and the type-2 AGN in diagnostics involving different X-ray parameters. Using the subsample with known z , we show that this separation between the AGN populations is a consequence of different absorption column densities. The two populations have the same average spectral index, 〈Γ〉=2±0.1. We present integrated spectra for the most distant type-2 QSO which shows strong X-ray absorption and a clear soft excess; we obtained the best fit for this object with a scattering model. We also confirm a progressive hardening for the combined integral spectra for faint objects which at first was noted by [Proceedings of the XXI Moriond Astrophysics Meeting (2001)]. Our results show a clear evolution of decrease of 〈Γ〉 with decreasing flux in the hard 2-10 keV band. However, we detect not only a regular increase in 〈Γ〉 for different subsamples of fluxes compared to Chandra results, but also a discrepancy of the values within our own data set, if we fitted in the different energy bands.

Identification of lapis-lazuli pigments in paint layers by PIGE measurements

Lapis-lazuli is a semi-precious stone used in the past to produce a blue pigment. Its main component is lazurite, 3Na 2O·3Al 2O 3·6SiO 2·2Na 2S. The possibility of using PIXE to identify this pigment in canvas and wood painting is severely limited by the strong absorption of low-energy X-rays in the protective varnish and – when, as typical, the pigment is mixed with lead white – by the overlapping of Pb M lines with S K α. In this work we discuss the possibility of identifying lapis-lazuli by detecting sodium with PIGE. PIXE and PIGE measurements have been performed on samples containing lapis-lazuli mixed to lead white in different percentage, covered with polymeric foils to simulate the presence of varnish. At a percentage of lapis-lazuli below 50%, Na X-rays are hardly detectable even with the thinner foil; on the contrary the characteristic γ-rays are clearly detected down to about 1%. A first application has been successfully performed on the “Madonna dei fusi” by Leonardo da Vinci, in the framework of an extensive scientific investigation on the painting techniques used by the Renaissance genius.

TheBeppoSAXView of the X‐Ray Active Nucleus of NGC 4258

BeppoSAX observed the Seyfert 1.9 galaxy NGC 4258 in 1998 December, when its 2-10 keV luminosity was about 1041 ergs s-1. Large amplitude (100%) variability is observed in the 3-10 keV band on timescales of a few tens of thousands of seconds, while variability of ~20% is observed on timescales as short as 1 hr. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5 ± 1.2) × 1022 cm-2; this component is detected at up to 70 keV with a signal-to-noise ratio of 3 and with a steep power-law energy spectral index of αE = 1.11 ± 0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in advection-dominated accretion flow models with strong winds, is ruled out by the data. The ratio between the nuclear radio (22 GHz) luminosity and the X-ray (5 keV) luminosity is consistent with that of radio-quiet quasars and Seyfert galaxies. X-ray variability, spectral shape, and radio/X-ray and near-IR/X-ray luminosity ratios suggest that the nucleus of NGC 4258 could be a scaled down version of a Seyfert nucleus and that the X-ray nuclear luminosity can be explained in terms of Comptonization in a hot corona. The soft (E 2 keV) X-ray emission is complex. There are at least two thermal-like components with temperatures of 0.6 ± 0.1 keV and 1.3 keV. The cooler (L0.1-2.4 keV~ 1040 ergs s-1) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI (Cecil et al. 1994). The luminosity of the second component, which can be modeled equally well by an unobscured power-law model with αE = 0.2, is L0.1-2.4 keV~ 7 × 1039 ergs s-1, consistent with that expected from discrete X-ray sources (binaries and supernova remnants) in the host galaxy. Observations of NGC 4258 and other maser active galactic nuclei (AGNs) show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of water maser emission from X-ray illumination. So a large column density absorbing gas may be a necessary property of masing AGNs.

