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NinjaSat Follow-up Observation of the X-ray Transient in the Galactic Center Region

ATel #17009; S. Watanabe, A. Aoyama, T. Takeda, S. Iwata, N. Ota, K. Yamasaki, T. Takahashi, A. Jujo (TUS/RIKEN), T. Mihara (RIKEN), W. Iwakiri (Chiba U.), T. Tamagawa (RIKEN/TUS), T. Enoto (Kyoto U./RIKEN), T. Kitaguchi, Y. Kato (RIKEN), C.-P. Hu (NCUE), and the NinjaSat team
on 31 Jan 2025; 23:42 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

Subjects: X-ray, Neutron Star, Transient

Referred to by ATel #: 17010

An X-ray increase from the Galactic center (GC) region was detected with MAXI (ATel #16975, #16983) in January. The intensity was about 250 mCrab from 2025 January 13 to 15. Triggered by this, NinjaSat performed a follow-up observation of the GC region for the X-ray transient. The observation started at 10:47 UT on 2025 January 16 (60691.4 MJD) to January 23 (60698.0 MJD).

The 2-10 keV flux of the GC region was almost constant at ~150 mCrab level on 60691.4 - 60693.0 MJD (hereafter, the “on-time” period) and then dropped to ~ 120 mCrab (the off-time period). We used the spectrum of the off-time (post-outburst) period on MJD 60693.3 (2025 Jan. 18) - 60695.2 (2025 Jan. 20) as the background, which includes particle background, galactic and extragalactic diffuse X-rays, contamination sources, and quiescent level of the transient. The average X-ray flux of the GC transient determined by NinjaSat GMC1 during on-time was 4.6×10-10 erg/s/cm2 (22 mCrab) in the 2-10 keV, which was derived from the spectral fitting after subtractions of the off-time spectrum.

Preliminary spectral fitting in the 2-15 keV by the power-law model gives the column density NH=(6.9 (+4.8, -2.3))×1023 cm-2 and the photon index of 3.1 (+1.2, -0.7) with errors in 90% CL. The spectrum is almost consistent with the MAXI result (ATel #16983). The X-ray spectrum is also consistent with Swift J174540.2-290037 which appeared in 2016 May (ATel #9109), although the flux with NinjaSat is about 6.6 times larger than SWIFT J174540.2-290037. Swift J174610-290018 which outbursted recently in 2024 February could be another candidate (ATel #16481), although the photon index of 1.8 and flux (75 times weaker) are not consistent.

There was no pulsation nor QPO between 0.01 and 1000 Hz.

The NinjaSat team plans to continue monitoring this GC transient.

NinjaSat ( https://astro.riken.jp/ninjasat ) is the 6U-size CubeSat X-ray observatory launched on 2023 November and operated by RIKEN, Japan.