Gravitational wave echoes from strange stars for various equations of state

The tentative Gravitational Wave Echo (GWE) at a frequency of about $72\,Hz$ has been recently claimed at $4.2\sigma$ significance level in the GW170817 event. GWEs can be used as a tool to study the characteristics of ultra-compact stellar objects. Considering the final ultra-compact, post-merger object as a strange star, the GWE frequency can be calculated. However, GWEs are observed for only those compact stellar structures whose compactness lies in between 0.33 and 0.44. Alternatively, GWE can be obtained for those compact stars which feature a photon sphere and compactness not crossing the Buchdahl's limit radius $R_{B}=9/4M$. A photon sphere is a surface located at $R=3M$, $R$ being the radius and $M$ is the total mass of the ultra-compact object. Recently using the simplest MIT Bag model Equation of State (EoS) it has been reported that strange stars can produce GWEs with frequencies of tens of kilohertz. In view of this, for a comparative study, we have calculated the respective echo frequencies associated with strange stars by considering three models of strange star EoSs, viz., MIT bag model, linear and polytropic EoSs \cite{JB}. We found that, not being too stiff the polytropic EoS can not emit GWE, whereas the MIT Bag model and the linear EoSs can emit GWEs at a frequency range of about tens of kilohertz. Also, GWE frequency increases with the increase in values of bag constant $B$ and decreases with the increasing values of linear constant $b$. So a model-dependent nature of GWE frequencies is observed.Comment: 3 pages, 1 figur

Quasinormal Modes and Optical Properties of 4-D black holes in Einstein Power-Yang-Mills Gravity

This paper explores the impact of the Yang-Mills charge parameter and the exponent term on a $4$D black hole solution in the Einstein Power-Yang-Mills theory. Through an investigation of the massless scalar quasinormal mode spectrum, black hole shadow, and emission rate, we have determined that the effects of these two parameters are opposite. Specifically, the Yang-Mills charge parameter causes an increase in the real quasinormal frequencies with a correspondingly smaller damping rate. It also results in a smaller black hole shadow and a lower evaporation rate.Comment: 13 pages, 9 figure

Impact of energy-momentum conservation violation on the configuration of compact stars and their GW echoes

This work investigates the impacts of energy-momentum conservation violation on the configuration of strange stars constraint with gravitational wave (GW) event GW190814 as well as eight recent observations of compact objects. The gravitational wave echoes from these interesting classes of compact objects are also calculated. To describe the matter of strange stars, we have used two different equations of state (EoSs): first an ad-hoc exotic EoS, the stiffer MIT Bag model and next realistic CFL phase of quark matter EoS. We choose Rastall gravity as a simple model with energy-momentum conservation violation with a set of model parameter values. Our results show that this gravity theory permits stable solutions of strange stars and the resulting structures can foster GW echoes. We illustrate the implication of the gravity theory and found that the negative values of the Rastall parameter result in more compact stellar configurations and lower GW echo frequency. With an increase in the Rastall parameter, both the compactness of the stellar configurations and echo time decrease. It is worth mentioning here that with the chosen set of some probable strange star candidates from observational data and also in light of GW 190814, we have evaluated the radii of stellar models. Also, the GW echo frequencies associated with strange stars are found to be in the range of $\approx 41-58$ kHz for both cases.Comment: 8 figures and 4 table

Deflection angle and quasinormal modes of a de Sitter black hole in f(T,B)f(\mathcal{T}, \mathcal{B}) gravity

This work is intended to investigate the influence of the boundary term on the bending angle of light for a static spherically symmetric black hole in $f(\mathcal{T}, \mathcal{B})$ modified gravity. To this end, we use the Ishihara et al. method which allows us to derive the deflection angle of light for an observer and source at finite distances from a lens object in a non-asymptotically flat spacetime. This method interprets the viewpoint of the receiver. The obtained deflection angle becomes divergent at far distances from the lens object, which is due to the non-asymptotically flat spacetime. However, the divergence of the deflection angle can be controlled with the boundary term parameter $c_0$. For small values of the parameter $c_0$ this divergence can be minimized within the finite range of the source and observer. We also calculate the quasinormal modes of axial gravitational perturbations in the background of the black hole using the Pad\'e averaged sixth order WKB approximation method. We observed that the boundary term of the model has notable influence on the quasinormal modes of the black hole. It is seen that for the physically perceptible quasinormal mode frequencies from the black hole, the value of the boundary term parameter $c_0$ should be less than $0.08$. This result in fact supports the outcome of our deflection angle analysis.Comment: 12 pages, 4 figure

Strange stars in f(R)f(\mathcal{R}) gravity Palatini formalism and gravitational wave echoes from them

The compact stars are promising candidates associated with the generation of gravitational waves (GWs). In this work, we study a special type of compact stars known as strange stars in the $f(\mathcal{R})$ gravity Palatini formalism. Here we consider three promising $f(\mathcal{R})$ gravity models viz., Starobinsky, Hu-Sawicki and Gogoi-Goswami models in the domain of MIT Bag model and linear equations of state (EoSs). We compute the stellar structures numerically and constrained the $f(\mathcal{R})$ model parameters with a set of probable strange star candidates. The study shows that the consideration of stiffer MIT Bag model and linear EoSs within a favourable set of $f(\mathcal{R})$ gravity model parameters may result in strange stars with sufficient compactness to produce echoes of GWs. Thus, we have computed the GWs echo frequencies and characteristic echo times for such stars. It is found that in compliance with the experimentally obtained possible strange star candidates, the obtained GW echo frequencies for all the models are in the range of $65 - 85$ kHz.Comment: 22 pages, 9 figure