Return of the clocked burster GS 1826-238

ATel #17245; S. Iwata, N. Ota, T. Kita, K. Yamasaki, S. Tsuchiya, Y. Nakano, M. Ichibakase, T. Takeda, A. Aoyama, T. Takahashi (TUS/RIKEN), T. Tamagawa (RIKEN/TUS), T. Enoto (Kyoto U./RIKEN), W. Iwakiri (Chiba U.), T. Kitaguchi (RIKEN), C.-P. Hu (NCUE), T. Mihara (RIKEN), and the NinjaSat team
on 25 Jun 2025; 10:32 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

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NinjaSat observed the neutron star low-mass X-ray binary GS 1826-238 from 2025-06-23 13:05 UT to 2025-06-24 13:53 UT with a total exposure of 15.3 ks, and detected six Type-I X-ray bursts. The intervals between the first five bursts were nearly constant at approximately 1.6 hours, suggesting that GS 1826-238 may have entered a clocked burster state for the first time in 10 years. This is the shortest burst recurrence period ever measured for GS 1826-238 in its clocked burster state.

GS 1826-238 remained in the hard state from 1996 until 2014. Between 1997 and 2007, it exhibited a clocked burster state with burst recurrence times of 3 to 6 hours, as reported by Thompson et al. (ApJ 681, 506, 2008), earning it the nickname "textbook burster". Around 2013, the source gradually brightened and transitioned to the soft state after 2014 (Fig. 12 in Asai et al., PASJ 74, 974, 2022). According to observations by Swift/BAT and MAXI, the intensity of GS 1826-238 gradually decreased since 2025-05-25, and transitioned back to the hard state on 2025-06-11. MAXI detected an X-ray burst from the source at 2025-06-20 14:37 UT.

The burst onset times in MJD detected by NinjaSat are as follows:
60849.5464
60849.6135
60849.6805
60849.7474
60849.8125
60850.1932

Follow-up observations are encouraged, as GS 1826-238 is expected to remain in the hard state, with further clocked X-ray bursts.

NinjaSat is a 6U-size X-ray CubeSat with Xe-filled proportional counters covering the energy range of 2-50 keV, focusing on agile observations of bright X-ray compact sources (Tamagawa et al., PASJ 77, 466, 2025).