Hard X-Ray Emission from the Galaxy Cluster A2256 (original) (raw)

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RXTE and ASCA Constraints on Nonthermal Emission from the A2256 Galaxy Cluster

The Astrophysical Journal, 1999

An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi X-ray Timing Explorer (RXTE) proportional counter array (PCA) produced a high quality spectrum in the 2 -30 keV range. Joint fitting with the 0.7 -11 keV spectrum obtained with the Advanced Satellite for Astrophysics and Cosmology (ASCA) Gas Imaging spectrometer (GIS) gives an upperlimit of ∼2.3×10 −7 photons cm −2 sec −1 keV −1 for non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic field of 0.36µG and an upperlimit of 1.8×10 −13 ergs cm −3 for the cosmic-ray electron energy density. The resulting lower limit to the central magnetic field is ∼1 -3 µG. While a magnetic field of ∼0.1 -0.2 µG can be created by galaxy wakes, a magnetic field of several µG is usually associated with a cooling flow or, as in the case of the Coma cluster, a subcluster merger. However, for A2256, the evidence for a merger is weak and the main cluster shows no evidence of a cooling flow. Thus, there is presently no satisfactory hypothesis for the origin of an average cluster magnetic field as high as > 0.36 µG in the A2256 cluster.

High and Low-Energy Nonthermal X-Ray Emission from the Abell 2199 Cluster of Galaxies

Astrophysical Journal, 1999

We report the detection of both soft and hard excess X-ray emission in the cluster of galaxies A 2199, based upon spatially resolved spectroscopy with data from the BeppoSAX, EUVE and ROSAT missions. The excess emission is visible at radii larger than 300 kpc and increases in strength relative to the isothermal component. The total 0.1-100 keV luminosity of this component is 15 % of the cluster luminosity, but it dominates the cluster luminosity at high and low energies. We argue that the most plausible interpretation of the excess emission is an inverse Compton interaction between the cosmic microwave background and relativistic electrons in the cluster. The observed spatial distribution of the non-thermal component implies that there is a large halo of cosmic ray electrons between 0.5-1.5 Mpc surrounding the cluster core. The prominent existence of this component has cosmological implications, as it is significantly changing our picture of a clusters's particle acceleration history, dynamics between the thermal and relativistic media, and total mass budgets.

Nonisothermal X-Ray--emitting Gas in Clusters of Galaxies

The Astrophysical Journal, 1996

We h a v e analyzed X-ray spectra from six galaxy clusters which contain cooling ows: A85, A478, A1795, A2142, A2147, & A2199. The X-ray spectra were taken with the HEAO1-A2 Medium and High Energy Detectors and the Einstein Solid State Spectrometer. For each cluster, we simultaneously t the spectra from these three detectors with models

Hard X-Ray Radiation in the Coma Cluster Spectrum

The Astrophysical Journal, 1999

Hard X-ray radiation has been detected for the first time in the Coma Cluster by BeppoSAX. Thanks to the unprecedented sensitivity of the Phoswich Detection System (PDS) instrument, the source has been detected up to ∼80 keV. There is clear evidence (4.5 j) for nonthermal emission in excess of thermal emission above ∼25 keV. The hard excess is very unlikely to be the result of X Comae, the Seyfert 1 galaxy that is present in the field of view of the PDS. A hard spectral tail that is due to inverse Compton scattering on cosmic microwave background photons is predicted in clusters, like Coma, with radio halos. Combining the present results with radio observations, a volume-averaged intracluster magnetic field of ∼0.15 mG is derived, while the electron energy density of the emitting electrons is ∼ ergs cm Ϫ3 .

Nonthermal Hard X‐Ray Emission in Galaxy Clusters Observed with the B eppo SAX PDS

