Short timescale variations of the H{\alpha} double-peaked profile of the
  nucleus of NGC 1097
  The broad (FWHM ~ 10,000 km/s) double-peaked H{\alpha} profile from the
LINER/Seyfert 1 nucleus of NGC 1097 was discovered in 1991, and monitored for
the following 11 years. The profile showed variations attributed to the
rotation of gas in a non-axisymmetric Keplerian accretion disk, ionized by a
varying radiatively inefficient accretion flow (RIAF) located in the inner
parts of the disk. We present and model 11 new spectroscopic observations of
the double-peaked profile taken between 2010 March and 2011 March. This series
of observations was motivated by the finding that in 2010 March the flux in the
double-peaked line was again strong, becoming, in 2010 December, even stronger
than in the observations of a decade ago. We also discovered shorter timescale
variations than in the previous observations: (1) the first, of ~7 days, is
interpreted as due to "reverberation" of the variation of the ionizing source
luminosity, and the timescale of 7 days as the light crossing time between the
source and the accretion disk; this new timescale and its interpretation
provides a distance between the emitting gas and the supermassive black hole
and as such introduces a new constraint on its mass; (2) the second, of
approximately 5 months, was attributed to the rotation of a spiral arm in the
disk, which was found to occur on the dynamical timescale. We use two accretion
disk models to fit theoretical profiles to the new data, both having
non-axisymmetric emissivities produced by the presence of an one-armed spiral.
Our modeling constrains the rotation period for the spiral to be approximately
18 months. This work supports our previous conclusion that the broad
double-peaked Balmer emission lines in NGC 1097, and probably also in other
low-luminosity active nuclei, originate from an accretion disk ionized by a
central RIAF.
