Return of the Clocked Burster: Exceptionally Short Recurrence Time in GS 1826-238

TL;DR

This paper reports the discovery of an exceptionally short 1.6-hour recurrence time for X-ray bursts in GS 1826-238, indicating unusual ignition conditions likely due to localized fuel accumulation on the neutron star surface.

Contribution

It presents the first observation of such a short recurrence time in GS 1826-238, revealing new insights into burst ignition physics and accretion behavior.

Findings

Detected 19 bursts during state transition with 14 in hard state

Identified a new epoch with 1.6-hour recurrence time, the shortest in this source

Suggests fuel accumulation on a smaller neutron star surface area causes the short recurrence

Abstract

We report the discovery of an exceptionally short burst recurrence time in the well-known clocked burster GS 1826$-$238, observed with the CubeSat X-ray observatory NinjaSat. In 2025 May, GS 1826$-$238 underwent a soft-to-hard state transition for the first time in 10 years. On June 23, NinjaSat began monitoring GS 1826$-$238 in the hard state and continued until it returned to a steady soft state. During this period, we detected 19 X-ray bursts: 14 during the hard state, 4 in the transitional state, and 1 in the soft state. In the hard state, we identified a new clocked bursting epoch, during which the burst recurrence time remained highly stable and unprecedentedly short among the clocked bursting phases of GS 1826$-$238, with $t_{\rm rec} = 1.603 \pm 0.040$ hr ($1\sigma$ error). Previous observations showed that the burst recurrence time in GS 1826$-$238 decreased with increasing mass accretion rate, reached its minimum value of $t_{\rm rec} \sim 3$ hr, and then increased again. The observed 1.6 hr recurrence time is therefore exceptionally short, indicating anomalous ignition conditions. We propose that this phenomenon reflects fuel accumulation over a smaller fraction of the neutron star surface, resulting in a higher local accretion rate compared to earlier epochs. This scenario naturally accounts for the exceptionally short recurrence time, together with the observed reductions during bursts in blackbody normalization (proportional to the emitting area) and fluence. We also discuss possible contributions from residual heat in the neutron star crust or the presence of an additional soft spectral component.