ScholarWorks@성균관대학교

초록

This study investigates a black hole surrounded by a cloud of strings and a cosmological dark fluid characterized by a modified Chaplygin-like equation of state (MCDF), p = Ap -B/p(beta) . We analyze its geodesic structure, shadow, and optical appearance. An analysis of the effective potential and epicyclic frequencies reveals that the existence of innermost/outermost stable circular orbits (ISCOs/OSCOs) for timelike particles is controlled by the parameters of the MCDF and the cloud of strings. The behavior of orbital conserved quantities and the Keplerian frequency are also examined. By equating the influence of the MCDF on the spacetime metric at spatial infinity with that of a cosmological constant, we constrain the MCDF parameters using the observed shadow radii of Sgr A* and M87*. We investigate the effects of the cloud of strings and MCDF on the shadows and optical images of the black hole, assuming various thin disk accretion profiles. Using the method developed by Wald and collaborators, light trajectories are classified by their impact parameters into direct emission, the lensing ring, and the photon ring. The presence of OSCOs can lead to the existence of outer edges in the direct emission and lensing ring images. The observed brightness primarily originates from direct emission, with a minor contribution from the lensing ring, whereas the contribution of the photon ring is negligible owing to extreme demagnification. The influence of the cloud of strings and MCDF parameters on all results is analyzed throughout the study.

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