Discovery of a 0.02 Hz QPO feature in the transient X-ray pulsar KS 1947+300

We report the discovery of quasi-periodic oscillations (QPOs) at 0.02 Hz in a transient high-mass X-ray binary pulsar KS 1947+300 using the RXTE Proportional Counter Array (PCA). The QPOs were detected during 2001 May–June, at the end of a long outburst. This is the ninth transient accretion-powered high magnetic field X-ray pulsar in which QPOs have been detected, and the QPO frequency of this source is lowest in this class of sources. The unusual feature of this source is that though the outburst lasted for more than 100 days, the QPOs were detected only during the last few days of the outburst when the X-ray intensity had decayed to 1.6 per cent of the peak intensity. The rms value of the QPO is large, ∼15.4 ± 1.0 per cent with a slight positive correlation with energy. The detection of QPOs and strong pulsations at a low luminosity level suggests that the magnetic field strength of the neutron star is not as high as was predicted earlier on the basis of a correlation between the spin-up torque and the X-ray luminosity.