X-ray emission from thin plasmas. Collisional ionization for atoms and ions of H to Zn

Every observation of astrophysical objects involving a spectrum requires atomic data for the interpretation of line fluxes, line ratios and ionization state of the emitting plasma. One of the processes which determines it is collisional ionization. In this study an update of the direct ionization (DI) and excitation-autoionization (EA) processes is discussed for the H to Zn-like isoelectronic sequences. In the last years new laboratory measurements and theoretical calculations of ionization cross sections have become available. We provide an extension and update of previous published reviews in the literature. We include the most recent experimental measurements and fit the cross sections of all individual shells of all ions from H to Zn. These data are described using an extension of Younger's and Mewe's formula, suitable for integration over a Maxwellian velocity distribution to derive the subshell ionization rate coefficients. These ionization rate coefficients are incorporated in the high-resolution plasma code and spectral fitting tool SPEX V3.0.Comment: Accepted for publication in A&A, 22 pages, 18 figure

Simultaneous X-ray and Ultraviolet spectroscopy of the Seyfert galaxy NGC 5548. III. X-ray time variability

The Seyfert 1 galaxy NGC 5548 was observed for a week by Chandra using both the HETGS and LETGS spectrometers. In this paper we study the time variability of the continuum radiation. During our observation, the source showed a gradual increase in flux over four days, followed by a rapid decrease and flattening of the light curve afterwards. Superimposed upon these relatively slow variations several short duration bursts or quasi-periodic oscillations occured with a typical duration of several hours and separation between 0.6-0.9 days. The bursts show a delay of the hard X-rays with respect to the soft X-rays of a few hours. We interprete these bursts as due to a rotating, fluctuating hot spot at approximately 10 gravitational radii; the time delay of the hard X-rays from the bursts agree with the canonical picture of Inverse Compton scattering of the soft accretion disk photons on a hot medium that is relatively close to the central black hole.Comment: 6 pages, 7 figures, accepted for publication in Astronomy and Astrophysic

X-ray narrow line region variability as a geometry probe: The case of NGC 5548

We study the long time scale variability of the gas responsible for the X-ray narrow emission lines in the Seyfert 1 galaxy NGC 5548, in order to constrain the location and geometry of the emitting gas. Using X-ray spectra taken with the Chandra-LETGS and HETGS instruments and with XMM-Newton RGS and combining them with long-term monitoring observations of the Rossi X-ray Timing Explorer (RXTE), we perform a correlation analysis in order to try constrain the time scale on which the narrow line emitting gas responds to variations of the continuum flux. With the inclusion of the 2007 Chandra-LETGS observation we have an additional observation at an historically low flux level. We conclude that the NLR in NGC 5548 is in the form of an ionization cone, compact in size, and located between 1 and 15 pc from the central source, depending on the exact geometry of the NLR.Comment: 6 pages, 4 figures, accepted for publication in A&

Oxygen emission in remnants of thermonuclear supernovae as a probe for their progenitor system

Recent progress in numerical simulations of thermonuclear supernova explosions brings up a unique opportunity in studying the progenitors of Type Ia supernovae. Coupling state-of-the-art explosion models with detailed hydrodynamical simulations of the supernova remnant evolution and the most up-to-date atomic data for X-ray emission calculations makes it possible to create realistic synthetic X-ray spectra for the supernova remnant phase. Comparing such spectra with high quality observations of supernova remnants could allow to constrain the explosion mechanism and the progenitor of the supernova. The present study focuses in particular on the oxygen emission line properties in young supernova remnants, since different explosion scenarios predict a different amount and distribution of this element. Analysis of the soft X-ray spectra from supernova remnants in the Large Magellanic Cloud and confrontation with remnant models for different explosion scenarios suggests that SNR 0509-67.5 could originate from a delayed detonation explosion and SNR 0519-69.0 from an oxygen-rich merger.Comment: 8 pages, 4 figures, MNRAS accepte

Simultaneous X-ray and UV spectroscopy of the Seyfert 1 galaxy NGC 5548.II. Physical conditions in the X-ray absorber

