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ASCA Guest Observer Facility

GIS Monitor Counts

There are GIS house-keeping parameters which are called "monitor counts" and can be used to estimate source and background flux.

ASCA GIS monitor

All the events whose pulse-heights exceeds the pulse-height lower discriminator is counted as the LD_HIT monitor count. The LD_HIT is divided into the six monitor counts, L0, L1, L2, H0, H1 and H2, according to the event pulse-height and rise-time discriminators, as shown above. Most X-ray events appear in L1, and only L1 counts are sent to the on-board CPU and become the GIS PH mode data.

Time resolution of the LD_HIT is 0.0625 sec for the high bit rate (8 and 32 times longer for the medium and low bit rate respectively). Time resolution of the other monitor counts is twice of that.

The deadtime for these monitor counts is small (25 micro-sec/event) and their telemetry limits are large (4096 cts/s/sensor for LD_HIT and 2048 cts/s/sensor for other monitor counts). Hence, the L1 monitor count is useful to estimate the flux of bright sources and study short-time variations. Other monitor counts may be used to estimate particle background rates. In particular, it is known that sum of H0 and H2 is a good indicater of the 'hard-flare' background events .

All the monitor counts are included in the filter file (mkf file) with a typical integration time of 32 seconds. Original monitor counts are available in the GIS HK files. Note that monitor counts are counted by 8-bit non-reset counters, and it is the increments of the HK parameters that give the actual monitor count rates. FITS format L1 light curve files can be made using the ftool ghkdump from GIS HK files. This may be effective for timing study of bright X-ray sources for which standard PH mode data might be hardly usable because of the significant deadtime and telemetry saturaitons.

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This file was last modified on Monday, 25-Jun-2001 11:48:13 EDT

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