Associated Spectral and Temporal State Transition of the bright ULX NGC1313 X-1

TL;DR

This study analyzes two different spectral and temporal states of the ULX NGC 1313 X-1, revealing distinct accretion disk geometries and spectral properties that resemble but also differ from Galactic black hole systems.

Contribution

The paper provides the first detailed analysis of spectral and temporal state transitions in NGC 1313 X-1, linking observed states to specific accretion disk geometries.

Findings

High flux state shows strong variability and a different spectral shape.

Low flux state is consistent with a truncated accretion disk model.

Spectral properties suggest a colder, optically thicker Comptonizing corona.

Abstract

(Abridged) 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 disk geometries. We present analysis of two long 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/temporal states. With a count rate of 1.5 counts/s and a fractional variability amplitude of ~15%, the ULX was in a high flux and strongly variable state in March 2006. In October 2006, 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%. Moreover, the spectral properties of the two states implies that the accretion disk geometry was different for them. The low flux state is consistent with a model where a standard accretion disk is truncated at a radius of ~17 Schwarzschild radius around a ~200 Msun black hole. The inner hot region Comptonizes photons from the outer disk 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 disk and Comptonizes the disk 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 colder Comptonizing temperature (~2 keV) and higher optical depth (~15) than what is observed for the Galactic ones.