On the origin of the gamma-ray emission from Omega Centauri: millisecond pulsars and dark matter annihilation

We explore two possible scenarios to explain the observed gamma-ray emission associated with the atypical globular cluster omega-Centauri: emission from millisecond pulsars (MSP) and dark matter (DM) annihilation. In the first case we predict the total number of MSPs in the globular cluster (n = 45(-16)(+22)) using a new Bayesian method that combines the observational uncertainty in gamma-ray counts with the intrinsic variation in individual pulsar luminosities. A DM interpretation is motivated by the possibility of omega-Centauri being the remnant core of an ancient dwarf galaxy hosting a surviving DM component. At least two annihilation channels, light quarks and muons, can plausibly produce the observed gamma-ray spectrum. We are able to constrain the DM particle mass despite substantial uncertainties in the shape of the density profile (10 GeV less than or similar to m(chi) less than or similar to 30 GeV for annihilation into light quarks and 5 GeV less than or similar to m(chi) less than or similar to 9 GeV for annihilation into muons). We translate upper limits on omega-Centauri's remnant dark matter content into lower limits on the annihilation cross section (<sigma v >) greater than or similar to 10(-29)cm(3)s(-1) and <sigma v > greater than or similar to 10(-28) cm(3)s(-1) for quarks and muons respectively), taking into account the spatial extension of the DM halo. Moreover, there is a relatively small range of DM density profiles which are consistent with omega-Centauri's kinematics and stellar mass and can explain its gamma-ray spectrum while simultaneously evading CMB constraints on dark matter annihilation. Further analysis of omega-Centauri's internal kinematics, and/or additional information on the resident MSP population will yield much stronger constraints and shed light about the origin of this otherwise mysterious gamma-ray source.