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.

Millisecond

Oscillation (cell signaling)

10.1088/0004-637x/706/1/417

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Citations (75)

Timing the Bursting LMXB KS 1731-260 in its Next Outburst

RXTE Proposal (2006)

Stratos Boutloukos

Bursting

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Star cluster formation and disruption time-scales - I. An empirical determination of the disruption time of star clusters in four galaxies

Monthly Notices of the Royal Astronomical Society (2003)

Stratos Boutloukos H. J. G. L. M. Lamers

We have derived the disruption times of star clusters from cluster samples of four galaxies, M51, M33, the Small Magellanic Cloud (SMC) and the solar neighbourhood. If the disruption time of clusters in a galaxy depends only on their initial mass as tdis(yr) =t4dis(Mcl/104 M⊙)γ, and if the cluster formation rate is constant, then the mass and age distributions of the observed clusters will each be given by double power-law relations. For clusters of low mass or young age the power law depends on the fading of the clusters below the detection limit due to the evolution of the stars. For clusters of high mass and old age the power law depends on the disruption time of the clusters. The samples of clusters in M51 and M33, observed with HST-WFPC2, indeed show the predicted double power-law relations in both their mass and age distributions. The values of tdis4 and γ can be derived from these relations. For the cluster samples of the SMC and the solar neighbourhood, taken from the literature, only the age distribution is known. This also shows the characteristic double power-law behaviour, which allows the determination of t4dis and γ in these galaxies. The values of γ are the same in the four galaxies within the uncertainty, and the mean value is γ= 0.62 ± 0.06. However, the disruption time t4dis of a cluster of 104 M⊙ is very different in the different galaxies. The clusters in the SMC have the longest disruption time, t4dis≃ 8 × 109 yr, and the clusters at 1–3 kpc from the nucleus of M51 have the shortest disruption time of t4dis≃ 4 × 107 yr. The disruption time of clusters 1–5 kpc from the nucleus of M33 is t4dis≃ 1.3 × 108 yr and for clusters within 1 kpc from the Sun we find t4dis≃ 1.0 × 109 yr.

10.1046/j.1365-8711.2003.06083.x

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Citations (134)

Implications of high-precision spectra of thermonuclear X-ray bursts for determining neutron star masses and radii

M. Coleman Miller Stratos Boutloukos Ka Ho Lo Frederick K. Lamb

X-ray burst spectra have long been used to estimate neutron star masses and radii.These estimates assumed that burst spectra are accurately described by the model atmosphere spectra developed over the last three decades.We compared RXTE data from a superburst with these spectra and found that the spectra predicted by previously published model atmospheres are strongly inconsistent with these high-precision measurements.In contrast, a simple Bose-Einstein spectrum is fully consistent with the data, as are recently published model atmosphere spectra.We discuss the implications of our results for determinations of neutron star masses and radii via constraints on their surface gravity and redshift, as originally suggested by Majczyna and Madej.Fast X-ray timing and spectroscopy at extreme count rates:

Thermonuclear Fusion

Surface gravity

10.22323/1.122.0024

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Space Telescopes and Instrumentation 2012

DSpace@MIT (Massachusetts Institute of Technology) (2012)

M. Feroci J. W. den Herder E. Bozzo D. Barret S. Brandt

Instrumentation

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Citations (152)