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tokens: We analyzed the burst spectra of the RB with the same model, and yielded an acceptable reduced χ 2 . However, the disk kT is up to 3 keV, which is too high for an NS X-ray binary. 4. Discussion and Summary We have analyzed the decay phase of the outburst experienced by the RB during 2017 observed by Insight-HXMT in a broad energy band. By stacking the type-II bursts with recurrence time of 100–300 s, we find for the first time a time delay of about 3 s at hard X-rays. The energy spectrum of the burst can be denoted by a Comptonization model, for which the temperature of the seed photons, the optical depth, and the temperature of the hot plasma are derived with values of 0.2 keV, 9–10, and 2–3 keV, respectively. These findings put more constraints upon the models at work for the RB. The tight relation between the burst strength and the waiting time (Lewin et al. 1976 ) leads to a series of models in history proposed via focusing on the sudden release of the accretion power in the disk. Type-II bursts can be caused by the instability of the disk on viscous (Lightman Eardley 1974 ) or thermal timescales (Shakura Sunyaev 1976 ), the magnetic barrier (D’Angelo Spruit 2010 , 2012 ), or the magnetic reconnection on the disk (Jiang et al. 2014 ). However, all models have their own shortages in covering all the features observed in the RB (Lewin et al. 1993 ). The possible breakthrough to discriminate different models may come from the joint observations carried out in 2015 by XMM, Swift/XRT, and NuSTAR (van den Eijnden et al. 2017 ). With this campaign, both the continuous and the mode-0 burst spectra were well measured in a rather broad energy band of 1–30 keV. It turns out that the inner radius keeps staying at round 41 R g ( R g = 2 GM /c 2 ) for both burst and pre-burst emissions, and the corona temperature of the pre-burst is 7.19 keV, which is significantly larger than 2.42 keV measured for the nonthermal temperature of the burst. Comparison for the corona temperatures between burst and pre-burst are always challenged by the contamination source 4U 1728–34, which is located only 05 away from the RB. We note that less change of the burst temperature with respect to the pre-burst was reported in ACSA (Mahasena et al. 2003 ) and BeppoSAX (Masetti et al. 2000 ), which is probably due to limited bandwidth (i.e., below 10 keV) of the former and the relatively poorer statistics of the latter that does not allow us to well constrain the parameters in the comptt model. If the burst is caused by the temporary accretion on the disk, via either disk instability or the magnetic barrier, the inner disk radius should evolve to be smaller than that of the pre-burst. Regarding the temperature difference of the corona between burst and pre-burst, the hot corona wherever it is located, either in the boundary layer or around the disk, can in principle be cooled by the seed photons of the burst shower. However, in history there are no hints for detecting such a