Coherence of burst oscillations and accretion-powered pulsations in the accreting millisecond pulsar XTE J1814-338
TL;DR: In this article, the burst oscillations of the pulsar XTE J1814-338 are coherent with the persistent pulsations, to the level where they track all of the observed fluctuations.
Abstract: X-ray timing of the accretion-powered pulsations during the 2003 outburst of the accreting millisecond pulsar XTE J1814-338 has revealed variation in the pulse time of arrival residuals. These can be interpreted in several ways, including spin-down and wandering of the fuel impact point around the magnetic pole. In this Letter we show that the burst oscillations of this source are coherent with the persistent pulsations, to the level where they track all of the observed fluctuations. Only one burst, which occurs at the lowest accretion rates, shows a significant phase offset. We discuss what might lead to such rigid phase-locking between the modulations in the accretion and thermonuclear burst emission, and consider the implications for spin variation and the burst oscillation mechanism. Wandering of the fuel impact hot spot around a fixed magnetic pole seems the most likely cause for the accretion-powered pulse phase variations. This means that the burst asymmetry is coupled to the hot spot, not the magnetic pole. If premature ignition at this point (due to higher local temperatures) triggers a burning front that stalls before spreading over the entire surface, the resulting localized nuclear hot spot may explain the unusual burst and burst oscillation properties of this source.
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TL;DR: In this article, the authors investigated a model of the accreting millisecond X-ray pulsars and showed that the magnetic poles of these stars are close to their spin axes, and the gas is channeled along different field lines to different locations on the stellar surface.
Abstract: We investigate further a model of the accreting millisecond X-ray pulsars we proposed earlier. In this model, the X-ray-emitting regions of these pulsars are near their spin axes but move. This is to be expected if the magnetic poles of these stars are close to their spin axes, so that accreting gas is channeled there. As the accretion rate and the structure of the inner disk vary, gas is channeled along different field lines to different locations on the stellar surface, causing the X-ray-emitting areas to move. We show that this "nearly aligned moving spot model" can explain many properties of the accreting millisecond X-ray pulsars, including their generally low oscillation amplitudes and nearly sinusoidal waveforms; the variability of their pulse amplitudes, shapes, and phases; the correlations in this variability; and the similarity of the accretion- and nuclear-powered pulse shapes and phases in some. It may also explain why accretion-powered millisecond pulsars are difficult to detect, why some are intermittent, and why all detected so far are transients. This model can be tested by comparing with observations the waveform changes it predicts, including the changes with accretion rate.
82 citations
TL;DR: In this article, a rotating circular hot spot model was proposed to determine neutron star parameters and their uncertainties, and the resulting best-fit parameters of the model favor stiff equations of state, as can be seen from the 3σ allowed regions in the mass-radius diagram.
Abstract: The accretion-powered (non-X-ray burst) pulsations of XTE J1814-338 are modeled to determine neutron star parameters and their uncertainties. The model is a rotating circular hot spot and includes: (1) an isotropic blackbody spectral component; (2) an anisotropic Comptonized spectral component; (3) relativistic time-delays and light-bending; and (4) the oblate shape of the star due to rotation. This model is the simplest possible model that is consistent with the data. The resulting best-fit parameters of the model favor stiff equations of state, as can be seen from the 3-sigma allowed regions in the mass-radius diagram. We analyzed all data combined from a 23 day period of the 2003 outburst, and separately analyzed data from 2 days of the outburst. The allowed mass-radius regions for both cases only allow equations of state (EOS) that are stiffer than EOS APR (Akmal et al. 1998), consistent with the large mass that has been inferred for the pulsar NGC 6440B (Freire et al. 2008). The stiff EOS inferred by this analysis is not compatible with the soft EOS inferred from a similar analysis of SAX J1808.
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Book•
01 Nov 2010
TL;DR: A decade of X-ray sources and their evolution is described in this paper, with a focus on the formation and evolution of super-soft sources and the formation of compact stellar sources.
