arXiv subjects

R. Lieu

Publications and source records attributed to R. Lieu.

At least 19 recordsLinked to original sources

arXiv2022

A WHIM origin for the soft excess emission in the Coma cluster

This paper provides a new analysis of ROSAT observations of the Coma cluster, to determine the amount of soft X--ray radiation in excess of the contribution from the hot intra--cluster medium. The re--analysis is made possible by a high--resolution study of the hot intra--cluster medium with the XMM-Newton and Planck telescopes out to the cluster's virial radius. The analysis confirms the original findings of a strong excess of soft X--ray radiation, which is likely to be of thermal origin. We find quantitative agreement between the detected soft excess and the physical characteristics of warm--hot intergalactic medium (WHIM) filaments seen in hydrodynamical simulations. We conclude that the most plausible explanation for the soft excess is the presence of $\sim 10$~Mpc--long filaments at $\log T(K)\simeq 6$, with a baryon overdensity $\sim 300$, converging towards the Coma cluster. This interpretation therefore provides support for the identification of the missing low--redshift baryons with WHIM filaments, as predicted by numerical simulations.

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arXiv2010

Non-thermal intracluster medium: a simultaneous interpretation of the central soft X-ray excess and WMAP's detection of reduced Sunyaev-Zel'dovich Effect

WMAP's detection of the Sunyaev-Zel'dovich effect at a much reduced level among several large samples of rich clusters is interpreted in terms of conventional physics. It has been suggested that the central soft X-ray and EUV excess found in some clusters cannot be of thermal origin, due to problems with rapid gas cooling and the persistent non-detection of the O VII line, but may arise from inverse-Compton scattering between intracluster relativistic electrons and the cosmic microwave background. In fact, recent XMM observations of the soft X-rays from Coma and Abell 3112 are equally well fitted by a power law or a thermal virialized gas. Therefore the missing Sunyaev-Zel'dovich flux could partly be due to an overestimate of the central density of virialized electrons which scatter the CMB. Synchrotron radiation in an intracluster magnetic field of strength of a few $μ$G is responsible for significant additional electron energy loss. Equipartition between relativistic particle and magnetic field energy densities is a realistic possibility. GHz radiation data from a Coma cluster halo yields information on the high energy steepening of the cluster relativistic electron spectrum. Cluster microwave emission in the WMAP passbands by higher energy cosmic ray electrons and gamma ray emission from an accompanying cosmic ray proton flux are also considered. The energetic electrons could originate from AGN jet injection, then distributed cluster-wide by Alfven wave sweeping, with accompanying {\it in situ} Fermi acceleration.

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arXiv2010

On the DM interpretation of the origin of non-thermal phenomena in galaxy clusters

(Abridged) We study the predictions of various annihilating Dark Matter (DM) models in order to interpret the origin of non-thermal phenomena in galaxy clusters. We consider three neutralino DM models with light (9 GeV), intermediate (60 GeV) and high (500 GeV) mass. The secondary particles created by neutralino annihilation produce a multi-frequency Spectral Energy Distribution (SED), as well as heating of the intracluster gas, that are tested against the observations available for the Coma cluster. The DM produced SEDs are normalized to the Coma radio halo spectrum. We find that it is not possible to interpret all non-thermal phenomena observed in Coma in terms of DM annihilation. The DM model with 9 GeV mass produces too small power at all frequencies, while the DM model with 500 GeV produces a large excess power at all frequencies. The DM model with 60 GeV and $τ^{\pm}$ composition is consistent with the HXR and gamma-ray data but fails to reproduce the EUV and soft X-ray data. The DM model with 60 GeV and $b{\bar b}$ composition is always below the observed fluxes. The radio halo spectrum of Coma is well fitted only in the $b{\bar b}$ or light and intermediate mass DM models. The heating produced by DM annihilation in the center of Coma is always larger than the intracluster gas cooling rate for an NFW DM density profile and it is substantially smaller than the cooling rate only for a cored DM density profile in DM model with 9 GeV. We conclude that the possibility of interpreting the origin of non-thermal phenomena in galaxy clusters with DM annihilation models requires a low neutralino mass and a cored DM density profile. If we then consider the multimessenger constraints to the neutralino annihilation cross-section, it turns out that such scenario would also be excluded unless we introduce a substantial boost factor due to the presence of DM substructures.

