NinjaSat detection of a Type I X-ray burst from EXO 0748-676

ATel #16678; A. Aoyama, S. Watanabe, S. Iwata, N. Ota, T. Takahashi, K. Yamasaki, T. Takeda (TUS/RIKEN), T. Tamagawa (RIKEN/TUS), T. Enoto (Kyoto U./RIKEN), T. Kitaguchi (RIKEN), W. Iwakiri (Chiba U.), Y. Kato (RIKEN), C.-P. Hu (NCUE), T. Mihara (RIKEN), and the NinjaSat team
on 28 Jun 2024; 15:19 UT
Credential Certification: Wataru Iwakiri (iwakiri@chiba-u.jp)

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The neutron star low-mass X-ray binary EXO 0748-676 had been in a quiescent state since 2008. On 2024-06-10 at 07:36 UT, Swift/BAT detected a type-I X-ray burst (GCN#26653, ATel#16655). Optical observations have confirmed that this object has entered the outburst phase (ATel#16646, #16648, #16649). MAXI also detected another type-I X-ray burst on 2024-06-13 at 19:23 UT (ATel#16654). NuSTAR observed eclipses and a type I X-ray burst (ATel#16673).
NinjaSat intermittently observed EXO 0748-676 from 2024-06-13 to 2024-06-23 with a total exposure of 56 ks, and detected one X-ray burst at 2024-06-23 08:58 UT (hereafter the time, T0). The burst duration was about 80 seconds. The peak flux of the burst was approximately 1 Crab in 2-30 keV. When we fit the decay phase of the burst by a simple exponential decay, there was an excess component from T0+30 s to T0+80 s. During the observation period of NinjaSat, the average persistent flux of EXO 0748-676 was approximately 3 mCrab.
The following link is the 5-s binned burst profile after subtracting the background and negligible persistent X-ray emission as the offset, and shown in the Crab unit.
http://maxi.riken.jp/plot/NinjaSat_EXO0748.png

NinjaSat is a 6U-size X-ray CubeSat launched on 2023-11-11 and operated by RIKEN. See the references in ATel#16495 for more information.

NinjaSat lightcurve of the X-ray burst from EXO 0748-676