Super-Eddington accretion on to a stellar mass ultraluminous X-ray source NGC 4190 ULX1

Abstract

We present the results of high-quality XMM-NEWTON observations of a ultraluminous X-ray source (ULX) in the galaxy NGC 4190. The detection of spectral cutoff in NGC 4190 ULX1 spectra rules out the interpretation of the ULX to be in a standard low/hard canonical accretion state. We report that the high quality EPIC spectra can be better described by broad thermal component, such as a slim disc. In addition we found long-term spectral and flux variability in the source using several XMM-NEWTON and Swift data. A clear anticorrelation between flux and power-law photon index is found which further confirms the unusual spectral state evolution of the ULX. Spectral properties of the ULX suggest that the source is in a broadened disc state with luminosities [≈(3 - 10) × 10³⁹ erg s^-1] falling in the ultraluminous regime. The positive luminosity-temperature relation further suggests that the multicolour disc model follows the L ∝ T⁴ relation that is expected for a blackbody disc emission from a constant area and the slim disc model seems to favour L ∝ T² relation consistent with an advection-dominated disc emission. From the broadened disc-like spectral feature at such luminosity, we estimated the upper limit of the mass of the central compact object from the inner disc radius and found that the ULX hosts a stellar mass black hole.