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arXiv2007

Soft and hard X-ray excess emission in Abell 3112 observed with Chandra

Chandra ACIS-S observations of the galaxy cluster A3112 feature the presence of an excess of X-ray emission above the contribution from the diffuse hot gas, which can be equally well modeled with an additional non-thermal power-law model or with a low-temperature thermal model of low metal abundance. We show that the excess emission cannot be due to uncertainties in the background subtraction or in the Galactic HI column density. Calibration uncertainties in the ACIS detector that may affect our results are addressed by comparing the Chandra data to XMM MOS and PN spectra. While differences between the three instruments remain, all detect the excess in similar amounts, providing evidence against an instrumental nature of the excess. Given the presence of non-thermal radio emission near the center of A3112, we argue that the excess X-ray emission is of non-thermal nature and distributed throughout the entire X-ray bandpass, from soft to hard X-rays. The excess can be explained with the presence of a population of relativistic electrons with ~7% of the cluster's gas pressure. We also discuss a possible thermal nature of the excess, and examine the problems associated with such interpretation.

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arXiv2007

Possible non-thermal nature of the soft-excess emission in the cluster of galaxies Sersic 159-03

We present an analysis of new Suzaku data and archival data from XMM-Newton of the cluster of galaxies Sersic 159-03, which has a strong soft X-ray excess emission component. The Suzaku observation confirms the presence of the soft excess emission, but it does not confirm the presence of redshifted OVII lines in the cluster. Radial profiles and 2D maps derived from XMM-Newton observations show that the soft excess emission has a strong peak at the position of the central cD galaxy and the maps do not show any significant azimuthal variations. Although the soft excess emission can be fitted equally well with both thermal and non-thermal models, its spatial distribution is neither consistent with the models of intercluster warm-hot filaments, nor with models of clumpy warm intracluster gas associated with infalling groups. Using the data obtained by the XMM-Newton Reflection Grating Spectrometers we do not confirm the presence of the warm gas in the cluster centre with the expected properties assuming the soft excess is of thermal origin. The observed properties of the soft excess emission are consistent with the non-thermal interpretation. While the high density of relativistic electrons associated with the peak of the soft emission in the cluster centre might have been provided by an active galactic nucleus in the central cD galaxy, the underlying population might have been accelerated in diffuse shocks.

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arXiv2005

Are the WMAP angular magnification measurements consistent with an inhomogeneous critical density Universe?

The propagation of light through a Universe of (a) isothermal mass spheres amidst (b) a homogeneous matter component, is considered. We demonstrate by an analytical proof that as long as a small light bundle passes {\it through} sufficient number of (a) at various impact parameters - a criterion of great importance - its average convergence will exactly compensate the divergence within (b). The net effect on the light is statistically the same as if all the matter in (a) is `fully homogenized'. When applying the above ideas towards understanding the angular size of the primary acoustic peaks of the microwave background, however, caution is needed. The reason is that most (by mass) of (a) are in galaxies - their full mass profiles are not sampled by passing light - at least the inner 20 kpc regions of these systems are missed by the majority of rays, while the rest of the rays would map back to unresolvable but magnified, randomly located spots to compensate for the loss in angular size. Therefore, a scanning pair of WMAP beams finds most frequently that the largest temperature difference occurs when each beam is placed at diametrically opposite points of the Dyer-Roeder collapsed sections. This is the {\it mode} magnification, which corresponds to the acoustic {\it peaks}, and is less than the mean (or the homogeneous pre-clumping angular size). Since space was seen to be Euclidean without taking the said adjustment into account, the true density of the Universe should be supercritical. Our analysis gives $Ω_m =$ 0.278 $\pm$ 0.040 and $Ω_Λ =$ 0.782 $\pm$ 0.040.

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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.

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arXiv2005

Thermal and non-thermal nature of the soft excess emission from Sersic 159-03 observed with XMM-Newton

Several nearby clusters exhibit an excess of soft X-ray radiation which cannot be attributed to the hot virialized intra-cluster medium. There is no consensus to date on the origin of the excess emission: it could be either of thermal origin, or due to an inverse Compton scattering of the cosmic microwave background. Using high resolution XMM-Newton data of Sersic 159-03 we first show that strong soft excess emission is detected out to a radial distance of 0.9 Mpc. The data are interpreted using the two viable models available, i.e., by invoking a warm reservoir of thermal gas, or relativistic electrons which are part of a cosmic ray population. The thermal interpretation of the excess emission, slightly favored by the goodness-of-fit analysis, indicates that the warm gas responsible for the emission is high in mass and low in metallicity.

