Testing strong gravitational lensing effects by supermassive compact objects with regular spacetimes

TL;DR

This paper investigates strong gravitational lensing effects by regular electrically charged black holes and no-horizon spacetimes, analyzing observable differences and constraints using astrophysical data from Sgr A*, M87*, and others.

Contribution

It provides a comparative analysis of lensing effects in regular black holes and no-horizon spacetimes, introducing new observable signatures and constraints based on recent astronomical observations.

Findings

Lensing observables vary with charge parameter b.

Time delays differ significantly from Schwarzschild predictions.

REC black holes are consistent with EHT observations, no-horizon spacetimes are ruled out.

Abstract

We compare and contrast gravitational lensing, in the strong-field limit, by photon sphere in spherically symmetric regular electrically charged (REC) black holes ($0<b\leq b_E$) and with those by corresponding REC no-horizon spacetimes ($b>b_E$). Here, $b$ is additional parameter due to charge and the value $b=b_E \approx 0.226$ corresponds to an extremal black hole with degenerate horizons. Interestingly, the spacetime admits photon sphere for $0<b\leq b_P \approx 0.247$ and an anti-photon sphere only for $b_E < b \leq b_P$. With no-horizon spacetime, images by lensing from the inside of the photon sphere ($u<u_{ps}$) can also appear. Interestingly, for the case $u<u_{ps}$ the deflection angle $\alpha_D$ increases with $u$. We analyse the lensing observables by modelling compact objects Sgr A*, M87*, NGC4649, and NGC1332 as black holes and no-horizon spacetimes. The angular position $\theta_{\infty}$ and photon sphere radius $x_{ps}$ decrease with increasing parameter $b$. Our findings suggest that the angular separations ($s$) and magnification ($r$) of relativistic images inside the photon sphere may be higher than those outside. Moreover, the time delay for Sgr A* and M87* can reach $\sim$ 8.8809 min and $\sim$ 12701.8 min, respectively, at $b = 0.2$, deviating from Schwarzschild black holes by $\sim$ 2.615 min and $\sim$ 4677 min. These deviations are insignificant for Sgr A* because it is too small, but they are sufficient for astronomical observation of M87* and some other black holes. With EHT bounds on $\theta_{sh}$ of Sgr A* and M 87*, within $1 \sigma$ region, placing bounds on the parameter $b$, our analysis concludes that the REC black holes agree with the EHT results in finite space, whereas the corresponding REC no-horizon spacetimes are completely ruled out.