Associated spectral and temporal state transition of the bright ULX NGC 1313 X-1

Abstract

Stellar mass black hole X-ray binaries exhibit X-ray spectral states which also have distinct and characteristic temporal properties. These states are believed to correspond to different accretion disc geometries. We present analysis of two XMM-Newton observations of the ultra-luminous X-ray source (ULX) NGC 1313 X^-1, which reveal that the system was in two different spectral states. While spectral variations have been observed in this source before, this data provides clear evidence that the spectral states also have distinct temporal properties. With a count rate of ~1.5 counts s^-1 and a fractional variability amplitude of ~15 per cent, the ULX was in a high flux and strongly variable state in 2006 March. In 2006 October, the count rate of the ULX had reduced by a factor of ~2 and the spectral shape was distinctly different with the presence of a soft component. No strong variability was detected during this low flux state with an upper limit on the amplitude < 3 per cent. Moreover, the spectral properties of the two states implies that the accretion disc geometry was different for them. The low flux state is consistent with a model where a standard accretion disc is truncated at a radius of ~17 Schwarzschild radius around a ~200 M_⊙ black hole. The inner hot region Comptonizes photons from the outer disc to give the primary spectral component. The spectrum of the high flux state is not compatible with such a geometry. Instead, it is consistent with a model where a hot corona covers a cold accretion disc and Comptonizes the disc photons. The variability as a function of energy is also shown to be consistent with the corona model. Despite these broad analogies with Galactic black hole systems, the spectral nature of the ULX is distinct in having a lower temperature (~2 keV) of the hot Comptonizing plasma and higher optical depth (~15) than what is observed for the Galactic ones.