On the Unusually High Temperature of the Cluster of Galaxies 1E 0657-56

A recent X-ray observation of the cluster 1E0657-56 (z=0.296) with ASCA implied an unusually high temperature of ~17 keV. Such a high temperature would make it the hottest known cluster and severely constrain cosmological since, in a Universe with critical density Omega=1 the probability of observing such a cluster is only 4e-5. Here we test the robustness of this observational result since it has such important implications. We analysed the data using a variety of different data analysis methods and spectral analysis assumptions and find a temperature of ~11-12 keV in all cases, except for one class of spectral fits. These are fits in which the absorbing column density is fixed at the Galactic value. We show that a high temperature of ~17 keV is artificially obtained if the true spectrum has a stronger low- energy cut-off than that for Galactic absorption only. The extra absorption may be astrophysical in origin, or it may be a problem with the low-energy CCD efficiency. Although significantly lower than previous measurements, this temperature of kT ~11-12 keV is still relatively high since only a few clusters have been found to have temperatures higher than 10 keV and the data therefore still present some difficulty for an Omega=1 Universe. Our results will also be useful to anyone who wants to estimate the systematic errors involved in different methods of background subtraction of ASCA data for sources with similar S/N to that of the 1E0657-56 data reported here.Comment: 14 pages plus 2 figures. Latex with separate postscript figure files. AASTEX 4.0 macro. Accepted for the Astrophysical Journal Letter

Monte Carlo simulations of the Nickel Kα\alpha fluorescent emission line in a toroidal geometry

We present new results from Monte Carlo calculations of the flux and equivalent width (EW) of the Ni Kalpha fluorescent emission line in the toroidal X-ray reprocessor model of Murphy & Yaqoob (2009, MNRAS, 397, 1549). In the Compton-thin regime, the EW of the Ni Kalpha line is a factor of ~22 less than that of the Fe Kalpha line but this factor can be as low as ~6 in the Compton-thick regime. We show that the optically-thin limit for this ratio depends only on the Fe to Ni abundance ratio, it being independent of the geometry and covering factor of the reprocessor, and also independent of the shape of the incident X-ray continuum. We give some useful analytic expressions for the absolute flux and the EW of the Ni Kalpha line in the optically-thin limit. When the reprocessor is Compton-thick and the incident continuum is a power-law with a photon index of 1.9, the Ni Kalpha line EW has a maximum value of ~3 eV and ~250 eV for non-intercepting and intercepting lines-of-sight respectively. Larger EWs are obtained for flatter continua. We have also studied the Compton shoulder of the Ni Ka line and find that the ratio of scattered to unscattered flux in the line has a maximum value of 0.26, less than the corresponding maximum for the Fe Kalpha line. However, we find that the shape of the Compton shoulder profile for a given column density and inclination angle of the torus is similar to the corresponding profile for the Fe Ka line. Our results will be useful for interpreting X-ray spectra of active galactic nuclei (AGNs) and X-ray binary systems in which the system parameters are favorable for the Ni Kalpha line to be detected.Comment: Accepted for publication in MNRAS. 8 pages, 5 figure