MAXI/GSC observations: the black hole X-ray nova 4U 1543-47 is reaching the Eddington luminosity

ATel #14708; H. Negoro, M. Nakajima, K. Kobayashi (Nihon U.), T. Mihara (RIKEN), N. Kawai (Tokyo Tech), K. Asakura, K. Seino (Nihon U.), 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.), M. Shidatsu (Ehime U.), 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 Jun 2021; 11:08 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The black hole binary 4U 1543-47 started a new outburst on June 11, 2021 (Negoro et al. ATel #14701). MAXI/GSC data show that the 2-20 keV X-ray flux continues to increase, and the 2-4 keV, 4-10 keV, and 10-20 keV X-ray fluxes at 06:37 on June 14 are 24.0 +/- 0.4 photons/cm2/s (~ 11.0 Crab), 9.5 +/- 0.2 photons/cm2/s (~ 7.6 Crab), and 0.22 +/- 0.05 photons/cm2/s (~ 0.55 Crab), respectively.

Preliminary analysis shows that the GSC spectrum obtained in the scan transit at 19:46 on June 13 is still represented by an absorbed disk blackbody model with a fixed column density of 4e21, an inner disk temperature Tin of 1.17 +/- 0.02 keV, and a disk normalization of 9446 +/- 654, converting to an inner disk radius Rin of 75 km. Here, we assume the distance of 7.5 kpc and the disk inclination angle of 21 deg often referred (see e.g., Tetarenko et al. 2016, ApJS, 222, 15). We note that the inner disk radius is significantly larger than that on June 11, ~64 km (we reexamined disk parameters by taking a more suitable source region to extract an energy spectrum for such a bright source). The 2-10 keV flux is 1.96e-7 erg/cm2/s (~8.2 Crab). This is the brightest in the X-ray novae MAXI detected in 11 years.

4U 1543-47 exhibited an outburst almost every 10 years in 1971 (Matilsky et al. 1972, ApJ, 174, L53; Li et al. 1976, ApJ, 203, 187), 1983 (Kitamoto et al. 1984, PASJ, 36, 799), 1992 (Harmon et al. 1992, IAUC, #5504), and 2002 (Park et al. 2004, ApJ, 610, 378; Kalemci et al. 2005, ApJ, 622, 508). However, an outburst in around 2012 was not observed. It took 2-3 days to reach the flux maximum in the outbursts in 1983 and 2002. The 2-20 keV flux in this outburst almost reaches the peak, and about twice higher than those in the previous outbursts, 2-4 Crab above 3 keV (e.g., see Park et al. 2004) and about 6 Crab in the 1.5-3.75 keV band in 1983 (Kitamoto et al. 1984). The twice brightness of this outburst may be due to the absence of the outburst in around 2012.

We also note that the bolometric luminosity calculated from 4pi Rin^2 Tin^4 (Makishima et al. 1986, PASJ, 308, 635) and the above parameters is 1.34e39 erg/s, which is comparable to the Eddington luminosity for the 9.4 solar mass black hole, 1.18e39 erg/s. For the black hole mass, we refer 9.4+/-1.0 solar mass by Orosz (2002, IAUS 212). On the other hand, the bolometric luminosity in the 0.5-60 keV band assuming isotropic radiation is 2.37e39 erg/cm2/s.

Followup observations with multi-wavelengths are encouraged.

MAXI GSC data for 4U 1543-47