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MAXI/GSC detection of a type-I X-ray burst from RX J1718.4-4029 or a new source

ATel #11907; W. Iwakiri,, Y. Tsuboi, R. Sasaki, H. Kawai, T. Sato (Chuo U.), H. Negoro, M. Nakajima, A. Sakamaki, W. Maruyama (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada (Kyoto U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.) report on behalf of the MAXI team
on 3 Aug 2018; 03:47 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)

Subjects: X-ray, Transient

The MAXI/GSC nova alert system triggered on a bright X-ray source at 22:05 UT on 2018 August 2. The spectrum of this scan transit was well reproduced by an absorbed blackbody model with a temperature of kT = 1.9 (+0.4, -0.3) keV (90% CL) and the hydrogen column density was 4.8 (-4.3, 7.2)x 1022 cm-2 (90% CL). The averaged X-ray flux in 2 - 10 keV band was 8.6(-1.4,+1.3) x 10-9ergs/cm2/s (90% CL). On the basis of the fitting results, this transient is probably a type-I X-ray burst.
Since the emission was detected only for a fraction of the triangular transit response, the uncertainty of its position along the scan direction is large. Without assuming the source constancy, we obtain a rectangular error box (90%C.L.) with the following corners:
(R.A., Dec) = (259.522, -39.888) deg = (17 18 05, -39 53 15) (J2000)
(R.A., Dec) = (259.363, -39.845) deg = (17 17 27, -39 50 40) (J2000)
(R.A., Dec) = (259.378, -42.084) deg = (17 17 30, -42 05 00) (J2000)
(R.A., Dec) = (259.543, -42.127) deg = (17 18 10, -42 07 35) (J2000).
The source was not detected significantly in the previous transit at 8/2 20:40 and in the next transit at 8/2 23:44 UT with an upper limit of 20 mCrab for each.
Since LMXB RX J1718.4-4029 located in the error region of the transient, we suggest that the transient is a type-I X-ray burst from RX J1718.4-4029 or a new source (we tentatively name the source MAXI J1718-409).