Hard X-ray Excess Research Articles

Nuclear obscuration in the high-ionization Seyfert 2 galaxy Tol 0109-383

We report the BeppoSAX detection of a hard X-ray excess in the X-ray spectrum of the classical high-ionization Seyfert 2 galaxy Tol0109-383. The X-ray emission of this source observed below 7 keV is dominated by reflection from both cold and ionized gas, as seen in the ASCA data. The excess hard X-ray emission is presumably due to the central source absorbed by an optically thick obscuring torus with N(H)~2e24 cm-2. The strong cold X-ray reflection, if it is produced at the inner surface of the torus, is consistent with the picture where much of the inner nucleus of Tol0109-383 is exposed to direct view, as indicated by optical and infrared properties. However, the X-ray absorption must occur at small radii in order to hide the central X-ray source but leave the optical high-ionization emission line region unobscured. This may also be the case for objects like the Seyfert 1 galaxy Mrk231.

Excess hard X-ray emission from the obscured low luminosity AGN in the nearby galaxy M 51 (NGC 5194)

We observed the nearby galaxy M 51 (NGC 5194) with BeppoSAX. The X-ray properties of the nucleus below 10 keV are almost the same as the ASCA results regarding the hard component and the neutral Fe K line, but the intensity is about half of the ASCA 1993 data. Beyond this, in the BeppoSAX PDS data, we detected a bright hard X-ray emission component which dominates above 10 keV. The 10{100 keV flux and luminosity of this component are respectively 2 10 11 erg s 1 cm 2 and 2 10 41 erg s 1 . These are about 10 times higher than the extrapolation from the soft X-ray band, and similar to the flux observed with Ginga, which found a bright power law component in 2{20 keV band. Considering other wavelength properties and the X-ray luminosity, together with strong neutral Fe K line, the hard X-ray emission most likely arises from a low luminosity active nucleus, which is obscured with a column density of 10 24 cm 2 . This suggests that hidden low luminosity AGNs may well be present in other nearby galaxies. We interpret the discrepancy between Ginga and other X-ray satellites to be due to a large variability of absorption column density toward the line of sight over several years, suggesting that the Compton thick absorption material may be present on a spatial scale of a parsec. Apart from the nucleus, several ultra-luminous o-nuclear X-ray sources detected in M 51 exhibit long-term time variability, suggesting the state transition similar to that observed in Galactic black hole candidates.

Chandra detection of reflected X-ray emission from the type 2 QSO in IRAS 09104+4109

We present X-ray imaging spectroscopy of the extremely luminous infrared galaxy IRAS 09104+4109 obtained with the Chandra X-ray Observatory. With the arcsec resolution of Chandra, an unresolved source at the nucleus is separated from the surrounding cluster emission. A strong iron K line at 6.4 keV on a very hard continuum is detected from the nuclear source, rendering IRAS 09104+4109 the most distant reflection-dominated X-ray source known. Combined with the BeppoSAX detection of the excess hard X-ray emission, it provides further strong support for the presence of a hidden X-ray source of quasar luminosity in this infrared galaxy. Also seen is a faint linear structure to the north, which coincides with the main radio jet. An X-ray deficit in the corresponding region suggests an interaction between the cluster medium and the jet driven by the active nucleus.

Detection of Excess Hard X-Ray Emission from the Group of Galaxies HCG 62

We detected an excess of hard X-ray emission at energies above ~4 keV from the group of galaxies HCG 62 using data from the ASCA satellite. The excess emission is spatially extended up to ~10' from the group center and somewhat enhanced toward the north. Its spectrum can be represented by either a power law of photon index 0.8-2.7 or a bremsstrahlung of temperature greater than 6.3 keV. In the 2-10 keV range, the observed hard X-ray flux, (1.0 ± 0.3) × 10-12 ergs cm-2 s-1, implies a luminosity of (8.0 ± 2.0) × 1041 ergs s-1 for a Hubble constant of 50 km s-1 Mpc-1. The emission is thus too luminous to be attributed to X-ray binaries in the member galaxies. We discuss possible origins of the hard X-ray emission.

