A NON-RADIAL OSCILLATION MODE IN AN ACCRETING MILLISECOND PULSAR?

作者：Strohmayer Tod^*; Mahmoodifar Simin

来源：Astrophysical Journal, 2014, 784(1): 72.

DOI：10.1088/0004-637X/784/1/72

• methods: data analysis
• stars: neutron
• stars: oscillations
• stars: rotation
• X-rays: binaries
• X-rays: individual (XTE J1751 305, XTE J1814, 338, NGC 6440 X-2)

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 fast Fourier transform power spectra of continuous light curves with up to 230 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 nu(spin) = 249.332609 Hz, and a fractional Fourier amplitude of 7.46x10(-4). We estimate the significance of this feature at the 1.6 x 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 7.8x10(-4) and 5.6x10(-3), respectively. We briefly discuss the prospects and sensitivity for similar searches with future, larger X-ray collecting area missions.

• 出版日期2014-3-20