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Register a new userNuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars
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Millisecond pulsars are neutron stars (NSs) that are thought to have been spun-up by mass accretion from a stellar companion. It is unknown whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many NSs that are accreting from a companion exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond brightness oscillations during bursts from ten NSs (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here, we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting NSs, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most NS models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.
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Nuclear-powered X-ray millisecond pulsars are the third type of millisecond pulsars, which are powered by thermonuclear fusion processes. The corresponding brightness oscillations, known as burst oscillations, are observed during some thermonuclear X-ray bursts, when the burning and cooling accreted matter gives rise to an azimuthally asymmetric brightness pattern on the surface of the spinning neutron star. Apart from providing neutron star spin rates, this X-ray timing feature can be a useful tool to probe the fundamental physics of neutron star interior and surface. This chapter presents an overview of the relatively new field of nuclear-powered X-ray millisecond pulsars.
We study theoretical X-ray light curves and polarization properties of accretion-powered millisecond pulsars. We assume that the radiation is produced in two antipodal spots at the neutron star surface which are associated with the magnetic poles. We compute the angle-dependent intensity and polarization produced in an electron-scattering dominated plane-parallel accretion shock in the frame of the shock. The observed flux, polarization degree and polarization angle are calculated accounting for special and general relativistic effects. The calculations also extended to the case of nuclear-powered millisecond pulsars -- X-ray bursts. In this case, we consider one spot and the radiation is assumed to be produced in the atmosphere of the infinite Thomson optical depth. The light curves and polarization profiles show a large diversity depending on the model parameters. Presented results can be used as a first step to understand the observed pulse profiles of accretion- and nuclear-powered millisecond pulsars. Future observations of the X-ray polarization will provide a valuable tool to test the geometry of the emission region and its physical characteristics.
In this Letter we re-examine the idea that gravitational waves are required as a braking mechanism to explain the observed maximum spin-frequency of neutron stars. We show that for millisecond X-ray pulsars, the existence of spin equilibrium as set by the disk/magnetosphere interaction is sufficient to explain the observations. We show as well that no clear correlation exists between the neutron star magnetic field B and the X-ray outburst luminosity Lx when considering an enlarged sample size of millisecond X-ray pulsars.
Rotation-powered millisecond radio pulsars have been spun up to their present spin period by a $10^8$ - $10^9$ yr long X-ray-bright phase of accretion of matter and angular momentum in a low-to-intermediate mass binary system. Recently, the discovery of transitional pulsars that alternate cyclically between accretion and rotation-powered states on time scales of a few years or shorter, has demonstrated this evolutionary scenario. Here, we present a thorough statistical analysis of the spin distributions of the various classes of millisecond pulsars to assess the evolution of their spin period between the different stages. Accreting sources that showed oscillations exclusively during thermonuclear type I X-ray bursts (nuclear-powered millisecond pulsars) are found to be significantly faster than rotation-powered sources, while accreting sources that possess a magnetosphere and show coherent pulsations (accreting millisecond pulsars) are not. On the other hand, if accreting millisecond pulsars and eclipsing rotation-powered millisecond pulsars form a common class of transitional pulsars, these are shown to have a spin distribution intermediate between the faster nuclear-powered millisecond pulsars and the slower non-eclipsing rotation-powered millisecond pulsars. We interpret these findings in terms of a spin-down due to the decreasing mass-accretion rate during the latest stages of the accretion phase, and in terms of the different orbital evolutionary channels mapped by the various classes of pulsars. We summarize possible instrumental selection effects, showing that even if an unbiased sample of pulsars is still lacking, their influence on the results of the presented analysis is reduced by recent improvements in instrumentation and searching techniques.
An understanding of spin frequency ($ u$) evolution of neutron stars in the low-mass X-ray binary (LMXB) phase is essential to explain the observed $ u$-distribution of millisecond pulsars (MSPs), and to probe the stellar and binary physics, including the possibility of continuous gravitational wave emission. Here, using numerical computations we conclude that $ u$ can evolve in two distinctly different modes, as $ u$ may approach a lower spin equilibrium value ($ u_{rm eq,per}$) for persistent accretion for a long-term average accretion rate ($dot{M}_{rm av}$) greater than a critical limit ($dot{M}_{rm av,crit}$), and may approach a higher effective spin equilibrium value ($ u_{rm eq,eff}$) for transient accretion for $dot{M}_{rm av} < dot{M}_{rm av,crit}$. For example, when $dot{M}_{rm av}$ falls below $dot{M}_{rm av,crit}$ for an initially persistent source, $ u$ increases considerably due to transient accretion, which is counterintuitive. We also find that, contrary to what was suggested, a fast or sudden decrease of $dot{M}_{rm av}$ to zero in the last part of the LMXB phase is not essential for the genesis of spin-powered MSPs, and neutron stars could spin up in this $dot{M}_{rm av}$-decreasing phase. Our findings imply that the traditional way of $ u$-evolution computation is inadequate in most cases, even for initially persistent sources, and may not even correctly estimate whether $ u$ increases or decreases.
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