We show that the superburst would be originated from thermonuclear burning ignited by accumulated fuels in the deep layers compared to normal X-ray bursts. Two cases are investigated for models related to superbursts by following thermal evolution of a realistic neutron star: helium flash and carbon flash accompanied with many normal bursts. For a helium flash, the burst shows the long duration when the accretion rate is low compared with the observation. The flash could become a superburst if the burning develops to the deflagration and/or detonation. For a carbon flash accompanied with many normal bursts, after successive 2786 normal bursts during 1.81？×？109？s, the temperature reaches the deflagration temperature. This is due to the produced carbon which amount reaches to ≈0.1 in the mass fraction. The flash will develop to dynamical phenomena of the deflagration and/or detonation, which may lead to a superburst. 1. Introduction Type I X-ray bursts have been identified to the thermonuclear explosions on the surface region of accreting neutron (compact) stars. As a consequence, the phenomenon has been studied from both nuclear reactions and nuclear structure inside the compact stars. However, there still remain many uncertainties concerning the elementary processes associated with the bursts [1–3]. Superbursts have been detected from 13 X-ray bursters by BeppoSAX and RXTE (see, e.g., Table？？2 in [4]). In particular, 4U 1636-536 exhibited four superbursts, where the shortest recurrence time is 1.5 years (http://www.astronomerstelegram.org/?read=2140) [5, 6]. Clearly, the light curve consists of a fast rise and slower power law-like decay [2, 7]. The spectrum hardens during the rise phase to the maximum in luminosity, whereas it softens in the decay phase. This is also reflected in the spectral fits to the time-resolved preburst subtracted from X-ray spectra. Each burst has energy of ergs and duration of a few hours. They are usually best described in terms of a black-body model. The effective temperature increases and decreases during the rise and decay phase, respectively. These superbursts are 1000 times luminous and 1000 times long in the duration compared with the normal bursts though the spectral evolution is similar. Even now, quantitative explanation and/or numerical simulation of superbursts using the stellar evolution code are limited. For example, Keek and Heger [8] do not self-consistently produce the carbon from hydrogen/helium burning in their calculation but instead accrete the carbon directly onto the neutron star, bypassing the
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