Long-term Behaviors of the Ultraluminous X-Ray Source NGC1313 X-1

Ultraluminous X-ray sources (ULXs) are known to vary X-ray luminosities with one order of magnitude. However, the behavior in just rising and decaying phases during outbursts is still unclear due mainly to the sparse data sampling. Here, we analyzed a long-term monitoring data set (since 2013 June) of NGC1313 X-1 obtained with Swift/XRT. We found that X-1 has experienced the low luminosity state (about 7 × 10^{39} erg/s, a quiescent stable phase) and two types of outbursts: One is month-scale outbursts with relatively modest brightening (to 9 × 10^{39} erg/s) and spectral softening. Contrastingly, the other type shows day-scale, remarkable brightening (to 4 × 10^{40} erg/s) with spectral hardening; such an extreme luminosity has been observed only once, with Suzaku reported by Mizuno et al. (2007). We also found a distinctive behavior just before some outbursts.Based on best-fit parameters of each phase and the hysteresis variation of NGC1313 X-1, we suggest a super-critical accretion flow model composed of two parts: a moderately massive inner disk and a highly massive outer one. The luminosity and spectral variations of X-1 may be caused by moves of the transition radius which separates these two disk regions. In addition, we will mention a notable behavior during a more long period by many archival data sets with other satellites.