Interplanetary Gamma-Ray Burst Timing Network (IPN)
The astronomical locations of GRBs are determined by the comparison of the observation times of an event at the locations of the detectors used on different space missions. The precision is, of course, proportional to the distances between the spacecraft, so that the localizational accuracy of a network with baselines of thousands of light-seconds can be equal to or superior to that of any other technique. The primary disadvantage of the IPN method is the delay of up to about 1 day in the acquisition of data from all the spacecrafts. Despite this fact, many GRB positions have been determined quickly enough to enable the identification of their fading counterparts, so that redshift measurements and other observations could be made. At one point, the IPN enabled nearly one-third of the total number of all GRB counterpart studies, including the event with the greatest redshift (z=4.5).
Interplanetary GRB networks have been in existence since 1976, contributing to the studies of various astrophysical gamma ray transients, most notably SGRs (soft gamma repeaters) and GRBs. The IPNs, however, are not necessarily planned and funded endeavors, but are existential in character. Networks evolved generally through the creative modifications of experiments planned for other purposes, rather than by direct execution. For example, the first mutually wide-baseline, three-cornered network consisted of the solar orbiter Helios-2, Pioneer-Venus Orbiter, the Franco-Soviet Venera-11 and -12 pair, launched in tandem, and included the Vela group that made the original GRB discovery. The chance that such separately and individually executed programs can successfully form a full network has not been and cannot be great. Even when an adequate number of units are simultaneously in operation, accidents of Fate have been known to interfere, such as the rough lineup in 1991-2 of the Earth, Venus and Jupiter (so that near-Earth missions, Pioneer-Venus Orbiter and the Ulysses space probe, then heading for Jovian encounter, formed a compromised triangle). Also, the launch failure and experiment failure rates for domestic and international planetary missions have been painfully high. On the other hand, a brilliant post-launch and in-flight modification of the Near Earth Asteroid Rendezvous mission (NEAR) made possible a very successful three-cornered, long-baseline interplanetary network several years ago.
This existing gamma-ray burst IPN consists primarily of the Ulysses solar orbiter, in a unique 5-AU orbit well out of the ecliptic plane, at distances up to 2500 light-seconds from the Earth, the Wind, in an actively adjusted trajectory up to several light-seconds from the Earth, and Mars Odyssey, presently orbiting Mars. Earth orbiters also contribute to the IPN, most notably HETE, INTEGRAL, RHESSI, and Swift. In the past other spacecraft that contributed to the IPN such as the interplanetary mission NEAR and earth orbiters such as CGRO, BeppoSAX and Rossi XTE. The Compton-GRO mission, carrying the BATSE that initiated the present era of GRB observations, was terminated in June 2000 and while BeppoSAX operations ended in April 2002.
On board Ulysses is the GRB/solar/Jovian-x-ray experiment, remaining from a pair of experiments originally selected for the Solar Polar Mission, a system that would have formed its own IPN, given any near-Earth vertex with GRB capability. Kevin Hurley at UC Berkeley is the GRB Principal Investigator on Ulysses GRB.
On board the Wind mission is the Konus experiment, the first Russian scientific package on a NASA spacecraft. Evgeny Mazets of St. Petersburg, Russia is the Konus Principal Investigator and Thomas Cline at Goddard is the Co-PI.
The Mars Odyssey 2001 mission has two on-board detectors with gamma-ray burst capability, a neutron detector (PI: Igor Mitrofanov, IKI, Moscow) and a gamma-ray detector (PI: William Boynton, U. Arizona).
Kevin Hurley is primarily responsible for disseminating the IPN calculations to the community. T. Cline, as the original IPN creator and as co-PI on Konus and co-I on Ulysses, NEAR, and TGRS is the principal Goddard coordinator for the IPN. S. Barthelmy is the creator and director of the Goddard GCN.
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