Is the X-ray spectrum of the Seyfert 2 galaxy NGC5252 intrinsically flat?

Abstract

The first X-ray observation of the Seyfert 2 galaxy NGC5252 is reported. ASCA collected $\sim 4000$ photons/detector enough to perform an accurate spectral analysis of this source. The luminosity of NGC5252 is L$_{\rm X}$(0.7-10 keV) $\simeq 2.6 \times 10^{43}$ erg s$^{-1}$, typical of a Seyfert 1 galaxy. A simple description of the spectrum with a single power law is ruled out by the SIS data which shows a strong soft excess at E $\lsimeq$ 1.2 keV. The spectrum is best fitted by models assuming either partial covering of the central source or scattering of the X-ray continuum. The best-fit partial covering model results in a flat ($Γ\simeq 1.45 \pm 0.2$) power law continuum emitted by a source almost completely covered (at $\sim$ 94-97\%) by neutral matter ($N_{\rm H} \simeq (4.3 \pm 0.6) \times 10^{22}$ cm$^{-2}$). The detected iron line is remarkably weaker (EW $\sim 90 \pm 60$ eV) than normally found from Seyfert 2 galaxies. In an effort to interpret the observed flat spectrum with an intrinsically steep power law as predicted by unified models, we also included in the models neutral reflection, ionized absorption (warm absorber), and non-uniform cold absorption (dual-absorber). We find that the observed flat continum {\bf and} weakness of the iron line poses a problem for neutral reflection models, whatever the assumed geometry is. The use of an ionized absorber (scattering + warm-absorber model) instead of a neutral absorber seems not justified with the present data. The ``dual-absorber'' model, representing nonuniform (density) absorption along the line of sight, may provide an alternative explanation for the flatness of the spectrum, with no constraints on the Fe K emission line. Another plausible interpretation of the present data is that the X-ray spectrum of NGC5252 is truly intrinsically flat. Its strong