2 Current and future missions

Figure 1 shows the approximate operating dates of the missions which are capable of providing GRB data to answer some of the questions listed above. Each of these missions will be reviewed briefly.

2.1 BeppoSAX

BeppoSAX has now observed 14 GRBs in the Wide Field Camera, with location accuracies in the $<10 \hbox{$^\prime$}$ range. The resulting detection rate is $\sim$ 10/year. Eleven of these have been followed up with Narrow Field Instrument observations, resulting in many cases in a reduction of the error circle radii to $\sim 1 \hbox{$^\prime$}$ (Costa 1999). There are delays of $\sim$ hours to obtain, analyze, and distribute the data. The approved lifetime of the mission is through 2001.

2.2 BATSE: GCN and locburst

The Global Coordinates Network (GCN: Barthelmy et al. 1999) distributes $\sim$ 300 GRB positions/year with delays of the order of seconds, determined directly onboard the CGRO spacecraft. The error circle radii are $\gt 4 \hbox{$^\circ$}$. The Locburst procedure (Kippen et al. 1998) distributes $\sim$ 100 of the stronger bursts/year. As Locburst relies on ground-based processing, the delays are longer, $\sim$ 15 min, but the accuracy is improved: the error circle radii are $\gt 1.6 \hbox{$^\circ$}$. These data are useful for follow-up searches with rapidly moving telescopes like LOTIS (Park 1999), and with the RXTE Proportional Counter Array, as well as for triangulation with the 3rd Interplanetary Network. BATSE will remain operational at least through 2002; its lifetime is limited by the available funding, and is reviewed every two years in NASA's "Senior Review'' process.

2.3 3rd interplanetary network

The 3rd IPN consists of the Ulysses and Near Earth Asteroid Rendezvous (NEAR) missions in interplanetary space, as well as numerous near-earth missions such as CGRO, RXTE, Wind, and BeppoSAX. The IPN observes and localizes $\sim$ 70 GRBs/year (Hurley 1999a,b). When a burst is observed by just two spacecraft, such as Ulysses and CGRO, the resulting error box is the intersection of the triangulation annulus with the BATSE error circle, with dimensions typically 5$^\prime$  by 5$^\circ$.When Ulysses, NEAR, and say, BATSE detect the burst, the resulting error box may be as small as 1$^\prime$ by 5$^\prime$ (Cline 1999). The delays involved are $\sim$ 1 day, imposed by the receipt of data from interplanetary spacecraft through NASA's Deep Space Network. The lifetime of the 3rd IPN will be through 2004, the nominal end of the Ulysses mission.

2.4 The Rossi X-ray timing explorer

The All-Sky Monitor aboard RXTE detects $\sim$ 4 GRBs/year; $\sim$2 of them can be localized to $\sim$ arcminute accuracy with delays of only minutes (Bradt 1999; Smith et al. 1999). In addition, the PCA performs about one target-of-opportunity observation per month of BATSE Locburst positions to search for fading X-ray counterparts (Takeshima et al. 1998). When successful, the counterpart position can be determined to $\sim 10 \hbox{$^\prime$}$ with a delay of hours. Like BATSE, RXTE's lifetime, determined by the Senior Review, will extend through 2002 at least.