Active Galactic Nuclei (PPT, PDF)
On the smallest scales, many AGN show signatures from the innermost accretion disk in the form of broad "relativistic" Fe K emission lines. These broad lines were discovered using ASCA in the early 1990s and have been confirmed by XMM-Newton and Suzaku (Reeves et al 2007). There is, however, a complex relationship between the Fe K line properties, the ionizing continuum, and signatures of cold material near the AGN.
Precise measurements of the complex Fe K line and absorption components require high spectral resolution. The optically thick material that produces the broad fluorescent Fe K line also creates a Compton peak at E > 20 keV detectable with the Astro-H HXI, providing multiple insights into the physics of the disk. SXS observations will provide high signal-to-noise measurements of the broad lines of hundreds of AGN up to z~2. These observations provide the first unbiased survey of broad Fe K line properties across all AGN.
XMM-Newton and Suzaku spectra frequently show time-variable absorption and emission features in the 5-10 keV band. If these features are due to Fe, they represent gas moving at very high velocities with both red and blue shifted components from material presumably near the event horizon. CCD resolution is too low and the required grating exposures are too long to properly characterize the velocity field and ionization of this gas and determine whether it is from close to the black hole or from high velocity winds. SXS, in combination with the HXI, will provide a dramatic increase in sensitivity over Suzaku, enabling measurements that probe the geometry of the central regions of ~50 AGN on the orbital timescale of the Fe producing region (for an AGN with a 3 x 107 Msun black hole, this is ~60GM/c2 = 10 ks).
In addition to its instrumental advantages, Astro-H is well-timed for the study of AGN. The availability of the eROSITA (launching as part of SRG in 2012, Predehl et al. 2006) all-sky survey will provide 10^4 potential AGN to study, while the Swift BAT survey will provide about 1000 hard X-ray-selected AGN by 2010. The availability of so many candidate sources, combined with the long lifetime of SXS, will produce the first synoptic study of AGN X-ray spectra at high resolution.
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| The source shown here on the left contains two slowly varying sinsusoidal absorption features and an Fe K line that becomes increasingly strong and broad. The SXS (middle panel) will measure velocity structures of 100 km/s on the 10 ksec orbital timescale of a 30 million solar-mass super-massive black hole, which cannot be seen with CCDs (right panel). | SXS can resolve the 2000 km/s Fe K emission line in NGC 3783 and measure the ionization parameter, redshift, and optical depth of the Fe K absorption lines. The spectrum shown (black) is a 100 ksec simulation using parameters from Reeves et al. (2007). The models are the Fe K emission (green), absorption (blue) and photo-ionized emission from the warm absorber region (red). SXS will measure both the velocity and width of the absorption features to better than 400 km/s. |
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