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