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

Abstract

We present the results of high-quality XMM-NEWTON observations of a 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 disk. In addition we found long term spectral and flux variability in the source using several XMM-NEWTON and Swift data. A clear anti-correlation 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 disk state with luminosities ($\approx (3-10)\times 10^{39}$ ergs s${}^{-1}$) falling in the ultraluminous regime. The positive Luminosity-temperature relation further suggests that the multi color disk model follows the $L\propto T^4$ relation which is expected for a black body disk emission from a constant area and the slim disk model seems to favour $L\propto T^2$ relation consistent with an advection dominated disk emission . From the broadened disk like spectral feature at such luminosity, we estimated the upper limit of the mass of the central compact object from the inner disk radius and found that the ULX hosts a stellar mass black hole.

10 pages, 10 figures; accepted for publication in MNRAS

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