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arXiv2004

WHIM emission and the cluster soft excess: a model comparison

The confirmation of the cluster soft excess (CSE) by XMM-Newton has rekindled interest as to its origin. The recent detections of CSE emission at large cluster radii together with reports of OVII line emission associated with the CSE has led many authors to conjecture that the CSE is, in fact, a signature of the warm-hot intergalactic medium (WHIM). In this paper we test the scenario by comparing the observed properties of the CSE with predictions based on models of the WHIM. We find that emission from the WHIM in current models is 3 to 4 orders of magnitude too faint to explain the CSE. We discuss different possibilities for this discrepancy including issues of simulation resolution and scale, and the role of small density enhancements or galaxy groups. Our final conclusion is that the WHIM alone is unlikely to be able to accout for the observed flux of the CSE.

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arXiv2004

Warm gas in the outskirts of galaxy clusters -- The cluster soft excess phenomenon

The cluster soft excess emission indicates the presence of large amounts of warm gas (T ~ 10^6 K) in the neighborhood of galaxy clusters. Among the clusters that display this phenomenon is the Coma cluster, the nearest rich galaxy cluster. The excess emission is more prominent at the cluster's outskirts than at its center. Detailed studies of its large-scale emission -- up to ~2.6 Mpc from the cluster's center -- reveal that these warm baryons are as massive as, or possibly more massive than, the well-known hot intra-cluster medium (T ~10^8 K). A possible interpretation of the excess emission from the Coma cluster is radiation from low-density filaments located in the neighborhood of the cluster. In this case, the filaments would extend for much larger distances, or feature higher density, than predicted by current cosmological simulations.

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arXiv2003

XMM-Newton discovery of O VII emission from warm gas in clusters of galaxies

XMM-Newton recently discovered O VII line emission from ~2 million K gas near the outer parts of several clusters of galaxies. This emission is attributed to the Warm-Hot Intergalactic Medium. The original sample of clusters studied for this purpose has been extended and two more clusters with a soft X-ray excess have been found. We discuss the physical properties of the warm gas, in particular the density, spatial extent, abundances and temperature.

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arXiv2002

Soft X-ray excess emission in clusters of galaxies observed with XMM-Newton

We present results on the spectroscopic analysis of XMM-Newton EPIC data of the central 0.5/h_50 Mpc regions of the clusters of galaxies Coma, A1795 and A3112. The temperature of the hot intracluster gas as determined by modeling the 2 - 7 keV PN and MOS data is consistent with that inferred from the FeXXV-FeXXVI line ratio. A significant warm emission component at a level above the systematic uncertainties is evident in the data and confirmed by ROSAT PSPC data for Coma and A1795. The non-thermal origin of the phenomenon cannot be ruled out at the current level of calibration accuracy, but the thermal model fits the data significantly better, with temperatures in the range of 0.6 -- 1.3 keV and electron densities of the order of 10^{-4} -- 10^{-3} cm^{-3}. In the outer parts of the clusters the properties of the warm component are marginally consistent with the results of recent cosmological simulations, which predict a large fraction of the current epoch's bayons located in a warm-hot intergalactic medium (WHIM). However, the derived densities are too high in the cluster cores, compared to WHIM simulations, and thus more theoretical work is needed to fully understand the origin of the observed soft X-ray excess.

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arXiv2002

A massive warm baryonic halo in the Coma cluster

Several deep PSPC observations of the Coma cluster reveal a very large-scale halo of soft X-ray emission, substantially in excess of the well known radiation from the hot intra-cluster medium. The excess emission, previously reported in the central region of the cluster using lower-sensitivity EUVE and ROSAT data, is now evident out to a radius of 2.6 Mpc, demonstrating that the soft excess radiation from clusters is a phenomenon of cosmological significance. The X-ray spectrum at these large radii cannot be modeled non-thermally, but is consistent with the original scenario of thermal emission from warm gas at ~ 10^6 K. The mass of the warm gas is on par with that of the hot X-ray emitting plasma, and significantly more massive if the warm gas resides in low-density filamentary structures. Thus the data lend vital support to current theories of cosmic evolution, which predict that at low redshift \~30-40 % of the baryons reside in warm filaments converging at clusters of galaxies.

