Frederik Paerels

High-Resolution X-Ray Spectroscopic Constraints on Cooling-Flow Models for Clusters of Galaxies

We present high-resolution X-ray spectra of 14 putative cooling-flow clusters of galaxies obtained with the Reflection Grating Spectrometer on XMM-Newton. The clusters in the sample span a large range of temperatures and mass deposition rates. Various of these spectra exhibit line emission from O VIII, Ne X, Mg XII and XI, Al XIII and XII, Si XIV and XIII, N VII, and C VI as well as all Fe L ions. The spectra exhibit strong emission from cool plasma at just below the ambient temperature, T0, down to T0/2, but also exhibit a severe deficit of emission relative to the predictions of the isobaric cooling-flow model at lower temperatures (<T0/3). In addition, the best-resolved spectra show emission throughout the entire X-ray temperature range, but increasingly less emission at lower temperatures than the cooling-flow model would predict. These results are difficult to reconcile with simple prescriptions for distorting the emission measure distribution, e.g., by including additional heating or rapid cooling terms. We enumerate some theoretical difficulties in understanding the soft X-ray spectra of cooling flows independent of the classic problem of the failure to detect the cooling-flow sink. Empirically, the differential luminosity distribution is consistent with being proportional to the temperature to the power of ≈1-2, instead of being independent of the temperature, as expected in the standard multiphase model. The primary differences in the observed low-temperature spectra are ascribed to differences in the ambient temperature.

X-ray imaging-spectroscopy of Abell 1835

We present detailed spatially-resolved spectroscopy results of the observation of Abell 1835 using the European Photon Imaging Cameras (EPIC) and the Reflection Grating Spectrometers (RGS) on the XMM- Newton observatory. Abell 1835 is a luminous (10 46 ergs s 1 ), medium redshift (z =0 :2523), X-ray emitting cluster of galaxies. The observations support the interpretation that large amounts of cool gas are present in a multi-phase medium surrounded by a hot (kTe =8 :2 keV) outer envelope. We detect O VIII Ly and two Fe XXIV complexes in the RGS spectrum. The emission measure of the cool gas below kTe =2 :7 keV is much lower than expected from standard cooling-flow models, suggesting either a more complicated cooling process than simple isobaric radiative cooling or dierential cold absorption of the cooler gas.

Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star

The fundamental properties of neutron stars provide a direct test of the equation of state of cold nuclear matter, a relationship between pressure and density that is determined by the physics of the strong interactions between the particles that constitute the star. The most straightforward method of determining these properties is by measuring the gravitational redshift of spectral lines produced in the neutron star photosphere. The equation of state implies a mass–radius relation, while a measurement of the gravitational redshift at the surface of a neutron star provides a direct constraint on the mass-to-radius ratio. Here we report the discovery of significant absorption lines in the spectra of 28 bursts of the low-mass X-ray binary EXO0748-676. We identify the most significant features with the Fe xxvi and xxv n = 2–3 and O viii n = 1–2 transitions, all with a redshift of z = 0.35, identical within small uncertainties for the respective transitions. For an astrophysically plausible range of masses (M ≈ 1.3–2.0 solar masses; refs 2–5), this value is completely consistent with models of neutron stars composed of normal nuclear matter, while it excludes some models in which the neutron stars are made of more exotic matter.

Spatially resolved X-ray spectroscopy of cooling clusters of galaxies

We present spatially resolved X-ray spectra taken with the EPIC cameras of XMM-Newton of a sample of 17 cooling clusters and three non-cooling clusters for comparison. The deprojected spectra are analyzed with a multi-temperature model, independent of any a priori assumptions about the physics behind the cooling and heating mechanisms. All cooling clusters show a central decrement of the average temperature, most of them of a factor of ∼2. Three clusters (Sersic 159−3, MKW 3s and Hydra A) only show a weak temperature decrement, while two others (A 399 and A 2052) have a very strong temperature decrement. All cooling clusters show a weak pressure gradient in the core. More important, at each radius within the cooling region the gas is not isothermal. The differential emission measure distribution shows a strong peak near the maximum (ambient) temperature, with a steep decline towards lower temperatures, approximately proportional to T 3 , or alternatively a cut-off at about a quarter to half of the maximum temperature. In general, we find a poor correlation between radio flux of the central galaxy and the temperature decrement of the cooling flow. This is interpreted as evidence that except for a few cases (like the Hydra A cluster) heating by a central AGN is not the most common cause of weak cooling flows. We investigate the role of heat conduction by electrons and find that the theoretically predicted conductivity rates are not high enough to balance radiation losses. The differential emission measure distribution has remarkable similarities with the predictions from coronal magnetic loop models. Also the physical processes involved (radiative cooling, thermal conduction along the loops, gravity) are similar for clusters loops and coronal loops. If coronal loop models apply to clusters, we find that a few hundred loops per scale height should be present. The typical loop sizes deduced from the observed emission measure distribution are consistent with the characteristic magnetic field sizes deduced from Faraday rotation measurements.

High-Resolution Observations of the Elliptical Galaxy NGC 4636 with the Reflection Grating Spectrometer on Board XMM-Newton

We present the first high spectral resolution X-ray observation of the giant elliptical galaxy NGC 4636, obtained with the reflection grating spectrometer on board the XMM-Newton Observatory. The resulting spectrum contains a wealth of emission lines from various charge states of oxygen, neon, magnesium, and iron. Examination of the cross-dispersion profiles of several of these lines provides clear, unambiguous evidence of resonance scattering by the highest oscillator strength lines, as well as a weak temperature gradient in the inner regions of the interstellar medium. We invoke a sophisticated new Monte Carlo technique that allows us to properly account for these effects in performing quantitative fits to the spectrum. Our spectral fits are not subject to many of the systematics that have plagued earlier investigations. The derived metal abundances are higher than have usually been inferred from prior, lower spectral resolution observations of this source, but are still incompatible with conventional chemical-enrichment models of elliptical galaxies. In addition, our data are incompatible with standard cooling-flow models for this system; our derived upper limit to the mass deposition rate is below the predicted value by a factor of 3-5.

