Suzaku Studies of the Central Engine in the Typical Type I Seyfert NGC 3227: Detection of Multiple Primary X-Ray Continua with Distinct Properties

Abstract

The type I Seyfert galaxy NGC 3227 was observed by Suzaku six times in 2008, with intervals of ~1 week and net exposures of ~50 ks each. Among the six observations, the source varied by nearly an order of magnitude; it was brightest in the first observation with a 2-10 keV luminosity of 1.2 × 10⁴² erg s^-1, while faintest in the fourth observation with 2.9 × 10⁴¹ erg s^-1. As it became fainter, the continuum in the 2-45 keV band became harder, while the narrow Fe-Kα emission line, detected on all occasions at 6.4 keV of the source rest frame, remained approximately constant in the photon flux. Through a method of variability-assisted broadband spectroscopy, the 2-45 keV spectrum of NGC 3227 was decomposed into three distinct components. One is a relatively soft power-law continuum with a photon index of ~2.3, weakly absorbed and highly variable on timescales of ~5 ks it was observed only when the source was above a threshold luminosity of ~6.6 × 10⁴¹ erg s^-1 (in 2-10 keV), and was responsible for further source brightening beyond. Another is a harder and more absorbed continuum with a photon index of ~1.6, which persisted through the six observations and varied slowly on timescales of a few weeks by a factor of ~2. This component, carrying a major fraction of the broadband emission when the source is below the threshold luminosity, is considered as an additional primary emission. The last one is a reflection component with the narrow iron line, produced at large distances from the central black hole.