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SUPER-EDDINGTON ACCRETION IN THE ULTRALUMINOUS X-RAY SOURCE NGC 1313 X-2: AN EPHEMERAL FEAST

2014    Weng, Shan-Shan; Zhang, Shuang-Nan; Zhao, Hai-Hui

We investigate the X-ray spectrum, variability, and the surrounding ionized bubble of NGC 1313 X-2 to explore the physics of super-Eddington accretion. Beyond the Eddington luminosity, the accretion disk of NGC 1313 X-2 is truncated at a large radius (similar to 50 times the innermost stable circular orbit), and displays the similar evolution track with both luminous Galactic black-hole and neutron star X-ray binaries (XRBs). In super-critical accretion, the speed of radiatively driven outflows from the inner disk is mildly relativistic. Such ultra-fast outflows would be overionized and might produce weak Fe K absorption lines, which may be detected by the coming X-ray mission Astro-H. If NGC 1313 X-2 is a massive stellar XRB, 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 similar to 10(4)-10(5) yr. The expansion of the surrounding bubble nebula with a velocity of similar to 100 km s(-1) might indicate that it has existed over similar to 10(6) yr and is inflated by the radiatively driven outflows from the transient with a duty cycle of activity of similar to 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 Stromgren 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.