  A search is made for periodic modulation in the X-ray flux from the low mass
X-ray binary GX13+1 using Rossi X-ray Timing Explorer All Sky Monitor data
collected over a period of almost seven years. From a filtered data set, which
excludes measurements with exceptionally large error bars and so maximizes
signal to noise, modulation is found at a period of 24.065 +/- 0.018 days. The
modulation is most clearly detectable at high energies (5 - 12 keV). Spectral
changes are revealed as a modulation in hardness ratio on the 24 day period and
there is a phase shift between the modulation in the 5 - 12 keV energy band
compared to the 1.5 - 5 keV band. The high-energy spectrum of GX13+1 is unusual
in displaying both emission and absorption iron line features and it is
speculated that the peculiar spectral and timing properties may be connected.

Astrophysics