Advanced Compton Telescope

- science goals -

The ACT science requirements (red lines) call for an ambitious improvement in gamma-ray line sensitivity, two orders of magnitude improvement over COMPTEL and INTEGRAL. This is necessary to allow detailed studies of supernova explosions. 

Scientific Objectives

ACT's main goal is to probe the nuclear fires where the chemical elements are created. Since gamma-ray lines probe the nucleosynthesis and dynamics of supernova (SN) explosions, a detailed study of gamma-ray line emission from supernovae (combined with optical observations) will lead to breakthroughs in the understanding of the explosion mechanisms and expansion dynamics of supernovae. Better understanding of Type Ia SNae will also enhance and validate their use as cosmological distance indicators. For all types of SNae, a measured inventory of newly created nuclei in individual explosions is necessary to understand their contribution to galactic chemical evolution.

As the ACT cornerstone science objective for the vision mission study, we define ACT's science requirements in terms of SN Ia and their 56Ni and 56Co decay line emission (158keV, 812 keV and 847keV, 1238keV lines, respectively). These lines are expected to be Doppler-broadened with typical widths of 3-4%. ACT must measure enough SN spectra with sufficient signal-to-noise to distinguish clearly among SN Ia models and probe the physics of the explosion. We deduce from this a goal of observing tje 847 keV line flux at 15sigma significance or better for five SN Ia per year. (This also corresponds to detecting 50 SN Ia per year in this line, and one with 40sigma significance per year.) An energy resolution of E/dE > 100 is required to derive information about the explosion geometry from the line profiles.
Simulated ACT observation of a Type Ia supernova at distance 24 Mpc, within which there is at least one per year. Three models are shown, together with an observation corresponding to the instrument sensitivity and energy resolution set as the goal for ACT. The observation would allow us to clearly distinguish between the models (delayed-detonation DD202c, deflagration W7, helium-triggered detonation HED6) . [Courtesy of M. Leising]

Supernova Remnants:
Due to the excellent sensitivity and wide field-of-view of an Advanced Compton Telescope, ACT will also detect the gamma-ray line emission from longer-lived radionuclides from a large number of galactic and Local Group supernova remnants (SNRs). This will permit a detailed study of the production of 44Ti, 26Al, 60Fe, and positrons in various types of SNRs (line detections are expected from at least 16 known individual SNRs in a 5-year mission lifetime).

Diffuse Galactic Emission:
Diffuse line emission from interstellar radionuclides, electron-positron annihilation, and nuclear excitation by accelerated particles afford the opportunity to study stellar evolution, the ongoing production of the elements, and energetic processes throughout the Milky Way. Main candidates for observation here are the diffuse 26Al and 60Fe emission and the 511 keV line emission from positron annihilation. 26Al, for example, is produced mainly in massive stars in explosive and/or hydrostatic burning, and 60Fe is co-produced with 26Al in neon burning core-collapse supernovae but not in other 26Al sources. The spatial differences between these two million-year-lifetime radionuclides will teach us about the nucleosynthesis processes in several classes of sources.

Additional Objectives:
Classical Novae are believed to emit gamma-rays both from positrons, and from nuclear lines from 7Be and 22Na decay. Detection of a nearby nova in gamma rays will provide unprecedented constraints on models of thermonuclear runaway, rapid convection, and ejection dynamics. Black Holes and AGN are bright at MeV energies, and display a wide range of spectral states. ACT will, e.g., test disk models for back holes accreting near the Eddington limit, and will search for broadened annihilation line features from nonthermal compact plasmas that are predicted to surround accreting black holes. With its large field-of-view, ACT will find highly obscured black holes surrounded by Thomson-thick material, extending searches by black-hole finder probes sensitive at hard X-ray energies only. Combining ACT and GLAST observations of Neutron Stars and Pulsars will help unravel the particle acceleration and photon production mechanisms in the outer reaches of pulsar magnetospheres. Polarization measurements will distinguish between polar cap and outer gap emission models. A gravitationally redshifted gamma-ray line (e.g. from H(n,gamma)D) originating from the NS atmosphere would constrain the nuclear equation of state at matter densities currently inaccessible in the terrestrial laboratory. ACT with its wide field-of-view will localize many Gamma-Ray Bursts promptly to allow ground-based follow-up observations, and will provide the flux and spectral measurements above a few hundred keV necessary to obtain the total bolometric luminosity of these GRBs. ACT can also measure the polarization of GRB emission, one of the few means available for probing the GRB central engine in the electromagnetic regime. Solar Flares constitute the efficient conversion of magnetic energy into the acceleration of high-energy particles, whose properties are constrained by gamma-ray lines and continuum. ACT will offer spectral resolution similar to that of RHESSI but more than 50 times higher effective area, and thus provide a wealth of information on acceleration and transport processes. The Diffuse Gamma-Ray Background, i.e. the extragalactic background radiation (EBR), provides a unique window on a variety of fundamental topics in cosmology, astrophysics, and particle physics. The least explored portion of the EBR is in the 100 keV to 10 MeV region. ACT will measure the details of the global emission, and better quantify the individual source contributors.