SIMBAD references

We present the ROSAT HRI X-ray measurements of 19 spectroscopically confirmed optically-selected QSOs in the 12' radius region in the Selected Area 57. The optical-to-X-ray spectral index α_ox ranges from 1.25 to 1.7, consistent with brighter QSOs. We have investigated the relationship between soft X-ray emission observed with the ROSAT HRI and optical colors (UJFN) of optically-selected QSOs in SA57. The optical to X-ray spectral index α_ox is found to be correlated with the U-J-F+N color, by a combined effect of α_ox and U, N excess correlations. Possible explanations are the physical link between broad emission-lines/Balmer continuum, and the ionizing radiation source seen in X-ray and/or a geometrical effect.