MAXI/GSC observations of the new X-ray transient SRGA J144459.2-604207

ATel #16469; T. Mihara (RIKEN), H. Negoro, M. Nakajima (Nihon U.), N. Nagashima (Chuo U.), K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Kawamuro, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto, E. Goto (Chuo U.), M. Shidatsu (Ehime U.), I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA), Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani, Y. Okada (Kyoto U.), M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC), W. Iwakiri (Chiba U.)
on 22 Feb 2024; 12:32 UT
Credential Certification: Tatehiro Mihara (mihara@crab.riken.jp)

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The bright X-ray transient SRGA J144459.2-604207 was discovered with SRG / Mikhail Pavlinsky ART-XC telescope (ATel #16464). Prior to this, the MAXI/GSC Nova-Alert system triggered on the source at 10:27 and 22:50 UT on February 17, 2024. Unfortunately, since the source region was contaminated by the particle events of the solar flares, the source was difficult to be recognized. MAXI/GSC data show that the 2-10 keV flux had been increasing since February 15, 2024 (MJD 60355), and reached 0.34 +/- 0.02 photons cm^-2 s^-1 (~100 mCrab) on Feb. 21. No spectral evolution was found so far. The GSC spectrum obtained during February 20 to 22 is roughly represented by a power-law with a photon index of 1.75 (+0.21, -0.20). The error is at the 90% confidence level. Although the slow-rise and the hard spectrum favor the hard state of the black hole or the X-ray binary pulsar, the nature of the source is still unknown. Further observations are encouraged.
MAXI data (3h update) : http://maxi.riken.jp/v7l3h/J1444-607/index.html