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A search is made for periodic modulation in the X-ray flux from the low massX-ray binary GX13+1 using Rossi X-ray Timing Explorer All Sky Monitor datacollected over a period of almost seven years. From a filtered data set, whichexcludes measurements with exceptionally large error bars and so maximizessignal to noise, modulation is found at a period of 24.065 +/- 0.018 days. Themodulation is most clearly detectable at high energies (5 - 12 keV). Spectralchanges are revealed as a modulation in hardness ratio on the 24 day period andthere is a phase shift between the modulation in the 5 - 12 keV energy bandcompared to the 1.5 - 5 keV band. The high-energy spectrum of GX13+1 is unusualin displaying both emission and absorption iron line features and it isspeculated that the peculiar spectral and timing properties may be connected.Comment: Accepted for publication in ApJ. 21 pages including 9 figure

Long‐Term X‐Ray Variability in GX 13+1: Energy‐dependent Periodic Modulation