The Summed Line Profiles for a sample of Seyfert 1 Galaxies
The Summed Line Profiles for a sample of Seyfert 1 Galaxies
The X-ray spectra of Seyfert 1 galaxies can contain numerous components. The continuum has an apparent power law form, which covers several decades in energy. This is attenuated at soft X-ray energies by absorption both in the interstellar medium in our Galaxy and by material local to the active galaxy. An emission line from iron K alpha has been observed at 6-7 keV in the vast majority of sources. A hard component affects the spectrum above 10keV which is probably produced by Compton scattering (``reflection'') of the continuum in the material which produces the line. In the soft X-ray regime, many sources have ``soft excess'' emission and can show emission features from oxygen and/or iron-L.
At least three nuclear emission/absorption regions are envisaged to account for these components. The region which emits the primary X-ray continuum may consist of
In many cases, the local absorption is due to a Compton-thin ``warm absorber'': highly-ionized gas identified by features due to oxygen and iron, which may also produce the soft X-ray lines.
Detailed measurement of the various emission and absorption features, and continua, can therefore yield information regarding the physical conditions of the regions, such their dynamics, geometry and ionization state. Of particular interest in this regard is the iron K alpha emission line. These lines were first discovered with HEAO-1 in heavily obscured Seyfert galaxies, such as NGC 4151 (Warwick et al 1989 and references therein), and a number of Narrow Emission Line Galaxies (Mushotzky 1982). Not until the launch of Ginga\ was iron K alpha emission recognized as an important, and possibly universal, property of Seyfert galaxies (Nandra & Pounds 1994, hereafter NP94, and references therein). From the time of their discovery in relatively unabsorbed AGN, the emission lines have been assigned an origin in material intimately associated with the accretion process, the most promising candidates being an accretion disk, or ``blobs'' of material surrounding the central source (Guilbert & Rees 1988; Fabian et al 1989; Nandra et al 1989; Pounds et al 1990). However, for individual sources, the properties of the line were not generally well determined, given the limited spectral resolution of Ginga. Nandra & Pounds (1994) have discussed the properties of the iron K alpha line in some detail, using a sample of Seyfert galaxies. The mean energy was found to be 6.4 keV, which indicates that the most likely origin for the line is fluorescence in near-neutral material. The mean energy was found to be 6.4 keV, which indicates that the most likely origin for the line is fluorescence in near-neutral material. Given this information, it can be deduced that the line cannot arise in material uniformly covering the source. The mean equivalent width of the line of 140eV requires a high optical depth for the material, which would produce considerably more soft X-ray absorption in these objects than is observed. Indeed, the equivalent width of the line is sufficiently high to indicate that the material may be optically thick to Compton scattering. Further evidence for a Compton-thick region exists in the form of the ``hard tail'', which can be produced by ``reflection'' from the line--producing region. Given this evidence, an accretion disk is an obvious source for these features, especially when considering that the strengths of both the line and ``reflection'' component imply a covering fraction of 50 per cent for the material. Nonetheless, the ``blob'' geometry was not ruled out by the Ginga data (Bond & Matsuoka 1993; Nandra & George 1994).
Further impetus for the study of iron K alpha lines in Seyfert galaxies has come from more recent ASCA data, which have indicated that this feature may be a key element in our understanding of AGN. The earliest data showed good evidence that the emission lines were resolved (Fabian et al 1994; Mushotzky et al 1995), confirming tentative suggestions from the Ginga data (NP94). Most dramatically, high signal-to-noise data for two sources have shown characteristic line profiles (MCG-6-30-15 Tanaka et al 1995; NGC 4151 Yaqoob et al 1995). The lines in these sources are extremely broad, with FWHM implying relativistic velocities of order 0.2c. Furthermore, there is a strong asymmetry to the red, which is indicative of the gravitational redshifts associated with the inner regions of an accretion disk surrounding the black hole. Fabian et al (1995) concluded that this was the most plausible explanation for such profiles, rejecting other interpretations such as Comptonization.
Nandra et al 1996 present evidence for widespread relativistic effects in the central regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies observed by ASCA 14 show an iron K alpha line which is is resolved, with mean width sigma=0.43+/- 0.12 keV for a gaussian profile FWHM ~ 50,000 km/s. However, many of the line profiles are asymmetric. A strong red wing is indicative of gravitational redshifts close to a central black hole and accretion disk models provide an excellent description of the data.
The peak energy of the line is 6.4 keV, indicating that it arises by fluorescence in near-neutral material. Our fits imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean of 30 degrees, consistent with orientation-dependent unification schemes. Differences in the line profiles from source-to-source imply slight variations in geometry, which cannot be accounted for solely by inclination. In most cases, we require that the line emission arises from a range of radii. Our data are fit equally well with rotating (Kerr) and non-rotating (Schwarzschild) black hole models. We find a mean spectral index in the 3-10 keV range of 1.91 +/- 0.07 after accounting for the effects of reflection.
Such observations probe the innermost regions of AGN, and arguably provide the best evidence yet obtained for the existence of super-massive black holes in the centers of active galaxies.