Testing Gravity And Observing Accretion Of The Supermassive Black Hole In The Galactic Center

Einstein's general relativity is a successful gravitational theory,which is tested precisely in the weak gravitational field,but poorly understood in the strong gravitational field.One of the fundamental predictions of general relativ-ity is the existence of black holes.Is the general relativity validated in the strong gravitational potential and the highly curved spacetime near the black hole?Is there a necessity of replacing general relativity with modified gravity?The su-permassive black hole candidate in the Galactic Center,Sagittarius A*(Sgr A*),is the closest one to the Earth,with the largest apparent diameter,which is a fundamental physical laboratory to answer all the above questions.On the theoretical aspect,gravitational lensing is one of the phenomena pre-dicted by general relativity.The lensing effects caused by the strong gravitational field near the black hole are quite different from that by the weak gravitational field.The light will bend exceed 2? and wind several loops before escape to the observer.Then,suppose a point source behind the black hole,we will see a series of discrete images on the both sides of the black hole,called relativistic images.Taking Sgr A*as the lens,we can estimate the observable,including the position,brightness,and time delay between the relativistic images,which can be used to quantitatively study the spacetime of the black hole.We use the Strong Deflection Limit(SDL)method to study the strong deflection gravitational lensing and test gravity of the charged Galileon black hole which can escape the hairless theorem and two kinds of black holes without singularity:the modified Hayward black hole and the Lee-Wick black hole.We find the difference of the relativistic images predicted by modified gravity and by general relativity is about 10nas level.On the observational aspect,the millimeter-waves Very Long Baseline Inter-ferometry(mm-VLBI)technique successfully images black holes.A non-illuminating shadow on the electromagnetic radiation released by the relativistic plasma accre-tion flow around the black hole,is expected a bit larger than the event horizon.Consider the shadow is formed by the projection of the innermost unstable or-bit of photons on the orthogonal plane of the sight,the size and the shape of the shadow are only affected by the spacetime but not the accretion flow,so the shadow measurement can be used to test gravity.General relativity predicts the size of the shadow for the Sgr A*is about 50 ?as.The first black hole image in human history has been published by the Event Horizon Telescope(EHT)on April 10,2019,which aims the black hole in the M87 galaxy center.The mea-sured shadow size is 42±3?as,but for the uncertainty of the mass measurements,the result is inadequate to test general relativity.Inspired by the first black hole image of M87,people are looking forward to see the first image of Sgr A*,as well as the shadow measurement result to constrain the gravity theory under the more precise observation of the mass and the distance.However,Sgr A*is different from M87 for its strong variation in the flux and the structure.Therefore,the black hole accretion holds a significant influence on the imaging observation for Sgr A*.Before testing gravity,we must have a good understanding of other black hole model parameters,including:black hole spin,accretion rate,magnetic field parameters,plasma thermodynamic parameters,viewing angle and position angle,etc.We focus on the simulated observation of EHT 2017 and the archival observation of EHT 2009,study the black hole accretion of Sgr A*.To do this,we use a model comparison frame,including the general relativistic magnetohydrodynamics(GRMHD)simulation,the ray-tracing,the observable calculation and the dynamical parameter estimation,under considering the scatter effect by the interstellar medium in our galaxy,to compare the model with the visibility amplitudes and the closure phases and estimate the accretion parameters under the Bayesian statistics.The result of comparing accretion model with the EHT 2009 observation shows that the Sgr A*is a edge-on crescent with strongly asymmetric structure where the bright part is on the east,and under the assumption of a*=0.9375,the electrons probably decouple from protons in the non-low magnetized regions,which need to be verified by the EHT 2017 observations in the future.