Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-02-24
Astronomy and Astrophysics
Astrophysics
8 pages, 8 figures, accepted for publication by Astronomy & Astrophysics
Scientific paper
10.1051/0004-6361:20010276
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 Line (BAL) objects. In this paper we study the X-ray emission of quasars selected through their emission line spectrum rather than their blue excess. X-ray 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 strongly underluminous in the X-rays. We discuss the physical nature of these sources and we propose an interpretation for their optical and X-ray properties 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 type 2 X-ray emission
Maiolino Roberto
Marconi Alessandro
Risaliti Guido
Salvati Marco
Severgnini Paola
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