At the end of the Wurzburg conference (circa 29 September 1995) 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 hotspot in a different corner of the detector. In January of 1997, a third hotspot was found about 5 arcminutes from the edge of the field in a different corner than the other two hotspots.
Although the precise cause of HRI hotspots are not known, they are thought to be a defect in a single pore of the micro channel plate. Some of the hotspots discovered before launch were subsequently incorporated into the HRI ``hotspot map'' which is used in the SASS processing to reject all events happening at given locations.
The original hotspot (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. HS#3 is different due to its low count rate, quasi-constant behavior and presence in almost all data sets since being noticed.
Since #1 and #2(the brighter hotspots) are 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 the hotspots 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.