Intermediate Missions ($75M - $300M)

The HIGHEST PRIORITY recommendation of the GRAPWG is: A next-generation 10 Mev to 100 GeV gamma-ray mission such as GLAST. One to two orders of magnitude improvement in sensitivity compared to EGRET are expected resulting in breakthroughs in our understanding of particle acceleration and nonthermal processes in AGN and galactic sources. The GRAPWG identified two other missions as very high priority for initiation within the next decade. These programs would serve pressing scientific needs and represent areas where prompt support for technology development and mission study promises great gains in the capabilities and efficiency of future missions.

• A focusing hard X-ray telescope. The expected two orders of magnitude improvement in sensitivity compare to RXTE would address questions such as the nature of accretion onto compact objects in galactic sources. A new multilayer mirror technology can extend the focusing range to ~100 keV. The HTXS concept includes a focusing hard X-ray telescope that extends up to 50 keV.
• A next-generation nuclear line and MeV continuum mission. A major step forward compared to INTEGRAL in both sensitivity and energy range would allow detailed studies of sites of nucleosynthesis in the galaxy and of nonthermal sources in the universe. The GRAPWG views this mission as a follow-on to INTEGRAL.

MIDEX and SMEX Missions

Future MIDEX and SMEX missions are crucial for NASA's gamma-ray and hard X-ray astronomy program. The two highest priorities for near-term SMEX and MIDEX missions are (of equal priority):

• A gamma-ray burst localization mission. Such a mission would address the question of the origin of gamma-ray bursts. Missions with coding apertures or an array of small telescopes would fill this need. Searches could also be made for a halo population of burst sources around M31 to test galactic halo models. Example mission concepts are BASIS, ETA, and BLAST.
• A hard X-ray all-sky survey and monitor mission such as EXIST. More than two orders of magnitude improvement over the HEAO-1 survey could be obtained in the 10 - 200 keV range. A significant fraction of the entire sky could be scanned every day for transient sources.

HETE

The loss of HETE is a major setback to the study of gamma-ray bursts. The objectives of that mission are still compelling; rapidly obtained precise positions are invaluable for multiwavelength counterpart searches. The HETE spacecraft can be rebuilt and reflown relatively quickly and inexpensively.

• We endorse this initiative and further recommend that support be provided for the construction of rapidly slewing ground-based telescopes.

Current and 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 are tremendously productive multi-instrument NASA missions that promise to remain scientifically exciting into the future. Adequate MO&DA funding should be made available to continue full operation and scientific utilization of these missions.
• The INTEGRAL will provide substantially better low-energy gamma-ray sensitivity than CGRO with major improvements in spectral and angular resolutions. NASA should continue to provide adequate support for U.S. participation.

Other Recommendations

• TECHNOLOGY: The future vitality of hard X-ray and gamma-ray astronomy depends critically on the development of new instrumentation. With opportunities for new technologies opening, there is increasing need for funding of basic technology development. The GRAPWG recommends that the SR&T funding level be increased and/or other funding identified for basic technology development.
• BALLOON PROGRAM: The GRAPWG views NASA's balloon program as highly important for the continuing vitality of our field. We strongly endorse continued support for ballooning and the development of a 100-day balloon capability at midlatitudes.
• DATA ANALYSIS & THEORY: Recent missions have left us with a number of outstanding puzzles. We recommend enhanced support for analysis of the rich trove of space data and for the theoretical work essential to interpretation of gamma-ray observations. At modest cost, this maintains the vitality of the field.

TeV ASTRONOMY: An important extension to high-energy gamma-ray studies is provided by ground-based observations in the TeV range. The GRAPWG endorses the development of new telescopes with low energy threshold for TeV astronomy.