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Session 45 - New Insights on X-ray Binaries.
Display session, Wednesday, June 11
South Main Hall,

[45.06] An Unusual X-ray Burst from the Globular Cluster M28

E. V. Gotthelf (GSFC/NASA), S. R. Kulkarni (Caltech)

We have discovered an unusual X-ray burst coincident with the Globular Cluster M28 (NGC 6626) using data acquired with the ASCA Observatory. The burst was recorded by all four ASCA telescopes and consists of a fast (\sim 1 msec) risetime pulse with a short exponential decay (T_1/e \sim 7 secs), followed by an afterglow lasting between 1000 - 2000 seconds. At its peak, the GIS--2 + GIS--3 count rate was \sim 28 cps, over 100 times the M28 rate, with an afterglow as bright as the quiescent M28 flux. This burst is unusual in two ways: it is intrinsically sub-luminous, \sim 0.02L_Edd and seems to originates from a LMXB fainter than the usual bursters by three orders of magnitude. We propose a connection with the mysterious low luminosity sources in clusters, and consider these as magnetized neutron stars accreting matter at a very low rate. The accreted material is probably confined to the polar cap. We suggest that each cluster low luminosity source emits a dozen such bursts every year. These should be detectable by sensitive large area monitor detectors that are planned in some future X-ray missions. Discovery of additional such bursts would validate our hypothesis and provide new points for comparison with the theory of X-ray bursters.

Program listing for Wednesday