| dc.description.abstract |
In the first part of this thesis, we investigate some tidal phenomena in the pre-merger stage of coalescing binaries with at least one neutron star (NS) involved.
In particular, during the last few minutes of coalescences, the tidal field exerted by the companion can force the primary strongly to excite certain quasi-normal modes thus resulting in various observable effects.
Among other things, resonance of low frequency modes (e.g., $g$- and $i$-modes) may result in crustal fracture, whereby unleash the energy used to be stored in the cracked area, possibly constituting a pre-emission of short gamma-ray burst (SGRB) if the NS is highly magnetised. In particular, we find it possible to associate two pre-emissions of SGRB 090510 with the resonantly excited $g_1$- and $g_2$-modes. We present, in addition, that the inferred frequencies of these two $g$-modes provide a novel avenue to estimate the spin of the NS, which can be applied to any SGRB preceded by two or more precursors. This presumably, $g$-mode-related phenomenon can also benefit in constraining the equation of state (EOS) since the EOS candidates can be grouped in terms of $g$-mode frequency.
On the other hand, $f$-mode excitation accelerates the merger course, leading to a ``tidal plunge'' phase; thereby, a phase shift is rendered in the associated gravitational waveform, which dictates the evolutionary track of the binary. Although the adiabatic tide attributes much more to the phase shift than the dynamical ones if the NS rotates slowly, the situation for fast spinning stars is different: a few hundred radiants of shift may be rendered.
The second part of this thesis is dedicated to the study of the dynamics of compact objects, viz.~NSs and black holes (BHs), in alternative gravity theories in the strong gravity regime. In particular, we consider some theories involving scalar field(s) as additional mediator(s) of gravitational interaction such as the (multi-)scalar-tensor theory and scalar-Gauss-Bonnet theory. In the former theory, it can happen that the scalar field of static stars dies out in the power of $-2$ of the distance, suppressing the scalar dipole radiation thus not constrained by pulsar experiments. In addition, these solutions are of discrete topological types, characterised by topological charge.
For the zero charge configurations, we show that up to three stable stars exist for a certain range of central energy density, and the stability is lost right at the occurrence of the most massive (either scalarized or non-scalarized) star. Accretions may therefore bring a stable scalarized NS into an unstable state, where a descalarization would be triggered, generating the gravitational phase transition (PT). This novel kind of PT leads to a sudden shrink in size of the star, mimicking well the traditional, material PT. However, the former transition will be accompanied by scalar-induced gravitational waves that are absent in material PT. In addition to the accreting process, we consider the spherically-symmetric core collapse for the scalar-tensor and the scalar-Gauss-Bonnet theories. Although a scalarized BH is absent in the former theory due to no-hair reason, we can construct one in the latter theory. In particular, we numerically demonstrate scalarization in a remnant BH behind stellar collapse, giving a first example on the production channel for scalarized BHs in the scalar-Guass-Bonnet theory. The scalar-induced gravitational waves generated along with (de)scalarization in both theories are also discussed. |
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