An Unusual X-Ray Burst from the Globular Cluster M28

We report the discovery of an unusual X-ray burst from the direction of theGlobular Cluster M28 using data acquired with the ASCA Observatory. The burstwas recorded by all four ASCA telescopes and displays a fast (~ 70 ms) risefollowed by an exponential decay (t = 7.5 s) and a steady afterglow which lastsbetween 800 - 3250 s. The image of the burst is consistent with an ASCA pointsource and is centered on quiescent X-ray emission from the core of M28. Theburst temporal profile is similar to Type-I bursts emitted by accreting neutronstars of low mass X-ray binaries (LMXB). We argue that the burst arises from anLMXB that is located in the core of M28. The burst is unique in two ways: it isintrinsically sub-luminous, ~ 0.02 L_Edd and more importantly, originates froma source whose quiescent luminosity is fainter than that of the known clusterbursters by three orders of magnitude. We suggest that this burst is from ahighly magnetized neutron star accreting at a low rate. These accreting systemsmay account for the mysterious low luminosity X-ray sources in globularclusters.Comment: 13 pages with 3 eps figures, LATEX, aastex, psfig. To appear in the Astrophysical Journal Letter