Hard X-ray flares and spectral variability in NGC 4395 ULX1

Abstract

We report the detection of flaring events in NGC 4395 ULX1, a nearby ultraluminous X-ray source(ULX), for the first time, using recent XMM-NEWTON observations. The flaring episodes are spectrally harder than the steady emission periods, resulting in higher fractional variability in the high energy regime. A thin Keplerian and a slim accretion disk provide the best-fit continuum for XMM-NEWTON spectra. In all observations, the presence of a broad Gaussian emission feature around ∼ 0.9 keV suggests a strong wind outflow in this ULX. The flaring spectra correspond to higher slim disk temperatures due to higher mass accretion rate under an advection-dominated accretion scenario. The luminosity-temperature profile in different flux states is consistent with the theoretical predictions for a slim accretion disk in the case of super-Eddington accretion onto a stellar-mass compact object. The unperturbed absorption column and wind outflow line emission during flaring events suggest that the site of origin of these flares is mainly the advection flow in the inner region of the accretion disk.