NinjaSat detection of MAXI J1752-457

ATel #16903; T. Takahashi, S. Watanabe, S. Iwata, N. Ota, K. Yamasaki, A. Jujo, T. Takeda, A. Aoyama (TUS/RIKEN), W. Iwakiri (Chiba U.), T. Tamagawa (RIKEN/TUS), T. Enoto (Kyoto U./RIKEN), T. Mihara, T. Kitaguchi, Y. Kato (RIKEN), C.-P. Hu (NCUE), and the NinjaSat team
on 12 Nov 2024; 12:00 UT
Credential Certification: Teruaki Enoto (teruaki.enoto@gmail.com)

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The new X-ray transient MAXI J1752-457 was discovered by MAXI on November 9, 2024 (ATel #16898). A further report (ATel #16902) noted that transient EP240809a (ATel #16765, #16767) was located within the MAXI error box. NinjaSat promptly started a ToO monitoring from November 10 at 12:53:55 UT (60624.54 MJD), approximately 2.5 hours after the ATel post of the MAXI discovery (ATel #16898).
The average X-ray flux with NinjaSat / GMC1 was 2.5×10^-10 erg/s/cm² (16 mCrab) in the 2-10 keV derived from the analysis after the background subtraction. The source flux gradually decreased from 30 mCrab to 6 mCrab between MJD 60624.6 and MJD 60625.4. There was no apparent strong pulsation in the 2-10 keV band and the frequency range of 0.1-1000 Hz.
We performed preliminary spectral fitting in the 2-20 keV band using the power-law and blackbody (bbodyrad in XSPEC) models. Since there is no apparent interstellar absorption in the soft band, we fixed the column density at NH=1×10²¹ cm^-2 using the value expected from the HI observations. The blackbody model is favored (chi2/d.o.f. = 83/79) over the power-law model (chi2/d.o.f=133/79, Photon index of about 2.5). The bbodyrad temperature is 1.05 (±0.04) keV (90% CL). The radius in the bbodyrad model is 3.9 (±1.5) × (D/8 kpc) km, where D is the distance to the source.
The NinjaSat team plans to continue this monitoring of MAXI J1752-457. The slightly large solar limitation angle at 40 degrees allows the NinjaSat to monitor this source at least until November 18, slightly later than other X-ray satellites.
NinjaSat is the 6U-size CubeSat X-ray observatory launched on 2023 November 11 and operated by RIKEN, Japan. It covers the 2-50 keV band. The NinjaSat webpage is https://astro.riken.jp/ninjasat . This ToO observation was performed on the first anniversary of NinjaSat.