The ultraviolet flare at the center of the elliptical galaxy NGC 4278
  A large fraction of otherwise normal galaxies shows a weak nuclear activity.
One of the signatures of the low-luminosity active galactic nuclei (LLAGNs) is
the ultraviolet variability which was serendipitously discovered in the center
of some low-ionization nuclear emission-line region (LINER) galaxies. There is
a pressing need to acquire better statistics about UV flaring and variability
in galaxy nuclei, both in terms of the number and monitoring of targets. The
Science Data Archive of the Hubble Space Telescope was queried to find all the
elliptical galaxies with UV images obtained in different epochs with the Wide
Field Planetary Camera 2 (WFPC2) and possibly with nuclear spectra obtained
with the Space Telescope Imaging Spectrograph (STIS) in the region of the
Halpha emission line. These data were found only for the elliptical radiogalaxy
NGC 4278. The UV flux of the nuclear source of NGC 4278 was measured by means
of aperture photometry on the WFPC2/F218W images obtained between June 1994 and
January 1995. The mass of the central supermassive black hole (SBH) was
estimated by measuring the broad components of the emission lines observed in
the STIS/G750M spectrum and assuming that the gas is uniformly distributed in a
sphere. The nucleus of NGC 4278 hosts a barely resolved but strongly variable
UV source. Its UV luminosity increased by a factor of 1.6 in a period of 6
months. The amplitude and scale time of the UV flare in NGC 4278 are remarkably
similar to those of the brightest UV nuclear transients which were earlier
found in other LLAGNs. The mass of the SBH was found to be in the range between
7x10^7 and 2x10^9 M_sun. This is in agreement with previous findings based on
different assumptions about the gas distribution and with the predictions based
on the galaxy velocity dispersion.
