Search

In this study, we undertake a spectral-timing analysis of the black hole X-ray binary source GRS 1915+105 using simultaneous observations carried out by AstroSat (Large Area X-ray Proportional Counter, LAXPC and Soft X-ray Telescope, SXT) and Neutron Star Interior Composition Explorer (NICER) in 2017. The source showed two flux levels (high and low), whose energy spectra can be described by the thermal comptonization of disk photons. The spectral parameters obtained by the joint fitting of SXT/LAXPC and NICER/LAXPC were consistent. The power density spectra from LAXPC and NICER revealed a broad, prominent feature at ∼2 Hz. The energy dependence of the fractional rms and time lag of this feature cannot be explained by only variations of coronal spectral parameters. Instead, a model where the coronal heating rate varies first and induces a change in the disk temperature and inner radius can explain the variation. Our results underline the importance of simultaneous observations by AstroSat and NICER and highlight the need for more sophisticated models to explain the spectral-temporal behavior of black hole systems.

We report simultaneous detection of twin kHz and ∼40 Hz quasiperiodic oscillations (QPOs) in the time-resolved analysis of the AstroSat/LAXPC observation of the neutron star low-mass X-ray binary, 4U 1728-34. The frequencies of the multiple sets of triplets are correlated with each other and are consistent with their identification as the orbital, periastron precession, and twice the nodal precession frequencies. The observed relations, along with the known spin of the neutron star, put constraints on the mass and the ratio of the moment of inertia to the mass of the neutron star to be ${M}_{\odot }^{* }=1.92\pm 0.01$ and ${I}_{45}/{M}_{\odot }^{* }=1.07\pm 0.01$ under the simplistic assumption that the metric is a Kerr one. We crudely estimate that the mass and moment of inertia values obtained may differ by about 1% and 5%, respectively, if a self-consistent metric is invoked. Using the Tolman–Oppenheimer–Volkoff equations for computing the moment of inertia of a neutron star in slow rotation approximation, having different equations of state, we find that the predicted values of neutron star parameters favor stiffer equations of state. We expect more stringent constraints would be obtained using a more detailed treatment, where the equation of state–dependent metric is used to compute the expected frequencies rather than the Kerr metric used here. The results provide insight into both the nature of these QPOs and the neutron star interior.