We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened emission lines of O VII and C VI in the spectra, similar to those observed in the optical and UV bands. The source was continuously variable, with a 30 % increase in luminosity in the second half of the observation. No variability in the warm absorber was detected between the spectra from the first 170 ks and the second part of the observation. The velocity structure of the X-ray absorber is consistent with the velocity structure measured simultaneously in the ultraviolet spectra. We find that the highest velocity outflow component, at -1040 km/s, becomes increasingly important for higher ionization parameters. This velocity component spans at least three orders of magnitude in ionization parameter, producing both highly ionized X-ray absorption lines (Mg XII, Si XIV) as well as UV absorption lines. A similar conclusion is very probable for the other four velocity components. Based upon our observations, we argue that the warm absorber probably does not manifest itself in the form of photoionized clumps in pressure equilibrium with a surrounding wind. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data. From the shape of this distribution and the assumption that the mass loss through the wind should be smaller than the accretion rate onto the black hole, we derive upper limits to the solid angle as small as 10^{-4} sr. From this we argue that the outflow occurs in density-stratified streamers. The density stratification across the stream then produces the wide range of ionization parameter observed in this source. Abridged.Comment: 21 pages, 12 figures accepted for publication in A&

Effective area calibration of the reflection grating spectrometers of XMM-Newton. II. X-ray spectroscopy of DA white dwarfs

White dwarf spectra have been widely used as a calibration source for X-ray and EUV instruments. The in-flight effective area calibration of the reflection grating spectrometers (RGS) of XMM-Newton depend upon the availability of reliable calibration sources. We investigate how well these white dwarf spectra can be used as standard candles at the lowest X-ray energies in order to gauge the absolute effective area scale of X-ray instruments. We calculate a grid of model atmospheres for Sirius B and HZ 43A, and adjust the parameters using several constraints until the ratio of the spectra of both stars agrees with the ratio as observed by the low energy transmission grating spectrometer (LETGS) of Chandra. This ratio is independent of any errors in the effective area of the LETGS. We find that we can constrain the absolute X-ray spectrum of both stars with better than 5 % accuracy. The best-fit model for both stars is close to a pure hydrogen atmosphere, and we put tight limits to the amount of helium or the thickness of a hydrogen layer in both stars. Our upper limit to the helium abundance in Sirius B is 4 times below the previous detection based on EUVE data. We also find that our results are sensitive to the adopted cut-off in the Lyman pseudo-continuum opacity in Sirius B. We get best agreement with a long wavelength cut-off. White dwarf model atmospheres can be used to derive the effective area of X-ray spectrometers in the lowest energy band. An accuracy of 3-4 % in the absolute effective area can be achieved.Comment: 15 pages, 7 figures, accepted for publication in Astronomy & Astrophysics, main journa

Estimating turbulent velocities in the elliptical galaxies NGC 5044 and NGC 5813

The interstellar and intra-cluster medium in giant elliptical galaxies and clusters of galaxies is often assumed to be in hydrostatic equilibrium. Numerical simulations, however, show that about 5-30% of the pressure in a cluster is provided by turbulence induced by, for example, the central AGN and merger activity. We aim to put constraints on the turbulent velocities and turbulent pressure in the ICM of the giant elliptical galaxies NGC 5044 and NGC 5813 using XMM-Newton RGS observations. The magnitude of the turbulence is estimated using the Fe XVII lines at 15.01 A, 17.05 A, and 17.10 A in the RGS spectra. At low turbulent velocities, the gas becomes optically thick in the 15.01 A line due to resonant scattering, while the 17 A lines remain optically thin. By comparing the (I(17.05)+I(17.10))/I(15.01) line ratio from RGS with simulated line ratios for different Mach numbers, the level of turbulence is constrained. The measurement is limited by systematic uncertainties in the atomic data, which are at the 20-30% level. We find that the line ratio in NGC 5813 is significantly higher than in NGC 5044. This difference can be explained by a higher level of turbulence in NGC 5044. The high turbulent velocities and the fraction of the turbulent pressure support of >40% in NGC 5044, assuming isotropic turbulence, confirm that it is a highly disturbed system, probably due to an off-axis merger. The turbulent pressure support in NGC 5813 is more modest at 15-45%. The (I(17.05)+I(17.10))/I(15.01) line ratio in an optically thin plasma, calculated using AtomDB v2.0.1, is 2 sigma above the ratio measured in NGC 5044, which cannot be explained by resonant scattering. This shows that the discrepancies between theoretical, laboratory, and astrophysical data on Fe XVII lines need to be reduced to improve the accuracy of the determination of turbulent velocities using resonant scattering.Comment: 11 pages, 5 figures, accepted for publication in A&

