Gravitational lensing and time delay by Kerr-Newman Black Hole
Tien Hsieh (National Dong Hwa University)
We study the strong gravitational lensing due to a Kerr-Newman black hole with angular momentum a and charge Q. We first derive the analytical expressions of the deflection angles of light rays that particularly diverge as they travel near the photon sphere. In this strong deflection limit (SDL), the light rays can circle around the black hole multiple times before reaching the observer, giving a series of relativistic images. The obtained analytical expressions are applied to compute the angular positions of relativistic images by lensing system due to the supermassive galactic black holes, and then we focus on the outermost image with reference to the optical axis and find the distinguishable angular separation between two relativistic images. Moreover, we study the time delay between two relativistic images due to strong gravitational lensing of the light rays caused by the Kerr-Newman black hole by using the expressions of the deflection angles in SDL allows us to analytically develop the formalism for the travel time of the light from the distant source winding around the black hole several times and reaching the observer, and there are some nontrivial and interesting time delay phenomena by rotating black holes.
On extreme space weather habitability impact of TRAPPIST-1 exoplanets
Yu-Hsiang Weng (National Central University)
TRAPPIST-1, an M8 dwarf star, has seven exoplanets in its planetary system; three (TRAPPIST-1e, f, g) are in the habitable zone (HZ). Both stellar wind and flares could be detrimental to the maintenance of a life-protecting atmosphere. Coronal mass ejections (CMEs) accompanying intense stellar flares may hit the exoplanets with a probability of about 4.96%, as estimated in our study. The atmospheric mass loss rate via MHD interaction with the stellar wind and CME of TRAPPIST-1 can be determined if the corresponding flare frequency distribution (FFD) can be derived from K2 or TESS observations. From this information, the total atmospheric mass losses of the individual habitable exoplanets in 10 Gyr (the approximate age of TRAPPIST-1) by adopting a Venus-like atmospheric interaction on TRAPPIST-1 exoplanets are estimated to be in the range of 10^21 g to 10^25 g.