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

TL;DR

This study models broadband spectra of MAXI J1820+070 during the soft state, revealing a rapidly spinning black hole with mild disk truncation and a warm corona, challenging previous soft excess origin assumptions.

Contribution

It provides new broadband spectral modeling results indicating a high black hole spin and a warm corona, and clarifies the soft X-ray excess origin in MAXI J1820+070.

Findings

Black hole spin estimated to be > 0.75.

Inner disk temperature declines mid-soft state.

Soft excess likely from warm corona beyond 10Rg.

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.