Instability of the 13.8 day period in X-ray emission from 4U1700-377

We present a new result on long-term periodicity searches of the X-ray emission from 4U1700-377 using RXTE ASM and CGRO BATSE data. The hard X-ray data (20 - 200 keV) from early BATSE observations (1780 days from JD 2448370, before RXTE observations started) show evidence of a 13.8 day periodicity. The long term periodicity became substantially less prominent in the data from later BATSE observations and in the soft X-ray data from the ASM (2 - 10 keV) observations. We demonstrate that disk precession models can explain the 13.8 day period and its instability in the X-ray emission from 4U1700-377.