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 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 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 × ν spin = 249.332609 Hz, and a fractional Fourier amplitude of 7.46 × 10 −4 . We estimate the significance of this feature at the 1.6 × 10 −3 level, slightly better than a 3σ 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 × 10 −4 and 5.6 × 10 −3 , respectively. We briefly discuss the prospects and sensitivity for similar searches with future, larger X-ray collecting area missions.

Gravitational Waves from Neutron Stars: A Review

“…This somewhat simple picture describing the r-mode instability window is inconsistent with observed spins and temperatures of lowmass X-ray binaries, implying the complete physical picture behind this mechanism is ill-understood (Ho, Andersson, & Haskell 2011;Haskell, Degenaar, & Ho 2012). Strohmayer & Mahmoodifar (2014) recently analysed Rossi X-ray Timing Explorer (RXTF) observations of the accreting millisecond pulsar XTE J1751-305, finding evidence for a coherent oscillation mode during outburst that they attributed to either an r-or g-mode (for which buoyancy is the restoring force). The r-mode interpretation is the most interesting in the context of gravitational wave emission; had the outburst occurred during the S5 LIGO run it would have been marginally observable (Andersson, Jones, & Ho 2014).…”

Seismology of adolescent neutron stars: Accounting for thermal effects and crust elasticity

“…There are, of course, issues to be resolved, especially concerning the dynamical role of the interior magnetic field in these systems [24]. A more indirect example comes from X-ray timing of fast spinning accreting neutron stars in Low-Mass X-ray Binaries [25,26]. The spin of these stars appears to be limited by some process.…”

M/R estimates for two neutron stars in LMXBs with possible r-mode frequencies detected

“…Now we are in possession of the best estimate for the rmode frequency as a function of the stellar compactness, radius and angular velocity, and we can use it to estimate the compactness of the two stars XTE J1751-305 and 4U 1636-536 for which it was reported that a frequency compatible with an r-mode was detected. Strohmayer & Mahmoodifar (2014a) found an oscillation with a frequency of 0.5727597 times the spin frequency of 435 Hz in the 2002 discovery outburst of XTE J1751-305. An interpretation of this frequency as an r-mode therefore implies κ = 0.5727597.…”

“…Their frequencies are tantalizingly close to the Newtonian estimate for the r-mode frequency of these stars. Strohmayer & Mahmoodifar (2014a) suggested that possible identifications of the observed frequency include rotationally modified g-modes or alternatively an inertial mode or core r-mode modified by the presence of a solid crust. However, the r-mode amplitude derived from the observed modulation amplitude would lead to a spindown rate too large to be consistent with the observations.…”

“…However, the r-mode amplitude derived from the observed modulation amplitude would lead to a spindown rate too large to be consistent with the observations. The spin evolution of XTE J1751-305 was rediscussed by Andersson et al (2014), who concluded from their analysis of different r-mode scenarios (stable/unstable and saturated/unsaturated) that the presence of an r-mode with the large amplitude suggested by Strohmayer & Mahmoodifar (2014a) cannot be reconciled with the observed spin-up of the star during the outburst and must therefore be considered unlikely. However, it has also been suggested by Lee (2014) that the frequency could be a surface g-mode or rmode, and alternatives like unstable toroidal crust modes or the amplification at the surface of a core r-mode with a lower amplitude were also discussed.…”