Testing gravitational lensing effects by supermassive massive black holes with superstring theory metric: Astrophysical implications and EHT constraints

We investigate gravitational lensing, both strong and weak, induced by charged black holes within heterotic superstring theory - a ' -corrected Reissner-Nordstr & ouml;m (RN) black holes characterized by an additional parameter, denoted as a , in addition to mass M and charge Q . Our findings reveal that as the parameter a increases, various crucial parameters such as the unstable photon orbit radius r ph , critical impact parameter u ph , deflection angle a D ( 0 ), and angular position 0 infinity also increase. Concurrently, the angular separation s decreases, while the relative magnitude r mag increases with the growing value of a . Utilizing supermassive black holes (SMBH) Sgr A* and M87* as gravitational lenses, the deflection angle resulting from the a ' -corrected RN black hole, in both strong and weak field limits, surpasses that of the RN black holes ( a = 0 ) and falls below that of Schwarzschild black holes ( a = 0, Q = 0 ). For Sgr A*, the angular position 0 infinity lies within (24.08, 26.38) mu as, while for M87* it ranges from (18.23, 19.96) mu as. Estimating the time delay between the first and second relativistic images using twenty supermassive galactic center black holes as lenses, we find, for instance, that the time delay for Sgr A* and M87*, with Q = 0 5 and a = 5, can reach approximately 9.81 min and 14835.6 min, respectively. Remarkably, the EHT results for Sgr A* impose more stringent limits on the parameter space of a ' -corrected RN black holes compared to those established by the EHT results for M87*. Our analysis deduces that, within the 1 a- region, a substantial portion of the parameter space for a ' -corrected RN black holes aligns with the Event Horizon Telescope (EHT) results for M87* and Sgr A*, and that and one cannot rule out the possibility of that heterotic superstring theory's a ' -corrected RN black holes being strong candidates for astrophysical black holes.