Glow-in-the-dark globular clusters: modelling their multiwavelength lanterns

Abstract: Globular clusters (GCs) are astronomical tapestries embroidered with an abundance of exotic stellar-type objects. Their high age promises a rich harvest of evolved stellar products, while the deep potential wells and high mass densities at their centres probably facilitate the formation of multiple-member stellar systems via increased stellar encounter rates. The ubiquity of low-mass X-ray binaries, thought to be the progenitors of millisecond pulsars (MSPs), explain the large number of observed GC radio pulsars and X-ray counterparts. The Fermi Large Area Telescope (LAT) recently unveiled the first gamma-ray GC pulsar (PSR J1823-3021A). The first observations of GCs in the GeV and TeV bands furthermore created much excitement, and in view of the above, it seems natural to explain these high-energy lanterns by investigating an MSP origin. An MSP population is expected to radiate several pulsed spectral components in the radio through gamma-ray wavebands, in addition to being sources of relativistic particles. The latter may interact with background photons in the clusters producing TeV excesses, while they may also radiate synchrotron photons as they traverse the cluster magnetic field. We present multiwavelength modelling results for Terzan 5. We also briefly discuss some alternative interpretations for the observed GC gamma-ray signals.