Abstract: 1. Accreting neutron stars and black holes: a decade of discoveries D. Psaltis 2. Rapid X-ray variability M. van der Klis 3. New views of thermonuclear bursts T. Strohmayer and L. Bildsten 4. Black hole binaries J. McClintock and R. Remillard 5. Optical, ultraviolet and infrared observations of X-ray binaries P. Charles and M. Coe 6. Fast X-ray transients and X-ray flashes J. Heise and J. in 't Zand 7. Isolated neutron stars V. Kaspi, M. Roberts and A. Harding 8. Globular cluster X-ray sources F. Verbunt and W. Lewin 9. Jets from X-ray binaries R. Fender 10. X-Rays from cataclysmic variables E. Kuulkers, A. Norton, A. Schwope and B. Warner 11. Super soft sources P. Kahabka and E. van den Heuvel 12. Compact stellar X-ray sources in normal galaxies G. Fabbiano and N. White 13. Accretion in compact binaries A. King 14. Soft gamma repeaters and anomalous X-ray pulsars: magnetar candidates P. Woods and C. Thompson 15. Cosmic gamma-ray bursts, their afterglows, and their host galaxies K. Hurley, R. Sari and S. Djorgovski 16. Formation and evolution of compact stellar X-ray sources T. Tauris and E. van den Heuvel.
3,100 citations
TL;DR: In this article, a rotating circular hot spot model was proposed to determine neutron star parameters and their uncertainties, and the resulting best-fit parameters of the model favor stiff equations of state, as can be seen from the 3σ allowed regions in the mass-radius diagram.
Abstract: The accretion-powered (non-X-ray burst) pulsations of XTE J1814-338 are modeled to determine neutron star parameters and their uncertainties. The model is a rotating circular hot spot and includes: (1) an isotropic blackbody spectral component; (2) an anisotropic Comptonized spectral component; (3) relativistic time-delays and light-bending; and (4) the oblate shape of the star due to rotation. This model is the simplest possible model that is consistent with the data. The resulting best-fit parameters of the model favor stiff equations of state, as can be seen from the 3-sigma allowed regions in the mass-radius diagram. We analyzed all data combined from a 23 day period of the 2003 outburst, and separately analyzed data from 2 days of the outburst. The allowed mass-radius regions for both cases only allow equations of state (EOS) that are stiffer than EOS APR (Akmal et al. 1998), consistent with the large mass that has been inferred for the pulsar NGC 6440B (Freire et al. 2008). The stiff EOS inferred by this analysis is not compatible with the soft EOS inferred from a similar analysis of SAX J1808.
3 citations
TL;DR: In this article, the authors investigated the effect of precession on the pulse properties of a millisecond pulsar and found that if the observed flux variations are due to precession, their model incompletely describes the relative precession phase offsets (e.g. the surface intensity map is over-simplified).
Abstract: Precession in an accretion-powered pulsar is expected to produce characteristic variations in the pulse properties. Assuming surface intensity maps with one and two hotspots, we compute theoretically the periodic modulation of the mean flux, pulse-phase residuals and fractional amplitudes of the first and second harmonic of the pulse profiles. These quantities are characterised in terms of their relative precession phase offsets. We then search for these signatures in 37 days of X-ray timing data from the accreting millisecond pulsar XTE J1814-338. We analyse a 12.2-d modulation observed previously and show that it is consistent with a freely precessing neutron star only if the inclination angle is < 0.1 degrees, an a priori unlikely orientation. We conclude that if the observed flux variations are due to precession, our model incompletely describes the relative precession phase offsets (e.g. the surface intensity map is over-simplified). We are still able to place an upper limit on \epsilon of 3.0 x 10^{-9} independently of our model, and estimate the phase-independent tilt angle \theta; to lie roughly between 5 and 10 degrees. On the other hand, if the observed flux variations are not due to precession, the detected signal serves as a firm upper limit for any underlying precession signal. We then place an upper limit on the product \epsilon cos(\theta) of \leq 9.9 x 10^{-10}. The first scenario translates into a maximum gravitational wave strain of 10^{-27} from XTE J1814-338 (assuming a distance of 8 kpc), and a corresponding signal-to-noise ratio of \leq 10^{-3} (for a 120 day integration time) for the advanced LIGO ground-based gravitational wave detector.
1 citations