The Astrophysical Journal, 2004

We study the X-ray emission in a sample of galaxy clusters using the BeppoSAX PDS instrument in the 20 -80 keV energy band. We estimate the non-thermal hard X-ray cluster emission (HXR) by modeling the thermal contribution from the cluster gas and the non-thermal contamination from the unobscured AGN in the clusters. We also evaluate the systematic uncertainties due to the background fluctuations. Assuming negligible contamination from the obscured AGN, the resulting non-thermal component is detected at a 2σ level in ∼50% of the non-significantly AGN-contaminated clusters: A2142, A2199, A2256, A3376, Coma, Ophiuchus and Virgo. The data are consistent with a scenario whereby relaxed clusters have no hard X-ray component of non-thermal origin, whereas merger clusters do, with a 20 -80 keV luminosity of ∼ 10 43−44 h −2 50 erg s −1 . The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR with a photon index of 2.8 +0.3 −0.4 in the 12 -115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGN, which have harder spectra and centrally concentrated distribution. These results are supportive of the assumption of the merger shock acceleration of electrons in clusters, which has been proposed as a possible origin of the non-thermal hard X-ray emission models. Assuming that the Cosmic Microwave Background photons experience Inverse Compton scattering from the mergeraccelerated relativistic electrons, and thus produce the observed HXR, the measured hard X-ray slope corresponds to a differential momentum spectra of the relativistic electrons with a slope of µ = 3.8 -5.0. In presence of cluster magnetic fields this relativistic electron population produces synchrotron emission with a spectral index of 1.4 -2.1, consistent with radio halo observations of merger clusters. Thus both hard X-ray and radio observations of merger clusters are consistent with the Inverse Compton model. The observed slope of HXR is also consistent with that predicted by the non-thermal bremsstrahlung, which thus cannot be ruled by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming centrally concentrated distribution of HXR, the data requires a harder slope for the HXR spectrum, which is consistent with secondary electron models, but this model yields a worse fit to the PDS data and thus seems to be disfavored over the primary electron Inverse Compton model.

Nonthermal emission from the radio relic of the galaxy cluster A2256

Astronomische Nachrichten, 2011

We aim to obtain a consistent description of non-thermal emissions from Abell 2256 and to give a prediction for a gamma-ray emission from this galaxy cluster. Assuming that a radio relic illuminates a localization of an ongoing merger, and that both radio and non-thermal part of hard X-ray emission are due to electron component of cosmic rays filling the relic, we derived from radio and hard X-ray properties of the relic in A2256 the magnetic field strength and number densities for relativistic electrons and protons. Due to the interpretation of the radio relic as a structure formed just where a shock front is, we discuss a gamma-ray emission at the cluster periphery. The estimated strength of the magnetic field in the relic is equal to 0.05 µG, while the amplitude of the electron number density varies from 3 • 10 −4 to 3 • 10 −5 cm −3 (respectively for the relic thickness of 50 to 500 kpc). We got a substantial degree of non-equipartition between cosmic rays and magnetic field in the relic region, where the CR pressure is approaching that of thermal gas. Our prediction for LOFAR is a synchrotron flux from the relic region of the order of ∼6 Jy at 60MHz and ∼10 Jy at 30MHz. The lower limit of the γ-ray flux from the relic region calculated for a hadronic channel is of the order of 10 −12 erg cm −2 .

Nonthermal emission from clusters of galaxies

Journal of Cosmology and Astroparticle Physics, 2009

We show that the spectral and radial distribution of the nonthermal emission of massive, M 10 14.5 M ⊙ , galaxy clusters may be approximately described by simple analytic expressions, which depend on the cluster thermal X-ray properties and on two model parameter, β core and η e . β core is the ratio of the cosmic-ray (CR) energy density (within a logarithmic CR energy interval) and the thermal energy density at the cluster core, and η e(p) is the fraction of the thermal energy generated in strong collisionless shocks, which is deposited in CR electrons (protons). Using a simple analytic model for the evolution of intra-cluster medium CRs, which are produced by accretion shocks, we find that β core ≃ η p /200, nearly independent of cluster mass and with a scatter ∆ ln β core ≃ 1 between clusters of given mass. We show that the hard X-ray (HXR) and γ-ray luminosities produced by inverse Compton scattering of CMB photons by electrons accelerated in accretion shocks (primary electrons) exceed the luminosities produced by secondary particles (generated in hadronic interactions within the cluster) by factors ≃ 500(η e /η p )(T /10keV) −1/2 and ≃ 150(η e /η p )(T /10keV) −1/2 respectively, where T is the cluster temperature. Secondary particle emission may dominate at the radio and very high energy ( 1 TeV) γ-ray bands. Our model predicts, in contrast with some earlier work, that the HXR and γ-ray emission from clusters of galaxies are extended, since the emission is dominated at these energies by primary (rather than by secondary) electrons. Our predictions are consistent with the observed nonthermal emission of the Coma cluster for η p ∼ η e ∼ 0.1. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) γray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed. In particular, we identify the clusters which are the best candidates for detection in γ-rays. Finally, we show that our model's results agree with results of detailed numerical calculations, and that discrepancies between the results of various numerical simulations (and between such results and our model) are due to inaccuracies in the numerical calculations.

