AstroSat’s Detection of QPO triplets: Contraining the neutron star parameters in 4U 1728-34

We report the detection of simultaneous QPO triplets, consisting of a low-frequency QPO at ~ 40 Hz and twin kHz QPOs at about 800 and 1000 Hz, from the AstroSat/LAXPC observation of 4U 1728-34. For the very first time, we report the detection of multiple sets of QPO triplets from a single observation in which the QPOs evolve with time, showing a remarkable correlation with each other and are consistent with their identification with the general-relativistic precession model. The observed correlation puts unprecedented constraints on the mass and moment of inertia of the neutron star to be M*⊙ = 1.91 ± 0.01 and I45/M*⊙ = 1.07 ± 0.01, respectively . We also estimate the mass and moment of inertia of neutron stars by solving the Tolman-Oppenheimer-Volkoff equations in conjunctions with the constraints from the Gravitational-wave event GW170817 and find that the parameters obtained from QPO observation favor relatively stiff equations of state (EOS) of the neutron star. Furthermore, the observed QPO frequencies show a tight correlation with the spectral with the spectral parameters of the source. The results provide insights into the nature of the QPOs and hold the promise of stringent constraints on the EOS of neutron stars.