Long-term X-ray variability in GX 13+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 7 yr. From a filtered data set, which excludes measurements with exceptionally large error bars and so maximizes signal-to-noise ratio, 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 and 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.