Estimates for Very High Energy Gamma Rays from Globular Cluster Pulsars

TL;DR

This paper predicts very high energy gamma-ray emission from globular clusters by modeling millisecond pulsars' particle injection and radiation processes, suggesting potential detectability with current telescopes like H.E.S.S.

Contribution

It introduces a method to estimate unpulsed gamma-ray emission from globular cluster pulsars using Monte Carlo simulations and MSP data, providing lower limit predictions.

Findings

IC radiation from 47 Tucanae may be detectable by H.E.S.S.

Predicted diffuse radiation could be challenging to distinguish from unresolved sources.

Results can be scaled to other globular clusters assuming similar MSP properties.

Abstract

Low-Mass X-ray Binaries (LMXRBs), believed to be the progenitors of recycled millisecond pulsars (MSPs), occur abundantly in globular clusters (GCs). GCs are therefore expected to host large numbers of MSPs. This is also confirmed observationally. The MSPs continuously inject relativistic electrons into the ambient region beyond their light cylinders, and these relativistic particles produce unpulsed radiation via the synchrotron and inverse Compton (IC) processes. It is thus possible, in the context of General Relativistic (GR) frame-dragging MSP models, to predict unpulsed very high energy radiation expected from nearby GCs. We use a period-derivative cleaned sample of MSPs in 47 Tucanae, where the effects of the cluster potential on the individual period derivatives have been removed. Using a Monte Carlo process to include effects of pulsar geometry, we obtain average injection spectra (with relatively small errors) of particles leaving the MSPs. These spectra are next used to predict cumulative synchrotron and IC spectra expected from 47 Tucanae, which is a lower limit, as no reacceleration is assumed. We find that the IC radiation from 47 Tucanae may be visible for H.E.S.S., depending on the nebular field B as well as the number of MSPs N in the GC. Telescopes such as Chandra and Hubble may find it difficult to test the SR component prediction of diffuse radiation if there are many unresolved sources in the field of view. These results may be rescaled for other GCs where less information is available, assuming universal GC MSP characteristics.