Estimates for very high energy gamma rays from globular cluster pulsars

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 impulsed 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. This MSP population is likely to have significant pair production inhibition, so that slot gaps and outer gaps are not expected to form in the pulsar magneto spheres. The utilisation of unscreened pulsar potentials is therefore justified, and fundamental tests for the predicted average single pulsar gamma‐ray luminosities and associated particle acceleration are simplified. 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