Spectral variability in NGC 1042 ULX1

TL;DR

This study analyzes the spectral variability of the ultraluminous X-ray source NGC 1042 ULX1 using archival and recent observations, revealing spectral state similarities to ultraluminous sources and potential super-Eddington accretion.

Contribution

It provides the first detailed spectral variability analysis of NGC 1042 ULX1, linking spectral changes to accretion processes and system inclination.

Findings

Spectral hardness varies over long-term observations.

The source exhibits a cool thermal disk component at ~0.2 keV.

Spectral hardness increases with luminosity, indicating possible accretion rate changes.

Abstract

We report X-ray spectral variability in an ultraluminous X-ray source NGC 1042 ULX1, using archival XMM-Newton and recent NuSTAR observations. In long-term evolution, the source has shown a trend of variation in spectral hardness. The variability in different XMM-Newton observations is prominent above $\sim 1$ keV. Cool thermal disk component with a characteristic temperature of $\sim 0.2$ keV manifests that the spectral state of NGC 1042 ULX1 in all epochs is similar to that of the ultraluminous state sources. An apparent anti-correlation between luminosity and powerlaw index demonstrates that the source becomes spectrally harder when it is in a brighter state. That is conceivably related to variation in accretion rate, strength of comptonization, wind/outflow in the system or a manifestation of varying disk occultation. Typical hard ultraluminous type spectra indicate that NGC 1042 ULX1 is a low inclination system in general. Spectral properties suggest that, like many other ULXs which show spectral curvature around $\sim 6-10$ keV, NGC 1042 ULX1 could be another stellar-mass super-Eddington accretor.