In-depth studies of the NGC 253 ULXs with XMM-Newton: remarkable variability in ULX1, and evidence for extended coronae

TL;DR

This study investigates the variability and spectral features of ULXs in NGC 253, revealing ULX1's remarkable variability, evidence for extended coronae, and insights into their accretion states and super-Eddington luminosities.

Contribution

It provides the first detailed variability analysis of NGC 253 ULXs, introduces an extended corona model for soft excesses, and compares spectral models to distinguish genuine spectral features.

Findings

ULX1 is three times more variable than ULX2 despite being dimmer.

ULX1 exhibits a high or very high state power density spectrum.

Extended coronae may explain the soft excess in ULX spectra.

Abstract

We examined the variability of three ultra-luminous X-ray sources (ULXs) in the 2003, 110 ks XMM-Newton observation of NGC253. Remarkably, we discovered ULX1 to be three times more variable than ULX2 in the 0.3--10 keV band, even though ULX2 is brighter. Indeed, ULX1 exhibits a power density spectrum that is consistent with the canonical high state or very high/steep power law state, but not the canonical low state. The 0.3--10 keV emission of ULX1 is predominantly non-thermal, and may be related to the very high state. We also fitted the ULX spectra with disc blackbody, slim disc and convolution Comptonization (SIMPL x DISKBB) models. The brightest ULX spectra are usually described by a two emission components (disc blackbody + Comptonized component); however, the SIMPL model results in a single emission component, and may help determine whether the well known soft excess is a feature of ULX spectra or an artifact of the two-component model. The SIMPL models were rejected for ULX3 (and also for the black hole + Wolf-Rayet binary IC10 X-1); hence, we infer that the observed soft-excesses are genuine features of ULX emission spectra. We use an extended corona scenario to explain the soft excess seen in all the highest quality ULX spectra, and provide a mechanism for stellar mass black holes to exhibit super-Eddington luminosities while remaining locally sub-Eddington.