The recent history of the X-ray absorber in NGC 3516

We present two BeppoSAX observations of the bright Seyfert 1 galaxy NGC 3516, performed four months apart (late 1996–early 1997). The earlier spectrum is considerably weaker and harder in the whole 0.1–50 keV energy range. In addition, the warm absorber oxygen features, which were already observed with ROSAT and ASCA, are much less pronounced. The most straightforward explanation is that in 1996 November NGC 3516 was being seen through a substantial (NH≃1022 cm−2) column of cold material. This is the first confirmation with modern instrumentation that NGC 3516 indeed undergoes phases of strong cold X-ray absorption. We speculate that these intervals may be connected to the onset of the remarkably variable UV absorption system, making NGC 3516 the best known example of a low-luminosity broad absorption line (BAL) quasar. The absorbing matter could be provided by clouds ablated from the rim of the circumnuclear molecular torus, seen at a rather high inclination angle.

Gallium K-edge x-ray absorption study on Mg-doped GaN

Ga K-edge x-ray absorption measurements were employed to investigate Mg-doping effects in GaN samples. Strong polarization-dependent x-ray absorption near-edge structures become less pronounced with increasing doping concentration, indicating the formation of a mixing-phase structure of cubic and hexagonal phases. Analysis of the extended x-ray absorption region of the spectra revealed doping-related defects such as vacancies, substitutions, and interstitial occupations. They were formed anisotropically in the crystal c axis direction and its perpendiculars. Disorderliness arising from phase mix and defects is believed to have lowered the Debye temperature of the doped GaN films and caused the destructive interference of the absorption fine-structure oscillation functions. These effects were taken into account for the observed large coordination number reductions in our samples.

The effect of large solid angles of collection on quantitative X-ray microanalysis in the AEM.

Increasing the solid angle of X-ray collection is a major factor in improving the analytical sensitivity of X-ray energy-dispersive spectrometry (XEDS) in the analytical electron microscope (AEM). A new scanning transmission electron microscope, the VG HB 603, is equipped with two XEDS detectors with the largest collection angles (0.30 and 0.17 sr) available in commercial AEMs. However, large collection angles result in a large range of take-off angles, from approximately 4 degrees to 36 degrees, and the low angles can cause strong X-ray absorption. In order to investigate possible detrimental effects of the low (and of the range of) take-off angles on quantitative microanalysis of specimens exhibiting significant absorption, a stoichiometric Ni3Al thin-film, in which the Al Kalpha line is significantly absorbed, was analysed. Furthermore, the effect of different values of the collection angle on X-ray intensities was theoretically evaluated by numerical calculations and spectral simulation. These theoretical approaches permitted correlation of changes in the X-ray take-off angle (and hence X-ray absorption) with changes in the collection angle. It is demonstrated that approximately 0.30 sr detectors, with minimum take-off angles as small as 4 degrees, only result in maximum errors of 4% in the quantification of Al in Ni3Al and, therefore, further increases in collection angle can be pursued while maintaining current levels of accuracy of quantification.

Absorption in 3C 212

3C 212 (Q0855+143) is a red quasar with ambiguous evidence for strong low-energy X-ray absorption in its ROSAT spectrum (Elvis et al. 1994). We combine Einstein and ROSAT data with ultraviolet absorption-line strengths to investigate the physical state of the absorber. We find that a highly ionized absorber (U approximately 0.3) provides a consistent fit to both X-ray and UV data as well as providing a significantly improved fit to the X-ray spectrum. The absorber is outflowing with a velocity of approximately 1600 km/sec with respect to the Mg II emission line, situated outside the broad emission-line region (BELR) and has low density. The alternative blackbody fit to the X-ray data is excluded if the source has not varied, and it is also unable to predict the UV absorption lines. This is the second case of an X-ray absorber identified with a UV absorber (the first one being 3C 351; Mathur et al. 1994) and suggests that UV/X-ray absorbers may be common. This is an exciting possibility as the combination of UV and X-ray constraints is very powerful in determining the physical conditions of an absorber.

