Low-frequency Divergebt X-ray Variability in the
Seyfert Galaxy NGC4051
A. Lawrence, M.G. Watson, K.A. Pounds, M. Elvis
Nature, 325, 694-696 (1987)
Abstract
The X-ray emission from NGC4051, a nearby low luminosity Type
1 Seyfert galaxy, exhibits variations that are both rapid and
apparently quasi-periodic. Many Seyfert galaxies are variable
but our knowledge of the form of variability has not advanced
substantially since early well-resolved observations and attempts
at statistical description. Here we report an uninterrupted
62-h observation of NGC4051 with the EXOSAT observatory, giving
an order of magnitude improvement in the quality of the time
series, allowing comparison with X-ray binaries. The observed
power spectral density is roughly proportional to 1/f, similar
to Cyg X-1, and characteristic of turbulent processes. No
preferred timescale or luminosity gradient has been seen so far.
If this behavior holds generally for active galactic nucleic,
quantities derived from short samples will be subject to
selection effects. Some naive interpretations of variability
are clearly invalid. Similar conclusions have recently been
arrived at by McHardy et. al. (see accompanying paper).
1/f in astronomy section