We present results of targeted searches for signatures of non-radial oscillation modes ( such as r- and g-modes ) in neutron stars using RXTE data from several accreting millisecond X-ray pulsars ( AMXPs ) .
We search for potentially coherent signals in the neutron star rest frame by first removing the phase delays associated with the star ’ s binary motion and computing FFT power spectra of continuous light curves with up to 2 ^ { 30 } time bins .
We search a range of frequencies in which both r- and g-modes are theoretically expected to reside .
Using data from the discovery outburst of the 435 Hz pulsar XTE J1751 - 305 we find a single candidate , coherent oscillation with a frequency of 0.5727597 \times \nu _ { spin } = 249.332609 Hz , and a fractional Fourier amplitude of 7.46 \times 10 ^ { -4 } .
We estimate the significance of this feature at the 1.6 \times 10 ^ { -3 } level , slightly better than a 3 \sigma detection .
Based on the observed frequency we argue that possible mode identifications include rotationally-modified g-modes associated with either a helium-rich surface layer or a density discontinuity due to electron captures on hydrogen in the accreted ocean .
In the latter case the presence of sufficient hydrogen in this ultracompact system with a likely helium-rich donor would present an interesting puzzle .
Alternatively , the frequency could be identified with that of an inertial mode or a core r-mode modified by the presence of a solid crust , however , the r-mode amplitude required to account for the observed modulation amplitude would induce a large spin-down rate inconsistent with the observed pulse timing measurements .
For the AMXPs XTE J1814 - 338 and NGC 6440 X-2 we do not find any candidate oscillation signals , and we place upper limits on the fractional Fourier amplitude of any coherent oscillations in our frequency search range of 7.8 \times 10 ^ { -4 } and 5.6 \times 10 ^ { -3 } , respectively .
We briefly discuss the prospects and sensitivity for similar searches with future , larger X-ray collecting area missions .