Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2

Super Eddington accretion is one of the most interesting and extreme processes in the life of black holes and neutron stars. Here tremendous amounts of matter transfer occur from the companion star to the compact objects with a significant impact onto their evolution. Ultraluminous X-ray sources (ULXs) are the ideal systems in which we can investigate this accretion regime.

In this work, we present our recent results on the spectral analysis of NGC 1313 X-2, whose baseline model consists of two thermal blackbody components with different temperatures plus an exponentially cutoff powerlaw. We studied the behaviour of the two thermal components, investigating how they evolve in the luminosity-temperature plane. We found a negative trend for the luminosity-temperature group of the cold component, which is not consistent with a Shakura-Sunayev sub-Eddington thin disc, nor with an advection-dominated disc, but would rather agree with a wind- dominated X-ray emission region. Instead, the (L, T) relationship for the hottest component is somewhere between the two theoretical scenarios, which argues in favour of super-Eddington accretion with a thicker disc. The exponentially cutoff powerlaw is significant and is likely from the NS accretion column.