The soft/medium gamma-ray regime is the least astrophysically explored range across the electromagnetic spectrum; the sensitivity of current missions is orders of magnitudes worse than neighboring bands due to high instrumental and atmospheric backgrounds, low interaction cross-sections, and inherent difficulty of imaging at these energies. It remains an extremely interesting range harboring the positron annihilation line, signatures of stellar nucleosynthesis, emission from the most extreme environments, and multimessenger astrophysics.
COMPTEL on the Compton Gamma-ray Observatory (CGRO) opened up the MeV gamma-ray band in the 1990’s, followed by INTEGRAL in 2002. Together they have made huge progress toward cataloging and understanding the gamma-ray sky, however there are still many open questions, and the following describes COSI’s four key science goals.
Uncover the Origin of Galactic Positrons
The 511 keV γ-ray line, signature of positron annihilation, was first discovered coming from the Galactic Center region in the 1970’s [Johnson et al. 1973], but the source of these positrons is still a mystery. Possible sources include β-decay from stellar nucleosynthesis, XRBs and microquasars, pulsars and magnetars, pion decay in cosmic rays, the supermassive black hole, and dark matter annihilation, decay or de-excitaion.
Spectral studies give information about the level of ionization and temperature of the annihlation medium: now thought to be neutral and ionized warm phases of the ISM from INTEGRAL/SPI measurements [Jean et al. 2006]. Spatial maps of the 511 line show the distribution of positron annihilations in the galaxy (not necessarily their orgin due to uncertainties in propagation [Jean et al. 2009]), and SPI’s intriguing image showing the majority of positron annihilating within 20° of the Galactic Center and along the plane, a spatial distribution unseen in other wavelengths (see the feature image [Bouchet et al. 2010]). The source of the bulge emission is unknown, however the plane emission is believed to come from the β-decay of stellar nucleosynthesis products, namely Al-26.
COSI will produce the first direct image of the 511 keV line from the galactic bulge and disk, as well as measure line profiles in these regions. By comparing the 511 keV map to the Al-26 map, COSI can determine if the positron emission in the disk traces the locations of these nuclei.
Reveal Galactic Element Formation
Observing nuclear-lines and comparing the measured flux of different isotopes reveals information about the evolution of massive stars and their SN explosions: measurements accessible only in the soft γ-ray range.
COMPTEL produced a map of the 1.8 MeV line emitted by 26Al [Oberlack et al. 1996; Knodlseder et al. 1999] (half-life ∼7.2×105 yr) which traces star forming regions in our galaxy through it’s production in massive stars. 60Fe (half-life ∼1.5×106 yr), not yet imaged in our galaxy, is released in significant quantities during ccSNe. 44Ti, with a half-life of ~60 yr, reveals young SNe remnants and is detectable with COSI through the 1.157 MeV photon produced in the β-decay of 44Sc. Together, these measurements can further our understanding of SN nucleosynthesis [Diehl 2013].
COSI will map 26Al and 60Fe emissions, enabling detailed studies of the 60Fe/26Al ratio, and search for 44Ti emission from young SNe.
Gain Insight into Extreme Environments with Polarization
As a Compton telescope, COSI measures the azimuthal scattering angle of incoming gamma rays, and this provides a measurement of the polarization of astrophysical sources. Polarization measurements provide a unique diagnostic tool to probe emission mechanisms and source geometries. Such measurements have the potential to provide new information about extreme environments in gamma-ray bursts (GRBs), pulsars, and accreting black holes (BHs), and COSI’s science goal focuses on measurements of accreting BHs.
COSI will be the first gamma-ray instrument with the sensitivity to make polarization measurements for accreting supermassive BHs in Active Galactic Nuclei (AGN), and the image above illustrates a hypothetical polarization measurement of the AGN Centaurus A. In addition to making polarization measurements for AGN, COSI will study accreting Galactic BHs such as Cygnus X-1 as well as bright Galactic BH transients.
Probe the Physics of Multimessenger Events
Besides photons, there are other types of cosmic signals, including cosmic rays of high-energy particles, neutrinos, and gravitational waves. We know of three types of sources that emit more than one of these signals: supernovae produce neutrinos and photons (for example, SN 1987A), merging neutron stars produce gravitational waves and photons (for example, the gravitational wave event GW170817 was detected as a short GRB), and AGN produce high-energy neutrinos and photons (for example, TXS 0506+056 was detected in high-energy neutrinos and gamma-rays).
COSI will detect and localize short GRBs to allow for follow-up observations with other telescopes, and the information will also potentially help the gravitational wave observatories focus in on the times when gravitational waves from merging neutron stars (illustrated in the image above) may be seen.