Abstract
Beginning in 1999 January, the bright, strongly variable narrow-line Seyfert 1 (NLS1) galaxy Akn 564 has been observed by RXTE once every ~4.3 days. It was also monitored every ~3.2 hr throughout 2000 July. These evenly sampled observations have allowed the first quantitative comparison of long- and short-timescale X-ray variability in an NLS1 galaxy and the derivation of an X-ray power density spectrum (PDS). The variability amplitude in the short-timescale light curve is very similar to that in the long-timescale light curve, in marked contrast to the stronger variability on longer timescales, which is characteristic of normal broad-line Seyfert 1 (BLS1) galaxies. Furthermore, the Akn 564 PDS power law cuts off at a frequency of 8.7 × 10-7 Hz, corresponding to a timescale of ~13 days, significantly shorter than that seen in the PDS of NGC 3516, a BLS1 galaxy of comparable luminosity. This result is consistent with NLS1 galaxies showing faster (as opposed to larger amplitude) variations than BLS1 galaxies, providing further evidence that NLS1 galaxies harbor lower mass black holes than BLS1 galaxies of similar luminosity, accreting at a correspondingly higher relative rate.
Highlights
A relatively recent development in the taxonomy of active galactic nuclei (AGNs) has been the emergence of narrow-line Seyfert 1 (NLS1) galaxies as an important subclass
The high accretion rate is directly responsible for the strong soft X-ray emission, by enhanced thermal radiation from the accretion disk, while the increased soft photon flux cools the hard X-ray source leading to the steeper power law frequently seen at higher X-ray energies in NLS1 galaxies (Brandt, Mathur, & Elvis 1997)
The straightforward deduction from a visual examination of the light curves is that Akn 564 exhibits most of its variability power on timescales substantially less than 1 month. We suggest this may well be a common feature of NLS1 galaxies and one that distinguishes the subclass from broad-line Seyfert 1 (BLS1) galaxies
Summary
A relatively recent development in the taxonomy of active galactic nuclei (AGNs) has been the emergence of narrow-line Seyfert 1 (NLS1) galaxies as an important subclass. Attempts to constrain a flattening or “cutoff” in the PDS (as required to avoid the variability power diverging) were made by combining EXOSAT data with earlier X-ray satellite observations of NGC 5506 (McHardy 1988) and NGC 4151 (Papadakis & McHardy 1995). Until now the only quantitative assessment of the rapid X-ray variability of NLS1 galaxies has been based on measurements of the excess variance, obtained from ASCA observations, typically of 1 day duration Based on these data, Turner et al (1999) and Leighly (1999) found the short-term variability of NLS1 galaxies to be substantially greater than for BLS1 galaxies, while in both cases the excess variance was anticorrelated with the Xray luminosity. 6.9 # 10Ϫ8–1.1 # 10Ϫ6 1.4 # 10Ϫ6–3.4 # 10Ϫ5 the start of our RXTE monitoring campaign, Akn 564 was chosen for simultaneous monitoring with the Hubble Space Telescope, ASCA, Chandra, and the AGN Watch network of ground-based optical telescopes
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