AbstractsAstronomy & Space Science

We study the multiwavelength properties of Ark 564 and Ton S180, two NLS1s that have been the object of extensive multiwavelength spectroscopic campaigns undertaken during the last four years to test the current hypothesis that Narrow-Line Seyfert 1 galaxies (NLS1s) have relatively lower black hole masses and higher accretion rates than normal broad-line Seyfert 1 galaxies (BLS1s) of the same luminosity. We present an analysis of the X-ray variability properties of Ark 564 and Ton S180, based on 35- and 12-day continuous observations with ASCA, respectively. The mean spectra are characterized by a very steep power-law continuum (Gamma = 2.54 and 2.44, respectively), a strong soft excess at energies lower than 2 keV, and an Fe Kalpha line that has a large equivalent width and originates in ionized material. Their spectral energy distributions peak at significantly higher energies in these objects than in BLS1s (15-100 eV for Ton S180 and approximately 50 eV in Ark 564). We also find that, based on available UV, FUV and X-ray data on absorption lines, the UV and X-ray absorbers in Ark 564 are physically related, and possibly identical. The multiwavelength properties of these two NLS1s are consistent with the predictions of the prevailing model for the structure of NLS1s, i.e. of AGNs with high accretion rate (Mdot approximately 0.1-0.9) onto a relatively low-mass black hole (M approximately 1-10 million solar masses).