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F. B. S. Paerels

Publications and source records attributed to F. B. S. Paerels.

At least 19 recordsLinked to original sources
arXiv2019

Shocks and non-thermal particles in clusters of galaxies

Galaxy clusters grow by gas accretion, mostly from mergers of substructures, which release powerful shock waves into cosmic plasmas and convert a fraction of kinetic energy into thermal energy, amplification of magnetic fields and into the acceleration of energetic particles. The modeling of the radio signature of cosmic shocks, combined with the lack of detected gamma-rays from cosmic ray (CR) protons, poses challenges to our understanding of how cosmic rays get accelerated and stored in the intracluster medium. Here we review the injection of CRs by cosmic shocks of different strengths, combining the detailed "microscopic" view of collisionless processes governing the creation of non-thermal distributions of electrons and protons in cluster shocks (based on analytic theory and particle-in-cell simulations), with the "macroscopic" view of the large-scale distribution of cosmic rays, suggested by modern cosmological simulations. We discuss time dependent non-linear kinetic models of particle acceleration by multiple internal shocks with large scale compressible motions of plasma. The models produce soft CR spectra containing a noticeable energy density in the super-thermal protons of energies below a few GeV which are difficult to constrain by Fermi observations. We consider the effect of plasma composition on CR injection and super-thermal particle population in the hot intracluster matter which can be constrained by fine high resolution X-ray spectroscopy of Fe ions.

astro-ph.HE↗
arXiv2015

The LOFT perspective on neutron star thermonuclear bursts

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper.

astro-ph.HE↗
arXiv2014

Calibration and in orbit performance of the reflection grating spectrometer onboard XMM-Newton

Context: XMM-Newton was launched on 10 December 1999 and has been operational since early 2000. One of the instruments onboard XMM-Newton is the reflection grating spectrometer (RGS). Two identical RGS instruments are available, with each RGS combining a reflection grating assembly (RGA) and a camera with CCDs to record the spectra. Aims: We describe the calibration and in-orbit performance of the RGS instrument. By combining the preflight calibration with appropriate inflight calibration data including the changes in detector performance over time, we aim at profound knowledge about the accuracy in the calibration. This will be crucial for any correct scientific interpretation of spectral features for a wide variety of objects. Methods: Ground calibrations alone are not able to fully characterize the instrument. Dedicated inflight measurements and constant monitoring are essential for a full understanding of the instrument and the variations of the instrument response over time. Physical models of the instrument are tuned to agree with calibration measurements and are the basis from which the actual instrument response can be interpolated over the full parameter space. Results: Uncertainties in the instrument response have been reduced to < 10% for the effective area and < 6 mA for the wavelength scale (in the range from 8 A to 34 A. The remaining systematic uncertainty in the detection of weak absorption features has been estimated to be 1.5%. Conclusions: Based on a large set of inflight calibration data and comparison with other instruments onboard XMM-Newton, the calibration accuracy of the RGS instrument has been improved considerably over the preflight calibrations.

astro-ph.IM↗
arXiv2013

A bright thermonuclear X-ray burst simultaneously observed with Chandra and RXTE

The prototypical accretion-powered millisecond pulsar SAX J1808.4-3658 was observed simultaneously with Chandra-LETGS and RXTE-PCA near the peak of a transient outburst in November 2011. A single thermonuclear (type-I) burst was detected, the brightest yet observed by Chandra from any source, and the second-brightest observed by RXTE. We found no evidence for discrete spectral features during the burst; absorption edges have been predicted to be present in such bursts, but may require a greater degree of photospheric expansion than the rather moderate expansion seen in this event (a factor of a few). These observations provide a unique data set to study an X-ray burst over a broad bandpass and at high spectral resolution (lambda/delta-lambda=200-400). We find a significant excess of photons at high and low energies compared to the standard black body spectrum. This excess is well described by a 20-fold increase of the persistent flux during the burst. We speculate that this results from burst photons being scattered in the accretion disk corona. These and other recent observations of X-ray bursts point out the need for detailed theoretical modeling of the radiative and hydrodynamical interaction between thermonuclear X-ray bursts and accretion disks.

