THE SOFT-X-RAY SPECTRUM OF NGC-4151 REVISITED

In a deep (similar to 30 ks) ROSAT PSPC observation, the Seyfert galaxy NGC 4151 is detected as a bright unresolved X-ray source in a very rich field. There is no clear evidence for soft X-ray variability during the 1-d ROSAT observation. In order to investigate further the very complex soft X-ray spectrum of NGC 4151, we combine the ROSAT PSPC data with near-simultaneous Ginga measurements to obtain wideband (0.1-20 keV) spectral coverage. We find that the X-ray emission observed below similar to 1 keV has a rather steep spectrum, consistent, in these low-resolution data, with either a thermal bremsstrahlung (kT similar to 0.5 keV) or power-law (photon index Gamma similar to 2.8) form. This soft X-ray emission can be attributed to a blend of thermal and electron-scattered components, although the exact contribution of each is uncertain. We also reassess the possible origin of the complex variable absorption which is most apparent between 1 and 4 keV in the X-ray spectrum of NGC 4151. The evidence now favours an absorbing medium subject to a modest level of photoionization, which we describe as a 'lukewarm' absorber. In this scenario it is possible to reconcile the column densities measured in the soft X-ray regime with those inferred from ultraviolet absorption-line studies.