Absorption Of Matter Field And Gravitational Lensing In Modified Black Hole Spacetime

Black holes are compact objects predicted by the general relativity.The spacetime around them is curved by strong gravity,which causes light rays to be unable to escape from their event horizons.Although we cannot directly observe black holes,we can verify the existence and absence of black holes and related physical information by studying the scattering,Hawking radiation,gravitational lensing and accretion disk,and so on.This thesis focuses on two aspects of the absorption cross section and weak-field gravitational lensing effect in modified black hole spacetime.In the first part of this thesis,we analyze the absorption of different matter fields of the different static spherically symmetric black holes for investigating the absorption cross section of different modified black holes and the influence of black hole parameters on the absorption cross section.The researches include:Firstly,we study the absorption cross section of the massless Dirac field for the ESTGB black hole with the magnetic charge.The results show that as the absolute value of magnetic charge increases,the graybody factor and absorption cross section gradually decrease.Additionally,compared to the absorption cross section of Reissner-Nordstr¨om black hole,we find that the absorption cross section of ESTGB black hole is strengthened in the low-to-high frequency region.Secondly,the absorption cross section of the massless scalar field for a Schwarzschild black hole surrounded by a cloud of strings in Rastall gravity is studied using the sinc approximation and the partial waves method.It is found that in the mid-to-high frequency region,the absorption cross section given by these two methods is consistent and oscillates around the geometric capture cross section.Then,we analyze the Hawking radiation of the scalar field for a Schwarzschild black hole surrounded by a cloud of strings in Rastall gravity.We find that the angular quantum number l,string parameter a,and Rastall gravity parameter β(β > 0)will suppress the power emission spectrum of Hawking radiation.However,when the Rastall gravity parameters β are less than zero,the power emission spectrum will be enhanced first and then suppressed as the parameters β decrease.In the second part of this thesis,the ESTGB black hole is used as the research background,and the gravitational lensing effect of the ESTGB black hole in the presence of plasma is analyzed.We discuss the effects of magnetic charge and plasma parameters on deflection angle and magnification of the image.The results show that the deflection angle and magnification of the image increase with the decrease of the absolute value of magnetic charge.However,with the increase of plasma parameters,the deflection angle and image magnification around the black hole in the uniform plasma increase exponentially,while in the case of non-uniform plasma,the deflection angle and image magnification decrease linearly.