Fresnel and refractive lenses for X-rays

We present a Gaussian beam analysis of X-ray refractive and Fresnel lenses. The X-ray refractive lens is featured by an intrinsic soft (Gaussian) aperture due to strong absorption of X-rays by materials. We defined a parameter N 0, the critical Fresnel number (CFN), to describe this optical property. The values of N 0 for all practical materials are below 1000 for photon energies exceeding 30 eV, still lower for high- Z materials. The maximum effective Fresnel number of a lens is determined by its material to be 2 N 0 and its maximum enhancement of X-ray intensity is limited to (2 πN 0) 2, independent of its shape. We found that the refractive lens is likely to be useful for manipulating nearly diffraction limited beam in the hard X-ray region and its application is severely restricted by available fabrication capabilities today. X-ray Fresnel lenses, both in cylindrical and linear forms, are proposed as superior focusing elements for hard X-rays. Their high efficiency, up to 100% in optimal construction, will enable us to manipulate beams with multiple lenses and obtain higher performance optics. Their design and fabrication are discussed in reference to those of X-ray Fresnel zone plates and micro Fresnel lenses for optoelectronics.

Quality control by PEXE of Al/Si alloys

Binary alloys of Al/Si with small amounts of other elements, either intentionally or accidentally added, are of frequent use for industrial purposes. The macroscopic homogeneity and the minor element concentrations can be reliably evaluated by thick target PIXE with protons at energies of about 1 MeV, except that Si concentrations are overestimated when computed with the conventional thick target PIXE formulae. This is shown to be a combined effect of the Si clustering in grains and the strong absorption of Si X-rays by aluminum. A simplified model is offered to explain quantitatively the effect and a strategy is presented whereby a correct evaluation of Si concentrations is deduced from two PIXE spectra produced with protons at two different energies.

Laser-stimulable transparent CsI:Na film for a high quality x-ray imaging sensor

Transparent films, which can be stimulated by laser beams after x-ray irradiation, have been searched to improve the spatial resolution of digital x-ray imaging sensors. As a result, evaporated CsI:Na films are found to be efficiently laser stimulable around 77 K. The modulation transfer function (MTF) of the film is evaluated using a scanning Ga1−xAlxAs semiconductor laser. The high MTF value (57% at 2 1p/mm), strong x-ray absorption, and high stimulation efficiency of the film ensure a high quality sensor for digital radiography.

Considerations for application of Si(Li) detectors in analysis of sub-keV, ion-induced X-rays

Spectroscopy of ion-induced X-rays is commonly performed using lithium-drifted, silicon detectors, Si(Li), with beryllium windows. Strong absorption of X-rays with energies below 1 keV occurs in even the thinnest commercially available beryllium windows and precludes useful analysis of sub-keV X-rays. Access to the sub-keV X-ray region, can be achieved using windowless (WL) and ultra-thin-windowed (UTW) Si(Li) detectors. These detectors have been shown to be useful for Spectroscopy of X-rays with energies above approximately 200 eV. The properties of such detectors are reviewed with regard to analysis of ion-induced X-rays. In particular, considerations of detection efficiency, output linearity, energy resolution, peak shapes and vacuum requirements are presented. The use of ion excitation for determination of many detector properties serves to demonstrate the usefulness of WL and UTW detectors for the Spectroscopy of sub-keV, ion-induced X-rays.

Position of caesium ions in crystalline B-form DNA determined by synchrotron radiation diffraction: Preliminary results

The diffraction patterns of CsDNA taken with copper X-irradiation are considerably impaired because of the strong X-ray absorption by caesium ions. The use of high power synchrotron radiation of wavelength λ = 1.2 Å has yielded photographs suitable for intensity measurements. The structure of phage T2 CsDNA at 76% relative humidity is isomorphous to the crystalline B-form of LiDNA, and the disposition of cations appears to conform to the 10-fold screw symmetry of B-DNA. The structure factor amplitudes of 20 reflections in the diffraction pattern of phage T2 CsDNA are noticeably different from those of the same reflections in the LiDNA diffraction pattern, and the positions of the Cs + ions could thus be found. The best model has the cations located significantly close to dyad axes lying between the planes of successive nucleotide pairs. One of the two cations “belonging” to a nucleotide pair touches the surface of the narrow groove of the B-DNA double helix, while the other is on the wide groove side, rather far from its “own” DNA molecule.