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although principally devoted to studies of compact galactic sources and active galactic nuclei , the beppo - sax satellite has devoted a small fraction of time to galaxy cluster observations .
the primary motivation for performing observations of galaxy clusters with beppo - sax is the high spatial resolution of the mecs instruments . in these proceedings
i briefly review the beppo - sax observational program for galaxy clusters . after having listed contributions on galaxy clusters to these proceedings by other authors
i describe results from the analysis of the perseus observation .
the mecs instrument on board beppo - sax is well suited for the analysis of extended sources .
as discussed in giommi et al .
( 1998 ) in these proceedings , the effective area of the 3 mecs units is comparable to that of the 2 gis units on board asca in the 2 - 10 kev ( see their figure 1 ) .
the point spread function of the mecs is significantly sharper than that of the asca gis ( see giommi et al . 1998 figure 2 ) . at 6.4
kev the half power radius ( hpr ) of the mecs psf is @xmath0 1 arcmin while the hpr of the asca gis psf is @xmath0 2 arcmin .
moreover , unlike the asca gis psf , the mecs psf does not vary strongly with energy ( see fig.1 of dacri et al .
1998 , these proceedings ) .
another important point is that the instrumental background for the mecs units on - board beppo - sax is quite low @xmath1 in the 1.5 - 10 kev band to be compared with @xmath2 in the same band for the gis units on board asca .
moreover , due to the low inclination orbit adopted by the beppo - sax satellite , the background is extremely stable .
the beppo - sax observing program includes observations of a few galaxy cluster .
a list of observations scheduled and performed is presented in these proceedings by pallavicini ( see his table 3 ) .
almost all these observations are devoted to extended objects , the main aim being spatially resolved spectroscopy . within these proceedings
beppo - sax observations of galaxy clusters are presented by a few authors .
kaastra et al .
( 1998 ) discuss spatially resolved spectroscopy of a2199 .
they find evidence of a spectral hardening in the outer parts of this cluster .
moreover , using pds data they confirm the presence of a hard tail in a2199 .
colafrancesco et al .
( 1998 ) discuss the analysis of 2 intermediate redshift clusters namely a33 and a348 .
they combine their data with previous asca measurements to place constraints on the @xmath3 relationship at @xmath4 .
dacri et al .
discuss an analysis technique which they have developed to correct for the spectral distortions introduced by the energy dependent mecs psf .
they apply this technique to the core of the virgo cluster finding a temperature decrement towards the inner parts of cluster , in agreement with previous measurements made with rosat and asca .
the central region of the perseus cluster was observed by the beppo - sax satellite ( boella , butler et al .
97 ) between the 20@xmath5 and the 21@xmath6 of september 1996 during the science verification phase ( svp ) .
the observation was rather long with a total effective exposure of 89 ks for the medium energy concentrator spectrometer ( mecs ) ( boella , chiappetti et al .
97 ) , 38 ks for the hpgspc instrument ( manzo et al . 1997 ) and 30 ks for the pds instrument ( frontera et al .
1997 ) .
i have accumulated a mecs spectrum from a region centered on the emission peak using an extraction radius of 6.4 arcmin .
i have fitted the mecs , hpgspc and pds spectra simultaneously using a thermal emission code ( mekal in xspec version 9.01 ) absorbed by an equivalent hydrogen column density of n@xmath7 @xmath8 the fit to the broad band spectrum ( figure 1 ) shows an excess of the data with respect to the model at energies e 20 kev .
this excess can not be fitted with a thermal component , unless an unphysical temperature of more than 20 kev is assumed .
a power - law component fits adequately the high energy excess .
the intensity of the power - law component is @xmath9 erg @xmath8s@xmath10 in the 20 - 100 kev band .
comparison of our measurement with previous detections by oso7 ( rothschild et al . 1981 ) , heao1 ( levine et al . 1984 ) and with the osse upper limit ( osako et al . 1994 ) indicate that the power - law component is undergoing a secular decrease in intensity .
an asca analysis of the central region of perseus ( fabian et al . 94 ) shows that the spectrum below 3 kev is strongly contaminated by a cooling flow . by comparing the surface brightness profile of perseus in the 0.5 - 2.0 kev band , derived from the analysis of rosat pspc archive data , with a king profile we find that the cooling flow extends out to @xmath0 8 arcmin , corresponding to 240 kpc .
