Long term Ultra-Violet Variability of Seyfert galaxies
  Flux variability is one of the defining characteristics of Seyfert galaxies,
a class of active galactic nuclei (AGN). Though these variations are observed
over a wide range of wavelengths, results on their flux variability
characteristics in the ultra-violet (UV) band are very limited. We present here
the long term UV flux variability characteristics of a sample of fourteen
Seyfert galaxies using data from the International Ultraviolet Explorer
acquired between 1978 and 1995. We found that all the sources showed flux
variations with no statistically significant difference in the amplitude of UV
flux variation between shorter and longer wavelengths. Also, the flux
variations between different near-UV (NUV, 1850 - 3300 A) and far-UV (FUV, 1150
- 2000 A) passbands in the rest frames of the objects are correlated with no
time lag. The data show indications of (i) a mild negative correlation of UV
variability with bolometric luminosity and (ii) weak positive correlation
between UV variability and black hole mass. At FUV, about 50% of the sources
show a strong correlation between spectral indices and flux variations with a
hardening when brightening behaviour, while for the remaining sources the
correlation is moderate. In NUV, the sources do show a harder when brighter
trend, however, the correlation is either weak or moderate.
