Revealing the Spectral State Transition of the Clocked Burster, GS 1826-238, with NuSTAR StrayCats

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 catalog 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 the source from the island atoll state to a banana branch. We also present the light-curve 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 duration than those from the island/hard state and have a higher burst fluence. From our analysis, we note an increase in the mass accretion rate of the source, and a decrease in burst frequency with the transition.

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“…There are three other known sources that have exhibited clocked bursting behavior. The canonical Clocked Burster (GS 1826−238) had regular burst recurrence timescales between 3.6 and 5.7 hr since its outburst onset in 1988 (Ubertini et al 1999;Chenevez et al 2016), though its bursting activity has dramatically reduced upon its transition into the spectrally softer "banana" state in 2016 (Chenevez et al 2016;Yun et al 2023). The other known clocked burster, 1RXS J180408.9−342058, was found to display clocked bursting behavior in its intermediate spectral state, with a recurrence timescales of around 0.9 hr (Marino et al 2019b).…”