8.1.2 MIDEX and SMEX MISSIONS

The GRAPWG strongly supports the current MIDEX and SMEX programs. Frequent flight opportunities are crucial for our field and the open peer review process should allow the best missions to be selected. There are several critical hard X-ray and gamma-ray measurements that can be accomplished within the weight and cost envelopes of these programs. The two highest priorities for near-term MIDEX and SMEX missions are (of equal priority): Gamma-ray burst localization mission. Such a mission would address the question of the origin of gamma-ray bursts. A large array of position sensitive CdZnTe or scintillation detectors combined with a wide-field coded mask or Fourier transform aperture could position bursts to arcsecond accuracy to allow deep unconfused searches for counterparts at other wavelengths. Another possibility for this measurement is an array of small-satellites spread around the Earth's orbit to accurately triangulate burst arrival times. Searches could also be made for a halo population of burst sources around M31 to test galactic halo models. Such missions would likely lead to a breakthrough in the understanding of gamma-ray bursts. Examples of NASA mission concepts for gamma-ray bursts are BASIS and ETA, as well as the MIDEX concept BLAST. Hard X-ray all-sky survey and monitor mission. Such a mission would address the questions of the nature and evolution of AGN, the origin of the diffuse background, the sites of recent galactic supernovae (through 44Ti line observations), and the nature of neutron stars, black hole systems, and the accretion process. For a mission concept like EXIST, a large array of position sensitive CdZnTe detectors combined with a wide-field coded mask aperture would give an all-sky survey at the 100 microCrab sensitivity level in the 10 - 200 keV range. This surpasses the HEAO 1 survey by more than two orders of magnitude. A significant fraction of the entire sky would be scanned every day for detection and arcminute positioning of transient sources.

8.1.3 HETE

The loss of HETE is a major setback to the study of cosmic gamma-ray bursts. The objectives of that mission are still compelling; rapidly obtained precise positions would be extremely valuable for multiwavelength counterpart searches. The HETE spacecraft can be rebuilt and reflown relatively quickly and inexpensively.  Therefore we endorse this initiative, and further recommend that additional support be provided for the construction of about ten small rapidly slewing ground-based telescopes to assure the maximum possible return on this investment.

8.2 CURRENTand APPROVED MISSIONS

While future missions are being developed, it is essential to continue scientific discovery with the existing and approved missions in gamma-ray and hard X-ray astronomy.

• CGRO and RXTE: CGRO and RXTE are tremendously productive multi-instrument NASA missions that promise to remain scientifically exciting for a long time into the future (5-10 years). The GRAPWG recommends that adequate MO&DA funding be made available to continue operations, data production and scientific investigations with these missions. The current biennial Senior Review peer evaluation of the missions is good and should be continued. The GRAPWG is concerned that the overall MO&DA budget beyond FY97 for astrophysics missions is not nearly adequate to realize the scientific potential of the existing missions.

• INTEGRAL: The INTEGRAL is an ESA-led mission with U.S. and Russian participation. It is scheduled for launch in 2001 and will operate in the 15 keV to 10 MeV range. The combined capability of its two main instruments will offer an order of magnitude better sensitivity than CGRO with significantly improved spectral and angular resolutions. This is the next major observatory for gamma-ray astronomy following CGRO. The GRAPWG recommends that NASA continue support for U.S. participation in the mission.

• The inclusion of small, inexpensive gamma-ray burst detectors aboard approved and future planetary missions is encouraged to provide an interplanetary network for burst localization: The GRAPWG recommends adequate MOEDA support to take advantage of fine localizations. A major step towards solving the gamma-ray burst mystery would be achieved by the identification of a counterpart at other wavelengths.