MAXI/GSC detection of short-term X-ray enhancement from a possible new X-ray transient MAXI J1724-298

ATel #12117; H. Negoro, W. Maruyama (Nihon U.), T. Mihara (RIKEN), N. Kawai (Tokyo Tech), M. Serino (AGU), M. Nakajima, A. Sakamaki, M. Aoki, K. Kobayashi (Nihon U.), S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), M. Sugizaki, Y. Tachibana, K. Morita (Tokyo Tech), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai, T. Midooka (JAXA), Y. Ueda, A. Tanimoto, T. Morita, S. Yamada, S. Ogawa (Kyoto U.), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo 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 16 Oct 2018; 14:07 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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MAXI/GSC detected a bright X-ray-burst-like event from the Galactic center region in the scan transit at 17:18 UT on 2018 October 14. Assuming that the source flux was constant over the transit, we obtain the source position at

(R.A., Dec) = (265.734 deg, -29.891 deg) = (17 42 56, -29 53 27) (J2000)
equivalently
(l, b) = (358.880 deg, -0.001 deg) = (358 52 49.2, -00 00 04)

with a statistical 90% C.L. elliptical error region with long and short radii of 0.38 deg and 0.19 deg, respectively. The roll angle of the long axis from the north direction is 59.0 deg counterclockwise. These errors are for the J2000 equatorial coordinate system. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 297 +- 38 mCrab (4.0-10.0keV, 1 sigma error).

Without assumptions on the source constancy, we obtain a rectangular error box for the transient source with the following corners:

(R.A., Dec) = (265.477, -28.919) deg = (17 41 54, -28 55 08) (J2000)
(R.A., Dec) = (264.748, -29.359) deg = (17 38 59, -29 21 32) (J2000)
(R.A., Dec) = (265.706, -30.556) deg = (17 42 49, -30 33 21) (J2000)
(R.A., Dec) = (266.439, -30.110) deg = (17 45 45, -30 06 35) (J2000)

There was no significant excess flux in the previous transit at UT 15:45 and in the next transit at 18:50 UT with an upper limit of 20 mCrab for each.

Because of the Galactic center region, there are a number of sources inside the error box. Our bright and transient X-ray source catalogue (see Negoro et al. 2016, PASJ, 68, S1 in detail) contains three sources in the error box, the black hole candidate 1E 1740.7-2942 (laying 0.26 deg from the above source position), the dim X-ray transient Swift J174444.9-295042 (0.40 deg; see Bandyopadhyay et al. ATel #4111), and the possible black hole transient EXS 1737.9-2952 (0.40 deg; Durouchoux et al. 1998, ApJ 507, 781). If this transient activity is really an X-ray burst (see below), it may be suggested that Swift J1744 is a neutron star and gave rise to this burst. We, however, tentatively name the source MAXI J1724-298 until the identification.

Preliminary spectral analysis shows the GSC spectrum of this event can be represented by an absorbed blackbody model with a temperature of 1.6 +/- 0.4 keV and a column density nH of (1.8 +/- 1.6)e22 atoms/cm^2. The absorbed and unabsorbed 2-10 keV fluxes are 5.4e-9 erg/cm^2/s and 1.1e-8 erg/cm^2/s, respectively. Thus, the spectrum is a typical spectrum of X-ray bursts. Significant time variations were, however, not recognized during the scan transit for about 100 sec. This may indicate that we observed a tail of a relatively long X-ray burst. The upper limit of the source distance calculating the above flux on the assumption of the Eddington luminosity of a neutron star with 1.4 solar mass is 10.2 kpc.

Followup observations are encouraged.

Information on MAXI J1742-298