Search

We present the results of a 55 ks NuSTAR observation of the core of the Coma Cluster. The global spectrum can be explained by thermal gas emission, with a conservative 90% upper limit to non-thermal inverse Compton (IC) emission of 5.1 × 10–12 erg cm–2 s–1 in a 12′ × 12′ field of view. The brightness of the thermal component in this central region does not allow more stringent upper limits on the IC component when compared with non-imaging instruments with much larger fields of view where claims of detections have been made. Future mosaic NuSTAR observations of Coma will further address this issue. The temperature map shows a relatively uniform temperature distribution with a gradient from the hot northwest side to the cooler southeast, in agreement with previous measurements. The temperature determination is robust given the flat effective area and low background in the 3-20 keV band, making NuSTAR an ideal instrument to measure high temperatures in the intracluster medium. [ABSTRACT FROM AUTHOR]

The search for diffuse non-thermal inverse Compton (IC) emission from galaxy clusters at hard X-ray energies has been undertaken with many instruments, with most detections being either of low significance or controversial. Because all prior telescopes sensitive at E > 10 keV do not focus light and have degree-scale fields of view, their backgrounds are both high and difficult to characterize. The associated uncertainties result in lower sensitivity to IC emission and a greater chance of false detection. In this work, we present 266 ks NuSTAR observations of the Bullet cluster, which is detected in the energy range 3-30 keV. NuSTAR's unprecedented hard X-ray focusing capability largely eliminates confusion between diffuse IC and point sources; however, at the highest energies, the background still dominates and must be well understood. To this end, we have developed a complete background model constructed of physically inspired components constrained by extragalactic survey field observations, the specific parameters of which are derived locally from data in non-source regions of target observations. Applying the background model to the Bullet cluster data, we find that the spectrum is well—but not perfectly—described as an isothermal plasma with kT = 14.2 ± 0.2 keV. To slightly improve the fit, a second temperature component is added, which appears to account for lower temperature emission from the cool core, pushing the primary component to kT ∼ 15.3 keV. We see no convincing need to invoke an IC component to describe the spectrum of the Bullet cluster, and instead argue that it is dominated at all energies by emission from purely thermal gas. The conservatively derived 90% upper limit on the IC flux of 1.1 × 10–12 erg s–1 cm–2 (50-100 keV), implying a lower limit on B ≳ 0.2 μG, is barely consistent with detected fluxes previously reported. In addition to discussing the possible origin of this discrepancy, we remark on the potential implications of this analysis for the prospects for detecting IC in galaxy clusters in the future. [ABSTRACT FROM AUTHOR]

After the positive detection by BeppoSAX of hard X-ray radiation up to approximately 80 keV in the Coma Cluster spectrum, we present evidence for nonthermal emission from A2256 in excess of thermal emission at a 4.6 sigma confidence level. In addition to this power-law component, a second nonthermal component already detected by ASCA could be present in the X-ray spectrum of the cluster, which is not surprising given the complex radio morphology of the cluster central region. The spectral index of the hard tail detected by the Phoswich Detection System on board BeppoSAX is marginally consistent with that expected for the inverse Compton model. A value of approximately 0.05 µG is derived for the intracluster magnetic field of the extended radio emission in the northern regions of the cluster, while a higher value of approximately 0.5 µG could be present in the central radio halo, which is likely related to the hard tail detected by ASCA.

We present results from a spatially resolved spectral analysis of the merging cluster Abell 2256. The long integration time (135 ks) and the good spatial resolution of the Medium-Energy Concentrator/Spectrometer on board BeppoSAX allow us to derive a new and substantially improved measurement of the temperature structure. We find that, within a central region of the cluster, where the effects of the merger are visible in the ROSAT surface brightness image, the azimuthally averaged projected temperature is remarkably flat; outside this region, the projected temperature rapidly declines. The BeppoSAX data also show clear evidence of an azimuthal temperature gradient in the 4&arcmin;-8&arcmin; (0.4-0.8 Mpc) radial bin, oriented in the same direction as the merger itself. Our metal abundance profile shows, for the first time, firm evidence of an abundance gradient in a rich merging cluster. Intriguingly, the abundance map shows, in the southeast sector, i.e., the one farthest away from the merger, a sharp factor of 2 drop in abundance at a radius comparable to the core radius of the cluster. A possible interpretation is that prior to the merger event, a cooling flow had already developed in the core of the infalling subgroup, as suggested by Fabian & Daines. The interaction between the substructures would have disrupted the cooling flow, thereby reheating and remixing the gas. Since the merger in A2256 is still in a relatively early stage, the gas located on the side opposite the merger event would still retain the low abundances associated with the intracluster medium prior to the cooling flow disruption.

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.

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.