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Probing spectral variability in NGC 4490 ULX-8 over 24 years of XMM-Newton, Chandra and Swift-XRT observations

Tarang Vashisht, Aru Beri, Tanuman Ghosh, Aman Upadhyay, Vikram Rana

TL;DR

This study analyzes NGC 4490 ULX-8 with a 24-year, multi-mission X-ray dataset (XMM-Newton, Chandra, Swift-XRT), fitting spectra with absorbed power-law and disk-blackbody models. It documents significant long-term luminosity variability without clear state transitions and identifies a softer-when-brighter trend alongside a possible L_X–T_in relation compatible with a standard thin disk. Disk normalization suggests Rin ≈ 140 km and a BH mass in the range ≈16–75 M_sun depending on spin, while an accreting NS scenario with B ≈ 3×10^{11} G is also considered. The results support either a stellar-mass BH with a conventional disk or a moderately magnetized NS as the accretor, highlighting the diverse long-term behavior of ULXs and implications for super-Eddington accretion physics.

Abstract

We present comprehensive spectral and timing results of 14 Chandra, 6 XMM-Newton and 19 Swift-XRT observations of the ultraluminous X-ray source NGC 4490 ULX-8, spanning from 2000 to 2024. We model the source spectra using absorbed power-law and absorbed multicolour disc blackbody models. The best-fit photon indices span 0.92-2.68, with typical uncertainties ranging from $\pm$0.1 to $\pm$1 depending on data quality. The inner disk temperature range from 0.97 to 1.69 keV, consistent with blackbody emission from an accretion disk. Our results reveal significant long-term variability in intrinsic X-ray source fluxes while the source remains relatively stable within individual observations. A Hardness-Intensity Diagram of the source shows no clear transition between hard and soft states, but an increase in brightness during two recent observations taken on 2022 December 1 and 2024 May 4. We find a positive correlation of X-ray luminosity and photon index that persists even when the hydrogen column density is tied across observations, suggesting a physical origin. The X-ray luminosity-inner disk temperature relation yields a weakly constrained slope owing to large temperature uncertainties, but a simpler fixed-slope test indicates consistency with a standard thin-disk. Using the derived disk parameters, we estimate the black hole mass to lie in the range of 16-75 $M_{\odot}$, under the assumption of a geometrically thin accretion flow, where the lower and upper bounds correspond to a Schwarzchild and a Kerr black hole respectively. Alternatively, we consider the scenario of ULX-8 hosting an accreting neutron star and estimate the corresponding magnetic field strength required to explain the observed properties.

Probing spectral variability in NGC 4490 ULX-8 over 24 years of XMM-Newton, Chandra and Swift-XRT observations

TL;DR

This study analyzes NGC 4490 ULX-8 with a 24-year, multi-mission X-ray dataset (XMM-Newton, Chandra, Swift-XRT), fitting spectra with absorbed power-law and disk-blackbody models. It documents significant long-term luminosity variability without clear state transitions and identifies a softer-when-brighter trend alongside a possible L_X–T_in relation compatible with a standard thin disk. Disk normalization suggests Rin ≈ 140 km and a BH mass in the range ≈16–75 M_sun depending on spin, while an accreting NS scenario with B ≈ 3×10^{11} G is also considered. The results support either a stellar-mass BH with a conventional disk or a moderately magnetized NS as the accretor, highlighting the diverse long-term behavior of ULXs and implications for super-Eddington accretion physics.

Abstract

We present comprehensive spectral and timing results of 14 Chandra, 6 XMM-Newton and 19 Swift-XRT observations of the ultraluminous X-ray source NGC 4490 ULX-8, spanning from 2000 to 2024. We model the source spectra using absorbed power-law and absorbed multicolour disc blackbody models. The best-fit photon indices span 0.92-2.68, with typical uncertainties ranging from 0.1 to 1 depending on data quality. The inner disk temperature range from 0.97 to 1.69 keV, consistent with blackbody emission from an accretion disk. Our results reveal significant long-term variability in intrinsic X-ray source fluxes while the source remains relatively stable within individual observations. A Hardness-Intensity Diagram of the source shows no clear transition between hard and soft states, but an increase in brightness during two recent observations taken on 2022 December 1 and 2024 May 4. We find a positive correlation of X-ray luminosity and photon index that persists even when the hydrogen column density is tied across observations, suggesting a physical origin. The X-ray luminosity-inner disk temperature relation yields a weakly constrained slope owing to large temperature uncertainties, but a simpler fixed-slope test indicates consistency with a standard thin-disk. Using the derived disk parameters, we estimate the black hole mass to lie in the range of 16-75 , under the assumption of a geometrically thin accretion flow, where the lower and upper bounds correspond to a Schwarzchild and a Kerr black hole respectively. Alternatively, we consider the scenario of ULX-8 hosting an accreting neutron star and estimate the corresponding magnetic field strength required to explain the observed properties.

Paper Structure

This paper contains 18 sections, 5 equations, 9 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. XMM-Newton
  4. Chandra
  5. Swift-XRT
  6. Analysis and Results
  7. Timing Analysis
  8. Hardness–intensity diagrams
  9. Spectral Analysis
  10. F-test
  11. Long-Term Spectral Variability
  12. Discussions
  13. Powerlaw Parameter Correlation Study
  14. Accretion Disk and Compact-Object Constraints
  15. Black Hole model
  16. ...and 3 more sections

Figures (9)

  • Figure 1: Latest X-ray images of NGC 4490. Source and background regions used for science product extraction are marked in green and cyan colours, respectively. Left: A mosaic created by stacking cleaned EPIC-pn and MOS1/2 images from the XM6 observation with XMM-Newton. Right: A cleaned Chandra image from observation C14.
  • Figure 2: Background-subtracted light curve from the XMM-Newton EPIC-pn module during the XM6 observation. The light curve is binned at 500 s and shows the intra-observational temporal evolution of the X-ray count rate.
  • Figure 3: The hardness-intensity diagram: Total 0.3--10 keV count rate vs hardness ratio (HR), combining observations from multiple instruments. Data points from Swift–XRT are shown in green, Chandra data in blue, and XMM-Newton data in red. WebPIMMS tool has been used to account for instrument differences and to insure comparison.
  • Figure 4: Intrinsic luminosity vs. time graphs for NGC 4490 ULX-8 with the best-fit horizontal line and its RMS error denoted by the shaded region. The oldest observation (C1 with MJD = 51851) is taken as the reference epoch. The Y axis is logarithmically scaled for better visualization. Upper:XMM-Newton and ChandraDISKBB luminosity values. Lower:XMM-Newton, Chandra and Swift-XRTPL luminosity values along with the two zoom in plots for the S1--S12 and C5--C14 observation samples.
  • Figure 5: LINMIX regression results of spectral parameters obtained from PL fits to XMM-Newton and Chandra observations. Left: photon index $\Gamma$ vs. intrinsic absorption $N_{\mathrm{H}}$. Middle: $L_{\mathrm{X}}$ vs. $\Gamma$ with $N_{\mathrm{H}}$ allowed to vary freely across observations. Right: $L_{\mathrm{X}}$ vs. $\Gamma$ with $N_{\mathrm{H}}$ tied across observations; three outliers (XM5, C1, C3) are excluded to better highlight the correlation, please refer to Section \ref{['Powerlaw Parameter Correlation Study']} for more details. Data points correspond to individual PL fit results, and error bars represent the 90 % confidence interval from spectral fitting. Best-fit regression lines are shown in black.
  • ...and 4 more figures