An extreme Ultraluminous X-ray source X-1 in NGC 5055

TL;DR

This study analyzes multi-epoch X-ray data of ULX NGC 5055 X-1, revealing its spectral and timing properties, suggesting it is in an ultraluminous soft state with high Eddington ratio accretion and possible outflows.

Contribution

It provides detailed spectral and timing analysis of NGC 5055 X-1, proposing its ultraluminous soft state and evidence for high Eddington accretion and outflows, which is novel for this source.

Findings

No significant variability in lightcurves.

Positive correlation between photon index and flux.

Inverse relation between luminosity and inner disk temperature.

Abstract

Aims. We analyzed multi-epoch X-ray data of the Ultraluminous X-ray source (ULX) NGC 5055 X-1, with luminosity up to $2.32\times10^{40}\ \rm erg\ s^{-1}$, in order to constrain the physical parameters of the source. Methods. We performed timing and spectral analysis of Chandra and XMM-Newton observations. We used spectral models which assume the emission is from an accreting black hole system. We fit the data with a multicolor disk (MCD) combined with a powerlaw (PL) or a thermal Comptonization (NTHCOMP) component, and compared those fits with a slim disk model. Results. The lightcurves of the source do not show significant variability. From the hardness ratios (3-10 keV/0.3-3 keV flux) we infer that the source is not spectrally variable. We found that the photon index is tightly, positively correlated with the unabsorbed 0.3-10 keV flux and the hydrogen column density. Furthermore, the temperature emissivity profile indicates a deviation from the standard sub-Eddington thin disk model. The source shows an inverse correlation between luminosity and inner disk temperature in all fitted models. Conclusions. Our analysis favors the source to be in an ultraluminous soft state. The positive correlations between the photon index and the flux, and between the photon index and the hydrogen column density may suggest the source is accreting at high Eddington ratios and might indicate the presence of a wind. The inverse luminosity relation with the inner disk temperature for all spectral models may indicate that the emission is geometrically beamed by an optically thick outflow.