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

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.