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arXiv2002

XMM-Newton confirmation of Soft X-ray excess emission in clusters of galaxies - the discovery of O VII emission from an extended warm baryonic component

We investigate a sample of 14 clusters of galaxies observed with XMM-Newton in a search for soft X-ray excess emission. In five of these clusters a significant soft excess is evident. This soft X-ray excess is compared with the thermal emission from both the hot intracluster gas and any cooling (flow) gas that may be present. A warm (kT=0.2 keV), extended (several Mpc), plasma component is particularly clear in the outer parts of the cluster, where the normal cluster X-ray emission is weak. This warm component causes both a thermal soft X-ray excess at low energies (below 0.4-0.5 keV), as well as O VII line emission with a redshift consistent with a cluster origin, and not easily interpreted as Galactic foreground emission. The intensity of this component is commensurate with what has been measured before with the ROSAT PSPC in the 1/4 keV band. We attribute this component to emission from intercluster filaments of the Warm-Hot Intergalactic Medium in the vicinity of these clusters. For the central regions of clusters the detection of lines in the soft X-ray spectrum is more difficult, due to the predominance of the X-ray emitting hot plasma there, hence we cannot discriminate between the thermal and nonthermal origin of the soft excess, leaving several options open. These include thermal emission from warm filaments seen in projection in front of or behind the cluster center, thermal or nonthermal emission in the cluster core itself related to magnetic reconnection, or Inverse Compton emission from the cosmic microwave background on relativistic electrons.

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arXiv2002

The soft X-ray emission in a large sample of galaxy clusters with ROSAT PSPC

The study of soft X-ray emission of 38 X-ray selected galaxy clusters observed by ROSAT PSPC indicates that the soft excess phenomenon may be a common occurrence in galaxy clusters. Excess soft X-ray radiation, above the contribution from the hot intra-cluster medium, is evident in a large fraction of sources, and is clearly detected with large statistical significance in the deepest observations. The investigation relies on new, high resolution 21 cm HI observations. The sample selection also features analysis of infra-red images, to further ensure reliability of results with respect to the characteristics of Galactic absorption. The possibility of background or calibration effects as cause of the excess emission is likewise investigated; a detailed analysis of the distribution of the excess emission with respect to detector position and Galactic HI column density shows that the excess emission is a genuine celestial phenomenon. We find evidence for a preferential distribution of the soft excess emission at distances larger than ~150-200 kpc from the centers of clusters; this behavior may be naturally explained in the context of a non-thermal Inverse-Compton scenario. Alternatively, we propose that the phenomenon may be caused by thermal emission of very large-scale `warm' filaments seen in recent hydrodynamic simulations. This new interpretation relieves the very demanding requirements of either the traditional intra-cluster `warm' gas and the non-thermal scenarios. We also investigate the possibility of the soft excess originating from unresolved, X-ray faint cluster galaxies.

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arXiv2001

Sersic 159-03: discovery of the brightest soft X-ray excess emitting cluster of galaxies

The soft X-ray excess emission in the southern cluster Sersic 159-03 represents hiterto the strongest effect of its kind. Emission in the ~0.2-0.4 keV passband is detected far in excess of the expected contribution from the hot phase of the intra-cluster medium, and extends to the X-ray signal limit of the cluster. Our analysis of ROSAT PSPC observations reveal that the soft excess can be interpreted as either a thermal or non-thermal effect, and the high data quality allows to place tight constraints on the two currently competing models. However, each model now implies major revisions to our understanding of clusters of galaxies: either `warm' gas masses in similar amounts to the hot gas, or relativistic particles in or above equipartition with the hot phase, appear to be unavoidable.

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arXiv2001

A Comparison of the EUV and X-Ray Emission Profiles in Six Clusters of Galaxies

We have performed the wavelet analysis and image reconstructions for six clusters of galaxies observed both at extreme ultraviolet and X-ray energies. The comparison of the profiles of the EUV and X-ray reconstructed images shows that they differ at a very large confidence level in five of the six clusters, and are similar only for the Coma cluster, which is by far the hottest one in the sample.

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arXiv2001

FUSE Detection of Diffuse Galactic O VI Emission toward the Coma and Virgo Clusters

We report the detection of diffuse O VI 1032, 1038 A emission in a 29-ksec observation centered on the Coma Cluster (l = 57.6, b = +88.0) and an 11-ksec observation toward Virgo (l = 284.2, b = +74.5) through the low-resolution (30"x30") aperture of the Far Ultraviolet Spectroscopic Explorer (FUSE). The emission lines have a redshift near zero and are thus produced by gas in our own Galaxy. Observed surface brightnesses are 2000 +/- 600 photons/cm2/s/sr for each of the O VI components in the Coma spectrum and 2900 +/- 700 and 1700 +/- 700 photons/cm2/s/sr for the 1032 and 1038 A lines, respectively, toward Virgo. These features are similar in strength to those recently observed in the southern Galactic hemisphere (l = 315.0, b =-41.3) in an ~ 200 ksec FUSE observation. From a FUSE spectrum of M87, we find that N(O VI) toward Virgo is (1.4 +/- 0.8) x 10^14 cm^{-2}. By combining emission- and absorption-line data for this sight line, we estimate the physical parameters of the emitting gas.

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