Correlation in the Hard Energy Range

Similarly to the correlation found in the soft energy range, a correlation between the slope of the 2-10 keV photon index, obtained from power-law fits to ASCA observations of broad- and narrow-line Seyfert 1 galaxies, and the width of the FWHM H^ line, has been discovered by [10]. Figure 22.3 shows the correlation between the hard photon index and the FWHM H^ line width. In NLS1s, a spectral flattening is seen above 2 keV. However, the 2-10 keV power-law photon indices are still steeper than previously thought, originating in a large diversity in the slopes in the hard continuum emission for NLS1s. Steep 2-10 keV X-ray continua, with values of the photon index between 1.9 and 2.6, are characteristic of NLS1s. As the power-law component is thought to arise primarily in the accretion disc corona, the distribution of data points in Fig. 22.3(b) allows in principle, statements regarding physical conditions in the accretion disc corona to be made. Detailed physical models to explain the large dispersion in the 2-10 keV slopes of Seyfert 1 galaxies are presently not available, and the origin for the different spectral continuum slopes requires further theoretical considerations. One possible explanation suggested is that NLS1s may exhibit a cooler accretion disc corona [42]. For inverse Compton scattering, the relation between the slope of the velocity distribution of the electrons in the accretion disc corona q is related to the energy index a by: a = (q + 1)/2. In this case, a cooler accretion disc results in steeper X-ray continua.

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