GCN Circular 1539

IPN GCN Circulars now to be issued without restriction
______________________________________________________


Up to now, the IPN collaboration has been filtering its GCN Circulars
and Notices on GRBs to include only localizations which satisfy three
criteria:

1. Relatively small error boxes (<~ 1000 arcmin2) 
2. Relatively rapidly determined error boxes (<~100 hours) 
3. Error boxes with relatively small maximum dimension in any direction (<~30 arcmin)

These filters were tailored to the requirements of astronomers
searching for long-wavelength GRB afterglows. The resulting GCN rate
has been roughly one per week over the past several years, with large
fluctuations in this rate due to solar activity and the start-up and
shutdown of various missions.

However, following discussions with members of the community, we
believe that, in order to better serve the needs of astronomers
searching for other GRB-related phenomena, including neutrinos,
gravitational radiation, and VHE gamma-rays, it is worthwhile to remove
all filters from IPN GRBs. Accordingly, we will proceed to issue GCN
Circulars which give information on bursts localized to annuli,
sections of annuli, double error boxes, and so on, regardless of the
area, maximum dimension, or delay in obtaining the error box.  These
Circulars will be issued as soon as possible after receipt of the
data.  The removal of filters should at least double the GCN message
rate; the majority of these Circulars will be issued within several
days of a burst, although outliers will always be present.  (We do not
plan to issue this information as GCN Notices, unless the circumstances
are exceptional, or a clear desire to have such Notices is expressed by
members of the community.)  As always, all such information should be
understood to be preliminary and subject to refinement when final data
(such as spacecraft ephemeris and timing) become available, which
takes up to about one month, depending on the spacecraft.   Apart from
exceptional cases, we will not issue final localizations as GCN
Circulars, but instead defer them to publications in the refereed
literature.

We will use several techniques in addition to arrival-time analysis to
constrain GRB arrival directions as much as possible.  They include
Earth-blocking, Mars-blocking, and ecliptic latitude determination.  A
brief discussion of some of these techniques may be found at
ssl.berkeley.edu/ipn3/statusreport.htm.

Unconfirmed events (i.e., events observed by a single detector, or by
several detectors on a single spacecraft) pose a particular problem.
In many cases, such events can safely be assumed to be cosmic due to
their energy spectra and time histories.  However, some cases are
harder to judge.  We will treat unconfirmed events in such a way as to
minimize the false positive rate, while maximizing the completeness of
the sample.  Roughly speaking, we expect the overall result to be a GRB
sample that is over 90% complete, with an over 90% probability that any
unconfirmed event is indeed cosmic.  (Note that some unconfirmed events
may be localized, and some confirmed events may be unlocalized, due to
the nature of the IPN and its experiments.)

We will attempt to specify the nature of the event and its localization
in the subject line of the circular, as far as possible.

Questions about specific events, or questions such as "did any
spacecraft observe a burst on day X at time Y?", can be addressed to
khurley@sunspot.ssl.berkeley.edu.