An unusual X-ray burst from the globular cluster M28

We report the discovery of an unusual X-ray burst from the direction of the globular cluster M28 using data acquired with ASCA. The burst was recorded by all four ASCA telescopes and displays a fast (less than or similar to 70 ms) rise followed by an exponential decay (tau = 7.5 s) and a steady afterglow that lasts between 800 and 3250 a. The image of the burst is consistent with an ASCA point source and is centered on quiescent X-ray emission from the core of M28. The burst temporal profile is similar to type I bursts emitted by accreting neutron stars of low-mass X-ray binaries (LMXBs). We argue that the burst arises from a LMXB that is located in the core of M28. The burst is unique in two ways: it is intrinsically subluminous, similar to 0.02L(E), and more importantly it originates from a source whose quiescent luminosity is fainter than that of the known cluster bursters by 3 orders of magnitude. We suggest that this burst is from a highly magnetized neutron star accreting at a low rate. These accreting systems may account for the mysterious low-luminosity X-ray sources in globular clusters.