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

TL;DR

This paper reports the discovery of a very low frequency QPO in the transient X-ray pulsar KS 1947+300, observed during the late stage of an outburst, indicating unique accretion and magnetic field properties.

Contribution

The study presents the first detection of a 0.02 Hz QPO in KS 1947+300, the lowest in its class, during the decay phase of an outburst, challenging previous magnetic field estimates.

Findings

QPO detected at 0.02 Hz during late outburst phase

QPO rms amplitude approximately 15.4% with energy dependence

Detection at low luminosity suggests lower magnetic field strength than previously estimated

Abstract

We report the discovery of Quasi Periodic Oscillations (QPO) at 0.02 Hz in a transient high mass X-ray binary pulsar KS 1947+300 using {\em RXTE}-PCA. The QPOs were detected during May-June 2001, at the end of a long outburst. This is the 9th 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% of the peak intensity. The rms value of the QPO is large, $\sim15.4\pm1.0%$ 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.