Growth rate of the tidal p-mode g-mode instability in coalescing binary neutron stars

We recently described an instability due to the nonlinear coupling of p-modes to g-modes and, as an application, we studied the stability of the tide in coalescing binary neutron stars. Although we found that the tide is p-g unstable early in the inspiral and rapidly drives modes to large energies, our analysis only accounted for three-mode interactions. Venumadhav, Zimmerman, and Hirata showed that four-mode interactions must also be accounted for as they enter into the analysis at the same order. They found a near-exact cancellation between three- and four-mode interactions and concluded that while the tide in binary neutron stars can be p-g unstable, the growth rates are not fast enough to impact the gravitational wave signal. Their analysis assumes that the linear tide is incompressible, which is true of the static linear tide (the m=0 harmonic) but not the non-static linear tide (m=+/- 2). Here we account for the compressibility of the non-static linear tide and find that the three- and four-mode interactions no longer cancel. As a result, we find that the instability can rapidly drive modes to significant energies (there is time for several dozen e-foldings of growth before the binary merges). We also show that linear damping interferes with the cancellation and may further enhance the p-g growth rates. The early onset of the instability (at gravitational wave frequencies near 50 Hz), the rapid growth rates, and the large number of unstable modes (> 10^3), suggest that the instability could impact the phase evolution of gravitational waves from binary neutron stars. Assessing its impact will require an understanding of how the instability saturates and is left to future work.Comment: 28 pages, 14 figures, matches version published in Ap

Super-Eddington winds from Type I X-ray bursts

We present hydrodynamic simulations of spherically symmetric super-Eddington winds from radius-expansion type I X-ray bursts. Previous studies assumed a steady-state wind and treated the mass-loss rate as a free parameter. Using MESA, we follow the multi-zone time-dependent burning, the convective and radiative heating of the atmosphere during the burst rise, and the launch and evolution of the optically thick radiation-driven wind as the photosphere expands outward to radii $r_{\rm ph} \gtrsim 100\text{ km}$. We focus on neutron stars (NSs) accreting pure helium and study bursts over a range of ignition depths. We find that the wind ejects $\approx 0.2\%$ of the accreted layer, nearly independent of ignition depth. This implies that $\approx 30\%$ of the nuclear energy release is used to unbind matter from the NS surface. We show that ashes of nuclear burning are ejected in the wind and dominate the wind composition for bursts that ignite at column depths $\gtrsim 10^9\text{ g cm}^{-2}$. The ejecta are composed primarily of elements with mass numbers $A> 40$, which we find should imprint photoionization edges on the burst spectra. Evidence of heavy-element edges has been reported in the spectra of strong, radius-expansion bursts. We find that after $\approx 1\text{ s}$ the wind composition transitions from mostly light elements ($^4$He and $^{12}$C), which sit at the top of the atmosphere, to mostly heavy elements ($A>40$), which sit deeper down. This may explain why the photospheric radii of all superexpansion bursts show a transition after $\approx 1\text{ s}$ from a superexpansion ($r_{\rm ph}>10^3\text{ km}$) to a moderate expansion ($r_{\rm ph}\sim 50\text{ km}$).Comment: 13 pages, 13 figures. Matches the version published in Ap

Weak Gravitational Lensing by Dark Clusters

We calculate the abundance of dark-matter concentrations that are sufficiently overdense to produce a detectable weak-gravitational-lensing signal. Most of these overdensities are virialized halos containing identifiable X-ray and/or optical clusters. However, a significant fraction are nonvirialized overdensities still in the process of gravitational collapse--these should produce significantly weaker or no X-ray emission. Our predicted abundance of such dark clusters are consistent with the abundance implied by the Erben et al. (2000) detection of a dark lens. Weak lensing by these nonvirialized objects will need to be considered when determining cosmological parameters with the lens abundance in future weak-lensing surveys. Such weak lenses should also help shed light on the process of cluster formation.Comment: 18 pages, 11 figures; a few sentences and a figure added, conclusions unchanged, published in MNRA

Ultraviolet Divergences in Cosmological Correlations

A method is developed for dealing with ultraviolet divergences in calculations of cosmological correlations, which does not depend on dimensional regularization. An extended version of the WKB approximation is used to analyze the divergences in these calculations, and these divergences are controlled by the introduction of Pauli--Villars regulator fields. This approach is illustrated in the theory of a scalar field with arbitrary self-interactions in a fixed flat-space Robertson--Walker metric with arbitrary scale factor $a(t)$. Explicit formulas are given for the counterterms needed to cancel all dependence on the regulator properties, and an explicit prescription is given for calculating finite regulator-independent correlation functions. The possibility of infrared divergences in this theory is briefly considered.Comment: References added on various regularization methods. Improved discussion of further issues. 26 pages, 1 figur

Einstein-Podolsky-Rosen correlations in a hybrid system

We calculate the relativistic correlation function for a hybrid system of a photon and a Dirac-particle. Such a system can be produced in decay of another spin-1/2 fermion. We show, that the relativistic correlation function, which depends on particle momenta, may have local extrema for fermion velocity of order 0.5 c. This influences the degree of violation of CHSH inequality.Comment: 9 pages, 6 figure

Nonlinear dynamical tides in white dwarf binaries

Compact white dwarf (WD) binaries are important sources for space-based gravitational-wave (GW) observatories, and an increasing number of them are being identified by surveys like ZTF. We study the effects of nonlinear dynamical tides in such binaries. We focus on the global three-mode parametric instability and show that it has a much lower threshold energy than the local wave-breaking condition studied previously. By integrating networks of coupled modes, we calculate the tidal dissipation rate as a function of orbital period. We construct phenomenological models that match these numerical results and use them to evaluate the spin and luminosity evolution of a WD binary. While in linear theory the WD's spin frequency can lock to the orbital frequency, we find that such a lock cannot be maintained when nonlinear effects are taken into account. Instead, as the orbit decays, the spin and orbit go in and out of synchronization. Each time they go out of synchronization, there is a brief but significant dip in the tidal heating rate. While most WDs in compact binaries should have luminosities that are similar to previous traveling-wave estimates, a few percent should be about ten times dimmer because they reside in heating rate dips. This offers a potential explanation for the low luminosity of the CO WD in J0651. Lastly, we consider the impact of tides on the GW signal and show that LISA and TianGO can constrain the WD's moment of inertia to better than 1% for deci-Hz systems.Comment: 21 pages, 18 figures. Submitted to MNRA

Quantum Corrections in Quintessence Models

We investigate the impact of quantum fluctuations on a light rolling quintessence field from three different sources, namely, from a coupling to the standard model and dark matter, from its self-couplings and from its coupling to gravity. We derive bounds for time-varying masses from the change of vacuum energy, finding \Delta m_e/m_e << 10^{-11} for the electron and \Delta m_p/m_p << 10^{-15} for the proton since redshift z~2, whereas the neutrino masses could change of order one. Mass-varying dark matter is also constrained. Next, the self-interactions are investigated. For inverse power law potentials, the effective potential does not become infinitely large at small field values, but saturates at a finite maximal value. We discuss implications for cosmology. Finally, we show that one-loop corrections induce non-minimal gravitational couplings involving arbitrarily high powers of the curvature scalar R, indicating that quintessence entails modified gravity effects.Comment: 10 pages + appendix, added reference