Long Term X-ray Variability in GX13+1: Energy Dependent Periodic Modulation

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.