# The nature of the 4th track in GX 5-1: discovery of Fe XXVI RRC in massive flares

## TL;DR

This paper analyzes high-quality RXTE data of GX 5-1, revealing that the 4th track is a continuation of flaring caused by unstable nuclear burning, with detection of Fe XXVI RRC indicating complex nuclear processes.

## Contribution

It provides the first spectral evidence of Fe XXVI RRC in the flaring and 4th branch of GX 5-1, linking spectral features to nuclear burning processes.

## Key findings

- Detection of Fe XXVI RRC at 9.28 keV during flaring.
- The 4th branch is a continuation of the flaring process with decreasing accretion rate.
- Large volume of unstable nuclear burning around the neutron star was observed.

## Abstract

We present an explanation of the 4th branch of the Z-track based on analysis of high-quality RXTE data on the source GX 5-1. Spectral analysis shows that the physical evolution on the 4th track is a continuation of the flaring branch which we previously proposed consists of unstable nuclear burning of the accretion flow on the neutron star. In flaring there is a huge increase of the neutron star emission from a volume that increases to a radius of 21 km. The 4th branch is shown to consist of flaring under conditions that the mass accretion rate and thus the total source luminosity is falling. We detect strong emission on the flaring and 4th branches at energies between 7.8 - 9.4 keV inconsistent with origin as Fe K emission, which we suggest is the radiative recombination continua (RRC) of iron Fe XXVI at 9.28 keV and of lower states. Evolution of the emission takes place, the energy falling but the flux increasing strongly, consistent with production in the large volume of unstable nuclear burning around the neutron star which eventually cools.