MAXI/GSC detection of renewed activity of the black hole candidate EXO 1846-031 after 34 years

ATel #12968; H. Negoro, M. Nakajima (Nihon U.), S. Sugita (AGU), R. Sasaki, W. Iwakiri (Chuo U.) T. Mihara (RIKEN), W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, H. Kawai, T. Sato (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Sugizaki, M. Oeda, K. Shiraishi (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, N. Isobe, R. Shimomukai, M. Tominaga (JAXA), Y. Ueda, A. Tanimoto, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, S. Iwahori, Y. Kurihara, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), and Y. Kawakubo (LSU)
on 31 Jul 2019; 04:15 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on a hard X-ray transient at 20:44 UT on July 23 (http://maxi.riken.jp/alert/novae/8686724981/8686724981.htm). We also received the "Alert" (Negoro+ 2016, PASJ, 68, S1) from the system triggered at 21:05 UT on July 30. The source position is consistent with those of the SFXT IGR J18483-0311 and EXO 1846-031, which are about 16 arcmin apart. We first regarded this flux increase as 18.54 days periodic activity from IGR J18483-0311 (Levine et al. 2006, ApJS, 196, 6; Negoro 2018, IAU Symposium S346, in printing). The 4-10 keV flux, however, reached about 100 mCrab on July 28, which exceeded the historically highest flux of IGR J18483-0311, and Swift-BAT data also showed a gradual increase of EXO 1846-031 since around July 23 (https://swift.gsfc.nasa.gov/results/transients/weak/EXO1846-031/). Thus, we conclude that EXO 1846-031 is in outburst. The 2-4 and 4-10 keV fluxes on July 30 are roughly 70 mCrab and 150 mCrab, respectively, implying that the source is still in a hard state.

EXO 1846-031 was discovered on April 3rd,1985 (Parmar and White 1985, IAUC 4051 , 1), and an ultrasoft energy component suggested the source to be a black hole binary (Parmar et al. 1 993, A&A, 279, 179). Any renewed activity has not been reported since the first discovery. We encourage followup observations of this historical black hole candidate.

MAXI data for EXO 1846-031 (and IGR J18483-0311)