The XMM-Newton long look of NGC 1365: lack of a high/soft state in its ultraluminous X-ray sources

TL;DR

This study analyzes a long XMM-Newton observation of NGC 1365, revealing that its ultraluminous X-ray sources do not exhibit the typical high/soft state, suggesting different accretion behaviors from stellar-mass black holes.

Contribution

It provides detailed spectral and timing analysis of ULXs in NGC 1365, highlighting the absence of a high/soft state and proposing alternative accretion scenarios.

Findings

ULXs remained in a hard power-law spectral state during observations.

They have never been observed in a high/soft state like stellar-mass black holes.

Persistent super-Eddington accretion may prevent standard disk formation.

Abstract

Based on our long (~ 300 ks) 2007 XMM-Newton observation of the Seyfert galaxy NGC 1365, we report here on the spectral and timing behaviour of two ultraluminous X-ray sources, which had previously reached isotropic X-ray luminosities L_X ~ 4 x 10^{40} erg/s (0.3-10 keV band). In 2007, they were in a lower state (L_X ~ 5 x 10^{39} erg/s, and L_X ~ 1.5 x 10^{39} erg/s for X1 and X2, respectively). Their X-ray spectra were dominated by power-laws with photon indices Gamma ~ 1.8 and Gamma ~ 1.2, respectively. Thus, their spectra were similar to those at their outburst peaks. Both sources have been seen to vary by a factor of 20 in luminosity over the years, but their spectra are always dominated by a hard power-law; unlike most stellar-mass BHs, they have never been found in a canonical high/soft state dominated by a standard disk. The lack of a canonical high/soft state seems to be a common feature of ULXs. We speculate that the different kind of donor star and/or a persistently super-Eddington accretion rate during their outbursts may prevent accretion flows in ULXs from settling into steady standard disks.