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### 9. "About Once a Day"

Scientists will tell you that the BATSE experiment aboard the Compton Gamma-Ray Observatory detects "about one GRB per day". What exactly does that mean? Once you detect a burst, do you not have to look for another 24 hours in order to see the next one? That would certainly be easier and cheaper! Or does "about one burst per day" mean no such thing? Below is given small part of the BATSE GRB catalog which scientists use to analyze the frequency of GRBs. We selected 20 bursts detected during August 1996 from the catalog of over 7500 bursts.

A note about GRB names. Each burst is given a unique name. They contain a 6 digit number "yymmdd" for the two-digit year, two-digit month, and two-digit day of month of the burst occurrence. When more than one GRB is detected on the same calendar day, the suffixes A, B, C, etc are added after the yymmdd, usually depending on how bright each burst was relative to the other bursts of that day. So for example, 920503B refers to the second brightest burst detected on May 3, 1992.

Catalog Entry #Burst NameSecs of Day (UT)

5561 4B 960803 67522.445

5562 4B 960804B 70360.781

5563 4B 960804 84535.750

5564 4B 960805 77347.148

5565 4B 960806B 58713.289

5566 4B 960806 80909.773

5567 4B 960807 71366.344

5568 4B 960808 60123.848

5569 4B 960810B 24571.594

5570 4B 960810 83459.078

5571 4B 960812 50665.098

5572 4B 960813 20769.994

5573 4B 960813B 78438.086

5574 4B 960814 58847.945

5575 4B 960815 36027.082

5576 4B 960816 8167.242

5580 4B 960818 57760.457

5581 4B 960819 74182.344

5585 4B 960824 6110.921

5586 4B 960825 62837.578

• From these data, determine the mean, median, and mode of the data set which represents the time intervals between GRB detections.

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• What do these measures of central tendency tell us about the data?

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• What is the maximum time interval between successive bursts?

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• What is the minimum interval between successive bursts?

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• What do all of these values tell you about how you must organize your observing program in order not to miss a burst?

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Extension:

Get on the World Wide Web and download other sections of the BATSE GRB catalog (the 4th version of the catalog, which is the most current through July 1999) is available online at http://www.batse.msfc.nasa.gov/batse/grb/catalog/4b/4br_basic.html). Do you obtain the same results, no matter which section of the catalog list you take? Caution: be careful about using the most recent data...before verification, it can contain many false triggers, i.e. listings for bursts that never really happened!