Variable Quasi Periodic Oscillations during an outburst of the transient X-ray pulsar XTE J1858+034

We have investigated the Quasi Periodic Oscillation (QPO) properties of the transient accreting X-ray pulsar XTE J1858+034 during the second outburst of this source in April-May 2004. We have used observations made with the Proportional Counter Array (PCA) of the Rossi X-ray Timing Explorer (RXTE) during May 14-18 2004, in the declining phase of the outburst. We detected the presence of low frequency QPOs in the frequency range of 140--185 mHz in all the RXTE-PCA observations. We report evolution of the QPO parameters with the time, X-ray flux, and X-ray photon energy. Though a correlation between the QPO centroid frequency and the instantaneous X-ray flux is not very clear from the data, we point out that the QPO frequency and the one day averaged X-ray flux decreased with time during these observations. We have obtained a clear energy dependence of the RMS variation in the QPOs, increasing from about 3% at 3 keV to 6% at 25 keV. The X-ray pulse profile is a single peaked sinusoidal, with pulse fraction increasing from 20% at 3 keV to 45% at 30 keV. We found that, similar to the previous outburst, the energy spectrum is well fitted with a model consisting of a cut-off power law along with an iron emission line.