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Register a new userX-ray/UV campaign on the Mrk 279 outflow: density diagnostics in Active Galactic Nuclei using O V K-shell absorption lines
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One of the main problems in modeling the ionised outflows in Active Galactic Nuclei is the unknown distance of the outflowing wind to the central source. Only if the density is known this distance can be determined through the ionisation parameter. Here we study density diagnostics based upon O V transitions. O V is known to have metastable levels that are density dependent. We study the population of those levels under photoionisation equilibrium conditions and determine for which parameter range they can have a significant population. We find that resonance line trapping plays an important role in reducing the critical densities above which the metastable population becomes important. We investigate the K-shell absorption lines from these metastable levels. Provided that there is a sufficient population of the metastable levels, the corresponding K-shell absorption lines are detectable and are well separated from the main absorption line originating from the ground state. We then present the Chandra LETGS spectrum of the Seyfert 1 galaxy Mrk 279 that may show for the first time the presence of these metastable level absorption lines. A firm identification is not yet possible due to both uncertainties in the observed wavelength of the strongest line as well as uncertainties in the predicted wavelength. If the line is indeed due to absorption from O V, then we deduce a distance to the central source of one light week to a few light months, depending upon the importance of additional heating processes.
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We present an analysis of the intrinsic UV absorption in the Seyfert 1 galaxy Mrk 279 based on simultaneous long observations with the Hubble Space Telescope (41 ks) and the Far Ultraviolet Spectroscopic Explorer (91 ks). To extract the line-of-sight covering factors and ionic column densities, we separately fit two groups of absorption lines: the Lyman series and the CNO lithium-like doublets. For the CNO doublets we assume that all three ions share the same covering factors. The fitting method applied here overcomes some limitations of the traditional method using individual doublet pairs; it allows for the treatment of more complex, physically realistic scenarios for the absorption-emission geometry and eliminates systematic errors that we show are introduced by spectral noise. We derive velocity-dependent solutions based on two models of geometrical covering -- a single covering factor for all background emission sources, and separate covering factors for the continuum and emission lines. Although both models give good statistical fits to the observed absorption, we favor the model with two covering factors because: (a) the best-fit covering factors for both emission sources are similar for the independent Lyman series and CNO doublet fits; (b) the fits are consistent with full coverage of the continuum source and partial coverage of the emission lines by the absorbers, as expected from the relative sizes of the nuclear emission components; and (c) it provides a natural explanation for variability in the Ly$alpha$ absorption detected in an earlier epoch. We also explore physical and geometrical constraints on the outflow from these results.
We investigate the applicability of inhomogeneous absorber models in the formation of AGN outflow absorption-troughs. The models we explore are limited to monotonic gradients of absorbing column densities in front of a finite emission source. Our main finding is that simple power-law and gaussian distributions are hard pressed to fit the Mrk 279 high-quality UV outflow data. An acceptable fit for the O VI troughs can only be obtained by assuming unrealistic optical depth values (upward of 100). The strongest constraints arise from the attempt to fit the Lyman series troughs. In this case it is evident that even allowing for complete freedom of both the power-law exponent and the optical depth as a function of velocity cannot yield an acceptable fit. In contrast, partial covering models do yield good fits for the Lyman series troughs. We conclude that monotonic inhomogeneous absorber models that do not include a sharp edge in the optical depth distribution across the source are not an adequate physical model to explain the trough formation mechanism for the outflow observed in Mrk 279.
Ionized outflows in Active Galactic Nuclei (AGN) are thought to influence their nuclear and local galactic environment. However, the distance of the outflows with respect to the central engine is poorly constrained, which limits our understanding of their kinetic power as a cosmic feedback channel. Therefore, the impact of AGN outflows on their host galaxies is uncertain. However, when the density of the outflows is known, their distance can be immediately obtained from their modelled ionization parameter. With the new self-consistent PhotoIONization (PION) model in the SPEX code, we are able to calculate detailed level populations, including the ground and metastable levels. This enables us to determine under what physical conditions the metastable levels are significantly populated. We then identify characteristic lines from these metastable levels in the 1 -- 2000 {AA} wavelength range. In the large density range of $n_H in (10^6, 10^{20} m^{-3}$, the metastable levels 2s 2p $(^3P_{0-2})$ in Be-like ions can be significantly populated. For B-like ions, merely the first excited level 2s$^2$ 2p $(^2P_{3/2})$ can be used as a density probe. For C-like ions, the first two excited levels 2s$^2$ 2p$^2$ ($^3P_1$ and $^3P_2$) are better density probes than the next two excited levels 2s$^2$ 2p$^2$ ($^1S_0$ and $^1D_2$). Different ions in the same isoelectronic sequence cover not only a wide range of ionization parameter, but also a wide range of density. On the other hand, within the same isonuclear sequence, less ionized ions probe lower density and smaller ionization parameter. Finally, we re-analyzed the high-resolution grating spectra of NGC 5548 observed with Chandra in January 2002, using a set of PION components to account for the ionized outflow. We derive lower (or upper) limits of plasma density in five out of six PION components, based on the presence (or absence) of the metastable absorption lines.
We review some of the main physical and statistical properties of the X-ray absorber in AGNs. In particular, we review the distribution of the absorbing column density inferred from X-ray observations of various AGN samples. We discuss the location of the X-ray absorber and the relation with the dust absorption at optical and infrared wavelengths. Finally, we shortly review the recent findings on X-ray absorption at high luminosities and at high redshift.
We discuss some topical issues related to the Fe K emission lines in AGNs. We show remarkable agreement between non-contemporaneous ASCA and Chandra grating data and explain why there has been terrible confusion about the ASCA and post-ASCA results on the relativistic Fe K lines. We point out that in fact the number of sources (not the percentage) that have been reported to exhibit relativistic Fe K lines is now larger than it was in the ASCA era. Thus, the case for Constellation-X as a probe of strong gravity is even more compelling than it was a decade ago. One of the primary goals of these studies is to establish the foundation for future missions to map the spacetime metric around black holes. A prerequisite first step is to measure the black hole angular momentum in a robust manner that does not rely on assumptions about the accreting system. In addition, probing the Fe K lines out to high redshifts will pave the way for studying the accretion history and evolution of supermassive black holes. However, we point out some issues that need to be resolved, pertaining to the spin measurement and to the relativistic Fe K line emission found from AGN in deep surveys.
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