Thermonuclear X-ray bursts detected in Cyg X-2 using AstroSat/LAXPC

TL;DR

This study reports the detection and detailed analysis of five thermonuclear X-ray bursts from Cyg X-2 using AstroSat, revealing properties consistent with Helium fuelled flashes and providing insights into the source's accretion state.

Contribution

First detection of multiple thermonuclear X-ray bursts from Cyg X-2 with comprehensive spectral and timing analysis using AstroSat/LAXPC.

Findings

Bursts are weak with peak-to-persistent ratios < 3

Decay times around 2 seconds and fluences ~10^-8 erg/cm^2

No significant burst oscillations or QPOs detected

Abstract

We report the detection of 5 Type-1 thermonuclear X-ray bursts and one burst-like event in the neutron star LMXB source, Cyg X-2 using X-ray data obtained with the Large Area X-ray Proportional Counter (LAXPC) instrument on board AstroSat. We carry out an energy resolved burst profile analysis as well as time resolved spectral analysis for each of the bursts and characterize their properties. All bursts are weak with burst peak-to-persistent intensity ratios $<$ 3, decay times $\sim$2 s, and with fluences $\sim$1$\times$10$^{-8}$ ergs/cm$^2$, indicating that the observed bursts are Helium fuelled flashes. An evolution of the blackbody temperature and radius is also observed during each burst. We carry out a search for Burst Oscillations (BO) and derive upper limits to the rms fractional amplitude for BO (for all the bursts) to be $\sim$1\%. We also carried out search for Quasi Periodic Oscillations (QPOs) in the power density spectra and we obtain upper limits to the fractional rms amplitude as $\sim$3.4\% at frequencies close to $\sim$5.6 Hz. We further carry out spectral and timing analysis of the non-burst persistent emission along with a study of the hardness-intensity and colour-colour diagrams. Using results from our analysis we infer that during this observation in 2016, Cyg X-2 can be characterized as being in the early Flaring Branch (FB) with a puffed up accretion disk and a clumpy coronal structure while undergoing medium-to-high levels of accretion.