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We report the detection of quasi-periodic oscillation (QPO) at ~41 mHz in thetransient high-mass Be/X-ray binary pulsar 4U~0115+634 using data from theRossi X-Ray Timing Explorer (RXTE) observatory. The observations used in thepresent work were carried out during X-ray outbursts in 1999 March-April, 2004September-October and 2008 March-April. This frequency of the newly detectedQPO was found to vary in 27-46 mHz range. This ~41 mHz QPO was detected in fourof the 36 pointed RXTE Proportional Counter Array (PCA) observations during1999 outburst where as during 2004 and 2008 outbursts, it was detected in fourand three times out of 33 and 26 observations, respectively. Though QPOs at ~2mHz, and ~62 mHz were reported earlier, the ~41 mHz QPO and its first harmonicwere detected for the first time in this pulsar. There are three RXTE/PCAobservations where multiple QPOs were detected in the power density spectrum of4U~0115+634. Simultaneous presence of multiple QPOs is rarely seen in accretionpowered X-ray pulsars. Spectral analysis of all the pointed RXTE/PCAobservations revealed that the 3-30 keV energy spectrum was well described byNegative and Positive Power-law with EXponential cutoff (NPEX) continuum modelalong with interstellar absorption and cyclotron absorption components. Duringthe three X-ray outbursts, however, no systematic variation in any of thespectral parameters other than the earlier reported anti-correlation betweencyclotron absorption energy and luminosity was seen. Presence of any systematicvariation of QPO frequency and rms of QPO with source flux were alsoinvestigated yielding negative results.

Detection of a variable QPO at ~41 mHz in the Be/X-ray transient pulsar 4U 0115+634