Quasi-periodic oscillations in XTE J0111.2-7317: highest frequency among the HMXB pulsars

Abstract

We report the discovery of quasi-periodic oscillations (QPOs) in the high-mass X-ray binary (HMXB) pulsar XTE J0111.2-7317 during a transient outburst of the source in 1998 December. Using observations made with the Proportional Counter Array of the Rossi X-Ray Timing Explorer during the second peak and the declining phase of the outburst, we have discovered a QPO feature at a frequency of 1.27 Hz. We have ruled out the possibility that the observed feature could instead be from the neighboring bright X-ray pulsar SMC X-1. This is the highest frequency QPO ever detected in an HMXB pulsar. In the absence of a cyclotron absorption feature in the X-ray spectrum, the QPO, along with the pulse period and X-ray flux measurement, helps us to constrain the magnetic field strength of the neutron star.