A Non-radial Oscillation Mode in an Accreting Millisecond Pulsar?

Abstract

We present results of targeted searches for signatures of non-radial oscillation modes (such as r- and g-modes) in neutron stars using {\it 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. 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. Alternatively, the frequency could be identified with that of an inertial mode or an 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.

17 pages, 11 figures. Additional discussion of r- and inertial mode interpretations. Accepted for publication in the Astrophysical Journal

References in corpus (4)

• Detecting gravitational wave emission from the known accreting neutron stars
• Oscillations of rapidly rotating stratified neutron stars
• Timing of the Accreting Millisecond Pulsar XTE J1814-338
• Coherence of burst oscillations and accretion-powered pulsations in the accreting millisecond pulsar XTE J1814-338

Cited by in corpus (25)

• Gravitational Waves from Neutron Stars: A Review
• Searches for Continuous-Wave Gravitational Radiation
• Seismology of adolescent neutron stars: Accounting for thermal effects and crust elasticity
• Recent searches for continuous gravitational waves
• The Spin Distribution of Fast Spinning Neutron Stars in Low Mass X-Ray Binaries: Evidence for Two Sub-Populations
• R-modes in neutron stars: Theory and observations
• R-mode frequencies of slowly rotating relativistic neutron stars with realistic equations of state
• Implications of an r-mode in XTE J1751-305: Mass, radius and spin evolution
• Discovery of a Neutron Star Oscillation Mode During a Superburst
• Gravitational waves within the magnetar model of superluminous supernovae and gamma-ray bursts
• Excitation of a nonradial mode in a millisecond X-ray pulsar XTE J1751-305
• Constraining equation of state groups from $g$-mode asteroseismology
• Searches for continuous gravitational waves from Scorpius X-1 and XTE J1751-305 in LIGO's sixth science run
• Oscillations of superfluid hyperon stars: decoupling scheme and g-modes
• R-mode frequencies of rapidly and differentially rotating relativistic neutron stars
• Observational signatures of neutron stars in low-mass X-ray binaries climbing a stability peak
• Constraining the nuclear equation of state via gravitational-wave radiation of short gamma-ray burst remnants
• Diffusion as a leading dissipative mechanism in superconducting neutron stars
• Evolving ONe WD+He WD systems to ultra-compact X-ray binaries
• Formulating the r-mode problem for slowly rotating neutron stars
• Non-analytic behavior of the relativistic r-modes in slowly rotating neutron stars
• $M/R$ estimates for two neutron stars in LMXBs with possible r-mode frequencies detected
• Gravitational waves from r-mode oscillations of stochastically accreting neutron stars
• Ultra-compact X-ray Binaries: A Review
• A search for the $835\text{Hz}$ superburst oscillation signal in the regular thermonuclear bursts of 4U 1636-536