Seminars
Limitations of Truncated Disk Models in Fitting Flux and Polarization X-ray Spectra of Cygnus X-1 in the Hard State
Sohee Chun (Washington University in St. Louis)
January 8(Wed), 2025
16:30
IBS HQ B109
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** Date: January 8th (Wed) 4:30 pm - 5:30 pm
** Place: B109
** Speaker: Sohee Chun (Washington University in St. Louis)
** Date: January 8th (Wed) 4:30 pm - 5:30 pm
** Place: B109
** Speaker: Sohee Chun (Washington University in St. Louis)
** TITLE : ``Limitations of Truncated Disk Models in Fitting Flux and Polarization X-ray Spectra of Cygnus X-1 in the Hard State"
** Abstracts:
The energy spectra of black hole X-ray binaries (BHXRBs) in the hard state are characterized by a hard power-law component, which is attributed to hot coronal plasma that Comptonizes photons from the accretion disk. The truncated disk-hot inner flow model assumes a standard geometrically thin, optically thick accretion disk transitioning into a geometrically thick, optically thin corona inside the disk truncation radius. In this article, we report on detailed general relativistic ray tracing simulations of the truncated disk model that we carried through to evaluate quantitatively if the model can describe the observed BHXRB energy spectra. We also present the spectropolarimetric analysis of archival Cygnus X-1 NuSTAR and NICER observations taken on May 15-21, 2022. Our result shows that the truncated disk model with disk seed photons fails to describe the observational data as it overpredicts the flux of the thermal component and underpredicts the flux of the non-thermal component of the spectral data. The reason is simple: although the disk intercepts some of the emission from the accretion disk, a good fraction of the emission escapes towards the observer (direct emission). The truncated disk model always produces too much direct emission. We end the paper with a discussion of which modifications can bring the truncated model predictions into agreement with the observational data, i.e., additional synchrotron seed photons inside the corona or shadowing of the direct emission.
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