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Issue A&A
Volume 371, Number 1, May III 2001
Page(s) 37 - 44
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010276



A&A 371, 37-44 (2001)
DOI: 10.1051/0004-6361:20010276

A new population of soft X-ray weak quasars

G. Risaliti1, A. Marconi2, R. Maiolino2, M. Salvati2 and P. Severgnini1

1  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

(Received 11 July 2000 / Accepted 15 February 2001)

Abstract
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

Offprint request: G. Risaliti, risaliti@arcetri.astro.it

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© ESO 2001


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