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 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 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^+2.7_-1.5 ×10^-8 h³ Mpc^-3 in a ΛCDM 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.