Detectability of Continuous Gravitational Waves from Magnetically Deformed Neutron Stars

Neutron stars are known to contain extremely powerful magnetic fields. Their effect is to deform the shape of the star, leading to the potential emission of continuous gravitational waves. The magnetic deformation of neutron stars, however, depends on the geometry and strength of their internal magnetic field as well as on their composition, described by the equation of state. Unfortunately, both the configuration of the magnetic field and the equation of state of neutron stars are unknown, and assessing the detectability of continuous gravitational waves from neutron stars suffers from these uncertainties. Using our recent results relating the magnetic deformation of a neutron star to its mass and radius—based on models with realistic equations of state currently allowed by observational and nuclear physics constraints—and considering the Galactic pulsar population, we assess the detectability of continuous gravitational waves from pulsars in the galaxy by current and future gravitational waves detectors.

Hybrid Stars with Color Superconducting Cores in an Extended FCM Model

We investigate the influence of repulsive vector interactions and color superconductivity on the structure of neutron stars using an extended version of the field correlator method (FCM) for the description of quark matter. The hybrid equation of state is constructed using the Maxwell description, which assumes a sharp hadron-quark phase transition. The equation of state of hadronic matter is computed for a density-dependent relativistic lagrangian treated in the mean-field approximation, with parameters given by the SW4L nuclear model. This model described the interactions among baryons in terms of σ, ω, ρ, σ*, and ϕ mesons. Quark matter is assumed to be in either the CFL or the 2SC+s color superconducting phase. The possibility of sequential (hadron-quark, quark-quark) transitions in ultra-dense matter is investigated. Observed data related to massive pulsars, gravitational-wave events, and NICER are used to constrain the parameters of the extended FCM model. The successful equations of state are used to explore the mass-radius relationship, radii, and tidal deformabilities of hybrid stars. A special focus lies on investigating consequences that slow or fast conversions of quark-hadron matter have on the stability and the mass-radius relationship of hybrid stars. We find that if slow conversion should occur, a new branch of stable massive stars would exist whose members have radii that are up to 1.5 km smaller than those of conventional neutron stars of the same mass. Such objects could be possible candidates for the stellar high-mass object of the GW190425 binary system.

Constraints on the Time Variation of the Gravitational Constant Using Gravitational Wave Observations of Binary Neutron Stars

We propose a method to constrain the variation of the gravitational constant G with cosmic time using gravitational wave (GW) observations of merging binary neutron stars. The method essentially relies on the fact that the maximum and minimum allowed masses of neutron stars at a particular cosmic epoch have a simple dependence on the value of G at that epoch. GWs carry an imprint of the value of G at the time of the merger. Thus, if the value of G at merger is significantly different from its current value, the masses of the neutron stars inferred from the GW observations will be inconsistent with the theoretically allowed range. This enables us to place bounds on the variation of G between the merger epoch and the present epoch. Using the observation of the binary neutron star system GW170817, we constrain the fractional difference in G between the merger and the current epoch to be in the range -1≲ΔG/G≲8. Assuming a monotonic variation in G, this corresponds to a bound on the average rate of change of -7×10^{-9}  yr^{-1}≤G[over ˙]/G≤5×10^{-8}  yr^{-1} between these epochs. Future observations will put tight constraints on the deviation of G over vast cosmological epochs not probed by other observations.

Nuclear Pairing Gaps and Neutron Star Cooling

We study the cooling of isolated neutron stars with particular regard to the importance of nuclear pairing gaps. A microscopic nuclear equation of state derived in the Brueckner-Hartree-Fock approach is used together with compatible neutron and proton pairing gaps. We then study the effect of modifying the gaps on the final deduced neutron star mass distributions. We find that a consistent description of all current cooling data can be achieved and a reasonable neutron star mass distribution can be predicted employing the (slightly reduced by about 40%) proton 1S0 Bardeen-Cooper-Schrieffer (BCS) gaps and no neutron 3P2 pairing.

On the origin of GW190425

The LIGO/Virgo collaborations recently announced the detection of a binary neutron star merger, GW190425. The mass of GW190425 is significantly larger than the masses of Galactic double neutron stars known through radio astronomy. We hypothesize that GW190425 formed differently from Galactic double neutron stars, via unstable ‘case BB’ mass transfer. According to this hypothesis, the progenitor of GW190425 was a binary consisting of a neutron star and a ∼4–$5\, {\mathrm{ M}_\odot }$ helium star, which underwent common-envelope evolution. Following the supernova of the helium star, an eccentric double neutron star was formed, which merged in ${\lesssim }10\, {\rm Myr}$. The helium star progenitor may explain the unusually large mass of GW190425, while the short time to merger may explain why similar systems are not observed in radio. To test this hypothesis, we measure the eccentricity of GW190425 using publicly available LIGO/Virgo data. We constrain the eccentricity at $10\, {\rm Hz}$ to be e ≤ 0.007 with $90{{\ \rm per\ cent}}$ confidence. This provides no evidence for or against the unstable mass transfer scenario, because the binary is likely to have circularized to e ≲ 10−4 by the time it was detected. Future detectors will help to reveal the formation channel of mergers similar to GW190425 using eccentricity measurements.