Implications of a Nonthermal Origin of the Excess Extreme‐Ultraviolet Emission from the Coma Cluster of Galaxies

The inverse Compton (IC) interpretation of the excess EUV emission that was recently reported from several clusters of galaxies suggests that the amount of relativistic electrons in the intracluster medium is highly signi—cant, ergs. Considering Coma as the prototype galaxy cluster of nonthermal radi- W e ( 1061 ation, with synchrotron and IC —uxes measured in the radio and EUV regions, and possibly also in the hard X-ray region, we discuss implications of the IC origin of the EUV —uxes in the case of low intra- cluster magnetic —elds of order 0.1 kG as required for the IC interpretation of the observed excess hard X-ray —ux, and in the case of high —elds of order 1 kG, as suggested by Faraday rotation measurements. Although for such high intracluster —elds the excess hard X-ray —uxes will require an explanation other than by the IC eUect, we show that the excess EUV —ux can be explained by the IC emission of a ii relic ˇˇ population of electrons driven into the incipient intracluster medium at the epoch of starburst activity by galactic winds and later on reenergized by adiabatic compression and/or large-scale shocks transmitted through the cluster as the consequence of more recent merger events. Radiative cooling will naturally produce a sharp cutoU in the spectrum of this relic electron population, which is required, in the case of microgauss —elds, in order to avoid a contradiction with the observed radio —uxes. For high magnetic —elds, B 1 kG, the interpretation of the radio —uxes of Coma requires a second population of electrons injected recently. They can be explained as secondaries produced by a population of relativistic protons. We calculate the —uxes of c-rays to be expected in both the low and high magnetic —eld sce- narios and discuss possibilities to distinguish between these two principal options by future c-ray obser- vations. Subject headings: cosmic raysdiUuse radiationgalaxies: clusters: individual (Coma) ¨ galaxies: starburstintergalactic mediumradiation mechanisms: nonthermal

Limits on the Diffuse Radio and Hard X‐Ray Emission of Abell 2199

The Westerbork Northern Sky Survey (WENSS) and the NRAO/VLA Sky Survey were used to determine an upper limit to the diffuse radio flux from the nearby cluster Abell 2199. For the entire cluster, this limit is less than 3.25 Jy at 327 MHz from WENSS; for the inner 15' radius, the limit is less than 168 mJy at 1.4 GHz. These limits are used to constrain the cluster magnetic field by requiring that the radio flux be consistent with the hard X-ray (HXR) flux observed by BeppoSAX, assuming that the observed HXR excess is the result of inverse Compton scattering of cosmic microwave background photons by relativistic electrons in the intracluster gas. We find that the magnetic field must be very weak (<0.073 μG) in order to avoid producing an observable radio halo. We also consider the possibility that the HXR excess is the result of nonthermal bremsstrahlung (NTB) by a population of suprathermal electrons that are being accelerated to higher energies. We find that a NTB model based on a power-law electron-momentum distribution with an exponent of μ ≈ 3.3 and containing about 5% of the number of electrons in the thermal ICM can reproduce the observed HXR flux.

Stochastic Acceleration and Nonthermal Radiation in Clusters of Galaxies

We study the possibility that the recently detected hard X-ray excess from some clusters of galaxies is due to bremsstrahlung emission of a nonthermal tail in the electron distribution produced by stochastic acceleration in a background of MHD waves. We find that this scenario requires an overall level of fluctuations which is only δB2/B2~ 3% for a magnetic fieldBon the order of μG. Despite the reasonable values for these parameters, a large rate of injection of waves is needed to compensate the damping of the waves.

A BeppoSAX Observation of the Merging Cluster Abell 3266.

We present results from a BeppoSAX observation of the rich cluster Abell 3266. The broadband spectrum (2-50 keV) of the cluster, when fitted with an optically thin thermal emission model, yields a temperature of 8.1+/-0.2 keV and a metal abundance of 0.17+/-0.02 in solar units, with no evidence of a hard X-ray excess in the Phoswich Detector System spectrum. By performing a spatially resolved spectral analysis, we find that the projected temperature drops with increasing radius, going from approximately 10 keV at the cluster core to approximately 5 keV at about 1.5 Mpc. Our BeppoSAX temperature profile is in good agreement with the ASCA temperature profile of Markevitch et al. From our two-dimensional temperature map, we find that the gradient is observed in all azimuthal directions. The temperature gradient may have been caused by a recent merger event that was also responsible for a velocity-dispersion gradient measured in the optical band. The projected metal abundance profile and the two-dimensional map are both consistent with being constant.

