O. Kosmas | Searcharxiv

arXiv2026

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

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.

astro-ph.HE↗

arXiv2024

A simplified approach for reproducing fully relativistic spectra in X-ray binary systems: Application to Cygnus X-1

General relativistic effects are strong near the black hole of an X-ray binary and significantly impact the total energy released at the innermost accretion disk's region. Our goal is to fully incorporate the black hole's spin and all the general relativistic effects on the observed spectra coming from X-ray binary systems while maintaining the simplicity of the standard disk model. That is possible by appropriately shifting only the disk's inner radius. We employ some of the most efficient pseudo-Newtonian potentials around Kerr black holes and derive two generalized disk temperature profiles, thus incorporating the spin's contribution to the thermal spectra. Then, we associate the observed radiative efficiency with the emission pattern featuring all the relativistic effects included in the kerrbb model, obtaining an expression about the modified inner radius of the disk. Moreover, we apply this method to Cygnus X-1 by fitting the observational data obtained during its high/soft and hard/low spectral states. The fully relativistic spectra are reproduced to a very good approximation with an error margin of 0.03-4%. The disk is parameterized by a modified innermost radius within the range of $(0.2-2)R_{ISCO}$, depending on the source's viewing angle and black hole spin. Relativistic effects near the black hole make an otherwise standard accretion disk with inclination $\theta <60^{\circ}$ seem truncated to larger radii to a distant observer. On the other hand, an edge-on view of the disk gives the perspective of being pulled closer to the central object than the respective ISCO radius. In addition, we show that the observational data of Cygnus X-1 can be satisfactorily fitted by employing a reasonably simple lepto-hadronic jet model and a hybrid thermal/non-thermal corona along with the Kerr-adjusted standard accretion disk.

astro-ph.HE↗