‘Bursts, beats, and beyond’: uncovering the landscape from accretion to ignition of 4U 1728–34 using AstroSat

Abstract

A comprehensive study on persistent and thermonuclear burst emission of 4U 1728–34, commonly known as ‘Slow Burster’, is performed using seven archival observations of AstroSat spanning from 2016 to 2019. The burst-free persistent spectra can be well fitted with a blackbody (bbody) and a powerlaw (powerlaw) components, with a powerlaw photon index (\(\Gamma \)) found to be \(\sim \)2, indicating the source was in ‘high/soft’ banana state or intermediate state. The time-averaged power density spectrum reveals the presence of twin kilohertz Quasi Periodic Oscillations (kHz QPOs) with centroid frequencies \(619\pm 10\) and \(965\pm 6\) Hz with a maximum fractional root mean squared amplitude of \(6.24\pm 1.31\)% at \(\sim \)16 keV. From the upper kHz QPO, we infer the magnetospheric disk radius to be \(\sim \)17 km, corresponding to a magnetic field strength of 0.35–\(1.27\times 10^7\) G. The burst spectral evolution indicates Photospheric Radius Expansion (PRE) in five bursts, yielding a touchdown radius of 3.1–5.47 km. These bursts reached near-Eddington luminosities, through which the distance of the source was calculated to be 5.18–5.21 kpc. Two of the bursts show coherent oscillations at 362.81–363.93 Hz. The presence of twin kHz QPOs and coherent Burst Oscillations allows us to provide two different estimates for the spin frequency of the Neutron Star in the system, for the first time using AstroSat.