arXiv subjects

Amnish Vachher

Publications and source records attributed to Amnish Vachher.

4 recordsLinked to original sources

arXiv2026

The Influence of Uniform Magnetic Fields on Strong Field Gravitational Lensing by Kerr Black Holes

We investigate strong gravitational lensing using magnetized Kerr black holes (MKBHs), which are accurate Kerr-Bertotti-Robinson solutions for Kerr black holes in a uniform magnetic field with additional magnetic field strength $B$ apart from mass $M$ and spin $a$. Unlike previous magnetized spacetimes, the MKBH geometry is Petrov type D, devoid of conical singularities, allowing photons to reach asymptotic infinity and making the concept astrophysically feasible. We use the strong deflection limit formalism to calculate the photon sphere radius, critical impact parameter, deflection angle, and lensing observables including the image position $θ_\infty$, angular separation $s$ and relative magnification $r_{\text{mag}}$, as well as their relationships with the parameters $a$ and $B$. Our results reveal that the relativistic image's photon sphere and angular size increase with $B$, whereas lensing observables deviate significantly from the Kerr scenario. For M87*, with $a=0.9$, the angular position of relativistic images increases from $10.8~μ$as (Kerr) to $12.02~μ$as, and the time delay between the first two images increases from $158.5$ h to $176$ h at $B=0.4$. Similarly, for Sgr A*, the image position increases from $14.4~μ$as to $16~μ$as, with time delays enhanced by approximately $0.7$ minutes. The relative magnification $r_{\text{mag}}$ grows with $B$ and deviates by $0.53$ from Kerr black holes at $B=0.4$. Our findings highlight strong gravitational lensing as a powerful tool to probe the presence of magnetic fields around astrophysical black holes, and in particular, we demonstrate that the MKBH spacetime enables constraints on the parameters $a$ and $B$.

gr-qc↗

arXiv2025

Testing Strong Gravitational Lensing Effects of Supermassive Black Holes with String-Inspired Metric: Observational Signatures and EHT Constraints

We analyze gravitational lensing in the strong field limit for spherically symmetric string-inspired Euler-Heisenberg black holes, characterized by magnetic charge ($q$) and Einstein-Maxwell-dilaton coupling constants ($α, β$) from the low-energy limit of heterotic string theory. Our results show that the string coupling has a weak impact on the positions of relativistic images, deflection angles, photon orbit radii, and shadow sizes, making these black holes indistinguishable from the Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) black holes with the same mass and charge. Compared to Schwarzschild black holes, the string-inspired Euler-Heisenberg black holes exhibit smaller deflection angles, decreasing with increasing charge. Moreover, the time delay for Sgr A * and M87 * can reach $~11.477$ and $~17349.8$ minutes, respectively, at $q=0.1$ and $η=-1$, deviating from Schwarzschild black holes by $~0.0198$ and $~28.9$ minutes, which are not very significant. For Sgr A* and M87*, we determine $θ_\infty$ range within $(11.52, 26.33)~μas$, and $(9.17, 19.78)~μas$ respectively, with angular separations $s$ ranging from $(3.29-6.85)~nas$ for Sgr A* and $(2.47-5.15)~nas$ for M87*. EHT bounds on the $θ_{sh}$ of Sgr A* and M87* within the $1σ$ interval bound the $q$ as: for Sgr A* $0.54109\le q \le 0.7796 $ and for M87* $0< q \le 0.29107$, while in both the cases, we did not find any bound on the parameter $η$. We show that string-inspired Euler-Heisenberg black holes and EHT observations agree in the finite parameter space. A discussion on the effective metric has been included.

gr-qc↗

arXiv2024

Strong Gravitational Lensing by Rotating Quantum-Corrected Black Holes: Insights and Constraints from EHT Observations of M87* and Sgr A*

We study gravitational lensing in the strong-field limit using the rotating quantum-corrected black hole (RQCBH) with an additional parameter $α$ besides mass $M$ and spin parameter $a$. We discover a decrease in the deflection angle $α_D$, the photon sphere radius $x_{ps}$, and the angular position $θ_{\infty}$. The flux ratio of the first image to all subsequent images, $r_{mag}$, decreases rapidly as $α$ increases. We compare RQCBH observables with those of Kerr black holes, using Sgr A* and M87* as lenses to observe the effect of the quantum-corrected parameter $α$. For Sgr A*, the angular position $θ_\infty$ in $\in~(14.8-26.3)~μas$, while for M87* $\in~(11.12-19.78)~μas$. The angular separation $s$, for supermassive black holes (SMBHs) SgrA* and M87*, differs significantly, with values ranging $\in~(0.033-0.79)~μas$ for Sgr A* and $\in~(0.033-0.59)~μas$ for M87*. The deviations of the lensing observables $|Δθ_\infty|$ and $|Δs|$ for RQCBH ($a=0.8,α=0.4$) from Kerr black holes can reach up to $1.6~μas$ and $0.41~μas$ for Sgr A*, and $1.2~μas$ and $0.31~μas$ for M87*. The relative magnitude $r_{mag}$ $\in~(1.81-6.82)~μas$. We also compared the time delays between the relativistic images in the 22 SMBHs at the center of various galaxies. We found that RQCBH can be quantitatively distinguished from Kerr black holes. Interestingly, the time delay for Sgr A* and M87* can reach approximately 24.95 min and 308.15 hrs, respectively. Our analysis concludes that, within the 1$σ$ region, a significant portion of the parameter space agrees with the EHT results of M87* and Sgr A*.

gr-qc↗

arXiv2024

Strong Gravitational Lensing by Static Black Holes in Effective Quantum Gravity

We investigate strong gravitational lensing by two static black hole models (Model-1 and Model-2) within the Effective Quantum Gravity (EQG) framework, characterized by mass $M$ and parameter $ζ$. For $ζ= 0$, they reduce to the Schwarzschild solution, and depending on the parameters, they describe black holes with an event and Cauchy horizon (Model-1), a single horizon (Model-2), or no horizons. Using SMBHs Sgr A* and M87* as lenses and integrating theoretical predictions with recent EHT data, we identify significant differences in lensing signatures due to quantum corrections. For Model-1, the deviations of the lensing observables: $|δθ_{\infty}|$ of black holes in EQG from Schwarzschild black hole, for SMBHs Sgr A* and M87, can reach as much as $1.75~μ$as and $1.32~μ$as, while $|δs|$ is about $30.12$~nas for Sgr A* and $22.63$~nas for M87*. The flux ratio of the first image to all subsequent packed images indicates that EQG black hole images are brighter than their Schwarzschild counterparts, with a deviation in the brightness ratio $|δr_{mag}|$ reaching up to 2.02. The time delays between the second and first images, denoted $|δT_{2,1}|$, exhibit substantial deviations from the GR counterpart, reaching up to 1.53 min for Sgr A* and 1159.9 min for M87*. The EHT constraints on $θ_{sh}$ of Sgr A* and M87* within the $1σ$ region limit the parameters $ζ$. Our analysis concludes that EQG black holes are consistent with the EHT observations within this finite space.

astro-ph.CO↗