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Register a new userGravitational Waves and the Maximum Spin Frequency of Neutron Stars
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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.
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With the remarkable advent of gravitational-wave astronomy, we have shed light on previously shrouded events: compact binary coalescences. Neutron stars are promising (and confirmed) sources of gravitational radiation and it proves timely to consider the ways in which these stars can be deformed. Gravitational waves provide a unique window through which to examine neutron-star interiors and learn more about the equation of state of ultra-dense nuclear matter. In this work, we study two relevant scenarios for gravitational-wave emission: neutron stars that host (non-axially symmetric) mountains and neutron stars deformed by the tidal field of a binary partner. Although they have yet to be seen with gravitational waves, rotating neutron stars have long been considered potential sources. By considering the observed spin distribution of accreting neutron stars with a phenomenological model for the spin evolution, we find evidence for gravitational radiation in these systems. We study how mountains are modelled in both Newtonian and relativistic gravity and introduce a new scheme to resolve issues with previous approaches to this problem. The crucial component of this scheme is the deforming force that gives the star its non-spherical shape. We find that the force (which is a proxy for the stars formation history), as well as the equation of state, plays a pivotal role in supporting the mountains. Considering a scenario that has been observed with gravitational waves, we calculate the structure of tidally deformed neutron stars, focusing on the impact of the crust. We find that the effect on the tidal deformability is negligible, but the crust will remain largely intact up until merger.
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.
The fastest-spinning neutron stars in low-mass X-ray binaries, despite having undergone millions of years of accretion, have been observed to spin well below the Keplerian break-up frequency. We simulate the spin evolution of synthetic populations of accreting neutron stars in order to assess whether gravitational waves can explain this behaviour and provide the distribution of spins that is observed. We model both persistent and transient accretion and consider two gravitational-wave-production mechanisms that could be present in these systems: thermal mountains and unstable $r$-modes. We consider the case of no gravitational-wave emission and observe that this does not match well with observation. We find evidence for gravitational waves being able to provide the observed spin distribution; the most promising mechanisms being a permanent quadrupole, thermal mountains and unstable $r$-modes. However, based on the resultant distributions alone it is difficult to distinguish between the competing mechanisms.
Dark matter could be composed of compact dark objects (CDOs). We find that the oscillation of CDOs inside neutron stars can be a detectable source of gravitational waves (GWs). The GW strain amplitude depends on the mass of the CDO, and its frequency is typically in the range 3-5 kHz as determined by the central density of the star. In the best cases, LIGO may be sensitive to CDO masses greater than or of order $10^{-8}$ solar masses.
We construct self-consistent equilibrium sequences of general relativistic, rotating neutron star models. Special emphasis in put on the determination of the maximum rotation frequency of such objects. Recently proposed models for the equation of state of neutron star matter are employed, which are derived by describing the hadronic phase within the many-body Brueckner--Bethe--Goldstone formalism, and the quark matter phase within the MIT bag model using a density dependent bag constant. We find that the rotational frequencies of neutron stars with deconfined quark phases in their cores rival those of absolutely stable, self-bound strange quark matter stars. This finding is of central importance for the interpretation of extremely rapidly rotating pulsars, which are the targets of present pulsar surveys.
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