An observational approach is presented to constrain the global structure and evolution of the intracluster medium based on the ROSAT and ASCA distant cluster sample .
From statistical analysis of the gas density profile and the connection to the L _ { X } - T relation under the \beta -model , the scaled gas profile is found to be nearly universal for the outer region and the luminosity evaluated outside 0.2 r _ { 500 } is tightly related to the temperature through \propto T ^ { \sim 3 } rather than T ^ { 2 } .
On the other hand , a large density scatter exists in the core region and there is clearly a deviation from the self-similar scaling for clusters with a small core size .
A direct link between the core size and the radiative cooling timescale , t _ { cool } and the analysis of X-ray fundamental plane suggest that t _ { cool } is a parameter to control the gas structure and the appearance of small cores in regular clusters may be much connected with the thermal evolution .
We derive the luminosity- ‘ ambient temperature ’ relation ( L _ { X } - T ^ { \prime } ) , assuming the universal temperature profile for the clusters with short cooling time and adopting a correction T ^ { \prime } = 1.3 T , and find the dispersion around the relation significantly decreases in comparison to the case of the L _ { X } - T and the slope becomes less steep from 3.01 ^ { +0.49 } _ { -0.44 } to 2.80 ^ { +0.28 } _ { -0.24 } . L _ { 1 keV } , which is defined as a normalization factor for each cluster , can be regarded as constant for a wide range of the cooling time .
We further examined the L _ { X } - T \beta and L _ { X } - T ^ { \prime } \beta relations and showed a trend that merging clusters segregate from the regular clusters on the planes .
A good correlation between the cooling time and the X-ray morphology on the L _ { 1 keV } - ( t _ { cool } / t _ { age } ) plane leads us to define three phases according to the different level of cooling , and draw a phenomenological picture : after a cluster collapses and t _ { cool } falls below the age of the universe , the core cools radiatively with quasi-hydrostatic balancing in the gravitational potential , and the central density gradually becomes higher to evolve from an outer-core-dominant cluster , which marginally follows the self-similarity , to inner-core-dominant cluster .