New Constraints on the Spin of the Black Hole Cygnus X-1 and the Physical Properties of its Accretion Disk Corona

Abstract

We present a new analysis of NuSTAR and Suzaku observations of the black hole Cygnus X-1 in the intermediate state. The analysis makes use of kerrC, a new model for analyzing spectral and spectropolarimetric X-ray observations of black holes. kerrC builds on a large library of simulated black holes in X-ray binaries. The model accounts for the X-ray emission from a geometrically thin, optically thick accretion disk, the propagation of the X-rays through the curved black hole spacetime, the reflection off the accretion disk, and the Comptonization of photons in coronae of different 3-D shapes and physical properties before and after the reflection. We present the results from using kerrC for the analysis of archival NuSTAR and Suzaku observations taken on May 27-28, 2015. We find that kerrC can account for the overall shape of the energy spectrum, but not for the broad spectral 6-7 keV feature, thought to be relativistically broadened Fe k-α emission from the inner region of the accretion flow. A wedge-shaped corona fits the data slightly better than a cone-shaped corona. The analysis indicates a black hole spin parameter a (−1 ≤ a ≤ 1) between 0.861 and 0.921. The kerrC model provides new insights about the radial distribution of the energy flux of returning and coronal emission irradiating the accretion disk. We use kerrC to predict the polarization signatures that the recently launched Imaging X-ray Polarimetry Explorer will detect and find small, but measurable, polarization fractions on the order of 1%.

Why is black hole entropy affected by rotation?

“…A more recent analysis[4] suggests slightly smaller, but still very large values, in the 0.86 to 0.92 range. Of course, this black hole is by no means unique: for example, the black hole associated with the GW200129 gravitational waves is thought[5] to have a value of a * which may exceed 0.9.…”

Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1

“…We used the general relativistic ray tracing code kerrC (14) to evaluate the polarization that cone-shaped coronae centered on the BH spin axes and wedge-shaped coronae sandwiching the accretion disk can produce. The code assumes a standard geometrically thin, optically thick accretion disk extending from the innermost stable circular orbit to 100 gravitational radii In both cases, we chose corona parameters which maximize the predicted polarization degrees, i.e., cone-shaped coronae very close to the accretion disk, and rather thin wedge-shaped coronae with a half opening angle of 10 • .…”

“…The theoretical expectation of the Cyg X-1 polarization degree in the 2-8 keV IXPE band was around ∼1% or lower depending on the state of the source (9,10,12,14). These expectations are motivated by the low inclination of the system i = 27.…”

“…The plasma of the corona has a temperature of ∼100 keV, much hotter than the ∼ 0.1 keV temperature of the accretion disk photosphere. The polarization of the Comptonized emission can be used to constrain the location and properties of the corona as well as the inner accretion disk (11)(12)(13)(14). The reflection of coronal emission off the accretion disk gives rise to an emission component that includes the iron Kα fluorescence line at ∼ 6.4 keV.…”

Are Binary Black Hole Mergers and Long Gamma-Ray Bursts Drawn from the Same Black Hole Population?

“…If the majority of LGRBs do not end their lives in BBH mergers, they cannot be produced in compact binary systems and must be emitted by single stars (e.g., Aguilera-Dena et al 2020) or in binary systems that do not merge. It may be natural to associate them with systems like the X-ray binaries (lowmass X-ray binaries and high-mass X-ray binaries) found in the galaxy, such as Cygnus X-1 (Zhao et al 2021;Krawczynski & Beheshtipour 2022), GRS 1915+105 (Shreeram & Ingram 2020), or EXO 1846-031 (Draghis et al 2020), which are rapidly spinning and could be the descendants of LGRBs (although Belczynski et al 2021 show that the BH spin determination can be subject to systematic effects). The detection of such systems, especially HMXBs, for which accretion spin-up is limited by the lifetime of the secondary star (e.g., Wong et al 2012), requires the birth of highly spinning BHs (Qin et al 2019;Miller-Jones et al 2021), an expected characteristic of LGRB descendants.…”