The complex soft X-ray spectrum of NGC 4151

Abstract

We present a detailed analysis of the complex soft X-ray spectrum of NGC 4151 measured by the RGS instruments on board XMM-Newton. The XMM-Newton RGS spectra demonstrate that the soft X-ray emission is extremely rich in X-ray emission lines and radiative recombination continua (RRC), with no clear evidence for any underlying continuum emission. Line emission and the associated RRC are clearly detected from hydrogen-like and helium-like ionization states of neon, oxygen, nitrogen and carbon. The measured lines are blueshifted with a velocity of ∼100-1000 km s-1, with respect to the systemic velocity of NGC 4151, approximately consistent with the outflow velocities of the absorption lines observed in the ultraviolet spectrum of NGC 4151 by Kriss et al. in the mid-1990s, suggestive of an origin for the ultraviolet and soft X-ray emission in the same material. Plasma diagnostics from the observed helium-like triplets imply a range of electron temperatures of ∼(1-5) × 104 K and electron densities of between 10 8 and 1010 cm-3. The soft X-ray spectrum of NGC 4151 is extremely similar to that of NGC 1068, in terms of both the atomic species present and the relative strengths of the observed emission lines and RRC (reported recently by Kinkhabwala et al.), suggesting that the soft X-ray excesses observed in many type 2 Seyfert galaxies may be composed of similar emission features. Modelling the RGS spectra in terms of emission from photoionized and photoexcited gas in an ionization cone reproduces all of the hydrogen-like and helium-like emission features observed in the soft X-ray spectrum of NGC 4151 in detail and confirms the correspondence between the soft X-ray emission in NGC 4151 and 1068. NGC 4151 shows somewhat lower individual ionic column densities than those measured for NGC 1068, indicating that the material in the ionization cones of NGC 4151 may be somewhat more dense than the material in the ionization cones of NGC 1068.