Frequency deviations in universal relations of isolated neutron stars and postmerger remnants

Abstract

We relate the fundamental quadrupolar fluid mode of isolated non-rotating NSs and the dominant oscillation frequency of neutron star merger remnants. Both frequencies individually are known to correlate with certain stellar parameters like radii or the tidal deformability, which we further investigate by constructing fit formulae and quantifying the scatter of the data points from those relations. Furthermore, we compare how individual data points deviate from the corresponding fit to all data points. Considering this point-to-point scatter we uncover a striking similarity between the frequency deviations of perturbative data for isolated NSs and of oscillation frequencies of rapidly rotating, hot, massive merger remnants. The correspondence of frequency deviations in these very different stellar systems points to an underlying mechanism and EoS information being encoded in the frequency deviation. We trace the frequency scatter back to deviations of the tidal Love number from an average tidal Love number for a given stellar compactness. Our results thus indicate a possibility to break the degeneracy between NS radii, tidal deformability and tidal Love number. We also relate frequency deviations to the derivative of the tidal deformability with respect to mass. Our findings generally highlight a possibility to improve GW asteroseismology relations where the systematic behavior of frequency deviations is employed to reduce the scatter in such relationships and consequently increase the measurement accuracy. In addition, we relate the f −mode frequency of static stars and the dominant GW frequency of merger remnants. We find an analytic mapping to connect the masses of both stellar systems, which yields particularly accurate mass-independent relations between both frequencies and between the postmerger frequency and the tidal deformability.

Impact of three-nucleon forces on gravitational wave emission from neutron stars

“…Exploring the post-merger signal is another way of gathering information on the f mode of static stars. This scenario consists of a rapidly evolving background spacetime, but even so the peak frequency of GW emission is related to the f mode [14,15] and moreover, we can establish relations between this frequency and the one of static stars [16][17][18]. In the next generation of GW detectors we would be able to detect the post-merger signal, which in turn would give us the possibility of constraining the f frequency of the inspiral phase, allowing to have another valuable information about the internal composition of NSs.…”

“…In these regimes, spacetime perturbations must be calculated with respect to a rapidly evolving background, but numerical simulations indicate the presence of high-frequency components related to the f mode [14,15]. In addition, it has been shown that preand post-merger features can be related [16][17][18]. The authors of Ref.…”

“…The authors of Ref. [17] have found an analytic mapping between the f -mode frequency of static stars and the dominant GW frequency of merger remnants, allowing to connect the masses in the two regimes.…”

Constraints on the maximum densities of neutron stars from postmerger gravitational waves with third-generation observations

“…In this Letter, we propose a new quasiuniversal relation connecting the NS maximum density to the peak PM frequency. The existence of a EOS-insensitive ρ TOV max (f 2 ) relation was suggested by previous work [29,30,41], but the relation obtained there involves the binary config- urations with the largest possible mass that does not promptly collapse to black hole. Here, by introducing an effective Keplerian radius associated to f 2 , we propose a more general approach that can be applied to any binary and delivers a strong constrain already with a single detection.…”

$f$-mode oscillations of compact stars in dynamical spacetimes: Equation of state dependencies and universal relations studies

“…They use the universal relations (UR) between 𝑓 -mode ( 2 𝑓 ) and tidal deformability (given as tidal coupling constant 𝜅 𝑇 2 ) given in Chan et al (2014) to discuss UR between 𝑓 𝑚𝑎𝑥 and 𝜅 𝑇 2 ). Frequency deviations in the universal relations of isolated neutron stars and postmerger remnants have been discussed by Lioutas et al (2021), whereas UR's for damping time have been carried out in Lioutas & Stergioulas (2018).…”