COMPARISON OF ASCA AND ROSAT CLUSTER TEMPERATURES: A2256, A3558, AND AWM 7

摘要

We address the consistency between ASCA and ROSAT spatially resolved cluster temperature measurements, which is of significant and immediate interest given the recent ASCA reports of temperature gradients in several hot clusters. We reanalyze ROSAT PSPC data on A2256 (originally analyzed by Briel & Henry) using the newer calibration and a technique less sensitive to the calibration uncertainties. We find a temperature decline with radius in good agreement with that of ASCA, although with much larger errors than those of ASCA. We also present ASCA temperature maps and radial profiles of two cooler clusters, A3558 and AWM 7. These are compared to the ROSAT results of Bardelli et al. and Neumann & Boehringer. In A3558, our analysis detects an asymmetric temperature pattern and a slight radial temperature decline at r ～ 0.5 h~(-1) Mpc, in addition to the spectral complexity of the cD galaxy region. We do not detect any significant spatial temperature variations in AWM 7, except around the cD galaxy, in agreement with the earlier ASCA analysis of this cluster. Radial temperature profiles of these two clusters are in a qualitative agreement with those from ROSAT. However, while ASCA average temperatures (5.5 and 3.9 keV, respectively) agree with those from other high-energy instruments (Ginga and EXOSAT for A3558 and Einstein MPC and EXOSAT for AWM 7), ROSAT temperatures are lower by factors of 1.7 and 1.25, respectively. We find that including realistic estimates of the current ROSAT systematic uncertainties enlarges the temperature confidence intervals so that ROSAT measurements are consistent with others for these two clusters as well. Because of the limited energy coverage of ROSAT PSPC, its results for the hotter clusters are highly sensitive to calibration uncertainties. We conclude that within the present calibration accuracy, there is no disagreement between ASCA and other instruments. This adds confidence to the ASCA results on the hotter clusters which can at present be studied with precision only with this instrument. On the scientific side, a ROSAT temperature underestimate for A3558 may be responsible for the anomalously high gas to total mass fraction found by Bardelli et al. Our spatially resolved temperature measurement may indicate a recent or ongoing subcluster merger in this core member of the Shapley supercluster.