Changes in the Long‐Term Intensity Variations in Cygnus X‐2 and LMC X‐3

We report the detection of changes in the long-term intensity variations intwo X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used thelong-term light curves obtained with the All-Sky Monitors (ASMs) of the RossiX-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanningmodulation collimator of HEAO 1. It is found that in the light curves of boththe sources, obtained with these instruments at various times over the last 30years, more than one periodic or quasi-periodic component is always present.The multiple prominent peaks in the periodograms have frequencies unrelated toeach other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days,and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks arealso observed in LMC X-3. The various strong peaks in the periodograms of LMCX-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and328 days (observed with Ginga-ASM). The present results, when compared with theearlier observations of periodicities in these two systems, demonstrate theabsence of any stable long period. The 78 day periodicity detected earlier inCyg X-2 was probably due to the short time base in the RXTE data that wereused, and the periodicity of 198 days in LMC X-3 was due to a relatively shortduration of observation with HEAO 1.Comment: 11 pages, 7 postscript figures include