A new population of soft X-ray weak quasars

Quasars selected in optical surveys by means of their blue or UV excess are known to be strong emitters in the X-rays, except for Broad Absorption Lin e (BAL) objects. In this paper we study the X-ray emission of quasars selec ted through their emission line spectrum rather than their blue excess. X-r ay data are obtained by cross-correlating two optical samples (the Hamburg Survey Catalogue and the Palomar Transit Catalogue) with the WGA catalogue of ROSAT observations. We find that a significant fraction of objects are s trongly underluminous in the X-rays. We discuss the physical nature of thes e sources and we propose an interpretation for their optical and X-ray prop erties based on a dust-poor and gas-rich absorber. Our results suggest the existence of a population of AGNs having a type 1 optical spectrum and a ty pe 2 X-ray emission.