Unstable Nonradial Oscillations on Helium‐burning Neutron Stars

Piro, Anthony L.; Bildsten, Lars

doi:10.1086/381431

Cited by 20 publications

(30 citation statements)

References 66 publications

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“…Based on its observed frequency it appears at least plausible that it could be related to a surface g-mode associated with a helium-rich layer on the neutron star surface (Piro & Bildsten 2004). Other possibilities include a g-mode associated with density discontinuities in the surface layers (Bildsten & Cumming 1998), an inertial mode (Passamonti et al 2009), or perhaps an r-mode modified by the presence of the neutron star crust (Yoshida & Lee 2001).…”

Section: Discussionmentioning

confidence: 99%

“…For magnetic fields B 0 > 10 5 G, these modified g-modes (magnetogravity modes) change with increasing B from predominantly gmodes with constant periods to predominantly magnetic modes with periods proportional to B −1 0 (see their Equation (42) and Figure 4). Piro & Bildsten (2004) estimated the maximum magnetic field before the shallow surface mode would be dynamically affected to be B dyn ≈ 5 × 10 7 G((ω/2π)/21.4 Hz) which is about 6 × 10 8 G for a rotationally modified shallow surface wave with a frequency of 249.33 Hz. This is close to the estimated value of the magnetic field of J1751 obtained from spin-down measurements due to magneto-dipole radiation which is about 4 × 10 8 G (Riggio et al 2011).…”

Section: G-modesmentioning

confidence: 99%

“…Piro & Bildsten (2004) studied non-radial oscillations at the surface of a helium-burning neutron star. They found one unstable mode that resides in the helium atmosphere and is supported by the buoyancy of the helium/carbon interface.…”

Section: G-modesmentioning

confidence: 99%

“…Moreover, the relevant scale height, h, will depend on details of the surface envelopes in question; however, a typical value for the He-rich envelopes of Piro & Bildsten (2004) is h ≈ 200 cm, thus, for a 10 km radius neutron star we would require N 2 2.2 × 10 10 Hz 2 in order to satisfy the assumptions associated with the traditional approximation. We note that this condition appears to be technically violated for these envelopes as N is everywhere less than about 1 × 10 5 Hz (see their Figures 2 and 3).…”

Section: G-modesmentioning

confidence: 99%

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

Strohmayer

Mahmoodifar

2014

ApJ

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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.

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Section: Discussionmentioning

confidence: 99%

Section: G-modesmentioning

confidence: 99%

Section: G-modesmentioning

confidence: 99%

Section: G-modesmentioning

confidence: 99%

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

Strohmayer

Mahmoodifar

2014

ApJ

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“…It is thought that excited surface modes generate nonradial oscillations in the burst tail (McDermott & Taam 1987;Cumming & Bildsten 2000;Lee 2004;Heyl 2004Heyl , 2005Lee & Strohmayer 2005;Piro & Bildsten 2005. In particular, Heyl (2004) suggests buoyant r-modes as the most promising candidate.…”

Section: Introductionmentioning

confidence: 99%

Damping of Type I X‐Ray Burst Oscillations by Convection

Cooper

2008

Astrophysical Journal

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I construct a simple model of the convective burning layer during a type I X-ray burst to investigate the effects convection has on the stability of the layer to nonradial oscillations. A linear perturbation analysis demonstrates that the region is stable to nonradial oscillations when energy transport is convection-dominated, but it is unstable when energy transport is radiation-dominated. Thus, efficient convection always dampens oscillations. These results may explain the nondetection of oscillations during the peak of some X-ray bursts.

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Short-period X-ray oscillations in super-soft novae and persistent super-soft sources

Ness

Beardmore

Osborne

et al. 2015

A&A

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Context. Transient short-period (<100 s) oscillations have been found in the X-ray light curves of three novae during their super-soft source (SSS) phase and in one persistent SSS. Aims. We pursue an observational approach to determine possible driving mechanisms and relations to fundamental system parameters such as the white dwarf mass. Methods. We performed a systematic search for short-period oscillations in all available XMM-Newton and Chandra X-ray light curves of persistent SSS and novae during their SSS phase. To study time evolution, we divided each light curve into short timesegments and computed power spectra. We then constructed a dynamic power spectrum from which we identified transient periodic signals even when only present for a short time. We base our confidence levels on simulations of false-alarm probability for the chosen oversampling rate of 16, corrected for multiple testing based on the number of time segments. From all time segments of each system, we computed fractions of time when periodic signals were detected. Results. In addition to the previously known systems with short-period oscillations, RS Oph (35 s), KT Eri (35 s), V339 Del (54 s), and Cal 83 (67 s), we found one additional system, LMC 2009a (33 s), and also confirm the 35 s period from Chandra data of KT Eri. The oscillation amplitudes are of about <15% of the respective count rates and vary without any clear dependence on the X-ray count rate. The fractions of the time when the respective periods were detected at 2σ significance (duty cycle) are 11.3%, 38.8%, 16.9%, 49.2%, and 18.7% for LMC 2009a, RS Oph, KT Eri, V339 Del, and Cal 83, respectively. The respective highest duty cycles found in a single observation are 38.1%, 74.5%, 61.4%, 67.8%, and 61.8%. Conclusions. Since fast rotation periods of the white dwarfs as origin of these transient oscillations are speculative, we concentrate on pulsation mechanisms. We present initial considerations predicting the oscillation period to scale linearly with the white dwarf radius (and thus mass), weakly with the pressure at the base, and luminosity. Estimates of the size of the white dwarf could be useful for determining whether these systems are more massive than typical white dwarfs, and thus whether they are growing from accretion over time. Signs of such mass growth may have implications for whether some of these systems are attractive as Type Ia supernova progenitors.

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Unstable Nonradial Oscillations on Helium‐burning Neutron Stars

Cited by 20 publications

References 66 publications

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

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

Damping of Type I X‐Ray Burst Oscillations by Convection

Short-period X-ray oscillations in super-soft novae and persistent super-soft sources

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