X-Ray Emission Lines from Cooling Flows

Abstract

Individual X-ray emission line strengths can be used to estimate the rate at which gas cools in the intracluster medium. We review the technique and show that departures from ionization equilibrium in the cooling plasma are not important at X-ray temperatures. We present data from 14 observations of 7 clusters performed with the Focal Plane Crystal Spectrometer on the Einstein Observatory, in addition to a brief review of earlier results on M87. About half the observations gave detections and half upper limits. Our estimates of the cooling rate Ṁ within the 3x30 arc min aperture are ≈4 M_⊙ yr^-1 for M87, ≈20–30 M_⊙ yr^-1 for Centaurus and ≈200 M_⊙ yr^-1 for Perseus. For four other cooling flow clusters, 0335+096, A262, A496 and A1060 we obtain non-restrictive upper limits. For M87, Cen and Per, our values of M are similar to those found by completely independent methods based on imaging or other spectral data. Although our estimates are formally only upper limits to the actual values of Ṁ if the gas is being heated by some mechanism, this agreement is circumstantial evidence in favor of the existence of substantial cooling flows and against the dominance of conduction or other heat sources. For Perseus, an estimate of the volume emission measure required to produce the Fe XVII line leads to the conclusion that the intracluster gas must be a multi-phase plasma. The unusually strong oxygen lines from M87 and Perseus suggest that the O/Fe abundance ratio is larger in the intracluster gas than in the Sun.