Relativistic time delay analysis of pulsar signals near ultra-compact objects

TL;DR

This paper analyzes the relativistic time delay of pulsar signals near ultra-compact objects like JMN-1 and JNW spacetimes, proposing a method to distinguish these from black holes based on signal delay differences.

Contribution

It introduces a relativistic time delay analysis for pulsar signals in specific ultra-compact spacetimes, comparing results with Schwarzschild black holes to identify potential observational signatures.

Findings

Propagation time delays differ significantly between ultra-compact objects and black holes.

Time delays are greater or lesser depending on the spacetime geometry and propagation path.

Results suggest possible observational methods to distinguish ultra-compact objects from black holes.

Abstract

The upcoming discoveries of pulsars orbiting the center of the Milky Way will present unparalleled opportunities to examine the causal structure of the spacetime geometry of Sagittarius A*. In this paper, we investigate the fully relativistic propagation time delay of pulsar signals in the Joshi-Malafarina-Narayan (JMN-1) and Janis-Newman-Winicour (JNW) spacetimes. This delay arises basically from the spacetime curvatures in the vicinity of these ultra-compact objects, induced by the intense gravitational field near the Galactic Center (GC). Using the principles of gravitational lensing, we compute the arrival time of photons originating from a pulsar in orbit around the GC. To validate our approach, we compare our time delay analysis of the Schwarzschild black hole with the corresponding delay in the post-Newtonian framework. Subsequently, we find that the propagation time of pulsar signal is greater and lesser for the given horizon-less ultra-compact objects for direct and indirect propagation respectively. Therefore, our results suggest quite significant propagation time delay differences in JMN-1 and JNW spacetimes, when compared to the Schwarzschild black hole case. This can be inferred as a possible distinguishing feature for these ultra-compact objects' geometries.