Examining the nature of the ultraluminous X-ray source Holmberg II X-1

Abstract

We present a comprehensive spectral analysis of the ultraluminous X-ray source Holmberg II X-1 using broadband and high-resolution X-ray spectra taken with the \ satellite over a period of 19 years black taking advantage of data from a recent campaign . We tested several models for the broadband spectra, including a double thermal component provided a reasonable description for the continuum black between 0.3-10 keV and enabled us to constrain the properties of the accretion disc. The luminosity-temperature trends of the inner and outer disc components black broadly agree with the expectations for a thin disc although the exact values of the slopes are slightly sensitive to the adopted model. However, all tested models show L-T trends that deviate from a black power law above a bolometric luminosity of about $5 erg/s$ black particularly for the hot thermal component associated with the inner accretion flow . Assuming that such deviations are due to the black accretion rate exceeding its Eddington limit or, most likely, the super-critical rate, a compact object with a mass of 16-36 M$_ odot $ was inferred, specifically, a stellar-mass black hole IX-X VII which indicate Ne-N-rich gas with non-solar abundances. This favours a nitrogen-rich donor star, such as a blue or red supergiant, which has escaped from its native stellar cluster black characterised by a low-metallicity environment.