GCN Circular 1085

INTRO:
There have been a lot of HETE triggers in the last week.  Many people
are wondering what is going on.  This announcement attempts to explain
the recent developments.

TRIGGERS:
Starting about a week ago, the HETE Operations team resumed sending
the HETE messages from the s/c to GCN -- there were gaps due to HETE and due
to GCN (the network problems).  And since the Galactic Center
is still in the HETE FOV, this has caused a marked step increase
in the amount of HETE Notices (from zero to several per day).
This increase has been further enhanced by the fact that Sco X-1 has left
the FOV of the HETE instruments two weeks ago.  The decrease in the background
due to the loss of Sco X-1 has increased the sensitivity to the the XRB
sources in the Galactic Center.

POSITIONS:
Since a larger fraction of these triggers is caused by X-ray Burst sources
which are very weak, they produce poor image detections and therefore
locations with large uncertainites.  The HETE Operations team
has commanded the s/c to suppress the positions if the "source"
in the image is not significant enough.  This results in the large
number of Notices with no positions.

FILTERING:
I have changed the GCN filtering system to compensate for this change
in the Notice content.  If there is no position in the S/C_Update,
S/C_Last, or even the Gnd_Analysis messages, then they are NOT distributed
the sites requesting HETE Notices.  The reasoning is that with no position
information in the Notice, what possible use could a follow-up observer
make of the notification.  This change was made Saturday, 28 Jul 01.

TOO MANY ALERTS?
However, the S/C_Alerts never were intended to have position information
(except for rare instances (~10%)), so they are still being distributed
to those sites that have requested the HETE S/C_Alert subtype.
Even with the filtering of the U/L/GA's that have no positions, there are
still a large number of Alert Notices being distributed to sites.  If you feel
that you are receiving too many Alerts (that subsequently have no U/L/GA
follow-ups), then you might consider requesting that the Alert subtype
be disabled for your site.  (Or you might consider "toughing it out"
for another 4-6 weeks until the Galactic Center is no longer in the HETE FOV 
in which case the trigger rate should drop dramatically.)

ACTION ITEM:
Sites can:
1) keep their current configurations,   or
2) request that S/C_Alerts be disabled,   or
3) request that S/C_Alerts/Updates/Lasts be disabled (say for the next month
or so, and then request they be turned back on),   or
4) request that all HETE subtypes (A/U/L/GA) be disabled (again for the next
month or so).
Given the no-positions-filtering incorporated last Saturday, choice "2"
seems like a reasonable approach if a site has any concern about "too many
Notices".

Sincerely,

Scott Barthelmy
NASA-GSFC
301-286-3106
scott@lheamail.gsfs.nasa.gov