Draft:Probe of Extreme Multi-Messenger Astrophysics (POEMMA)

The Probe of Extreme Multi-Messenger Astrophysics (POEMMA) is a conceptual design for a next generation astroparticle physics observatory. Designed to measure ultra-high energy cosmic rays (UHECRs) and astrophysical neutrinos, POEMMA was submitted to the 2020 Astronomy and Astrophysics Decadal Survey as a potential probe-class mission. Two satellites flying in formation in a low-earth orbit will house 4-m diamter telescopes focusing onto a hybrid focal surface designed for the detection of extensive air showers. Prototype instruments have been deployed as superpressure balloon payloads.

Science Goals
The main science goals of POEMMA are to study the nature and origin of the highest energy particles in the universe, UHECRs, and to observe neutrino emission above 20 PeV from extreme astrophysical transients.

Observations of UHECRs
Originating from astrophysical sources believed to bne outside the galaxy \cite{}, UHECRs are single particles with macroscopic energies beyond 10^{18} eV. When UHECRs reach Earth's atmosphere, they induce a cascade of billions of secondary particles, including electrons, positrons and photons. As these particles pass through the atmosphere, collisional excitation creates ultraviolet light which can be observed.

The flux of UHECRs is relatively low, with a cosmic ray above 10^{19} eV occurring roughly once per square kilometer per year, and falls proportional to E^{-2.7}. Therefore, the highest energy UHECRs require very large collection areas in order to acquire a sufficient number of observations. The basic idea of POEMMA is to achieve this large collection area by looking down at the atmosphere from above. This will allow for an area to be observed that is larger than the current state-of-the-art UHECR detectors the Pierre Auger Observatory and the Telescope Array.

In addition to achieving a larger effective observational area compared to ground based detectors without the need for a physically larger detector, POEMMA presents several advantages for the observation of UHECRs. Being a fluorescence detector, all observations will include information on the shower maximum as well as calometric energy, features relevant for interpretting observations in anisotropy studies. Further, utilizing an inclined orbit around the Earth, POEMMA will have sensitivity to arrival directions spanning the entire sky.

Observations of astrophysical neutrinos
The existence of UHECRs implies the presence of very-high energy astrophysical neutrinos, with energies greater than 20 PeV, that have not yet been observed. These very-high energy neutrinos would not only provide information about their production, but could also be correlated with gravity waves and electromagnetic signals to preform multi-messenger astronomy.

The proposed detection technique for astrophysical neutrinos is to detect Earth-skimming tau-neutrinos which interact in the crust of the Earth and produce a tau lepton which decays in the atmosphere and produces an upward going air shower (physics). Different from the fluorescence emission that will be observed to detect UHECRs, which occurs along the axis of the shower's momentum and is emitted isotropically, these tau-induced showers will be detected via Cherenkov radiation emitted in a cone around the shower axis. Since this radiation is strongly beamed in the direction of the primary particle, a lower energy is required for observation. However, since the detector needs to be roughly aligned with the direction of the shower, the exposure realized with this observation technique is lower than that of fluorescence observations.

A great strength of POEMMA would be the ability to preform follow up observations of astrophysical target-of-opportunity (ToO). By utilizing the slewing capability of the spacecraft, the detector can be aligned with astrophysical transient events, such as binary neutron star mergers or gamma-ray bursts. Combined with the large observing volume, this leads to instantaneous sensitivity which is orders of magnitudes greater than kilometer scale ground based detectors, such as IceCube.

Secondary Science Goals
POEMMA can test models of physics beyond the standard model (BSM) through observations of neutrinos over a wide range of energies that are not reachable by terrestrial accelerators. POEMMA will also be able to test hadronic interaction models through the observation of high-altitude horizontal air showers.

Being positioned above the atmosphere, POEMMA will also be sensitive to transient luminous events, meteorites and exotic particles such as nuclearites.

Design
The conceptual design consists of two identical spacecrafts observing in two modes. In stereo configuration, separated by roughly 300 km and looking straight down, precision measurements of UHECRs will be made. In limb mode, separated by roughly 25 km, the two spacecraft will point towards the limb of the Earth allowing for the maximum exposure to UHECRs and follow up of astrophysical ToO events.

Each of POEMMA's two spacecraft will house schmidt telescopes with an entrance diameter of 4 m and a 45° field of view. At the focus of these telescopes will be a hybrid focal surface consisting of silicon photomultipliers (SiPMs) and multi-anode photomultiplier tubes (MAPMTs). The MAPMTs, capable of discriminating single photons, will be digitized with a frequency of 1 MHz and be used for the observation of fluorescence signals from UHECR induced air showers. The SiPMs will be digitized with a frequency of 100 MHz and be used for the observation of Cherenkov Radiation from tau-induced air showers.

Current Status
POEMMA was submitted as a conceptual design to the 2020 Astronomy and Astrophysics Decadal Survey as a potential probe-class mission. The survey highlighted probe class missions as a particular interest for future investment, but did not select astroparticle physics as an area of interest.

A prototype instrument was flown as a mission of opportunity on a NASA superpressure balloon in 2023. A second payload is under development planning for a 2027 launch.