SIMBAD references

We explore the X-ray properties of a subset of the optically selected Sloan Digital Sky Survey (SDSS) cluster sample of Goto et al. by analyzing seven public XMM-Newton pointings, with exposure times ranging from ∼4 to 46 ks. There are in total 17 SDSS clusters out of which only eight are detected at X-ray wavelengths with f_0.5–2keV≳1.2x10^–14 ergs/cm2/s. For the remaining nine SDSS clusters, we estimate their 3 σ luminosity upper limits (corresponding to L_X≲5x10⁴² ergs/s in the 0.5-2 keV band). This relatively low luminosity suggests that if they are real structures, these galaxy aggregations correspond to poor groups of galaxies. Using the SDSS photometric catalog, we also derive the cluster optical r-band luminosities. The resulting scaling relations (L_opt-L_X, L_opt-T_X) are consistent with those of other recent studies.