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

TL;DR

This paper reports the first detection of flaring events in NGC 4395 ULX1, revealing spectral hardening during flares, evidence of winds, and insights into accretion processes consistent with slim disk models.

Contribution

It presents the first observation of flares in NGC 4395 ULX1 and analyzes their spectral and luminosity-temperature behavior, supporting a super-Eddington slim disk accretion scenario.

Findings

Flaring events are spectrally harder than steady emission.

Presence of a broad emission feature around 0.9 keV indicating winds.

Luminosity-temperature relation consistent with L∝T^2, supporting slim disk models.

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 intervals, 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. All observations show a broad hump-like feature around $\sim 0.9$ keV, which can be associated with a collection of blended emission lines, and suggests the presence of a wind/outflow in this ULX through comparison with other ULXs that show a similar feature. The flaring spectra correspond to higher slim disk temperatures due to higher mass accretion rate under an advection-dominated accretion scenario. The luminosity-temperature (L-T) values in different flux states show a positive trend. When characterized with a powerlaw relation, the L-T profile is broadly consistent with both $L\propto T^2$ and $L\propto T^4$ relations for the analysed data. The empirical predictions for a slim accretion disk in the case of super-Eddington accretion onto a stellar-mass compact object is $L \propto T^2$ which is a possible scenario in ULX1. The origin of the flaring events is understood as an intrinsic change of accretion rate or presence of variable clumpy wind in the inner region of the accretion disk.