arXiv subjects

Siddharth Kumar Sahoo

Publications and source records attributed to Siddharth Kumar Sahoo.

5 recordsLinked to original sources

arXiv2026

A note on methods for computing the critical curve of Kerr-like black holes

This study systematically compares Bardeen's, de Vries's, and Grenzebach et al.'s celestial coordinate definitions of the critical curve ("shadow") of Kerr-like black holes. We find that all three definitions agree for black holes in vacuum or surrounded by inhomogeneous plasma observed from large distances. However, they diverge for observers located at a finite distance: Bardeen's definition yields the smallest critical curve, while de Vries's yields the largest. When homogeneous plasma is considered, critical curve computed using Bardeen's definition deviates from the other two even at large distances and contracts compared to the vacuum case with increasing plasma density. This is in clear contradiction with the behaviour predicted by de Vries's, Grenzebach et al.'s definitions, and previous gravitational lensing studies. We derive de Vries's definition assuming a critical curve on the observer's sky plane and explain its discrepancy with Grenzebach et al.'s definition. We further explore the effect of the change of tetrad on the critical curve. Using Bardeen and Carter tetrads, we plot the critical curve for Schwarzschild and Kerr black holes in the presence of plasma, highlighting that tetrad changes introduce only a horizontal shift in the critical curve.

gr-qc↗

arXiv2026

Investigating the interplay of the braneworld gravity and the plasma environment on the black hole shadow

We investigate the shadow of a rotating braneworld black hole in dispersive plasma environments and assess the potential of the Event Horizon Telescope (EHT) observations to constrain braneworld gravity. The spacetime around a rotating braneworld black hole is modelled by a Kerr-Newman-like metric determined by its mass $M$, spin $a$, and tidal charge $q$, which encodes the gravitational effects of the bulk spacetime. We consider both inhomogeneous and homogeneous plasma environments characterised by plasma parameters $α_i$ ($i=1,2\text{ and }3$) to study light propagation and the interplay of the background spacetime and the plasma environment in influencing the shadow size and shape. We find that as the plasma density increases, inhomogeneous plasma environments decrease the shadow size, however homogeneous plasma enlarges it. On studying the effect due to the background spacetime, we find that $q<0$ (negative tidal charge) increases the shadow diameter, while $q>0$ decreases it. Using the EHT measurements of M87* and Sgr A*, we constrain the $(q,α_i)$ parameter space. The EHT data constrains the tidal charge in the range $-1.15 \lesssim q \lesssim 0.45$ for M87* and $-0.65 \lesssim q \lesssim 0.8$ for Sgr A* in the low density plasma limit, which is indeed the case for M87* and Sgr A*. However, for black holes surrounded by high density plasma, the shadow size is governed both by the background geometry as well as by the plasma environment. In such cases, joint constraints from plasma density estimates and observed shadow angular diameters can provide valuable insights into the underlying spacetime geometry.

gr-qc↗

arXiv2025

Deciphering signatures of Kerr-Sen black holes in presence of plasma from the Event Horizon Telescope data

The present work explores the role of the dilaton charge $r_2$ and the plasma environment in explaining the observed images of M87* and Sgr A*. Dilaton charges are associated with Kerr-Sen black holes, the stationary, axi-symmetric black hole solution in the Einstein-Maxwell-dilaton-axion (EMDA) gravity which arise in the low energy effective action of superstring theories. We investigate the impact of the background spacetime (here dilaton charge and spin) and the plasma environment in modifying the shape and size of the black hole shadow. The theoretically derived shadow is compared with the observed images of M87* and Sgr A* which enable us to constrain the background spacetime in presence of the plasma environment. { Our analysis reveals that the shadow of M87* favors the Kerr scenario and rules out $r_2>0.48$, while the shadow of Sgr A* exhibits a marginal preference towards the Kerr-Sen scenario (although GR is allowed within 1-$σ$) and rules out $r_2>1$. Thus, large values of dilaton charge are disfavored for M87* and Sgr A* and this result holds good irrespective of the inhomogeneous plasma environment. Moreover, the shadows of M87* and Sgr A* rule out very dense inhomogeneous plasma environments surrounding these objects but the plasma density is further constrained from the electron number density and accretion rate estimates. As a consequence, with the current level of precision of the shadow related data we cannot distinguish between the Kerr and mildly charged Kerr-Sen black holes. }

gr-qc↗

arXiv2023

Imprints of Einstein-Maxwell dilaton-axion gravity in the observed shadows of Sgr A* and M87*

Einstein-Maxwell dilaton-axion (EMDA) gravity provides a simple framework to investigate the signatures of string theory. The axion and the dilaton fields arising in EMDA gravity have important implications in inflationary cosmology and in addressing the late time acceleration of the universe. It is therefore instructive to explore the implications of such a model in explaining the astrophysical observations. In this work we explore the role of EMDA gravity in explaining the observed shadows of black holes (M87* and Sgr A*) released by the Event Horizon Telescope (EHT) collaboration. The Kerr-Sen metric represents the exact, stationary and axisymmetric black hole solution of EMDA gravity. Such a black hole is characterized by the angular momentum $a$ acquired from the axionic field and the dilatonic charge $r_2$ arising from string compactifications. We study the role of spin and the dilaton charge in modifying the shape and size of the black hole shadow. We note that black holes with larger dilaton charge cast a smaller shadow. We investigate the consequences of such a result in addressing the EHT observations of M87* and Sgr A*. Our analysis reveals that the shadow of M87* exhibits a preference towards the Kerr scenario. However, when 10% offset in the shadow diameter is considered, $0.1\lesssim r_2\lesssim 0.3$ is observationally favored within 1-$σ$. The shadow of Sgr A* on the other hand shows a preference towards the Kerr-Sen scenario since the central value of its shadow can be better explained by a non-zero dilaton charge $0.1 \lesssim r_2 \lesssim 0.4$. However, when the 1-$σ$ interval is considered the Kerr scenario is included. We discuss the implications of our results.

gr-qc↗

arXiv2022

Quasar continuum spectrum disfavors black holes with a magnetic monopole charge

Black holes carrying a magnetic monopole charge are a subject of interest for a long time. In this work we explore the possibility of an observational evidence of such black holes carrying a magnetic monopole, namely the Bardeen rotating black holes. We derive the theoretical spectrum from the accretion disk surrounding a Bardeen black hole using the thin-disk approximation. We compare the theoretically derived spectrum in comparison to the optical data of eighty Palomar Green quasars to constrain the monopole charge parameter $g$ and the spin parameter $a$ of the quasars. From our analysis we note that the Kerr-scenario in \gr\ is observationally more favored than black holes with a monopole charge. We arrive at such a conclusion using error estimators like $χ^2$, the Nash-Sutcliffe efficiency, the index of agreement and their modified forms. In particular, black holes with $g \geq 0.03$ are outside $99\%$ confidence interval. The implications are discussed.

gr-qc↗