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Register a new userGravitational Waves from Compact Dark Objects in Neutron Stars
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Added by
Charles J. Horowitz
Publication date
2019
fields
Physics
and research's language is
English
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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.
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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.
As is well known, gravitational wave detections of coalescing binaries are standard sirens, allowing a measurement of source distance by gravitational wave means alone. In this paper we explore the analogue of this for continuous gravitational wave emission from individual spinning neutron stars, whose spin-down is driven purely by gravitational wave emission. We show that in this case, the distance measurement is always degenerate with one other parameter, which can be taken to be the moment of inertia of the star. We quantify the accuracy to which such degenerate measurements can be made. We also discuss the practical application of this to scenarios where one or other of distance or moment of inertia is constrained, breaking this degeneracy and allowing a measurement of the remaining parameter. Our results will be of use following the eventual detection of a neutron star spinning down through such gravitational wave emission.
Neutron stars may harbour the true ground state of matter in the form of strange quark matter. If present, this type of matter is expected to be a color superconductor, a consequence of quark pairing with respect to the color/flavor degrees of freedom. The stellar magnetic field threading the quark core becomes a color-magnetic admixture and, in the event that superconductivity is of type II, leads to the formation of color-magnetic vortices. In this Letter we show that the volume-averaged color-magnetic vortex tension force should naturally lead to a significant degree of non-axisymmetry in systems like radio pulsars. We show that gravitational radiation from such color-magnetic `mountains in young pulsars like the Crab and Vela could be observable by the future Einstein Telescope, thus becoming a probe of paired quark matter in neutron stars. The detectability threshold can be pushed up toward the sensitivity level of Advanced LIGO if we invoke an interior magnetic field about a factor ten stronger than the surface polar field.
We investigate a simple holographic model for cold and dense deconfined QCD matter consisting of three quark flavors. Varying the single free parameter of the model and utilizing a Chiral Effective Theory equation of state (EoS) for nuclear matter, we find four different compact star solutions: traditional neutron stars, strange quark stars, as well as two non-standard solutions we refer to as hybrid stars of the second and third kind (HS2 and HS3). The HS2s are composed of a nuclear matter core and a crust made of stable strange quark matter, while the HS3s have both a quark mantle and a nuclear crust on top of a nuclear matter core. For all types of stars constructed, we determine not only their mass-radius relations, but also tidal deformabilities, Love numbers, as well as moments of inertia and the mass distribution. We find that there exists a range of parameter values in our model, for which the novel hybrid stars have properties in very good agreement with all existing bounds on the stationary properties of compact stars. In particular, the tidal deformabilities of these solutions are smaller than those of ordinary neutron stars of the same mass, implying that they provide an excellent fit to the recent gravitational wave data GW170817 of LIGO and Virgo. The assumptions underlying the viability of the different star types, in particular those corresponding to absolutely stable quark matter, are finally discussed at some length.
Gravitational waves from coalescences of neutron stars or stellar-mass black holes into intermediate-mass black holes (IMBHs) of $gtrsim 100$ solar masses represent one of the exciting possible sources for advanced gravitational-wave detectors. These sources can provide definitive evidence for the existence of IMBHs, probe globular-cluster dynamics, and potentially serve as tests of general relativity. We analyse the accuracy with which we can measure the masses and spins of the IMBH and its companion in intermediate-mass ratio coalescences. We find that we can identify an IMBH with a mass above $100 ~ M_odot$ with $95%$ confidence provided the massive body exceeds $130 ~ M_odot$. For source masses above $sim200 ~ M_odot$, the best measured parameter is the frequency of the quasi-normal ringdown. Consequently, the total mass is measured better than the chirp mass for massive binaries, but the total mass is still partly degenerate with spin, which cannot be accurately measured. Low-frequency detector sensitivity is particularly important for massive sources, since sensitivity to the inspiral phase is critical for measuring the mass of the stellar-mass companion. We show that we can accurately infer source parameters for cosmologically redshifted signals by applying appropriate corrections. We investigate the impact of uncertainty in the model gravitational waveforms and conclude that our main results are likely robust to systematics.
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