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At the end of the Würzburg conference (circa 1995 September 29) Thomas Preibisch alerted MPE about a feature near the edge of the HRI field. Approximately two weeks later, many datasets were examined at MPE and 3 sequences which showed this effect were identified. SAO was notified on 16 October. and subsequently examined this sporadic feature. Many people at MPE, GSFC, and at SAO have contributed to this report.
During the course of the ROSAT Results Archive visual inspection, J. Silverman discovered a second hot spot in a different corner of the detector.
Although the precise cause of HRI hot spots are not known, they are thought to be a defect in a single pore of the micro channel plate. Some of the hot spots discovered before launch were subsequently incorporated into the HRI ``hot spot map'' which is used in the SASS processing to reject all events happening at given locations.
The original hot spot (aka #1) and the second are similar in several respects but differ in their PHA distribution, their variability characteristics, and the length of time they have been active. Because of their respective locations, HS#1 appears as a radial streak (in a normally wobbled observation), often appearing to extend into the region where there are few or no background counts. HS#2 being in an adjacent corner, appears as a tangential streak.
While it is somewhat surprising that both hot spots appeared within a few months of each other after years of no occurrences, they seem to be isolated events; no additional hot spots have been found. Since they are both at the very edge of the detector and their intensity is small enough that they do not encroach on the telemetry limit, they have essentially no impact on the quality of the scientific returns of the HRI. Monitoring of both hot spots has been incorporated into the verification software running at SAO; MPE continues to watch current activity on quick look data from the satellite; and the ROSAT Results Archive quality checking serves as a backup monitor for any new occurrences.