This is the first part of a study of the detailed X‐ray properties of the cores of nearby clusters. We have used the flux‐limited sample of 55 clusters listed by Edge et al., and archival and proprietary data from the ROSAT observatory. In this paper an X‐ray spatial analysis based on the surface‐brightness‐deprojection technique is applied to the clusters in the sample with the aim of studying their cooling flow properties. We determine the fraction of cooling flows in this sample to be 70–90 per cent, and estimate the contribution of the flow region to the cluster X‐ray luminosity. We show that the luminosity within a strong cooling flow can account for up to 70 per cent of a cluster X‐ray bolometric luminosity. Our analysis indicates that about 40 per cent of the clusters in the sample have flows depositing more than 100 M⊙ yr−1 throughout the cooling region, and that these possibly have been undisturbed for many Gyr, confirming that cooling flows are the natural state of cluster cores. New cooling flows in the sample are presented, and previously ambiguous ones are clarified. We have constructed a catalogue of some intracluster medium properties for the clusters in this sample. The profiles of the mass deposited from cooling flows are analysed, and evidence is presented for the existence of breaks in some of the profiles. Comparison is made to recent optical and radio data. We cross‐correlate our sample with the Green Bank, NVSS and FIRST surveys, and with the volume‐limited sample of brightest cluster galaxies presented by Lauer &38; Postman. Although weak trends exist, no strong correlation between optical magnitude or radio power of the brightest cluster galaxy and the strength of the flow is found.

The behavior of uniformly accelerated detectors in the Minkowski and Rindler vacua is analyzed when the detector is coupled to a scalar field during a finite amount of time T. We point out that the logarithmic ultraviolet divergences reported in the literature are due to the instantaneous switching of the detector. We explicitly show this by considering a detector switched on and o8' continuously.The usual Planckian spectrum for the excitation probability is recovered in the limit T~+oo. PACS number(s): 04.60.+n, 03.70.+kThe development of quantum-Beld theory in curved space-time has raised a number of questions related to the particle concept and its frame dependence. As a consequence, the behavior of detectors moving in various ways has been a continuous source of renewed investigation (see [1] for a comprehensive discussion). It is known that a uniformly accelerated detector in the Minkowski vacuum feels a thermal bath characterized by a temperature proportional to its proper acceleration. This thermal bath is often called the Fulling-Davies-Unruh (FDU) thermal bath [2,3], and it is composed of Rindler particles. Thus, a uniformly accelerated detector prepared in the ground state has a finite probability of being excited to a higher level in the Minkowski vacuum. This transition is followed by the emission of a Minkowski particle as described by inertial observers, and by the absorption of a Rindler particle from the FDU thermal bath as described by uniformly accelerated observers [4]. The introduction of the FDU thermal bath is also crucial in analyzing other quantum phenomena such as the bremsstrahlung eKect in the accelerated frame [5].The FDU thermal bath is usually analyzed with an idealized detector which is turned on forever. In this paper we show that no divergence appears in the excitation rate for finite-time detectors provided that they are turned on and off continuously. (Actually, this is a more realistic procedure for modeling physical detectors. ) Moreover, no "renormalization" procedure is used here, and the excitation probability obtained. is positive definite. We also obtain an expression for the excitation rate in the limit where the detector is kept switched on for a long time. The usual Planckian spectrum is recovered when we keep the detector permanently turned on (T -+ +oo). The analysis is carried out in the uniformly accelerated frame, where the calculations are simpler. Finally, we show that our conclusions do not depend sensitively on the way the detector is switched on and oK We will carry out our analysis with Unruh-DeWitt detectors.The metric signature adopted is (+ ---), and natural units (Ii = c = k~= 1) are used.In order to analyze the behavior of a uniformly accelerated finite-time detector in its rest frame, we will first quantize the massless scalar Beld in the Rindler wedge, i.e. , the portion z )~t of the Minkowski space-time. We use Rindler coordinates (r, x, y, ()

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