Spectro-temporal Evolution of 4U 1702-429 Using AstroSat-NICER

Abstract We present the broadband spectral and timing properties of the atoll source 4U 1702-429 using two observations of AstroSat with the second one having simultaneous NICER data. For both observations, the spectra can be represented by a Comptonizing medium with a blackbody seed photon source that can be identified with the surface of the neutron star. A disk emission along with a distant reflection is also required for both spectra. For the first observation, the coronal temperature (∼7 keV) is smaller than the second (∼13 keV), and the disk is truncated at a larger radius, ∼150 km, compared to the second, ∼25 km, for an assumed distance of 7 kpc. A kHz quasiperiodic oscillation (QPO) at ∼800 Hz is detected in the first and is absent in the second observation. Modeling the energy-dependent rms and time lag of the kHz QPO reveals a corona size of ≤30 km. A similar model can explain the energy dependence of the broadband noise at ∼10 Hz for the second observation. The results suggest that kHz QPOs are associated with a compact corona surrounding the neutron star and may occur when the disk is truncated at large distances. We emphasize the need for more wide-band observations of the source to confirm these results.