At the end of the Wurzburg conference (circa 29 September 95), 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.
Although the precise cause of HRI hotspots are not known, they are thought to be defects 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.
While it is somewhat surprising that both hotspots appeared within a few months of each other after years of no occurrences, they appear to be isolated events; no additional hotspots 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 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.
Both hotspots have quasi power law distributions (sigma = 1.4"), with most of the photons falling within a circle of r=6 pixels. This size circle does not appear to be a function of time; even for short bursts, the spatial distribution is maintained.
HOTSPOT #1 HOTSPOT #2 detector coordinates: rawx = 440.5; rawy = 484.5 rawx = 3636 rawy = 552
For #1, an examination of light curves and statistical time variability tests show that there are many intervals and obis when no activity is seen. #2 is quite different, being more steady (less "sparking" or "flickering"). For both, the average countrate drops quickly as the length of the time bin is increased.
HOTSPOT #1 HOTSPOT #2 First known occurrence 8Jul95/rh800694 mid-Mar95 Last known occurrence continues (Jan96) 19Jun95 Largest known c/s (1 sec bins) 57(rh300399) much less than #1 Largest known <c/s> for a seq: 0.76 (17.2ksec) 0.0177(28 ksec) Incidence of Detection 120 out of 240 seqs; 7/240 (Jul-Sep 1995)
The PHA distribution differs for the two hotspots.
HOTSPOT #1 HOTSPOT #2 85% in channel 1 peaks in channel 1 15% in channel 2 then decreases with <1% in channel 3 about 1/2 counts in channels 4-10.
5.0 VIEWING AND DELETING
To view the hotspot in PROS; > xdisplay "rh123456n00.qp[key=(rawx,rawy][186:697,230:741]" (change coordinates for HS#2) OR > xexamine rh...qp[key=(rawx,rawy)] To make a KS plot, in the xtiming package of PROS, > timsort rh....qp[key=(rawx,rawy)] region: box 441 485 20 20 > vartst > ksplotTo mask the hotspot, use qpcopy with the same [key=...] as above and specify a region (box or circle) to be excluded.
To view the hotspots in MIDAS/EXSAS:
in EXSAS the following commands will produce "detector images (200x200)" with hotspot No.1 and No. 2 in the center with the maximum resolution:
bin/image XDET,YDET 1 pixel ,340,384,540,584 *events *hs1-area
bin/image XDET,YDET 1 pixel ,3536,452,3736,652 *events *hs2-area
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