The VLBA Extragalactic Proper Motion Catalog and a Measurement of the
  Secular Aberration Drift
  We present a catalog of extragalactic proper motions created using archival
VLBI data and our own VLBA astrometry. The catalog contains 713 proper motions,
with average uncertainties of ~ 24 muas/yr, including 40 new or improved proper
motion measurements using relative astrometry with the VLBA. The observations
were conducted in X-band and yielded positions with uncertainties ~ 70 muas. We
add 10 new redshifts using spectroscopic observations taken at Apache Point
Observatory and Gemini North. With the VLBA Extragalactic Proper Motion
Catalog, we detect the secular aberration drift - the apparent motion of
extragalactic objects caused by the solar system's acceleration around the
Galactic center - at a 6.3 sigma significance. We model the aberration drift as
a spheroidal dipole, with the square root of the power equal to 4.89 +/- 0.77
muas/yr, an amplitude of 1.69 +/- 0.27 muas/yr, and an apex at (275.2 +/- 10.0
deg, -29.4 +/- 8.8 deg). Our dipole model detects the aberration drift at a
higher significance than some previous studies (e.g., Titov & Lambert 2013),
but at a lower amplitude than expected or previously measured. The full
aberration drift may be partially removed by the no-net-rotation constraint
used when measuring archival extragalactic radio source positions. Like the
cosmic microwave background dipole, which is induced by the observer's motion,
the aberration drift signal should be subtracted from extragalactic proper
motions in order to detect cosmological proper motions, including the Hubble
expansion, long-period stochastic gravitational waves, and the collapse of
large-scale structure.
