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 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 ∼0.9 keV, which can be associated with a collection of blended emission lines, and suggests the presence of a wind or 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 a 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 power-law relation, the L-T profile is broadly consistent with both L ∝ T 2 and L ∝ T 4 relations for the analyzed data. The empirical predictions for a slim accretion disk in the case of super-Eddington accretion onto a stellar-mass compact object is L ∝ T 2, which is a possible scenario in ULX1. The origin of the flaring events is understood as an intrinsic change in accretion rate or presence of variable clumpy wind in the inner region of the accretion disk.

Constraint on accretion of NGC 6946 X-1 using broadband X-ray data

“…In this paper, we mainly focus on the continuum spectral fitting of 2020-2021 broadband X-ray observations and undertake a simpler approach to fit the ∼ 0.9 keV feature by a Gaussian model (see discussion on such a similar feature in other ULXs, e.g., Middleton et al 2014Middleton et al , 2015aGhosh et al 2022). We investigate the continuum fitting with different models relevant to the physical scenarios expected in ULX systems.…”

“…There are ULX sources, e.g., NGC 247 ULX1, NGC 55 ULX, NGC 4395 ULX1, NGC 1313 X-1, NGC 5408 ULX1, NGC 300 X-1, Ho IX X-1, Ho II X-1, and NGC 5408 X-1 (Middleton et al 2015b;Pinto et al 2016Pinto et al , 2017Earnshaw & Roberts 2017;Pinto et al 2021;Ghosh et al 2022) where the strong ∼ 0.9 keV feature is detected. In fact, most of the bright ULXs where strong ∼ 0.9 keV line emission/absorption features have been discovered are found to have predominantly softer spectra (Kosec et al 2021).…”

Constraint on the Accretion of NGC 6946 X-1 Using Broadband X-Ray Data

“…In this paper, we mainly focus on the continuum spectral fitting of 2020-2021 broadband X-ray observations and undertake a simpler approach to fit the ∼0.9 keV feature by a Gaussian model (see discussions on such a similar feature in other ULXs in, e.g., Middleton et al 2014Middleton et al , 2015aGhosh et al 2022). We investigate the continuum fitting with different models relevant to the physical scenarios expected in ULX systems.…”

“…There are ULX sources, e.g., NGC 247 ULX-1, NGC 55 ULX, NGC 4395 ULX-1, NGC 1313 X-1, NGC 5408 ULX-1, NGC 300 X-1, Ho IX X-1, Ho II X-1, and NGC 5408 X-1 (Middleton et al 2015b;Pinto et al 2016Pinto et al , 2017Earnshaw & Roberts 2017;Pinto et al 2021;Ghosh et al 2022) where the strong ∼0.9 keV feature is detected. In fact, most of the bright ULXs where strong ∼0.9 keV line emission/absorption features have been discovered are found to have predominantly softer spectra (Kosec et al 2021).…”