3 The next big step

At present we rely on spacecraft instrumentation to provide X-ray positions which are accurate to arcminutes, and on rapid ground-based photometry from small to moderate-sized telescopes to identify optical counterparts to arcsecond accuracy. Only at that point can a large telescope be used to determine the redshift (for example, the spectrometer slits on the Keck Low Resolution Imaging Spectrometer are only 1-8$^{\prime\prime}$). The next big step will be to determine GRB positions directly on the spacecraft to arcsecond accuracy, eliminating the delays involved in refining the positions on the ground. Some of the future missions discussed below will be capable of accomplishing this.

  

Figure 1: Approximate dates of current and future GRB missions

3.1 HETE-II

The High Energy Transient Explorer-II combines a Wide Field X-ray Monitor and a Soft X-ray Camera to localize up to $\sim$50 GRBs/y to accuracies of 10$^{\prime\prime}$ to 5$^\prime$ (Ricker 1999). Locations will be transmitted to the ground in near real-time. The HETE-II mission is planned for a two year lifetime starting in January 2000.

  

Table 1: Capabilities of current and future GRB missions

3.2 CATSAT

The Cooperative Astrophysics and Technology Satellite (Forrest et al. 1995) will contain a soft X-ray spectrometer consisting of 190 cm² of Si avalanche photodiodes to measure the 0.5-20 keV spectra of GRBs and their afterglows. From these spectral measurements, the hydrogen column along the line of sight may be determined. CATSAT has only coarse localization capability, but measurements of $N_{\rm H}$ will help to answer the question of the locations of GRBs with respect to their host galaxies. $\sim 12$ GRBs/year should be detected, with data available $\sim$ 5 hours after the bursts. A nominal one year mission in 2000 is planned.

3.3 INTEGRAL

The International Gamma-Ray Laboratory can detect bursts with its Ge spectrometer array (the SPI), as well as with IBIS (the Imager on-Board the INTEGRAL Satellite), and with the BGO anticoincidence shield around the spectrometer. IBIS, a CdTe array with a coded mask, provides the most accurate, rapid locations. It can detect $\sim$ 20 GRBs/year and localize them to arcminute accuracy (Kretschmar et al. 1999). These positions can be distributed to observers within 5-100 s. The nominal INTEGRAL mission is two years long, starting in April 2001.

3.4 Future IPN

A future Interplanetary Network, consisting of Mars Surveyor Orbiter 2001, INTEGRAL, BATSE, and Ulysses, may exist around the year 2002. MSO has two GRB two instruments which will detect GRBs with good sensitivity and time resolution, a Ge spectrometer and a neutron detector. The BGO anticoincidence shield of the INTEGRAL SPI is similarly equipped to detect bursts (Hurley 1999). With such a network, $\sim$ 70 GRBs/year could be localized to arcminute accuracies, with delays of the order of a day. This IPN might remain in place for one or two years, bridging the gap to a possible dedicated GRB MIDEX.

3.5 A dedicated MIDEX

The results of the recent MIDEX competition have just been announced. The SWIFT GRB proposal (N. Gehrels, P.I.) was selected for a 6 month study phase. SWIFT can localize perhaps 300 GRBs/year to arcsecond accuracy onboard the spacecraft, and transmit the locations to the ground in near real-time. The final mission selection will be announced in September 1999. If selected for flight, SWIFT could fly in the years 2003-2005.