The Interplanetary Network Supplement to the Fermi GBM Catalog of Cosmic Gamma-Ray Bursts

We present Interplanetary Network (IPN) data for the gamma-ray bursts in the first Fermi Gamma-Ray Burst Monitor (GBM) catalog. Of the 491 bursts in that catalog, covering 2008 July 12 to 2010 July 11, 427 were observed by at least one other instrument in the nine-spacecraft IPN. Of the 427, the localizations of 149 could be improved by arrival time analysis (or "triangulation"). For any given burst observed by the GBM and one other distant spacecraft, triangulation gives an annulus of possible arrival directions whose half-width varies between about 0.'4 and 32°, depending on the intensity, time history, and arrival direction of the burst, as well as the distance between the spacecraft. We find that the IPN localizations intersect the 1σ GBM error circles in only 52% of the cases, if no systematic uncertainty is assumed for the latter. If a 6° systematic uncertainty is assumed and added in quadrature, the two localization samples agree about 87% of the time, as would be expected. If we then multiply the resulting error radii by a factor of three, the two samples agree in slightly over 98% of the cases, providing a good estimate of the GBM 3σ error radius. The IPN 3σ error boxes have areas between about 1 arcmin² and 110 deg², and are, on the average, a factor of 180 smaller than the corresponding GBM localizations. We identify two bursts in the IPN/GBM sample that did not appear in the GBM catalog. In one case, the GBM triggered on a terrestrial gamma flash, and in the other, its origin was given as "uncertain." We also discuss the sensitivity and calibration of the IPN.