astro-ph.HE↗
arXiv2009

Properties of the ionised plasma in the vicinity of the neutron-star X-ray binary EXO 0748-676

We present the spectral analysis of a large set of XMM-Newton observations of EXO 0748-676, a bright dipping LMXB. In particular, we focus on the dipping phenomenon as a result of changes in the properties of the ionised gas close to the source. Using the high-resolution spectra collected with the RGS, we explored two simple geometrical scenarios for which we derived physical quantities of the absorbing material like the density, size, and mass. We find that the continuum is absorbed by a neutral gas, and by both a collisionally (temperature T~70 eV) and photoionised (ionisation parameter logξ~2.5) absorbers. Emission lines from OVII and OVIII are also detected. This is the first time that evidence of a collisionally ionised absorber has been found in a low-mass X-ray binary. The collisionally ionised absorber may be in the form of dense (n>10^14 cm^-3) filaments, located at a distance r>10^11 cm. During dips, the photoionised absorber significantly increases its column density (factor 2--4) while becoming less ionised. This strengthens the idea that the colder material of the accretion stream impinging the disc is passing on our line of sight during dips. We find that the distance from the neutron star to the impact region (~ 5x10^10 cm) is similar to the size of the neutron star's Roche lobe. The gas observed during the persistent state may have a flattened geometry. Finally, we explore the possibility of the existence of material forming an initial, hotter portion of a circumbinary disc.

astro-ph.GA↗
arXiv2009

HDE 245059: A Weak-Lined T Tauri Binary Revealed by Chandra and Keck

We present the Chandra High Energy Transmission Grating Spectrometer (HETGS) and Keck observations of HDE 245059, a young weak-lined T Tauri star (WTTS), member of the pre-main sequence group in the Lambda Orionis Cluster. Our high spatial resolution, near-infrared observations with Keck reveal that HDE 245059 a binary separated by 0.87". Based on this new information we have obtained an estimate of the masses of the binary components; 3M_{sun} and 2.5M_{sun} for the north and south components, respectively. We have estimated the age of the system to be ~2-3 Myr. We detect both components of the binary in the zeroth order Chandra image and in the grating spectra. Our fits to the spectrum of the binary have shown that the emission is dominated by a plasma between 8 and 15 MK, a soft component at 4 MK and a hard component at 50 MK are also detected. The value of the hydrogen column density was low, 8 x 10^{19} cm^{-2}, likely due to the clearing of the inner region of the Lambda Orionis cloud. The abundance pattern shows an inverse First Ionization Potential (FIP) effect for all elements from O to Fe, the only exception being Ca. A 3-T model was fitted to the individual zeroth order spectra using the abundances derived for the binary. We have also obtained several lines fluxes from the grating spectra. The fits to the triplets show no evidence of high densities. We conclude that the X-ray properties of the weak-lined T Tau binary HDE 245059 are similar to those generally observed in other weak-lined T Tau stars. Although its accretion history may have been affected by the clearing of the interstellar material around Lambda Ori, its coronal properties appears not to have been strongly modified.

astro-ph.SR↗
arXiv2009

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.

