  Results of observations of the Galactic bulge X-ray source GX 9+9 by the
All-Sky Monitor (ASM) and Proportional Counter Array (PCA) onboard the Rossi
X-ray Timing Explorer are presented. The ASM results show that the 4.19 hour
X-ray periodicity first reported by Hertz and Wood in 1987 was weak or not
detected for most of the mission prior to late 2004, but then became strong and
remained strong for approximately 2 years after which it weakened considerably.
When the modulation at the 4.19 hour period is strong, it appears in folded
light curves as an intensity dip over slightly less than 30% of a cycle and is
distinctly nonsinusoidal. A number of PCA observations of GX 9+9 were performed
before the appearance of strong modulation; two were performed in 2006 during
the epoch of strong modulation. Data obtained from the earlier PCA observations
yield at best limited evidence of the presence of phase-dependent intensity
changes, while the data from the later observations confirm the presence of
flux minima with depths and phases compatible with those apparent in folded ASM
light curves. Light curves from a Chandra observation of GX 9+9 performed in
the year 2000 prior to the start of strong modulation show the possible
presence of shallow dips at the predicted times. Optical observations performed
in 2006 while the X-ray modulation was strong do not show an increase in the
degree of modulation at the 4.19 hour period. Implications of the changes in
modulation strength in X-rays and other observational results are considered.
