MAXI/GSC detection of a possible hard-to-soft state transition in the initial phase of the outburst and further brightening of MAXI J1828-249

ATel #5483; H. Negoro (Nihon U.), M. Sugizaki, T. Mihara, M. Matsuoka (RIKEN), S. Nakahira, S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), M. Serino, M. Morii, J. Sugimoto, T. Takagi (RIKEN), N. Kawai, R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Nakano (AGU), H. Tsunemi, M. Sasaki (Osaka U.), M. Nakajima, H. Sakakibara, K. Fukushima, T. Onodera, K. Suzuki (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.), M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada (Miyazaki U.), K. Yamaoka (Nagoya U.) report on behalf of the MAXI team
on 18 Oct 2013; 13:06 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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After the discovery of MAXI J1828-249 (Nakahira et al. ATel #5474), we made light curves of the source with MAXI data, excluding the contamination of a nearby bright source GS 1826-238. The 4-10 keV light curve shows a slight count excess, comparable to the flux of ~10 mCrab, since the beginning of October 15th, 2013 (UT). A rapid count increase in the 2-4 keV band is clearly seen since the scan transit at 18:49 on Oct. 15th (MJD 56580.784). The 4-10 keV to 2-4 keV count ratios, however, tend to gradually change from ~1 at the beginning the outburst to ~0.2 on 16 March, implying the source has undergone a hard-to-soft state transition during these two days. These properties are very similar to those of other black hole candidates previously observed with MAXI.

The latter ratio on Oct. 16th is consistent with the result of the Swift observation performed at 15:04UT (Kennea et al. ATel #5478, #5479). On the other hand, the hard power-law component initially obtained with INTEGRAL/ISGRI from 18:03 on Oct. 15th to 09:10 on Oct. 16th (Filippova et al. ATel #5476) and this MAXI result suggest that the state transition was undergoing, or might be in the intermediate state.

The 2-4 keV flux is still increasing, and reaching ~ 100 mCrab at the end of October 17th. We also note that the hardness ratios are still highly variable even on Oct. 17th. The source might be still under the transition as suggested by Kennea et al. (ATel #5479).

Suzaku observation is also planed. Multi-wavelength followup observations are encouraged.

Public Data Product of MAXI J1828-249