Faint X-ray Sources in the Core of the Globular Cluster M28 the Globular Cluster M28 Was Observed with the High Resolution Imager

Globular clusters, the most ancient stellar groups in our Galaxy, are known to contain bright X-ray sources, faint X-ray sources and millisecond pulsars. The bright X-ray sources are neutron stars accreting matter from a companion. The millisecond pulsars are believed to be descendents of these sources. At the present time, the origin of the faint sources is unclear. Here we report X-ray satellite observations of the globular cluster M28 with high angular and good spectral resolution. The two datasets allow us to determine the spectrum of faint X-ray cluster sources for the rst time. The high angular resolution image shows an extended source, slightly ooset from the center of the cluster and a point source. The point source pulsates at the same period as the well known 3-ms pulsar 1, 2]. We nd that the extended source has a spectrum which extends to 10 keV, i.e. is \hard". We discuss plausible models and conclude that this source is either a collection of low luminosity accreting neutron star binaries, or, a synchrotron nebula powered by a recent outburst of energy from an unknown source. Sensitive optical and X-ray observations have the potential to distinguish between the two models.