MAXI-GSC detection of a sudden drop in X-ray intensity from GX 13+1

ATel #6433; H. Negoro (Nihon U.), S. Nakahira (JAXA), M. Matsuoka (RIKEN), Y. Ueda (Kyoto U.), N. Kawai (Tokyo Tech), S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), M. Sugizaki, T. Mihara, M. Morii, M. Serino, J. Sugimoto, T. Takagi, A. Yoshikawa (RIKEN), T. Yoshii, Y. Tachibana (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, D. Uchida (Osaka U.), M. Nakajima, K. Fukushima, T. Onodera, K. Suzuki, T. Namba, M. Fujita, F. Honda (Nihon U.), M. Shidatsu, T. Kawamuro, T. Hori (Kyoto U.), Y. Tsuboi, A. Kawagoe (Chuo U.), M. Yamauchi, Y. Morooka D. Itoh (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team:
on 1 Sep 2014; 07:54 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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MAXI-GSC detected a sudden drop in X-ray intensity from the low-mass X-ray binary GX 13+1 on August 30, 2014. Base on the on-demand analysis, the 2-10 keV flux once decreased from 0.97+-0.04 photons/s/cm2, which was the average one at 7 scan transits from 01:20 to 10:37 UT on August 30, to 0.31+-0.07 photons/s/cm2 at the scan transit at 12:10, and recovered to 0.87+-0.10 photons/s/cm2 at 13:43. At the following scan transits at 15:16, the flux dropped again, and remained low until 21:28. The average flux over these five scans was 0.05+-0.03 photons/s/cm2, and no significant variation could be recognized among them. The energy spectrum during the drop is relatively soft, suggesting that X-rays come from a dust scattering halo (e.g., Smith et al. 2002). At and after the scan transit at 02:06 on August 31, the flux recovered to the previous flux level. (The fluxes at 06:45 and 08:18 on August 31, however, decreased again approximately by 50 %.)

Such a deep and long intensity drop has not been reported in this system yet. A preliminary analysis of all the past MAXI data showed that a similar deep intensity drop was only recognized on July 10, 2013. Recently, Iaria et al. (A&A 561, a99, 2014) deeply investigated the orbital ephemeris of this source. Using their ephemeris (either eq. (1) or (2) in their paper), this and the past events occurred at the phases of 0.76 and 0.78 of the 24.527 day orbital period, respectively. Dip events far from expected (zero) dip phases are, however, not rare (see Table 3 in Iaria et al. 2014). Thus, the intensity drop might be due to the occultation of an unstable accretion disk. The observed properties, however, also suggest that the drop could be caused by the occultation of another star, happening to across or being one of the triplet.

Further long-term monitoring is expected to reveal the nature of the sudden, deep intensity drop.

(Note: data of the MAXI public data archive shown below are slightly different from what we have obtained and shown here by the on-demand analysis.)

MAXI GX 13+1