The BeppoSAX View of the Hot Cluster Abell 2319.

We present results from a BeppoSAX observation of the rich cluster Abell 2319. The broadband spectrum (2-50 keV) of the cluster can be adequately represented by an optically thin thermal emission model with a temperature of 9.6+/-0.3 keV and a metal abundance of 0.25+/-0.03 in solar units and with no evidence of a hard X-ray excess in the PDS spectrum. From the upper limit to the hard-tail component, we derive a lower limit of approximately 0.04 µG for the volume-averaged intracluster magnetic field. By performing spatially resolved spectroscopy in the medium energy band (2-10 keV), we find that the projected radial temperature and metal abundance profiles are constant out to a radius of 16&arcmin; (1.4 Mpc). A reduction of the temperature of one-third, when going from the cluster core out to 16&arcmin;, can be excluded in the present data at the 99% confidence level. From the analysis of the temperature and abundance maps, we find evidence of a temperature enhancement and of an abundance decrement in a region localized 6&arcmin;-8&arcmin; northeast of the core, where a merger event may be taking place. Finally, the temperature map indicates that the subcluster located northwest of the main cluster may be somewhat cooler than the rest of the cluster.

Cosmic rays, radio halos and nonthermal X-ray emission in clusters of galaxies

We calculate the flux of radio, hard X-ray and UV radiation from clusters of galaxies as produced by synchrotron emission and Inverse Compton Scattering of electrons generated as secondaries in cosmic ray interactions in the intracluster medium. Both the spatial distribution of cosmic rays due to their diffusion and the spatial distribution of the intracluster gas are taken into account. Our calculations are specifically applied to the case of the Coma cluster. The fluxes and spectra of the radio halo emission and of the hard X-ray excess from Coma can be explained in this model if an average magnetic field B ∼ 0.1 μG is assumed. However, such a low value for the intracluster magnetic field implies a large cosmic ray energy density which in turn is responsible, through neutral pion decay, for a gamma ray flux above 100 MeV which exceeds the EGRET upper limit. This gamma ray bound can be relaxed if the hard X-ray excess and the radio halo emission from Coma are not due to the same population of electrons. We finally stress the unique role that the new generation gamma ray satellites will play to discriminate among different models for the nonthermal emission in clusters of galaxies.

Clusters of galaxies: magnetic fields and nonthermal emission

The nonthermal particle content of galaxy clusters should in part have a cosmological component generated during the early starburst phase of the member galaxies. This is reviewed in the framework of a simple cluster formation model suggested previously. It implies a nonthermal energy fraction of about 10 percent for the Intracluster gas. We also propose a mechanism for the early generation of Intracluster magnetic fields in terms of Galactic Winds. It results in typical field strenghts of 10 −7 G. Such comparatively weak fields are consistent with an inverse Compton origin of the excess EUV and hard X-ray emission of the Coma cluster, given the radio synchrotron emission. The required relativistic electrons must have been accelerated rather recently, less than a few billion years ago, presumably in cluster accretion shocks. This is in contrast to the hadronic nonthermal component which accumulates on cosmological time scales, and whose π 0-decay TeV γ-ray emission is expected to be larger, or of the same order as the inverse Compton TeV emission. This γ-radiation characterizes the energetic history of cluster formation and should be observable with future arrays of imaging atmospheric Cherenkov telescopes.

A binary model for SN 1987 A and the hard X-ray excess

We demonstrate the existence of a companion to SN 1987 A by an analysis of the colour of the field SK69° 202. Assuming the binary model we discuss the accretion of the ejecta from SN 1987 A by the companion and the formation of an axisymmetric quasistationary shock wave structure. The spectrum detected by the satellite GINGA is reproduced and the hard X-ray emission below 10 keV and the early luminosity variation are explained.