Speaker
Description
We report the detection of three simultaneous quasi-periodic oscillations (QPOs) at ∼ 40 Hz, ∼ 800 Hz, and ∼ 1100 Hz, along with a broad feature at ∼ 150 Hz from the AstroSat-LAXPC observation of NS-LMXB 4U 1728-34. For the very first time, we obtain thirteen sets of QPO triplets in time-resolved power density spectra for a particular observation. These QPO triplet frequencies are found to evolve with time, showing a remarkable correlation with each other. Using the relativistic precession model, in which low-frequency QPO and lower kHz QPO frequencies are identified as twice the nodal precession and periastron precession frequencies, respectively, and upper kHz QPO frequency as orbital frequency, we determine a well-constrained mass and moment of
inertia of the neutron star to be $M^*_\odot = 1.92\pm 0.01$ and $I_{45}/M^*_\odot = 1.07\pm 0.01$, respectively. We also numerically compute the moment of inertia of a neutron star spinning at 300 Hz for ten different equations of state by solving the Tolman-Oppenheimer-Volkoff equations in a slow rotation approximation, and we find that the predicted values of neutron star parameters from QPO triplets favor a few stiffer equations of state.
