36th Annual Report (2023-24)

25 waveform models. The absence of lensing in the models can lead to a false deviation in GR tests using lensed signals. We restricted our results f o r t he I MR c ons i s t enc y t e s t and parameterised tests. [arXiv 2311.08446] Addressing issues in defining the Love numbers for blackholes: In their recent work [Phys. Rev. D 108, 084013 (2023); arXiv:2306.13627 [gr-qc]], Rajendra Prasad Bhatt, Sumanta Chakraborty, and Sukanta Bose presented an analytic method for calculating the tidal response function of non-rotating and slowly rotating black holes from the Teukolsky equation in the small frequency and the near horizon limit. They also pointed out the two possible ways to define the tidal Love numbers and dissipative part from the tidal response function (in relativistic context), and calculated them for non- rotating and slowly rotating black holes. They also discussed a procedure to calculate the tidal response function and consequently the tidal Love numbers for an arbitrary rotating black hole. For future work, they proposed an interesting exercise of finding the effect of the tidal response function of a black hole (or compact objects in general) to the inspiral part of the gravitational wave waveform fromWeyl scalar. Which will allow us to directly relate the tidal Love numbers and the dissipative part to the gravitational wave signal. Constraining Neutron Star Equations of State with Gravitational Waves: Neutron stars are among the densest objects in the Universe and serve as astrophysical laboratories for testing properties of extremematter. In a series of recent investigations, information frommulti-disciplinary physics (nuclear theory at low densities, heavy-ion data at intermediate densities, electromagnetic + gravitational wave astrophysical data at high densities) was imposed on the composition of the neutron star interior for ¬ ¬ ExtremeMatter 1. nucleonic matter only [S. Ghosh, D. Chatterjee+ EPJA 58,37 (2022)] 2. matter composed of strange baryons (“hyperons”) [S. Ghosh, B.K. Pradhan, D Chatterjee+ FrASS 9,864294 (2022)] 3. matter with phase transition to d e c o n f i n e d q u a r k s , t h e fundamental constituents of matter [S. Shirke, S. Ghosh, and D. Chatterjee ApJ 944, 7 (2023)] Effect of NS viscosity on the Gravitationalwaveandtidalheatingof NS in binary: The presence of hyperon matter in the NS interior can lead to a substantial effect heating the NS to temperatures of up to 1MeV by damping the tidally excited modes in binary and introduce an additional phase shift in the GW waveform which is relevant for upcoming next-generation detectors. This was shown by S. Ghosh, B.K. Pradhan and D. Chatterjee [Accepted in Phys. Rev. D (2024)]. Neutron Star oscillation modes to study NS interior as well as effects on GWs: The effect of the Neutron star interior composition on fluid oscillation modes is being studied extensively. Using future GW detection from unstable oscillation modes, the NS interior models can be constrained. Exotic matter in NSs can lead to changes in oscillation modes which impact the observed GW frequencies. The effect of hyperons on f-mode oscillations of neutron stars was studied by B. K. Pradhan & D. Chatterjee [Phys. Rev. C 103, 035810 (2021)] using Cowling approximation. This was extended to include general-relativistic effects in B. K. Pradhan, D. Chatterjee, M. Lanoye and P. Jaikumar [Phys. Rev. C 106, 015805 (2022)]. The effect of the admixture of dark matter in NSs on these oscillation modes was then explored in detail in S. Shirke, B. K. Pradhan, D. Chatterjee, L. Sagunski, and J. Schaffner-Bielich [arXiv:2403.18740 (2024)]. Its impact on r-mode oscillations, interesting for continuous waves, was explored by S. Shirke, S. Ghosh, D. Chatterjee, L. Sagunski, and J. ¬ ¬ estimated the corresponding redshift of the GW events using two methods: the redshifted masses and a galaxy catalogue. Using the binary black hole (BBH) redshifted masses, they simultaneously inferred the source mass distribution and the Hubble constant. Their results show that the source mass distribution displays a peak around 34 M , followed by a drop- ☉ off. Assuming this mass scale does not evolve with the redshift results in a measurement of the Hubble constant, the H0 measurement from the binary neutron star GW170817 and its electromagnetic counterpart. In the second method the researchers associated each GW event with its probable host galaxy in a galaxy catalogue, statistically marginalising over the redshifts of each event’s potential hosts and could obtain tighter constraints, however those constraints can be strongly impacted by the assumption about the unknown mass distribution of the BBH sources. [Astrophys. J., 949, 76 (2023)] Expansion rate from Binary black holes: The statistical host identification method used to infer the expansion rate of the Universe with the help of binary black hole events suffers from galaxy catalogues whose incompleteness increases as we go to higher and higher redshifts, severely affecting the constraints. IUCAA researchers have developed methods to utilise the clustering of the visible population of brighter galaxies with the gravitational wave events in order to infer the Hubble constant from data at higher redshifts. [arXiv:2312.16305]. B i ases i n t es t s o f GR due t o microlensed signals: Anuj Mishra and Apratim Ganguly have developed a method to investigate how isolated microlensed GW signals can introduce biases in tests of general relativity due to the non-inclusion of lensing effects in ¬ ¬ Testing General Relativity (TGR)