astro-ph.HE↗
arXiv2008

Equilibration processes in the Warm-Hot Intergalactic Medium

The Warm-Hot Intergalactic Medium (WHIM) is thought to contribute about 40-50 % to the baryonic budget at the present evolution stage of the universe. The observed large scale structure is likely to be due to gravitational growth of density fluctuations in the post-inflation era. The evolving cosmic web is governed by non-linear gravitational growth of the initially weak density fluctuations in the dark energy dominated cosmology. Non-linear structure formation, accretion and merging processes, star forming and AGN activity produce gas shocks in the WHIM. Shock waves are converting a fraction of the gravitation power to thermal and non-thermal emission of baryonic/leptonic matter. They provide the most likely way to power the luminous matter in the WHIM. The plasma shocks in the WHIM are expected to be collisionless. Collisionless shocks produce a highly non-equilibrium state with anisotropic temperatures and a large differences in ion and electron temperatures. We discuss the ion and electron heating by the collisionless shocks and then review the plasma processes responsible for the Coulomb equilibration and collisional ionisation equilibrium of oxygen ions in the WHIM. MHD-turbulence produced by the strong collisionless shocks could provide a sizeable non-thermal contribution to the observed Doppler parameter of the UV line spectra of the WHIM.

astro-ph↗
arXiv2008

Thermal radiation processes

We discuss the different physical processes that are important to understand the thermal X-ray emission and absorption spectra of the diffuse gas in clusters of galaxies and the warm-hot intergalactic medium. The ionisation balance, line and continuum emission and absorption properties are reviewed and several practical examples are given that illustrate the most important diagnostic features in the X-ray spectra.

astro-ph↗
arXiv2008

FUV and X-ray absorption in the Warm-Hot Intergalactic Medium

The Warm-Hot Intergalactic Medium (WHIM) arises from shock-heated gas collapsing in large-scale filaments and probably harbours a substantial fraction of the baryons in the local Universe. Absorption-line measurements in the ultraviolet (UV) and in the X-ray band currently represent the best method to study the WHIM at low redshifts. We here describe the physical properties of the WHIM and the concepts behind WHIM absorption line measurements of H I and high ions such as O VI, O VII, and O VIII in the far-ultraviolet and X-ray band. We review results of recent WHIM absorption line studies carried out with UV and X-ray satellites such as FUSE, HST, Chandra, and XMM-Newton and discuss their implications for our knowledge of the WHIM.

astro-ph↗
arXiv2008

Clusters of galaxies: beyond the thermal view

We present the work of an international team at the International Space Science Institute (ISSI) in Bern that worked together to review the current observational and theoretical status of the non-virialised X-ray emission components in clusters of galaxies. The subject is important for the study of large-scale hierarchical structure formation and to shed light on the "missing baryon" problem. The topics of the team work include thermal emission and absorption from the warm-hot intergalactic medium, non-thermal X-ray emission in clusters of galaxies, physical processes and chemical enrichment of this medium and clusters of galaxies, and the relationship between all these processes. One of the main goals of the team is to write and discuss a series of review papers on this subject. These reviews are intended as introductory text and reference for scientists wishing to work actively in this field. The team consists of sixteen experts in observations, theory and numerical simulations.

astro-ph↗
arXiv2007

Resonance scattering in the X-ray emission line profiles of Zeta Puppis

We present XMM-Newton Reflection Grating Spectrometer observations of pairs of X-ray emission line profiles from the O star Zeta Pup that originate from the same He-like ion. The two profiles in each pair have different shapes and cannot both be consistently fit by models assuming the same wind parameters. We show that the differences in profile shape can be accounted for in a model including the effects of resonance scattering, which affects the resonance line in the pair but not the intercombination line. This implies that resonance scattering is also important in single resonance lines, where its effect is difficult to distinguish from a low effective continuum optical depth in the wind. Thus, resonance scattering may help reconcile X-ray line profile shapes with literature mass-loss rates.

astro-ph↗
arXiv2006

The O VII X-ray forest toward Markarian 421: Consistency between XMM-Newton and Chandra

Recently the first detections of highly ionised 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 towards the bright blazar Mrk 421. We investigate the robustness of this detection by a re-analysis 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-alpha absorption system also lacks sufficient statistical evidence. We conclude that there is insufficient observational proof for the existence of the two proposed WHIM filaments towards Mrk 421, the brightest X-ray blazar on the sky. Therefore, the highly ionised component of the WHIM still remains to be discovered.

