"Bursts, Beats, and Beyond": Uncovering the landscape from accretion to ignition of 4U 1728-34 using AstroSat

TL;DR

This study analyzes AstroSat observations of 4U 1728-34, revealing its spectral states, kHz QPOs, burst oscillations, and estimating neutron star properties like magnetic field and spin frequency.

Contribution

First AstroSat-based analysis of 4U 1728-34 providing detailed spectral, timing, and burst oscillation insights, including neutron star spin and magnetic field estimates.

Findings

Detection of twin kHz QPOs at 619 and 965 Hz.

Observation of burst oscillations near 363 Hz.

Estimated neutron star magnetic field strength between 3.5 and 12.7 million Gauss.

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 2016-2019. The burst-free persistent spectra can be well fitted with a blackbody (bbody) and a powerlaw (powerlaw) components, with a powerlaw photon index (Gamma) was found to be ~2 indicating the source was in "high/soft" bananna 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+/-10 Hz and 965+/-6 Hz with a maximum fractional root mean squared amplitude of 6.24+/-1.31% at ~16 keV. From the upper kHz QPO, we infer the magnetospheric disk radius to be ~17 km, corresponding to a magnetic field strength of 0.35-1.27 * 10^7 G. The burst spectral evolution indicates Photospheric Radius Expansion (PRE) in five bursts, yeilding 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.