X-ray Polarimetry as a Tool to Measure the Black Hole Spin in Microquasars: Simulations of IXPE Capabilities (2301.04002v2)

Abstract: Measurements of the angular momentum (spin) of astrophysical black holes are extremely important, as they provide information on the black hole formation and evolution. We present simulated observations of a X-ray binary system with the Imaging X-ray Polarimetry Explorer (IXPE), with the aim to study the robustness of black hole spin and geometry measurements using X-ray polarimetry. As a representative example, we used the parameters of GRS 1915+105 in its former unobscured, soft state. In order to simulate the polarization properties, we modeled the source emission with a multicolor blackbody accounting for thermal radiation from the accretion disk, including returning radiation. Our analysis shows that the polarimetric observations in the X-ray waveband will be able to estimate both spin and inclination of the system with a good precision (without returning radiation we obtained for the lowest spin $\Delta a \leq 0.4$ (0.4/0.998 $\sim$ 40%) for spin and $\Delta i \leq 30^\circ$ (30$^\circ$/70$^\circ$ $\sim$ 43%) for inclination, while for the higher spin values we obtained $\Delta a \leq 0.12$ ($\sim$ 12%) for spin and $\Delta i \leq 20^\circ$ ($ \sim $ 29%) for inclination, within 1$ \sigma $ errors). When focusing on the case of returning radiation and treating inclination as a known parameter, we were able to successfully reconstruct spin and disk albedo in $\Delta a \leq 0.15$ ($\sim$ 15%) interval and $\Delta$ albedo $\leq 0.45$ (45%) intervals within 1$ \sigma $ errors. We conclude that X-ray polarimetry will be a useful tool to constrain black hole spins, in addition to timing and spectral-fitting methods.