Nonthermal Hard X‐Ray Emission in Galaxy Clusters Observed with theBeppoSAXPDS

Abstract

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 nonthermal hard X-ray (HXR) cluster emission by modeling the thermal contribution from the cluster gas and the nonthermal contamination from the unobscured active galactic nuclei (AGNs) in the clusters. We also evaluate the systematic uncertainties due to the background fluctuations. Assuming negligible contamination from the obscured AGNs, the resulting nonthermal component is detected at a 2 level in $50% of the nonsignificantly 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 nonthermal origin, whereas merger clusters do, with a 20-80 keV luminosity of $10 43 -10 44 h À2 50 ergs s À1 . The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR emission with a photon index of 2.8 þ0:3 À0:4 in the 12-115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGNs, which have harder spectra and a 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 nonthermal hard X-ray emission models. Assuming that the cosmic microwave background photons experience inverse Compton scattering from the merger-accelerated 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 ¼ 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 the HXR emission is also consistent with that predicted by the nonthermal bremsstrahlung, which thus cannot be ruled out by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming a centrally concentrated distribution of HXR emission, the data require 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.

GALAXY CLUSTERS IN THESWIFT/BURST ALERT TELESCOPE ERA: HARD X-RAYS IN THE INTRACLUSTER MEDIUM

“…The 99% CL limit on the nonthermal luminosity is 6.1 × 10 43 erg s −1 . The marginal (∼ 2σ ) BeppoSAX detection of a nonthermal emission (Nevalainen et al 2004) is a factor of 5 larger than our upper limit and is, thus, incompatible with our data.…”

“…According to many authors (e.g., Sauvageot et al 2005;Finoguenov et al 2006, and references therein), A3266 recently underwent a major merger, probably with a subcluster that was stripped during the encounter with the A3266 dense core. De and Nevalainen et al (2004) observed A3266 with BeppoSAX. The first group modeled the BeppoSAX broad-band spectrum (2-50 keV) with a simple, optically thin, thermal emission model at the temperature of 8.1 ± 0.2 keV, while the second group found marginal evidence (0.8 σ ) of nonthermal X-ray excess.…”

“…This was done by adding a power law to the thermal model used. We fixed the power-law index to 2.0, which is a value generally accepted for the nonthermal hard X-ray component generated by IC of relativistic electrons off CMB photons (e.g., Reimer et al 2004;Nevalainen et al 2004). We then let the power-law normalization vary until the Δχ 2 increment was larger than 2.7 (6.64).…”

“…A hard X-ray component has been detected with HEAO 1 by Primini et al (1981). Nevalainen et al (2004) used BeppoSAX and previous Rossi X-Ray Timing Explorer (RXTE) measurements to prove that this nonthermal component is variable and must therefore be connected to the central bright AGN. Sanders & Fabian (2007) reported, using Chandra, the presence of nonthermal X-ray emission in the core of Perseus in correspondence to the radio mini-halo (Gisler & Miley 1979;Gitti et al 2002).…”

“…In the above derivation, we do not make any assumption on the mechanism generating the nonthermal flux. However, in most cases, IC scattering is believed to be the principal emission process (e.g., Sarazin 1999;Nevalainen et al 2004;Fusco-Femiano et al 2007;Eckert et al 2008b). If this is true, then the IC emission can be modeled as a power law with a photon index of ∼2 in the 1-200 keV energy range (see, e.g., Reimer et al 2004).…”