MAXI/GSC detection of a new X-ray outburst from Swift J1922.7-1716

ATel #16451; M. Nakajima, H. Negoro, K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto, E. Goto (Chuo U.), M. Shidatsu (Ehime U.), I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA), Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani, Y. Okada (Kyoto U.), M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC), W. Iwakiri (Chiba U.)
on 15 Feb 2024; 07:06 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered an X-ray transient source on 2024-02-10. Assuming that the source fluxes were constant over the transits, we obtain the source position at
(R.A., Dec) = (290.595 deg, -17.484 deg) = (19 22 22, -17 29 02) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.67 deg and 0.44 deg, respectively. The roll angle of long axis from the north direction is 74.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). This position is consistent with the neutron star low-mass X-ray binary Swift J1922.7-1716 (Falanga et al. 2006; Degenaar et al. 2012). The averaged X-ray flux observed from 2024-02-09T12:42(UT) to 2024-02-11T09:51(UT) was 21 ± 3 mCrab (4.0-10.0keV, 1 sigma error). This is the second detection by MAXI/GSC since the outburst in 2011 (ATel #3548, #3567, #3740, #3741, #3742; GCN #12522, #12526).

The current outburst had started from 2024-02-08 (MJD 60348). The flux increase rate of 2.0 mCrab/day in the 2-4 keV band is obtained from the onset to the latest data. The 2-20 keV spectrum observed from 2024-02-06 to 2024-02-13 can be described with a power-law model with a photon index of 2.2^+1.1_-0.8 at a 90% confidence level. These obtained results are consistent with the beginning phase of the previous outburst in 2011. If the flux variation of the present outburst traces the last outburst behavior (ATel #3807), this event will last several months and a hard-to-soft state transition might be observed about 4 months later. Follow-up observations are encouraged.

The latest X-ray light-curve can be checked at the following pages;
MAXI ( http://maxi.riken.jp/v7l3h/J1922-172/index.html ), and
Swift/BAT ( https://swift.gsfc.nasa.gov/results/transients/SWIFTJ1922.7-1716/ ).