Super-Eddington Accretion in the Ultraluminous X-ray Source NGC1313 X-2: An Ephemeral Feast

Abstract: We investigate the X-ray spectrum, variability and the surrounding ionized bubble of NGC1313 X-2 to explore the physics of super-Eddington accretion. Beyond the Eddington luminosity, the accretion disk of NGC1313 X-2 is truncated at a large radius ($\sim$ 50 times of innermost stable circular orbit), and displays the similar evolution track with both luminous Galactic black-hole and neutron star X-ray binaries. In super-critical accretion, the speed of radiatively driven outflows from the inner disk is mildly relativistic. Such ultra-fast outflows would be over ionized and might produce weak Fe K absorption lines, which may be detected by the coming X-ray mission {\it Astro-H}. If the NGC1313 X-2 is a massive stellar X-ray binary, the high luminosity indicates that an ephemeral feast is held in the source. That is, the source must be accreting at a hyper-Eddington mass rate to give the super-Eddington emission over $\sim 10^{4}-10^{5}$ yr. The expansion of the surrounding bubble nebula with a velocity of $\sim$ 100 km~s$^{-1}$ might indicate that it has existed over $\sim 10^{6}$ yr and is inflated by the radiatively driven outflows from the transient with a duty cycle of activity of $\sim$ a few percent. Alternatively, if the surrounding bubble nebula is produced by line-driven winds, less energy is required than the radiatively driven outflow scenario, and the radius of the Str\"{o}mgren radius agrees with the nebula size. Our results are in favor of the line-driven winds scenario, which can avoid the conflict between the short accretion age and the apparently much longer bubble age inferred from the expansion velocity in the nebula.