The warm absorber in NGC 5548: The lean years

We study the variability of the warm absorber and the gas responsible for the emission lines in the Seyfert 1 galaxy NGC 5548, in order to constrain the location and physical properties of these components. Using X-ray spectra taken with the \textit{Chandra}$-$LETGS in 2002 and 2005, we study variability in the ionic column densities and line intensities. We find a lower \ion{O}{vii} forbidden emission line flux in 2005, while the Fe K$\alpha$ line flux stays constant. The warm absorber is less ionized in 2005, allowing us to constrain its location to within 7 pc of the central source. Using both the observed variability and the limit on the FWHM of the \ion{O}{vii} f line, we have constrained the location of the narrow line region to a distance of 1 pc from the central source. The apparent lack of variability of the Fe K$\alpha$ line flux does not allow for a unique explanation.Comment: 6 pages, 6 figures, accepted by A&

Active Galaxies and Cluster Gas

Two lines of evidence indicate that active galaxies, principally radio galaxies, have heated the diffuse hot gas in clusters. The first is the general need for additional heating to explain the steepness of the X-ray luminosity--temperature relation in clusters, the second is to solve the cooling flow problem in cluster cores. The inner core of many clusters is radiating energy as X-rays on a timescale much shorter than its likely age. Although the temperature in this region drops by a factor of about 3 from that of the surrounding gas, little evidence is found for gas much cooler than that. Some form of heating appears to be taking place, probably by energy transported outward from the central accreting black hole or radio source. How that energy heats the gas depends on poorly understood transport properties (conductivity and viscosity) of the intracluster medium. Viscous heating is discussed as a possibility. Such heating processes have consequences for the truncation of the luminosity function of massive galaxies.Comment: 14 pages, 16 fig, Feb 2004 talk for Phil Trans Roy So

Simultaneous UV and X-ray Spectroscopy of the Seyfert 1 Galaxy NGC 5548. I. Physical Conditions in the UV Absorbers

We present new UV spectra of the nucleus of the Seyfert 1 galaxy NGC 5548, which we obtained with the Space Telescope Imaging Spectrograph at high spectral resolution, in conjunction with simultaneous Chandra X-ray Observatory spectra. Taking advantage of the low UV continuum and broad emission-line fluxes, we have determined that the deepest UV absorption component covers at least a portion of the inner, high-ionization narrow-line region (NLR). We find nonunity covering factors in the cores of several kinematic components, which increase the column density measurements of N V and C IV by factors of 1.2 to 1.9 over the full-covering case; however, the revised columns have only a minor effect on the parameters derived from our photoionization models. For the first time, we have simultaneous N V and C IV columns for component 1 (at -1040 km/s), and find that this component cannot be an X-ray warm absorber, contrary to our previous claim based on nonsimultaneous observations. We find that models of the absorbers based on solar abundances severely overpredict the O VI columns previously obtained with the Far Ultraviolet Spectrograph, and present arguments that this is not likely due to variability. However, models that include either enhanced nitrogen (twice solar) or dust, with strong depletion of carbon in either case, are successful in matching all of the observed ionic columns. These models result in substantially lower ionization parameters and total column densities compared to dust-free solar-abundance models, and produce little O VII or O VIII, indicating that none of the UV absorbers are X-ray warm absorbers.Comment: 33 pages, 5 figures (Figures 3 and 4 are in color), Accepted for publication in the Astrophysical Journa