BATSE Location Errors

The BATSE burst catalogs list statistical location errors obtained from the location fitting process. Earlier catalogs specified a simple error model in which the total location error of a burst was obtained by summing a systematic error in quadrature with the statistical error of the burst. It was noted that the error distribution might have a non-Gaussian tail. By comparing 411 BATSE 4B locations with IPN locations an improved error model has been developed. We implement the idea of a distribution with an extended tail with the sum of two Fisher distributions (the analogue on the sphere of the Gaussia distribution), one representing the "core" of the error distribution, the other the "tail".

For each burst, 78% of the probability content is in the core term with a systematic error of 1.85 degrees, while the remaining probability is in a tail term with systematic error 5.1 degrees. These values also apply to post-4B bursts and will remain appropriate until such time as LOCBURST is significantly improved.

two-component error distribution This figure depicts the appearance of the two-component model for a burst with negligible statistical error. The solid curve shows the total model, while the dashed curves show the core and tail terms separately. The azimuthal angle has already been integrated over and the curves include the solid angle factor. The integral of the curve from 0 degrees to 180 degrees is one. A postscript version of this figure is also available.

confidence radii This figure gives the confidence radii for BATSE GRB locations as a function of the statistical error quoted in the catalog. The solid curves are for the error model described here; the dotted and dashed curves are for a model with a datatype dependence (see astro-ph/9901111). The lowest trio of curves gives the 68% confidence radii, the middle trio the 90% confidence radii and the top trio the 95% confidence radii. A postscript version of this figure is also available.

Further information on the derivation and use of the error model will appear in The Astrophysical Journal Supplement Series as a companion paper to the 4B catalog. In the meantime, a preprint may be obtained from the LANL preprint server as astro-ph/9901111. The two figures above were extracted from that paper.