Unobscured QSO 2: A new class of objects?

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 Telescopic Nazionale Galileo, confirm the widths of the Hβ 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 Γ ∼ 1.7 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 -1.5+2.7 × 10-8 h3 Mpc -3 in a ACDM model with h = 0.7. 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. © ESO 2005.