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The hard ultraluminous state of NGC 5055 ULX X-1

N. Cruz-Sanchez, E. A. Saavedra, F. A. Fogantini, F. García, J. A. Combi

TL;DR

This study presents the first simultaneous broadband (0.3–20 keV) X-ray analysis of NGC 5055 ULX X-1 using XMM-Newton and NuSTAR, revealing a hard ultraluminous state characterized by a two-temperature, disk-dominated continuum and a weak high-energy tail. Timing analysis finds no coherent pulsations, with pulsed-fraction upper limits of $10\%$ and $32\%$ for XMM-Newton and NuSTAR, respectively. Spectral modeling prefers a super-Eddington, wind-inflated inner disk around a stellar-mass black hole, yielding an unabsorbed luminosity of $L_{0.3-20\mathrm{keV}}\approx 2\times10^{40}$ erg s$^{-1}$ and significant partial-covering absorption. A black-hole mass of $M_{\rm BH}=17.5^{+8.5}_{-6.2}\ M_\odot$ is inferred under a nonmagnetic BH scenario, though a neutron-star accretor cannot be ruled out; the results place NGC 5055 X-1 among archetypal hard ultraluminous ULXs and demonstrate the power of broadband, coordinated observations for disentangling accretion physics in extreme systems.

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$_{\odot}$.

The hard ultraluminous state of NGC 5055 ULX X-1

TL;DR

This study presents the first simultaneous broadband (0.3–20 keV) X-ray analysis of NGC 5055 ULX X-1 using XMM-Newton and NuSTAR, revealing a hard ultraluminous state characterized by a two-temperature, disk-dominated continuum and a weak high-energy tail. Timing analysis finds no coherent pulsations, with pulsed-fraction upper limits of and for XMM-Newton and NuSTAR, respectively. Spectral modeling prefers a super-Eddington, wind-inflated inner disk around a stellar-mass black hole, yielding an unabsorbed luminosity of erg s and significant partial-covering absorption. A black-hole mass of is inferred under a nonmagnetic BH scenario, though a neutron-star accretor cannot be ruled out; the results place NGC 5055 X-1 among archetypal hard ultraluminous ULXs and demonstrate the power of broadband, coordinated observations for disentangling accretion physics in extreme systems.

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 XMMNewton and NuSTAR missions, with a combined exposure time of 100 ks across the 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 XMMNewton 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 keV luminosity is erg s, 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 M.

Paper Structure

This paper contains 12 sections, 4 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. XMM--Newton
  4. NuSTAR
  5. Analysis and results
  6. Timing analysis
  7. Spectral analysis
  8. Discussion
  9. Spectral state and inner-flow geometry
  10. Black-hole mass estimation
  11. Conclusions
  12. Posterior distributions from the spectral models

Figures (4)

  • Figure 1: Background-subtracted light curves of NGC 5055 X-1 extracted from the XMM--Newton and NuSTAR calibrated data in the 0.3–12 keV and 3–78 keV energy bands, respectively, both using a binning length of 800 s.
  • Figure 2: Residuals from fitting two absorption configurations to the joint XMM--Newton and NuSTAR spectra (0.3--20 keV). Top panel: Model A = tbnew*(diskbb+diskpbb) (single neutral absorber; Galactic column only), which reveals a feature around ${\sim}0.95$ keV. Bottom panel: Model B = tbnew*tbnew_pcf*(diskbb+diskpbb) (additional partial-covering neutral absorber), in which this feature is unnoticeable. Data have been rebinned for visualization only.
  • Figure 3: XMM--Newton+ NuSTAR unfolded spectra of NGC 5055. The top panel shows the best-fit cutoffpl model. The second panel displays the simpl model fit. The bottom three panels show the residuals for each tested scenario with their corresponding $\chi^2$ values: cutoffpl (third panel), simpl (fourth panel), and diskbb+diskpbb (fifth panel).
  • Figure 4: Corner plots showing the posterior probability distributions of the best-fit parameters obtained from the MCMC analysis of the three tested models: cutoffpl (top left), simpl (top right), and diskbb+diskpbb (bottom). N$_{\rm H}$ and T$_{\rm in}$ are expressed in units of $10^{22}$ cm$^{-2}$ and keV, respectively. Each distribution is based on $10^{6}$ converged MCMC iterations.