MAXI/GSC observes a hard-to-soft transition in MAXI J1803-298

ATel #14627; M. Shidatsu (Ehime U.), H. Negoro (Nihon U.), N. Kawai (Tokyo Tech), M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), R. Adachi, M. Niwano, R. Hosokawa (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 14 May 2021; 14:59 UT
Credential Certification: Megumi Shidatsu (shidatsu@kusastro.kyoto-u.ac.jp)

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We report the detection of a hard-to-soft state transition of the new X-ray transient MAXI J1803-298 (ATels #14587, #14588, #14591, #14594, #14597, #14598, #14601, #14602, #14606, #14607, #14613) with MAXI/GSC. MAXI was unable to observe the source for 8 days from MJD 59338 (May 4). On MJD 59346, the source returned to MAXI's field of view and its 1-day averaged intensity in 2-20 keV was ~340 mCrab, which was ~4 times higher than that 8 days before. Meantime, the hardness ratio between 4-10 keV and 2-4 keV decreased by ~3 times, suggesting spectral softening. The time averaged MAXI/GSC spectrum in MJD 59446 - MJD 59448 can be modeled by a strong disk blackbody component and a weak power-law tail with a photon index fixed at 2.0, with interstellar absorption. The inner disk radius (R_in) is strongly coupled with the column density (N_H), which is difficult to determine with the MAXI/GSC data alone. Allowing N_H to vary, we obtain T_in = 0.8 +/- 0.2 keV and R_in = 54^+56_-14 km (where D = 8kpc and and from the detection of dips (ATels #14606, #14609) i = 60 deg are assumed), but N_H is not well constrained (< 2e22 /cm2). When N_H is fixed at 3e21 /cm2 (estimated with NICER; ATel #14606) or 2e22 /cm2, the model gives T_in = 0.8 +/- 0.1 keV and R_in = 61^+26_-17 km, or T_in = 0.7 +/- 0.1 keV and R_in = 114^+49_-33 km, respectively, with similar fit qualities. Despite the large uncertainty, the inner radius is relatively large compared with those of the neutron star X-ray binaries, and hence the system is likely to host a black hole. Followup observations are encouraged.