MAXI/GSC detection of a bright hard X-ray outburst probably from AX J1631.9-4752

ATel #12320; K. Kobayashi, W. Maruyama, H. Negoro (Nihon U.), S. Sugita, M. Serino (AGU), M. Nakajima, A. Sakamaki, M. Aoki, (Nihon U.), T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), T. Sakamoto, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita, T. Oeda, K. Shiraishi (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.)
on 21 Dec 2018; 13:59 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 bright hard X-ray transient source at 04:33 UT on December 21, 2018. Assuming that the source flux was constant over each scan transit at 04:33 and 06:06, we obtain the source position at
(R.A., Dec) = (247.770 deg, -47.920 deg) = (16 31 04, -47 55 12) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.17 deg and 0.17 deg, respectively. The roll angle of the long axis from the north direction is 18.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scans was 209 +- 27 mCrab (4.0-10.0keV, 1 sigma error).

The error region includes the 1309 second X-ray pulsar AX J1631.9-4752/IGR J16320-4751 (Sugizaki et al. 2001, ApJS, 134, 77; Rodriguez et al. 2003, A&A, 407, L41l; Lutovinov et al. 2005, A&A 433, L41). In previous observations, AX J1631.9-4752 showed hard energy spectra (e.g., Rodriguez et al. 2006, A&A, 366, 274), but the fluxes were below several tens of mCrab.

The first trigger with the alert system was at 18:25 on December 18, 2018. At that time, however, the source was near the edge of a detector and it was difficult to identify the source. The source went into an invisible sky area soon after that, and entered the field of view again on December 21. An interpolated light curve suggests that the source flux has been increasing since the first detection. On the other hand, the Swift/BAT light curve of AX J1631.9-4752 (https://swift.gsfc.nasa.gov/results/transients/AXJ1631.9-4752/) showed flare-like behavior on December 21.

Because of the brightest outburst never seen before from AX J1631.9-4752, and relatively poor spatial resolution of MAXI/GSC, we tentatively name the source MAXI J1631-479. Followup observations are encouraged though the source is relatively close to the Sun (~ 30 deg).

Information on the outburst of AX J1631.9-4752/MAXI J1631-479