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Register a new userThe Future of Fe-K Line Diagnostics for Probing Strong Gravity
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T. Yaqoob
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We review what we have learnt with ASCA from studying the Fe-K lines in AGN and describe a program to deconvolve the narrow, non-disk components of the lines with Chandra. This is necessary to derive the correct profiles of the broad, relativistic lines obtained using data from XMM and other high-throughput instruments. Since reverberation techniques are now not looking promising, we present Constellation-X simulations showing an alternative way we might be able to measure black-hole mass and spin.
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We systematically analyzed the high-quality Suzaku data of 88 Seyfert galaxies. We obtained a clear relation between the absorption column density and the equivalent width of the 6.4 keV line above 10$^{23}$ cm$^{-2}$, suggesting a wide-ranging column density of $10^{23-24.5}$ cm$^{-2}$ with a similar solid and a Fe abundance of 0.7--1.3 solar for Seyfert 2 galaxies. The EW of the 6.4 keV line for Seyfert 1 galaxies are typically 40--120 eV, suggesting the existence of Compton-thick matter like the torus with a column density of $>10^{23}$ cm$^{-2}$ and a solid angle of $(0.15-0.4)*4pi$, and no difference of neutral matter is visible between Seyfert 1 and 2 galaxies. An absorber with a lower column density of $10^{21-23}$ cm$^{-2}$ for Compton-thin Seyfert 2 galaxies is suggested to be not a torus but an interstellar medium. These constraints can be understood by the fact that the 6.4 keV line intensity ratio against the 10--50 keV flux is almost identical within a range of 2--3 in many Seyfert galaxies. Interestingly, objects exist with a low EW, 10--30 eV, of the 6.4 keV line, suggesting that those torus subtends only a small solid angle of $<0.2*4pi$. Ionized Fe-K$alpha$ emission or absorption lines are detected from several percents of AGNs. Considering the ionization state and equivalent width, emitters and absorbers of ionized Fe-K lines can be explained by the same origin, and highly ionized matter is located at the broad line region. The rapid increase in EW of the ionized Fe-K emission lines at $N_{H}>10^{23}$ cm$^{-2}$ is found, like that of the cold material. It is found that these features seem to change for brighter objects with more than several $10^{44}$ erg/s such that the Fe-K line features become weak. We discuss this feature, together with the torus structure.
New laboratory measurements using an Electron Beam Ion Trap (EBIT) and an x-ray microcalorimeter are presented for the n=3 to n=2 Fe XVII emission lines in the 15 {AA} to 17 {AA} range, along with new theoretical predictions for a variety of electron energy distributions. This work improves upon our earlier work on these lines by providing measurements at more electron impact energies (seven values from 846 to 1185 eV), performing an in situ determination of the x-ray window transmission, taking steps to minimize the ion impurity concentrations, correcting the electron energies for space charge shifts, and estimating the residual electron energy uncertainties. The results for the 3C/3D and 3s/3C line ratios are generally in agreement with the closest theory to within 10%, and in agreement with previous measurements from an independent group to within 20%. Better consistency between the two experimental groups is obtained at the lowest electron energies by using theory to interpolate, taking into account the significantly different electron energy distributions. Evidence for resonance collision effects in the spectra is discussed. Renormalized values for the absolute cross sections of the 3C and 3D lines are obtained by combining previously published results, and shown to be in agreement with the predictions of converged R-matrix theory. This work establishes consistency between results from independent laboratories and improves the reliability of these lines for astrophysical diagnostics. Factors that should be taken into account for accurate diagnostics are discussed, including electron energy distribution, polarization, absorption/scattering, and line blends.
Recent state-of-the-art calculations of A-values and electron impact excitation rates for Fe III are used in conjunction with the Cloudy modeling code to derive emission line intensity ratios for optical transitions among the fine-structure levels of the 3d$^6$ configuration. A comparison of these with high resolution, high signal-to-noise spectra of gaseous nebulae reveals that previous discrepancies found between theory and observation are not fully resolved by the latest atomic data. Blending is ruled out as a likely cause of the discrepancies, because temperature- and density-independent ratios (arising from lines with common upper levels) match well with those predicted by theory. For a typical nebular plasma with electron temperature $T_{rm e} = 9000$ K and electron density $rm N_{e}=10^4 , cm^{-3}$, cascading of electrons from the levels $rm ^3G_5$, $rm ^3G_4$ and $rm ^3G_3$ plays an important role in determining the populations of lower levels, such as $rm ^3F_4$, which provide the density diagnostic emission lines of Fe III, such as $rm ^5D_4$ - $rm ^3F_4$ at 4658 AA. Hence further work on the A-values for these transitions is recommended, ideally including measurements if possible. However, some Fe III ratios do provide reliable $N_{rm e}$-diagnostics, such as 4986/4658. The Fe III cooling function calculated with Cloudy using the most recent atomic data is found to be significantly greater at $T_e$ $simeq$ 30000 K than predicted with the existing Cloudy model. This is due to the presence of additional emission lines with the new data, particularly in the 1000--4000 AA wavelength region.
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.
We have used an electron beam ion trap to measure electron-density-diagnostic line-intensity ratios for extreme ultraviolet lines from F XII, XIII, and XIV at wavelengths of 185-205 255-276 Angstroms. These ratios can be used as density diagnostics for astrophysical spectra and are especially relevant to solar physics. We found that density diagnostics using the Fe XIII 196.53/202.04 and the Fe XIV 264.79/274.21 and 270.52A/274.21 line ratios are reliable using the atomic data calculated with the Flexible Atomic Code. On the other hand, we found a large discrepancy between the FAC theory and experiment for the commonly used Fe XII (186.85 + 186.88)/195.12 line ratio. These FAC theory calculations give similar results to the data tabulated in CHIANTI, which are commonly used to analyze solar observations. Our results suggest that the discrepancies seen between solar coronal density measurements using the Fe XII (186.85 + 186.88)/195.12 and Fe XIII 196.54/202.04 line ratios are likely due to issues with the atomic calculations for Fe XII.
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