The Variability of Hardness Ratio 1 observed by ROSAT: Narrow-Line Seyfert 1 Galaxies versus Broad-Line Seyfert 1 Galaxies

Abstract

In order to understand how the soft X-ray spectra vary we present the Hardness Ratio 1 versus Count Rates (HR1-CTs) correlation of 8 Narrow-line Seyfert 1 Galaxies (NLS1s) and 14 Broad-line Seyfert1 Galaxies (BLS1s) obtained during the ROSAT PSPC pointing observations. According to our criteria, six of the NLS1s show a positive HR1-CTs correlation, and seven of the BLS1s display an anti-correlation of HR1 versus CTs. The other 2 NLS1s and 7 BLS1s do not show a clear HR1-CTs correlation. From these we can see that the NLS1s statistically show a different spectral shape variability with flux change from the BLS1s: the spectra of NLS1s become harder as total flux increases while those of BLS1s soften. We attribute the different spectral variations to a strong stable 'soft excess' in NLS1s, while it is weak in BLS1s. For two types of objects, the power law component similarly becomes softer with increasing intensity. These imply that the soft excess originates from the Big Blue Bump and power law emission is from Compton upscattering of UV or Soft X-ray photons. Our results are consistent with what is widely accepted that NLS1s have smaller black hole masses and higher accretion rates than BLS1s.