Follow-up analyses of the binary-neutron-star signals GW170817 and GW190425 by using post-Newtonian waveform models

We reanalyze the binary-neutron-star signals, GW170817 and GW190425, focusing on the inspiral regime to avoid uncertainties on waveform modeling in the postinspiral regime. We use post-Newtonian waveform models as templates, which are theoretically rigid and efficiently describe the inspiral regime. We study potential systematic difference in estimates of the binary tidal deformability $\tilde{\Lambda}$ by using different descriptions for the point-particle dynamics and tidal effects. We find that the estimates of $\tilde{\Lambda}$ show no significant systematic difference among three models for the point-particle parts: TF2, TF2g, and TF2+, when they employ the same tidal model. We compare different tidal descriptions given by different post-Newtonian orders in the tidal phase. Our results indicate that the estimates of $\tilde{\Lambda}$ slightly depend on the post-Newtonian order in the tidal phase and an increase in the tidal post-Newtonian order does not lead to a monotonic change in the estimate of $\tilde{\Lambda}$. We also compare the estimate of $\tilde{\Lambda}$ obtained by the post-Newtonian tidal model and numerical-relativity calibrated tidal models. We find that the post-Newtonian model gives slightly larger estimate of $\tilde{\Lambda}$ and wider posterior distribution than the numerical-relativity calibrated models. According to Bayesian model comparison, it is difficult to identify a preference among the post-Newtonian orders by relying on the GW170817 and GW190425 data. Our results indicate no preference among numerical-relativity calibrated tidal models over the post-Newtonian model. Additionally, we present constraints on equation-of-state models for neutron stars with the post-Newtonian model, which show that the GW170817 data disfavor less compact models, though they are slightly weaker constraints than the numerical-relativity calibrated tidal models.Comment: 18 pages, 9 figures, Accepted for publication in Physical Review

Searching for gravitational wave echoes from black hole binary events in the third observing run of LIGO, Virgo, and KAGRA collaborations

Gravitational wave echo signals have been proposed as evidence for the modification of the spacetime structure near the classical event horizon. These signals are expected to occur after the mergers of compact binaries as a sequence of weak pulse-like signals. Some studies have shown evidence of the echo signals from several binary black hole merger events. On the other hand, the other studies have shown the low significance of such signals from various events in the first, second and third observing runs (O1, O2 and O3). Our previous study also shows the low significance of echo signals from events in O1 and O2, though, we observe that more than half of the events have p-value smaller than 0.1 when the simply modeled waveform is used for the analysis. Since there are only nine events appropriate for this analysis in O1 and O2, it is necessary to analyze more events to evaluate the significance statistically. In this study, we search for echo signals from binary black hole events observed during O3 operated by LIGO, Virgo and KAGRA collaborations. We perform the template-based search by using two different models for echo signal templates: simply modeled one and physically motivated one. Our results show that the distributions of p-values for all events analyzed in this study are consistent with the noise distribution. This means that no significant echo signals are found for both models from O3 events.Comment: 11 page

Quasinormal modes of a massless charged scalar field on a small Reissner-Nordstr\"om-anti-de Sitter black hole

We investigate quasinormal modes of a massless charged scalar field on a small Reissner-Nordstr\"om-anti-de Sitter (RN-AdS) black hole both with analytical and numerical approaches. In the analytical approach, by using the small black hole approximation (r_+ << L), we obtain the quasinormal mode frequencies in the limit of r_+/L -> 0, where r_+ and L stand for the black hole event horizon radius and the AdS scale, respectively. We then show that the small RN-AdS black hole is unstable if its quasinormal modes satisfy the superradiance condition and that the instability condition of the RN-AdS black hole in the limit of r_+/L -> 0 is given by Q>(3/eL)Q_c, where Q, Q_c, and e are the charge of the black hole, the critical (maximum) charge of the black hole, and the charge of the scalar field, respectively. In the numerical approach, we calculate the quasinormal modes for the small RN-AdS black holes with r_+ << L and confirm that the RN-AdS black hole is unstable if its quasinormal modes satisfy the superradiance condition. Our numerical results show that the RN-AdS black holes with r_+ =0.2L, 0.1L, and 0.01L become unstable against scalar perturbations with eL=4 when the charge of the black hole satisfies Q > 0.8Q_c, 0.78Q_c, and 0.76Q_c, respectively.Comment: 13 pages, 11 figure