Abstract
We present a detailed analysis of the complex soft X-ray spectrum of NGC 4151 measured by the RGS instruments aboard 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 between ~100-1000 km/s, with respect to the systemic velocity of NGC 4151, approximately consistent with the outflow velocities of the absorption lines observed in the UV, suggestive of an origin for the UV and soft X-ray emission in the same material. Plasma diagnostics imply a range of electron temperatures of ~1-5x10^4 K and electron densities of between 10^8-10^10 cm^-3. The soft X-ray spectrum of NGC 4151 is extremely similar to that of NGC 1068, suggesting that the soft X-ray excesses observed in many 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 NGC 1068.
Highlights
Prior to the advent of XMM–Newton and Chandra, the origin of the soft X-ray emission from heavily obscured active galactic nuclei (AGN) remained very uncertain
Chandra observations of several heavily obscured AGN have revealed considerable extended soft X-ray emission in each source, which is tightly correlated with the [O III] (λ5007) emission observed with the Hubble Space Telescope (HST; NGC 1068, Young, Wilson & Shopbell 2001; Mrk 3, Sako et al 2000; NGC 4151, Ogle et al 2000; Circinus, Sambruna et al 2001)
These observations constitute a growing body of evidence that suggests that the soft X-ray emission from heavily obscured Seyfert galaxies is largely composed of soft Xray emission lines that originate in gas that is photoionized by the central AGN
Summary
Prior to the advent of XMM–Newton and Chandra, the origin of the soft X-ray emission from heavily obscured active galactic nuclei (AGN) remained very uncertain. Recent spectral observations of these AGN with the Xray grating instruments on board Chandra (NGC 1068, Brinkman et al 2002; Ogle et al 2003; Mrk 3, Sako et al 2000; NGC 4151, Ogle et al 2000; Circinus, Sambruna et al 2001) and XMM–Newton (Kinkhabwala et al 2002) have revealed soft X-ray spectra dominated by prominent emission lines from a range of light elements (e.g. C, N, O, Ne, etc.) These observations constitute a growing body of evidence that suggests that the soft X-ray emission from heavily obscured Seyfert galaxies is largely composed of soft Xray emission lines that originate in gas that is photoionized (and photoexcited) by the central AGN.
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