Unobscured QSO 2: a new class of objects?

¹ INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy e-mail: anna@brera.mi.astro.it
² INAF – CNR – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy e-mail: gioia@ira.cnr.it
³ Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA e-mail: henry@ifa.hawaii.edu
⁴ Department of Astronomy, University of Michigan, 918 Dennison Building, Ann Arbor, MI 48109, USA e-mail: cmullis@umich.edu

Received: 16 May 2005
Accepted: 18 August 2005

Abstract

We present in this paper optical and X-ray follow up observations for three X-ray selected objects extracted from the ROSAT North Ecliptic Pole survey which is a flux-limited, completely identified survey. All three objects have X-ray luminosities in the 10⁴⁴ erg s^-1 regime and show narrow emission lines in their optical discovery spectra, typical of QSO 2 type objects. Spectroscopic data for the three QSO 2 candidates, obtained with the Telescopio Nazionale Galileo, confirm the widths of the Hα or Hβ emission lines are less than 750 km s^-1. On the other hand XMM-Newton data do not show any sign of obscuration as expected for this class of objects. The X-ray spectra of the three objects are all well fit by a single power law model with with low energy absorption fixed to the Galactic value along the line of sight to each object. Most observational evidence supports the scenario where optical and X-ray obscurations are linked, contrary to our findings. We discuss the unanticipated results of these observations, and compute the space density in soft X-ray surveys of this possibly new class of objects. Their spatial density in the ROSAT NEP survey is 2.8 h³ Mpc^-3 in a ΛCDM model with . Unobscured QSO 2 candidates could go unrecognized in current X-ray surveys where the low hydrogen column density is inferred by a hardness ratio rather than a more precise X-ray spectrum measurement.