A Hard Medium Survey with ASCA. III.: a Type 2 AGN revealed from X-ray Spectroscopy

In this paper we report the discovery of an hard X-ray selected Type 2 Seyfert galaxy and we present and discuss its X-ray and optical spectrum together with the radio to X-ray energy distribution. The X-ray source - AXJ2254+1146 - is part of the ASCA Hard Serendipitous Survey (HSS). What makes this discovery particularly noteworthy is the fact that the Type 2 classification of this Seyfert galaxy has resulted directly from the X-ray data and has been confirmed by optical spectroscopy only subsequently. The X-ray spectrum of AXJ2254+1146 is best described by a model consisting of an unresolved Gaussian line at $6.43\pm 0.1$ keV plus the so called "leaky-absorber" continua having an intrinsic power law photon index of $\Gamma$ = $2.51^{2.76}_{2.17}$ (1 $\sigma$ confidence interval). The best fit values of the absorbing column density ($N_H$ = $1.85^{2.24}_{1.47} \times 10^{23}$ cm$^{-2}$), of the line equivalent width ($0.6^{0.84}_{0.36}$ keV) and of the scattering fraction ($0.7^{1.4}_{0.1} %$), lead us to classify it as a Type 2 AGN from an X-ray point of view. Inspection of the POSS II image reveals the presence, within the ASCA X-ray error circle, of the nearby Sbc spiral galaxy UGC 12237 ($m_{B_o}=14.26$) that, even on positional ground considerations alone, is the most likely optical counterpart of AXJ2254+1146. Subsequent optical spectroscopy of UGC 12237 has confirmed its Seyfert 2 optical nature.