On the Possibility of Observing the Shapiro Effect for Pulsars in Globular Clusters

TL;DR

This paper explores the potential to observe the Shapiro effect in pulsars within globular clusters to detect invisible objects and study matter distribution, providing models and event rate estimates.

Contribution

It introduces a method to observe the Shapiro effect in pulsars in globular clusters and estimates event rates considering Galactic and cluster matter distributions.

Findings

Event probabilities depend on Galactic position and mass distribution models.

Ratios of event probabilities for certain clusters are comparable to those within the clusters.

Estimated event rates include contributions from brown dwarfs in the Galactic disk.

Abstract

For pulsars in globular clusters, we suggest using observations of the relativistic time delay of their radiation in the gravitational eld of a massive body (the Shapiro effect) located close to the line of sight to detect and identify invisible compact objects and to study the distribution of both visible and dark matter in globular clusters and various components of the Galaxy. We have derived the dependences of the event probability on the Galactic latitude and longitude of sources for two models of the mass distribution in the Galaxy: the classical Bahcall-Soneira model and the more recent Dehnen-Binney model. Using three globular clusters (M15, 47 Tuc, Terzan 5) as an example, we show that the ratios of the probability of the events due to the passages of massive Galactic objects close to the line of sight to the parameter f2 for pulsars in the globular clusters 47 Tuc and M15 are comparable to those for close passages of massive objects in the clusters themselves and are considerably higher than those for the cluster Terzan 5. We have estimated the rates of such events. We have determined the number of objects near the line of sight toward the pulsar that can produce a modulation of its pulse arrival times characteristic of the effect under consideration; the population of brown dwarfs in the Galactic disk, whose concentration is comparable to that of the disk stars, has been taken into account for the first time.