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At about 4 hours UT on 2003 January 11, one of the four WXM counters, YB, exhibited an increase in the gain on all of its three anodes, followed by excessive noise half an hour later. The behavior of the YB counter matches that expected for a slow gas leak that began suddenly. Based on detailed analyses of the data, we have concluded that the most likely cause of this failure is the impact of a small piece of space debris or a micrometeorite with a size of tens of microns on the beryllium window of the counter. At the epoch of the event, the WXM window was indeed facing in the direction of the spacecraft's orbital motion. The anticipated probablity of impact by a piece of orbital debris or a micrometeorite of this size within a few years of operation in orbit is of the order of ten percent, which is small, but not negligible.
On 2003 January 15, we resumed normal operation of the XB and YA counters, which use a high voltage power supply that is independent of the high voltage supply for the YB counter. The localization of GRB030115 was actually carried out with these two counters. On 2003 January 22, WXM was back in normal operation with the three remaining counters, XA, XB and YA. The three remaining counters are not affected at all by the failure of the YB counter.
As a result of the loss of the YB counter, the total effective area of
the WXM is reduced by 25%, and the solid angle in which a localization
in the Y-direction can be made is reduced by 20%. Analysis of past WXM
localizations indicates that the effect on the accuracy of WXM
localizations is expected to be small, but the rate of two-dimensional
localizations is expected to be affected. The net effect is that the
expected rate of two-dimensnional localizations by WXM is reduced by
20%. We therefore expect ~ 20 two-dimensional WXM localizations per
year now, instead of ~ 25 localization per year.