4B Cmax / Cmin Table Description

The CMAXMIN Table specifies the peak count rate in units of the threshold count rate, and the threshold count rate. The table contains 912 triggered bursts observed from launch until 29 August 1996. Many bursts are not included, particularly after March of 1992, since insufficient data exist to determine either the peak counts or the threshold.

The BATSE on-board software tests for bursts by comparing the count rates on the eight large-area detectors to threshold levels for three separate time intervals: 64 ms, 256 ms, and 1024 ms. A burst trigger occurs if the count rate is above threshold in two or more detectors simultaneously. The thresholds are set by command to a specified number of standard deviations above back- ground (nominally 5.5 sigma). Background rates are recomputed every 17 seconds. The thresholds exhibit a coarse quantization that results from truncating the square root of the 64 ms count rate. Since we require that rates be above the thresholds of at least two detectors, the trigger threshold is determined by the threshold of the second most brightly illuminated detector.

When a burst trigger occurs, subsequent triggers are disabled during the accumulation period when the BATSE burst memories accumulate data. These data are then transmitted. During this readout period, the 64 ms threshold is revised to correspond to the maximum rate attained by the current burst, and triggering is disabled on the 256 ms and 1024 ms timescales. Bursts intense enough to trigger during this readout period are termed "over- writes". They are recognized in the table by the value of -999 in the threshold columns for 256 ms and 1024 ms.

The CMAXCMIN Table is available for download.

The format of the file is as follows:

1. The first column specifies the BATSE trigger number, which is a key for all of the BATSE Burst Catalog tables.
2. The second column is the maximum counts in the second most brightly illuminated detector divided by the threshold count rate on the 64 ms timescale.
3. The third column is trigger threshold on the 64 ms timescale. It is the number of counts in 64 ms required to trigger the second most brightly illuminated detector for this particular burst.
4. The fourth column is the maximum counts in the second most brightly illuminated detector divided by the threshold count rate on the 256 ms timescale.
5. The fifth column is trigger threshold on the 256 ms timescale. It is the number of counts in 256 ms required to trigger the second most brightly illuminated detector for this particular burst.
6. The sixth column is the maximum counts in the second most brightly illuminated detector divided by the threshold count rate on the 1024 ms timescale.
7. The seventh column is trigger threshold on the 1024 ms timescale. It is the number of counts in 1024 ms required to trigger the second most brightly illuminated detector for this particular burst.

Since a trigger can occur on any of the three timescales, there are often cases in which the maximum rate will be below threshold on one or two of the timescales. The value of V/Vmax can be determined for any burst by selecting the maximum of the three peak rates, raised to the -3/2 power. Many bursts have unknown counts or thresholds on one or more timescales. These are marked by a "-999" in the table. This can happen for one of the following reasons:

1. If the trigger occurs on the 64 ms timescale during the peak 256 ms rate, then the peak 256 ms rate is not found.
2. If the 64 ms peak rate never exceeds the 64 ms threshold, and it occurs before the trigger time, then the peak 64 ms rate is not found.
3. If the 256 ms peak rate never exceeds the 256 ms threshold, and it occurs before the trigger time, then the peak 256 ms rate is not found.

Note that items 2 and 3 do not affect V/Vmax, since these peak rates do not exceed threshold. Item 1 can on rare occasions lead to an overestimate of V/Vmax.

Author	BATSE GRB Team
Responsible Manager	Charles Meegan
Site Curator	Valerie Connaughton