2025PASJ...77L..24T

The CubeSat X-ray observatory NinjaSat was launched on 2023 November 11 and has provided opportunities for agile and flexible monitoring of bright X-ray sources. On 2024 February 23, the NinjaSat team started long-term observation of the new X-ray source SRGA J144459.2-604207 as the first scientific target, which was discovered on 2024 February 21 and recognized as the sixth clocked X-ray burster. Our 25-day observation covered almost the entire decay of this outburst from two days after the peak at ${\sim}100\:$mCrab on February 23 until March 18 at a few mCrab level. The Gas Multiplier Counter on board NinjaSat successfully detected 12 Type I X-ray bursts with a typical burst duration of ${\sim}20\:$s, shorter than other clocked burster systems. As the persistent X-ray emission declined by a factor of five, X-ray bursts showed a notable change in its morphology: the rise time became shorter from $4.4(7)\:$s to $0.3(3)\:$s ($1\sigma$ errors), and the peak amplitude increased by $44\%$. The burst recurrence time $\Delta t_{\rm rec}$ also became longer, from 2 to $10\:$hr, following the relation of $\Delta t_{\rm rec} \propto F_{\rm per}^{-0.84}$, where $F_{\rm per}$ is the persistent X-ray flux, by applying a Markov chain Monte Carlo method. The short duration of bursts is explained by the He-enhanced composition of accretion matter and the relation between $\Delta t_{\rm {rec}}$ and $F_{\rm per}$ by a massive neutron star. This study demonstrated that CubeSat pointing observations can provide valuable astronomical X-ray data.