The hard ultraluminous state of NGC 5055 ULX X-1

TL;DR

This study presents the first broadband X-ray analysis of NGC 5055 ULX X-1, revealing a stable, high-luminosity source with spectral features consistent with the hard ultraluminous state, and constraining its black hole mass.

Contribution

It provides the first broadband spectral analysis of NGC 5055 ULX X-1, characterizing its spectral state and estimating the black hole mass under specific accretion assumptions.

Findings

Source exhibits stable flux with no pulsations detected.

Spectrum dominated by two thermal components, with a weak high-energy tail.

Estimated black hole mass between 11 and 26 solar masses.

Abstract

We present the results of the first broadband X-ray analysis of the ultraluminous X-ray source NGC 5055 ULX X-1, combining simultaneous data from XMM$-$Newton and NuSTAR missions, with a combined exposure time of $\sim$100 ks across the $0.3-20$ keV energy range. The source exhibits a stable flux across the entire exposure with no detectable pulsations by any instrument on their X-ray light curves, placing pulsed-fraction upper limits of 10% and 32% for XMM$-$Newton and NuSTAR, respectively. The X-ray spectrum is dominated by two thermal components consistent with the emission from an accretion disk, and shows a weak high-energy tail above 10 keV, with no statistical requirement for an additional nonthermal component. The unabsorbed $0.3-20$ keV luminosity is ${\sim}2\times10^{40}$ erg s$^{-1}$, evidencing the ULX nature of the source. The parameters obtained from spectral modeling are consistent with the hard ultraluminous state. Despite the fact that a neutron-star accretor cannot be ruled out by the available data, under the assumption that the compact object in NGC 5055 ULX X-1 is a black hole accreting through a geometrically thick, radiation-pressure-supported disk that drives an optically thick wind, we constrained its putative mass to $11-26$ M$_{\odot}$.