Probing the origin of the extended flaring branch of Z-type X-ray binaries GX 340+0 and GX 5-1 using AstroSat

TL;DR

This study uses AstroSat data to analyze the extended flaring branch in two Z-type X-ray binaries, revealing it is caused by rapid expansion and cooling of the boundary layer, with significant RRC features detected.

Contribution

First broadband spectral analysis of EFB in GX 340+0 and GX 5-1, identifying RRC components and clarifying the origin of EFB as boundary layer expansion.

Findings

Detection of RRC features during EFB but not FB.

Observation of flaring events during EFB dips.

Increase in blackbody radius during FB to EFB transition.

Abstract

`Z' type neutron star low-mass X-ray binaries typically show a `Z'-like three-branched track in their hardness intensity diagram. However, a few such `Z' sources show an additional branch known as the extended flaring branch (EFB). EFB has been poorly studied, and its origin is not known. It is thought to be an extension of the flaring branch (FB) or associated with Fe K$\alpha$ complex or an additional continuum due to the radiative recombination continuum (RRC) process. Using AstroSat observations, we have detected the EFB from two `Z' sources, GX 340+0 and GX 5-1, and performed a broadband spectral analysis in the 0.5-22 keV energy range. During EFB, both sources show the presence of a significant RRC component with absorption edges at $7.91^{+0.16}_{-0.15}$ keV and $8.10^{+0.16}_{-0.17}$ keV, respectively along with blackbody radiation and thermal Comptonisation. No signature of RRC was detected during the FB, which is adjoint to the EFB. No Fe K$\alpha$ complex is detected. Interestingly, inside EFB dips of GX 5-1, for the first time, we have detected flaring events of 30-60s, which can be modelled with a single blackbody radiation. During the FB to EFB transition, an increase in the blackbody radius by a factor of 1.5-2 is observed in both sources. Our analysis strongly suggests that EFB is not an extension of FB or caused by the Fe K$\alpha$ complex. Rather, it is caused by a sudden expansion of the hot, thermalised boundary layer and subsequent rapid cooling.