Volume 371, Number 1, May III 2001
|Page(s)||37 - 44|
|Section||Cosmology (including clusters of galaxies)|
|Published online||15 May 2001|
A new population of soft X-ray weak quasars
Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
Corresponding author: G. Risaliti, email@example.com
Accepted: 15 February 2001
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.
Key words: galaxies: active / galaxies: X-rays: galaxies
© ESO, 2001
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