A comparison of the hard ASCA spectral slopes of broad- and narrow-line Seyfert 1 galaxies

Abstract

The soft (≈ 0.1–2.0 keV) X-ray spectra of narrow-line Seyfert 1 galaxies are known to be generally steeper than those of Seyfert 1 galaxies with broader optical permitted lines. This has been attributed to the presence of strong soft X-ray excesses, over the hard X-ray power law, in many narrow-line Seyfert 1s. Here we use the currently available ASCA data to systematically compare the harder ( ≈ 2–10 keV) X-ray continua of soft ROSAT narrow-line Seyfert 1s with those of Seyfert 1s with larger Hβ FWHM. Our robust and non-parametric testing suggests, with high statistical significance, that soft ROSAT narrow-line Seyfert 1s have generally steeper intrinsic hard X-ray continua than Seyfert 1s with larger Hβ FWHM. The hard photon index trend appears similar to the previously known soft photon index trend, although with a reduced photon index spread. If the soft X-ray excesses of all Seyfert 1s are confined to below ≈ 1 keV they cannot directly affect the ≈ 2–10 keV spectra studied here. However, as suggested for the extreme narrow-line Seyfert 1 RE J 1034 + 393, a strong soft X-ray excess may affect the accretion disc corona which creates the underlying hard X-ray power law. If this is occurring, then more detailed study of this physical process could give clues about the formation of the underlying continua of all Seyferts. Other effects, such as intrinsic 2–10 keV continuum curvature, could also lead to the observed photon index trend and need further study.