Spectro-temporal evolution of 4U 1702-429 using AstroSat-NICER

TL;DR

This study analyzes the spectral and timing behavior of the neutron star source 4U 1702-429 using AstroSat and NICER data, revealing correlations between corona size, disk truncation, and kHz QPO presence.

Contribution

It provides the first detailed broadband spectral and timing analysis of 4U 1702-429 with simultaneous AstroSat and NICER observations, linking QPOs to corona and disk properties.

Findings

kHz QPO at 800 Hz detected in one observation

Corona size estimated to be ≤ 30 km

Disk truncation varies between observations

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 black body seed photon source which 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 ($\sim 7$ keV) is smaller than the second ($\sim 13$ keV), and the disk is truncated at a larger radius, $\sim 150$ km, compared to the second, $\sim 25$ km, for an assumed distance of 7 kpc. A kHz QPO at $\sim 800$ Hz is detected in the first and is absent in the second observation. Modeling the energy-dependent r.m.s and time lag of the kHz QPO reveals a corona size of $\leq$ 30 km. A similar model can explain the energy dependence of the broadband noise at $\sim 10$ 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.