MAXI/GSC detection of a possible new short X-ray transient MAXI J1216-655

ATel #13946; H. Negoro, K. Kobayashi (Nihon U.), K. Komachi (AGU), T. Mihara (RIKEN), M. Nakajima, M. Aoki, R. Takagi, K, Asakura, K, Seino (Nihon U.), C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano (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 (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team
on 15 Aug 2020; 11:50 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 faint uncatalogued X-ray transient source at 1:41 UT on 2020 August 9 in one-day time bins. Assuming that the source flux was constant over the transit, we obtain the source position at
(R.A., Dec) = (184.247 deg, -65.509 deg) = (12 16 59, -65 30 32) (J2000)
with a 90% C.L. statistical error of 0.47 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius). Since there is no bright catalogued source in the error region, we tentatively name the source MAXI J1216-655.

Because the high-mass X-ray binary 4U 1210-64 lies 0.75 arc-deg from the above position, we performed the PSF-fit image analysis (Morii et al. 2016, PASJ, 68, S11) to obtain light curves of both objects. The transient source is detectable only in the 4-10 keV band, and the source flux gradually increased on August 8 (MJD 59069), and decreased on August 9. The peak flux in the 4-10 keV band is 0.046 (0.020, -0.017) c/cm^2/s (39 mCrab) in the scan transit at 00:08 on Aug. 9, or 0.031 (+0.009, -0.008) c/cm^2/s (26 mCrab) in the four scan transits from 00:08 to 04:47 on Aug. 9. On the other hand, the 2-4 keV fluxes in the above periods are 0.004 (+0.012, -0.008) c/cm^2/s (4 mCrab) and 0.003 (+0.005, -0.004) c/cm^2/s, respectively. The one-day average 4-10 keV fluxes on Aug. 8 and Aug. 9 are 0.010 +/- 0.004 c/cm^2/s (9 mCrab) and 0.012 +/- 0.004 c/cm^2/s (10 mCrab), respectively. From August 10, it is difficult to see the transient source in X-ray images, and distinguish the source from 4U 1210-64.

The nature of the source is unknown. The short-lived outburst suggests that the source is a kind of a very fast X-ray transient (VFXT) like MAXI J1957+032 (Negoro et al. ATel #7504, also see Mata Sanchez et al. 2017, MNRAS, 468, 564; Beri et al. 2019, MNRAS, 486, 1620). The non-detection of soft X-rays may be due to absorption by accretion gas.

Trigger Information about MAX J1216-655