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MAXI J1820+070: A rapidly spinning black hole with mild disk truncation in the soft state and a warm corona

Th. V. Papavasileiou, T. S. Kosmas, O. Kosmas, I. Sinatkas

Abstract

Our study seeks to address the debate over the spin of MAXI J1820+070 through broadband spectral modeling of NuSTAR observations obtained during the soft state. We further compare our results with previous spin estimates and examine the source variability across the soft state. In addition, we investigate the origin of the soft X-ray excess, which we argue does not originate from the plunge region as previously suggested. To further investigate the origin of this excess, we calculate spin-dependent radial disk temperature profiles across all epochs. Our results indicate that the black hole in MAXI J1820+070 is rapidly spinning, with spin $a$ > 0.75, potentially powering the relativistic jets. Our analysis reveals a significant decline in the inner disk temperature midway through the soft state, accompanied by a modest increase in the inferred inner disk radius up to 3.5Rg. This behavior is consistent with slight disk truncation, possibly associated with a reduction in gas ionization and nonthermal processes. Furthermore, the soft excess emission below 10 keV is well described by a blackbody component with kT=0.5 keV, approximately 38% cooler than the inner disk. This suggests that the emission may originate from a warm corona layer located beyond 10Rg, analogous to warm Comptonization models proposed to explain the soft X-ray excess in active galactic nuclei.

MAXI J1820+070: A rapidly spinning black hole with mild disk truncation in the soft state and a warm corona

Abstract

Our study seeks to address the debate over the spin of MAXI J1820+070 through broadband spectral modeling of NuSTAR observations obtained during the soft state. We further compare our results with previous spin estimates and examine the source variability across the soft state. In addition, we investigate the origin of the soft X-ray excess, which we argue does not originate from the plunge region as previously suggested. To further investigate the origin of this excess, we calculate spin-dependent radial disk temperature profiles across all epochs. Our results indicate that the black hole in MAXI J1820+070 is rapidly spinning, with spin > 0.75, potentially powering the relativistic jets. Our analysis reveals a significant decline in the inner disk temperature midway through the soft state, accompanied by a modest increase in the inferred inner disk radius up to 3.5Rg. This behavior is consistent with slight disk truncation, possibly associated with a reduction in gas ionization and nonthermal processes. Furthermore, the soft excess emission below 10 keV is well described by a blackbody component with kT=0.5 keV, approximately 38% cooler than the inner disk. This suggests that the emission may originate from a warm corona layer located beyond 10Rg, analogous to warm Comptonization models proposed to explain the soft X-ray excess in active galactic nuclei.
Paper Structure (9 sections, 3 equations, 7 figures, 4 tables)

This paper contains 9 sections, 3 equations, 7 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Data reduction of NuSTAR observations
  3. Best-fit results and spectral analysis
  4. Source evolution across the soft state
  5. Spin implications and comparison with previous estimates
  6. Indications of mild disk truncation
  7. Disk truncation or gas thermalization?
  8. Origin of blackbody excess emission
  9. Conclusions

Figures (7)

  • Figure 1: The merged spectra recorded by the two NuSTAR modules from MAXI J1820+070 in the soft state during the 2018 outburst.
  • Figure 2: Spectra from epochs nu23-nu31 unfolded to the best-fit models along with the respective residuals and data-to-model ratios. Top panel: kerrbb$+$bbody$+$cutoffpl (Model A), bottom panel: kerrbb$+$bbody$+$nthComp (Model B).
  • Figure 3: Contribution of each model component for both Model A (left panel) and Model B (right panel) in the nu27 spectrum. The parameter values employed are the best-fit results of Table \ref{['Table3']}.
  • Figure 4: Source evolution during the soft state based on the variation of the Model B best-fit parameters across all epochs.
  • Figure 5: The evolution of the inner disk radius according to the spin estimations of Model B over epochs nu23-nu31. For comparison, the ISCO radius corresponding to $a$ = 0.988 from Draghis_2023 is also plotted.
  • ...and 2 more figures