Nonthermal Bremsstrahlung and Hard X‐Ray Emission from Clusters of Galaxies

The Astrophysical Journal, 2000

We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray (HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these models, the HXR emission is due to suprathermal electrons with energies of ∼10-200 keV. We consider models in which these transrelativistic suprathermal particles are the low energy end of a population of electrons which are being accelerated to high energies by shocks or turbulence ("accelerating electron" models). We also consider a model in which these electrons are the remnant of an older nonthermal population which is losing energy and rejoining the thermal distribution as a result of Coulomb interactions ("cooling electron" models). The suprathermal populations are assumed to start at an electron kinetic energy which is 3kT , where T is the temperature of the thermal intracluster medium (ICM). The nonthermal bremsstrahlung spectra flatten at low photon energies because of the lack of low energy nonthermal particles. The accelerating electron models have HXR spectra which are nearly power-laws from ∼20-100 keV. However, the spectra are brighter and flatter than given by the nonrelativistic bremsstrahlung cross-section because of transrelativistic effects. The HXR spectrum of the cooling electron model is very flat, and most of the X-ray emission in the HXR energy range (10-100 keV) actually arises from electrons with much higher energies (∼100 MeV). Under the assumption that the suprathermal electrons form part of a continuous spectrum of electrons including highly relativistic particles, we have calculated the inverse Compton (IC) extreme ultraviolet (EUV), HXR, and radio synchrotron emission by the extensions of the same populations. For accelerating electron models with powerlaw momentum spectra (N[p] ∝ p −µ ) with µ ∼ < 2.7, which are those expected from strong shock acceleration, the IC HXR emission exceeds that due to NTB. Thus, these models are only of interest if the electron population is cut-off at some upper energy ∼ <1 GeV. Similarly, flat spectrum accelerating electron models produce more radio synchrotron emission than is observed from clusters if the ICM magnetic field is B ∼ > 1 µG. The cooling electron model produces vastly too much EUV emission as compared to the observations of clusters. We have compared these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB models require a nonthermal electron population which contains about 3% of the number of electrons in the thermal ICM. If the suprathermal electron population is cut-off at some energy above 100 keV, then the models can easily fit the observed HXR fluxes and spectral indices in both clusters. For accelerating electron models without a cutoff, the electron spectrum must be rather steep ∼ > 2.9 to avoid producing too much IC HXR emission. The model HXR spectra are then rather steep, but marginally consistent with observations of the HXR spectrum in Abell 2199 and Coma or the radio spectrum in Coma. These models can account for the HXR and radio properties of these two clusters, but do not produce enough EUV emission.

South-West extension of the hard X-ray emission from the Coma cluster

Astronomy and Astrophysics, 2007

Aims. We explore the morphology of hard (18-30 keV) X-ray emission from the Coma cluster of galaxies. Methods. We analyse a deep (1.1 Ms) observation of the Coma cluster with the ISGRI imager on board the INTEGRAL satellite. Results. We show that the source extension in the North-East to South-West (SW) direction (∼ 17 ′ ) significantly exceeds the size of the point spread function of ISGRI, and that the centroid of the image of the source in the 18-30 keV band is displaced in the SW direction compared to the centroid in the 1-10 keV band. To test the nature of the SW extension we fit the data assuming different models of source morphology. The best fit is achieved with a diffuse source of elliptical shape, although an acceptable fit can be achieved assuming an additional point source SW of the cluster core. In the case of an elliptical source, the direction of extension of the source coincides with the direction toward the subcluster falling onto the Coma cluster. If the SW excess is due to the presence of a point source with a hard spectrum, we show that there is no obvious X-ray counterpart for this additional source, and that the closest X-ray source is the quasar EXO 1256+281, which is located 6.1 ′ from the centroid of the excess. Conclusions. The observed morphology of the hard X-ray emission clarifies the nature of the hard X-ray "excess" emission from the Coma cluster, which is due to the presence of an extended hard X-ray source SW of the cluster core.

Soft X-Ray and Extreme Ultraviolet Excess Emission from Clusters of Galaxies

Space Science Reviews, 2008

An excess over the extrapolation to the extreme ultraviolet and soft X-ray ranges of the thermal emission from the hot intracluster medium has been detected in a number of clusters of galaxies. We briefly present each of the satellites (EUVE, ROSAT PSPC and BeppoSAX, and presently XMM-Newton, Chandra and Suzaku) and their corresponding instrumental issues, which are responsible for the fact that this soft excess remains controversial in a number of cases. We then review the evidence for this soft X-ray excess and discuss the possible mechanisms (thermal and non-thermal) which could be responsible for this emission.