MAXI/GSC detection of the brightest transient from Terzan 1 in more than 13 years
ATel #16124; H. Negoro, M. Nakajima, K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, 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 (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 (Kyoto U.), M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC), and W. Iwakiri (Chiba U.)
on 10 Jul 2023; 17:12 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)
Subjects: X-ray, Neutron Star
The MAXI/GSC nova-alert system triggered on an X-ray transient positionally consistent with the globular cluster Terzan 1 several times since June 21, 2023. We found that X-ray count rates from the cluster on July 8 is 0.16 +/- 0.02 photons/cm2/s at 2-4 keV (~ 78 mCrab),
0.14 +/- 0.01 ph/cm2/s at 4-10 keV (~110 mCrab), and 0.029 +/- 0.008 ph/cm2/s at 10-20 keV (~ 73 mCrab), which are the highest count rates at least in 13 years, for the MAXI mission life. An absorbed 2-10 keV luminosity is about 1.2e37 erg/s (6.7 kpc distance). An X-ray burst was also detected during the scanning observation at 10:51 on July 8.
Long term MAXI light curves from Terzan 1 showed that the 4-10 keV base-line count rate has been increased from < 10 mCrab to 20-30 mCrab since around MJD 58950 (September 28, 2022). An outburst with a peak of 70-80 mCrab is recognized for MJD 60020-60050 (March 17-Aprill 16, 2023) as reported on March 21 (ATel #15955, #15957). The recent activity was also detected by LEIA in June (ATel #16108).
This transient is most likely to be the low-mass X-ray binary X1732-304 (a.k.a. XB 1732-304, 1RXS J173546.9-302859) (see ATel #13983; #13991). The globular cluster Terzan 1, however, contains 12 or 14 X-ray sources in a radius of 1.4 arcmin (Cackett et al. 2006, MNRAS, 369, 407). We tentatively name the source MAXI J1735-304b. Followup observations are encouraged.
MAXI GSC light curves of Terzan 1
