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Astron. Astrophys. 353, 498-506 (2000)

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1. Introduction

High redshift clusters of galaxies have long served as valuable tools to test theories of cosmic structure formation (e.g. Press and Schechter 1974, Peebles et al. 1989, Eke et al. 1996, to mention just a few). Measurements of the temperature of the intra-cluster medium (ICM) in distant clusters are particularly important since cluster temperatures are closely related to their mass, as long as these systems are in virial equilibrium. Physical cluster parameters of the highest redshift clusters, measurable by means of deep X-ray observations (e.g. the Luminosity-Temperature relation), set also strong constraints on the thermodynamical evolution of their gaseous component. In addition, the measurement of the metal abundance from X-ray spectra in remote bound systems can constrain the mode and epoch of the enrichment of the intracluster gas.

Despite their importance, ICM properties at high redshift ([FORMULA]) are still largely unexplored. This is due to the limited effective area and spatial resolution of the past X-ray missions, as well as to the difficulty of finding X-ray luminous clusters at [FORMULA]. Indeed the Extended Medium Sensitivity Survey (EMSS; Gioia et al., 1990) carried out with the Einstein Observatory has detected clusters at redshift up to [FORMULA] (e.g. MS1137+6625 at 0.78 and MS1054-0321 at z=0.83, Gioia & Luppino, 1994; Donahue et al., 1998) but the statistics of high-z clusters in the EMSS is still low with only 6 clusters (out of a total of about 100) at [FORMULA]. Over the last few years, however, several X-ray cluster surveys based on ROSAT-PSPC data (the RDCS: Rosati et al., 1995, 1998; the WARPS: Scharf et al., 1997; the SHARC: Burke et al., 1997; the 160o survey of Vikhlinin et al., 1998; the NEP: Mullis et al., 1998) have changed the observational scenario by collecting sizable samples of distant ([FORMULA]) systems, thus extending the pioneering work of the EMSS. The ROSAT Deep Cluster Survey (RDCS, Rosati et al. 1995, 1998) has pushed these studies to the faintest fluxes and highest redshifts, with 33 clusters spectroscopically identified to date at [FORMULA], out to [FORMULA] (Rosati et al. 1999).

As an alternative method, several studies have tried to locate distant clusters around high redshift radio galaxies (e.g. 3C 324 at [FORMULA], Smail & Dickinson, 1995; 3C 184 at [FORMULA], Deltorn et al. 1997). ROSAT observations of distant radio galaxies (Crawford & Fabian 1996; Dickinson 1997) have provided some evidence that the observed X-ray emission might originate from hot intra-cluster gas. It remains difficult, however, to reliably measure ICM physical parameters in these objects, since one needs to discriminate an absorbed power-law component, due to the radio galaxy's AGN, from the thermal component of the putative diffuse ICM.

Although the number of bona-fide high-z clusters has significantly increased, only long ASCA observations, with a broad energy response, have allowed a few sketchy studies of their ICM properties to date. A recent compilation of cluster temperatures (Wu et al. 1999) reports a total of 168 clusters with good broad band X-ray spectroscopy, but only 5 (3) of these have been studied at [FORMULA] ([FORMULA]).

In this paper we present Beppo SAX observations of two high redshift clusters: MS 2053.7-0449 from the EMSS sample at [FORMULA] and RXJ 0152.7-1357 from the RDCS sample at [FORMULA]. The Beppo SAX MECS instrument (see Sect. 2) has a slightly smaller (larger) effective area than the ASCA GIS for [FORMULA] keV ([FORMULA] keV) and it is characterized by a significantly sharper PSF if compared with the ASCA GIS or ASCA SIS instruments, especially at high energy. This latter property implies a reduced background, making Beppo SAX MECS the best instrument to date for distant clusters investigations. RXJ 0152.7-1357 is the highest redshift cluster with good quality broad band X-ray spectroscopy to date and among the most X-ray luminous known at [FORMULA] together with the well known MS1054.4-0321 (Gioia & Luppino, 1994; Donahue et al., 1998). Our data provide a rather accurate ([FORMULA]) temperature measurement for RXJ 0152.7-1357, which allows an interesting comparison with MS1054.4-0321 with very similar redshift and luminosity. In Table 1 the main properties of the two clusters, in their own discovery bands, are summarized.


Table 1. Observed clusters.
a) We note that in the case of RXJ 0152.7-1357 we have reported the ROSAT PSPC [FORMULA] (luminosity) measured over a circle of 3 arcmin radius, corresponding to [FORMULA] Mpc at [FORMULA], while for MS 2053.7-0449 we have reported the total EMSS "corrected" [FORMULA] (luminosity) i.e. the flux (luminosity) corrected for the effect of the finite EMSS detection cell.

This paper is organized as follows. In Sect. 2 we present the Beppo SAX observations. The spectral analysis for RXJ 0152.7-1357 is presented in Sect. 3 and for MS 2053.7-0449 in Sect. 4. Sect. 5 contains a discussion of the results in the context of the high redshift [FORMULA] relationship. Finally, summary and conclusions are presented in Sect. 6. We adopt [FORMULA] km s-1 Mpc-1 and [FORMULA] throughout, unless otherwise noted.

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Online publication: December 17, 1999