First Light Measurements of Capella with the Low-Energy Transmission Grating Spectrometer aboard the Chandra X-Ray Observatory

We present the first X-ray spectrum obtained by the Low-Energy Transmission Grating Spectrometer (LETGS) aboard the Chandra X-Ray Observatory. The spectrum is of Capella and covers a wavelength range of 5-175 Å (2.5-0.07 keV). The measured wavelength resolution, which is in good agreement with ground calibration, is Deltalambda approximately 0.06 Å (FWHM). Although in-flight calibration of the LETGS is in progress, the high spectral resolution and unique wavelength coverage of the LETGS are well demonstrated by the results from Capella, a coronal source rich in spectral emission lines. While the primary purpose of this Letter is to demonstrate the spectroscopic potential of the LETGS, we also briefly present some preliminary astrophysical results. We discuss plasma parameters derived from line ratios in narrow spectral bands, such as the electron density diagnostics of the He-like triplets of carbon, nitrogen, and oxygen, as well as resonance scattering of the strong Fe xvii line at 15.014 Å.

First light measurements with the XMM-Newton reflection grating spectrometers: Evidence for an inverse first ionisation potential effect and anomalous Ne abundance in the Coronae of HR 1099

The RS CVn binary system HR 1099 was extensively observed by the XMM-Newton observatory in February 2000 as its first-light target. A total of 570 ks of exposure time was accumulated with the Reflection Grating Spectrometers (RGS). The integrated X-ray spectrum between 5-38A is of unprecedented quality and shows numerous features attributed to transitions of the elements C, N, O, Ne, Mg, Si, S, Fe. Ni, and probably others. We perform an in-depth study of the elemental composition of the average corona of this system, and find that the elemental abundances strongly depend on the first ionisation potential (FIP) of the elements. But different from the solar coronal case, we find an inverse FIP effect, i.e., the abundances (relative to oxygen) increase with increasing FIP. Possible scenarios, e.g., selective enrichment due to Ne-rich flare-like events, are discussed.

Simultaneous X-ray and UV spectroscopy of the Seyfert 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 (full width half maximum = 8000 km s −1 ) emission lines of O  and C  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. The gradual increase in luminosity occurred over a timescale of ∼300 ks. No variability in the warm absorber was detected between the spectra from the first 170 ks and the second part of the observation. The longer wavelength range of the LETGS resulted in the detection of absorption lines from a broad range of ions, in particular of C, N, O, Ne, Mg, Si, S and Fe. 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 −1 , 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  ,S i) 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. We determine an upper limit of 0.3 Myr −1 for the mass loss from the galaxy due to the observed outflows.

X-Ray Spectral Study of the Photoionized Stellar Wind in Vela X-1

We present results from quantitative modeling and spectral analysis of the high-mass X-ray binary system Vela X-1 obtained with the Chandra HETGS. The spectra exhibit emission lines from H- and He-like ions driven by photoionization, as well as fluorescent emission lines from several elements in lower charge states. The properties of these X-ray lines are measured with the highest accuracy to date. In order to interpret and make full use of the data, we have developed a simulator, which calculates the ionization structure of a stellar wind and performs Monte Carlo simulations of X-ray photons propagating through the wind. From comparisons of the observed spectra with results from the simulator, we are able to find the ionization structure and the geometrical distribution of material in the stellar wind that can reproduce the observed spectral line intensities and continuum shapes remarkably well. We find that the stellar wind profile can be represented by a CAK model with a star mass-loss rate of (1.5-2.0) × 10-6 M☉ yr-1, assuming a terminal velocity of 1100 km s-1. It is found that a large fraction of emission lines from highly ionized ions are formed in the region between the neutron star and the companion star. We also find that the fluorescent lines must be produced in at least three distinct regions: the extended stellar wind, reflection off the stellar photosphere, and in a distribution of dense material partially covering and possibly trailing the neutron star, which may be associated with an accretion wake. Finally, from detailed analysis of the emission-line profiles, we demonstrate that the stellar wind dynamics is affected by X-ray photoionization.

The O VII X-Ray Forest toward Markarian 421: Consistency between XMM-Newton and Chandra

Recently, the first detections of highly ionized gas associated with two warm-hot intergalactic medium (WHIM) filaments have been reported. The evidence is based on X-ray absorption lines due to O vii and other ions observed by Chandra toward the bright blazar Mrk 421. We investigate the robustness of this detection by a reanalysis of the original Chandra LETGS spectra, the analysis of a large set of XMM-Newton RGS spectra of Mrk 421, and additional Chandra observations. We address the reliability of individual spectral features belonging to the absorption components, and assess the significance of the detection of these components.We also use Monte Carlo simulations of spectra.We confirm the apparent strength of several features in the Chandra spectra, but demonstrate that they are statistically not significant. This decreased significance is due to the number of redshift trials that are made and that are not taken into account in the original discovery paper. Therefore, these features must be attributed to statistical fluctuations. This is confirmed by the RGS spectra, which have a higher signal-to-noise ratio than the Chandra spectra, but do not show features at the same wavelengths. Finally, we show that the possible association with a Ly absorption system also lacks sufficient statistical evidence.We conclude that there is insufficient observational proof for the existence of the two proposed WHIM filaments toward Mrk 421, the brightest X-ray blazar in the sky. Therefore, the highly ionized component of the WHIM still remains to be discovered.