Revealing the spectral state transition of the Clocked Burster, GS 1826-238 with NuSTAR StrayCats

TL;DR

This study analyzes long-term NuSTAR stray light data of the neutron star X-ray binary GS 1826-238, revealing its spectral state transition from island to banana, and correlates burst properties with accretion rate changes.

Contribution

First long-term analysis of GS 1826-238 using NuSTAR stray light data, demonstrating spectral state transition and its impact on burst characteristics.

Findings

Source transitions from island to banana state.

Shorter, more energetic bursts in the banana state.

Increase in mass accretion rate with state change.

Abstract

We present the long term analysis of GS 1826-238, a neutron star X-ray binary known as the "Clocked Burster", using data from NuSTAR StrayCats. StrayCats, a catalogue of NuSTAR stray light data, contains data from bright, off-axis X-ray sources that have not been focused by the NuSTAR optics. We obtained stray light observations of the source from 2014-2021, reduced and analyzed the data using nustar-gen-utils Python tools, demonstrating the transition of source from the "island" atoll state to a "banana" branch. We also present the lightcurve analysis of Type I X-Ray bursts from the Clocked Burster and show that the bursts from the banana/soft state are systematically shorter in durations than those from the island/hard state and have a higher burst fluence. From our analysis, we note an increase in mass accretion rate of the source, and a decrease in burst frequency with the transition.