i have attempted to spatially resolve the cooling flow component with the mecs by analyzing spectra from 4 concentric annuli centered on the emission peak and with bounding radii of 0 - 2 arcmin , 2 - 4 arcmin , 4 - 6 arcmin and 6 - 8 arcmin respectively .
i have fitted each spectrum in the 3.5 - 10 kev range with a thermal model ( mekal ) allowing the temperature and the abundances to be free .
i have then reintroduced the data in the 1.5 - 3.5 kev range and plotted the residuals to the fits ( see figure 2 ) .
as can be clearly seen the relative intensity of the cooling flow component decreases as the radial distance from the peak of the emission increases ( the increase observed when going from panel 1 to panel 2 is due to instrumental effects , more specifically the detector psf , below 3 kev , redistributes a large fraction of photons coming from a circular region with a radius of 2 arcmin over a larger region ) .
as described in the previous subsection i have fitted spectra accumulated in concentric annuli with a thermal emission model . in figure 3
i plot the temperature derived from the fits . in order to avoid contamination from the cooling flow
the fits were performed in the 3 - 10 kev band .
our data clearly indicates that the temperature presents a strong gradient in perseus over a region of 8 arcmin ( corresponding to 240 kpc ) in radius .
i have accumulated a mecs spectrum from a circular region centered on the emission peak using an extraction radius of 6.4 arcmin , corresponding to @xmath0 200 kpc .
i have modeled the spectrum in the 3 - 10 kev band using a bremsstrahlung for the continuum and two gaussian lines , one for the 6.8 kev iron complex , the other for the 8 kev iron - nickel complex .
data at energies below 3 kev has been excluded because significantly contaminated by the cooling flow .
i find a temperature of @xmath11 kev for the continuum component , an energy e@xmath12 kev and a width @xmath13 kev for the lower energy line and an energy e@xmath14 kev and a width @xmath15 kev for the higher energy line , where both e@xmath16 and e@xmath17 are given in the source rest - frame .
the 8 kev feature is clearly broad and most likely due to a blend of different lines from highly ionized iron ( fe xxv and fe xxvi ) and nickel ( ni xxvii ) .
fits with thermal emission codes such as meka , mekal or raymond & smith do not reproduce satisfactorily the 8 kev feature , all these models underestimate significantly the intensity of the emission feature , the reason being that all the above codes predict an 8 kev over 6.8 kev line intensity ratio in the range 0.11 - 0.13 against an observed ratio of @xmath18 .
the most convincing explanation i have found for the observed anomalous ratio involves a process known as resonant scattering ( a detailed discussion of alternative explanations and of other aspects of the line ratio analysis is presented in molendi et al .
the inter - galactic medium in clusters , while optically thin to the continuum and many lines , may be optically thick in the center of some lines . indeed the process describing the absorption of an fe k@xmath19 line photon by an iron ion followed by the immediate re - emission , which is known as resonant scattering , can be quite effective for typical cluster gas densities and temperatures .
gilfanov et al .
( 86 ) have shown that the cores of rich cluster , such as perseus and virgo , should have optical depths , for the above process , of the order of a few .
if the gas is optically thick to resonant scattering , the line emission coming from the core of the cluster will be attenuated because of the photons which are scattered out of the line of sight .
if this is the case then the abundances measured using standard thermal emission codes , which assume optically thin thermal emission , can be significantly underestimated . in figure 4
i show the abundance profiles computed under the assumption of optically thin line emission ( open circles ) and of optically thick emission , using 2 different methods ( filled circles and open squares ) .
i find that the the fe abundance in the innermost circular region , with radius 2 arcmin , corresponding to @xmath0 60 kpc , is @xmath0 0.9 solar and consistent with 1 .
the obvious implication is that a very large fraction of the gas in the core of perseus has been processed in stars .
i acknowledge support of various nature from my colleagues in the beppo - sax team .
i am particularly grateful to the loc for the exquisite choice of restaurant for the social dinner .
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the high spatial resolution of the mecs experiment on board beppo - sax has encouraged a few scientists , including the author , to perform observations of galaxy clusters . 
 results from the analysis of the first few observed objects are encouraging . 
 after having reviewed the beppo - sax observing program for galaxy clusters and referenced contributions to these proceedings by other authors on the same topic , i present results from the analysis of the perseus cluster .