astro-ph↗
arXiv2005

Ionization structure of the warm wind in NGC 5548

We present the results from our 140 ks XMM-Newton and 500 ks Chandra observation of NGC 5548. The velocity structure of the X-ray absorber is consistent with the velocity structure measured in the simultaneous UV spectra. In the X-rays we can separate the highest outflow velocity component, -1040 km/s, from the other velocity components. 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) and UV absorption lines. A similar conclusion is very probable for the other four velocity components. We show that the lower ionized absorbers are not in pressure equilibrium with the rest of the absorbers. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data.

astro-ph↗
arXiv2005

An XMM-Newton Study of the Coronae of $σ^2$ Coronae Borealis

(Abridged) We present results of XMM-Newton observations of the RS CVn binary $σ^2$ Coronae Borealis. The RGS and EPIC MOS2 spectra were simultaneously fitted with collisional ionization equilibrium plasma models to determine coronal abundances of various elements. Contrary to the solar first ionization potential (FIP) effect in which elements with a low FIP are overabundant in the corona compared to the solar photosphere, and contrary to the ``inverse'' FIP effect observed in several active RS CVn binaries, coronal abundance ratios in $σ^2$ CrB show a complex pattern as supported by similar findings in the Chandra HETGS analysis of $σ^2$ CrB with a different methodology (Osten et al. 2003). Low-FIP elements ($<10$ eV) have their abundance ratios relative to Fe consistent with the solar photospheric ratios, whereas high-FIP elements have their abundance ratios increase with increasing FIP. We find that the coronal Fe abundance is consistent with the stellar photospheric value, indicating that there is no metal depletion in $σ^2$ CrB. However, we obtain a higher Fe absolute abundance than in Osten et al. (2003). Except for Ar and S, our absolute abundances are about 1.5 times larger than those reported by Osten et al. (2003). However, a comparison of their model with our XMM-Newton data (and vice versa) shows that both models work adequately in general. We find, therefore, no preference for one methodology over the other to derive coronal abundances. Despite the systematic discrepancy in absolute abundances, our abundance ratios are very close to those obtained by Osten et al. (2003). Finally, we confirm the measurement of a low density in \ion{O}{7} ($< 4 \times 10^{10}$ cm$^{-3}$), but could not confirm the higher densities measured in spectral lines formed at higher temperatures.

astro-ph↗
arXiv2005

The temperature structure in the core of Sersic 159-03

We present results from a new 120 ks XMM-Newton observation of the cluster of galaxies Sersic 159-03. In this paper we focus on the high-resolution X-ray spectra obtained with the Reflection Grating Spectrometer (RGS). The spectra allow us to constrain the temperature structure in the core of the cluster and determine the emission measure distribution as a function of temperature. We also fit the line widths of mainly oxygen and iron lines.

astro-ph↗
arXiv2005

Tentative detection of warm intervening gas towards PKS 0548-322 with XMM-Newton

We present the results of a long (~93 ksec) XMM-Newton observation of the bright BL-Lac object PKS 0548-322 (z=0.069). Our Reflection Grating Spectrometer (RGS) spectrum shows a single absorption feature at an observed wavelength lambda=23.33 A, which we interpret as OVI Ka absorption at z=0.058, i.e., ~3000 km/s from the background object. The observed equivalent width of the absorption line ~30 mA, coupled with the lack of the corresponding absorption edge in the EPIC pn data, implies an OVI column density ~ 2 10^{16} cm-2 and turbulence with a Doppler velocity parameter b>100 km/s. Within the limitations of our RGS spectrum, no OVII or OV Ka absorption are detected. Under the assumption of ionisation equilibrium by both collisions and the extragalactic background, this is only marginally consistent if the gas temperature is ~250,000 K, with significantly lower or higher values being excluded by our limits on OV or OVII. If confirmed, this would be the first X-ray detection of a large amount of intervening warm absorbing gas through OVI absorption. The existence of such a high column density absorber, much stronger than any previously detected one in OVI, would place stringent constraints on the large-scale distribution of baryonic gas in the Universe.

astro-ph↗