MAXI/GSC detection of successive mini-outbursts from the black hole candidate MAXI J1348-630

ATel #13994; H. Negoro, M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino (Nihon U.), T. Mihara, C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 8 Sep 2020; 07:28 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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At 05:28 UT on 2020 September 05, the MAXI/GSC nova alert system triggered on a faint X-ray transient source positionally consistent with the black hole candidate MAXI J1348-630 discovered on 2019 January 26 (Yatabe et al. ATel #12425, also see Tominaga et al. 2020, ApJ, 899, L20; Belloni et al. 2020, MNRAS, 496, 4366; Jana et al. 2020, ApJ, 897, a3). The X-ray flux averaged over the 6 scan transits from 20:57 UT on September 5 to 4:42 on September 6 was 28 +- 6 mCrab (4.0-10.0keV, 1 sigma error).

This is likely the 5th or 6th mini-outburst from MAXI J1348-630. After the rapid decrease in the X-ray flux around MJD 58700 (2019 August 5) at the end of the second large outburst (e.g., Tominaga et al. 2020), we detected 5 or 6 mini-outbursts including this detection. If we fit a 2-10 keV (1d) light curve of each mini-outburst with a Lorentzian function (currently no physical meaning), we obtain the following centroid time and peak fluxes.

1. MJD 58747.6 +/- 0.4 (+48 d from the end of the outburst), 0.038 +/- 0.004 c/cm2/s (17 mCrab)
2. MJD 58812.7 +/- 0.3 (+65 d from the 1st mini-outburst), 0.035 +/- 0.005 c/cm2/s (16 mCrab)
3. MJD 58886.8 +/- 0.4 (+74 d from the 2nd), 0.027 +/- 0.004 c/cm2/s (12 mCrab) (ATel #13451, #13459, #13467)
4. MJD 58975.8 +/- 1.0 (+89 d from the 3rd), 0.026 +/- 0.006 c/cm2/s (12 mCrab) (ATel #13710)
5. MJD 59033.3 +/- 1.6 (+57 d from the 4th), 0.015 +/- 0.004 c/cm2/s ( 7 mCrab)
6. MJD 59098.0 (fixed) (+65 d from 5th or 122 d from the 4th), 0.036 +/- 0.012 c/cm2/s (16 mCrab)

The durations of the 1st to 4th mini-outbursts are almost the same (about 6 days at FWHM), and that of the 5th is 3+/-7 days. (The 6th is still going on.) Because the source during the 5th mini-outburst is not visible on GSC images, the count excess of the 5th may be due count fluctuations of the background. The 4-10 keV to 2-4 keV count ratios of all the mini-bursts are consistent with those in the hard state in the main outburst. Except for the 5th, it is interesting to note that the peak fluxes are similar (14 +/- 3 mCrab), and that the time intervals between the mini-outbursts are longer and longer. The similar peak flux suggests the existence of some critical mass in the accretion disk to trigger a mini-outburst, and the longer time interval does the decline in the accretion rate from the companion.

Finally, we also note that such successive mini-outbursts after the main outburst were sometimes observed in various binary systems (also see Yan and Yu 2017, MNRAS, 470, 4298). For instance, the black hole binaries/candidates GRS 1739-278 (two mini-outbursts), XTE J1650-500 (5 times, Yan and Yu 2017), and MAXI J1535-571 (5 times, Parikh et al. 2019, ApJ, 878, L28), the milli-second pulsar SAX J1808.4-3658 (4-5 times, Burderi et al. 2006, ApJ, 653, L133), and the dwarf nova WZ Sge (12 times in optical, Ishioka et al. 2002, A&A, 381, L41).

Monitoring observations of this source are encouraged.

MAXI data for MAXI J1348-650