NinjaSat detection of X-ray bursts from SRGA J144459.2-604207

ATel #16495; T. Takeda, N. Ota, S. Watanabe, A. Aoyama, S. Iwata (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 1 Mar 2024; 07:18 UT
Credential Certification: Wataru Iwakiri (iwakiri@chiba-u.jp)

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SRGA J144459.2-604207 is a millisecond accreting X-ray pulsar reported by SRG/ART-XC on 2024 February 21 (ATel #16464) and later confirmed by MAXI (ATel #16469), Swift/XRT (ATel #16471), NICER (ATel #16474) and INTEGRAL (ATel #16485). Type I bursts were reported by NICER, Swift, MAXI, and INTEGRAL, and a quasi-periodic burst activity of about 1.69 hours was reported based on the INTEGRAL observations (ATel #16485).

The newly launched 6U-size X-ray CubeSat “NinjaSat” observed SRGA J144459.2-604207 on February 23 and started a monitoring campaign on February 26. NinjaSat detected three X-ray bursts at 4:45, 17:32 UT on February 27, and 16:28 UT on February 28. The duration of all three bursts was about 20 seconds. The timing information was tagged with GPS onboard the satellite. The X-ray intensities of these three bursts were roughly 1 Crab at the peak, although the calibration is ongoing. The burst timings observed by NinjaSat slightly deviated from the expected time derived from the quasi-clocked period (~1.69 hr) reported by INTEGRAL. As the persistent flux weakens, the burst interval appears to increase significantly from the extrapolated line. The NinjaSat team continues the monitoring campaign.

NinjaSat is a 6U-size X-ray CubeSat with Xe-filled proportional counters covering the energy range of 2-50 keV. It was launched to the sun-synchronous orbit on 2023 November 11 and operated by RIKEN. The satellite focuses on agile observations of bright X-ray compact sources. NinjaSat finished the satellite and payload commissioning on February 22, and SRGA J144459.2-604207 was the first science target. See the following papers for more information:
Proceeding of SPIE, Enoto et al., 114441V, 2020 https://doi.org/10.1117/12.2561152.
Proceedings of Small Satellite Conference 2023
Tamagawa et al., WIV-06 https://digitalcommons.usu.edu/smallsat/2023/all2023/33/
Takeda et al., WIII-01 https://digitalcommons.usu.edu/smallsat/2023/all2023/21/
Kato et al., WIII-08 https://digitalcommons.usu.edu/smallsat/2023/all2023/23/.