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Newsgroups: sci.space,sci.astro
Subject: Re: Gamma Ray Bursters. Where are they?
Message-ID: <[e-mail]>
From: [e-mail]
Date: 26 Apr 93 19:25:35 -0600
References: <1radsr$[e-mail]> <[e-mail]> 
 <[e-mail]> <[e-mail]>
Organization: Mankato State University
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In article <[e-mail]>, [e-mail] (it's enrico palazzo!) writes:
>> = From: Graydon <[e-mail]>
> 
>> If all of these things have been detected in space, has anyone
>> looked into possible problems with the detectors?
> 
>> That is, is there some mechanism (cosmic rays, whatever) that
>> could cause the dector to _think_ it was seeing one of these
>> things?
> 
>> Graydon
> 
> That would not explain why widely separated detectors, such as on Ulysses
> and PVO and Ginga et al., would see a burst at the same time(*).  In fact, be-
> fore BATSE, having this widely separated "Interplanetary Network" was the
> only sure way to locate a random burst.  With only one detector, one cannot
> locate a burst (except to say "It's somewhere in the field of view.").  With
> two detectors, one can use the time that the burst is seen in each detector
> to narrow the location to a thin annulus on the sky.  With three detectors,
> one gets intersecting annuli, giving two possible locations.  If one of these
> locations is impossible (because, say, the Earth blocked that part of the 
> sky), voila, you have an error box.
> 
> BATSE, by having 8 detectors of its own, can do its own location determination,
> but only to within about 3 degrees (would someone at GSFC, like David, like
> to comment on the current state of location determination?).  Having inde-
> pendent sightings by other detectors helps drive down the uncertainty.
> 
> You did touch on something that you didn't mean to, though.  Some believe
> (in a reference that I have somewhere) that absorption-like features seen
> in a fraction of GRBs can actually be caused by the detector.  It would be
> a mean, nasty God, though, that would have a NaI crystal act like a 10^12 Gauss
> neutron star...but this is getting too far afield.
> 
> Peter
> [e-mail]
> 

        All of this is VERY valid and very true.  But to add to this
explaniation, each individual detector also has a built in fail-safe, just so
the detector does not read the background radiation(i.e. cosmic rays), 
if I remember right, the detectors go off about 3 to 5 sigma above the 
background.  This is so they don't catch particularly energetic cosmic rays
that would normally set it off. Even with this buffer, they still have to throw
out something like 1/2 of the bursts that they DO get, because of the Earth's
Van Allen Belts, the South Atlantic Anomaly, the Sun,  if I remember right,
there is either a radar station, or a radio station in Australia, and there are
a couple other sources as well.  
                                                -jeremy
                                                [e-mail]