2019-05-17 11:00  P7F

Physical Origin of the Asymmetric Ring in the Event Horizon Telescope Result

Masanori Nakamura

The Event Horizon Telescope (EHT) has mapped the central compact radio source of the giant elliptical galaxy M87 in 2017. With unprecedented angular resolution of ~ 20 micro arcsecond, the first image reveals an asymmetric ring-like morphology with a central brightness depression. A black hole (BH) is expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. EHT 2017 observations give a shadow diameter of ~ 42 micro arcsecond, indicating that the region contains a supermassive black hole with 6.5 billion solar masses. We briefly overview the physical implications of the asymmetric ring. EHT theory working group constructed a large library of models based on general relativistic magnetohydrodynamic simulations of magnetize accretion flows and synthetic images produced by GR ray tracing. Overall, the observed image is consistent with expectations for the shadow of a BH. The asymmetry in brightness of the ring can be explained regarding the relativistic beaming of the emission from a plasma rotating close to the speed of light around a BH. Relativistic jets in M87 are well-known during last century, but EHT 2017 observation could not reveal the signature. This allows us a wider variety of models for the asymmetric ring. We discuss the next procedure toward a further constraint on the parameter space. It will provide more clear picture of the physical condition in the vicinity of the BH.