High redshift X – ray galaxy clusters . II . The L X − T relationship revisited

Aims. In this paper we re-visit the observational relation between X–ray luminosity and temperature for high-z galaxy clusters and compare it with the local L X –T and with theoretical models. Methods. To these ends we use a sample of 17 clusters extracted from the Chandra archive supplemented with additional clusters from the literature, either observed by Chandra or XMM–Newton, to form a final sample of 39 high redshift (0.25 < z <1.3) objects. Different statistical approaches are adopted to analyze the L X –T relation. Results. The slope of the L X –T relation of high redshift clusters is steeper than expected from the self–similar model predictions and steeper, even though still compatible within the errors, than the local L X –T slope. The distant cluster L X –T relation shows a significant evolution with respect to the local Universe: high-z clusters are more luminous than the local ones by a factor ≈ 2 at any given temperature. The evolution with redshift of the L X –T relation cannot be described by a single power law nor by the evolution predicted by the self–similar model. Conclusions. We find a strong evolution, similar or stronger than the self–similar model, from z = 0 to z ≤ 0.3 followed by a much weaker, if any, evolution at higher redshifts. The weaker evolution is compatible with non-gravitational models of structure formation. According to us a statistically significant sample of nearby clusters (z<0.25) should be observed with the current available X-ray telescopes to completely exclude observational effects due to different generation detectors and to understand this novel result.