Performance and science reach of the Probe of Extreme Multimessenger Astrophysics for ultrahigh-energy particles

The Probe of Extreme Multimessenger Astrophysics (POEMMA) is a potential NASA Astrophysics Probe-class mission designed to observe ultrahigh-energy cosmic rays (UHECRs) and cosmic neutrinos from space. POEMMA will monitor colossal volumes of the Earth’s atmosphere to detect extensive air showers (EASs) produced by extremely energetic cosmic messengers: UHECRs above 20 EeV over the full sky and cosmic neutrinos above 20 PeV. We focus most of this study on the impact of POEMMA for UHECR science by simulating the detector response and mission performance for EAS from UHECRs. We show that POEMMA will provide a significant increase in the statistics of observed UHECRs at the highest energies over the entire sky. POEMMA will be the first UHECR fluorescence detector deployed in space that will provide high-quality stereoscopic observations of the longitudinal development of air showers. Therefore it will be able to provide event-by-event estimates of the calorimetric energy and nuclear mass of UHECRs. The particle physics in the interactions limits the interpretation of the shower maximum on an event-by-event basis. In contrast, the calorimetric energy measurement is significantly less sensitive to the different possible final states in the early interactions. POEMMA will increase by a factor of 30 fluorescence observations, with accurate measurements of the shower maximum. We study the prospects to discover the origin and nature of UHECRs using expectations for measurements of the energy spectrum, the distribution of arrival direction, and the atmospheric column depth at which the EAS longitudinal development reaches maximum. We also explore supplementary science capabilities of POEMMA through its sensitivity to particle interactions at extreme energies and its ability to detect ultrahigh-energy neutrinos and photons produced by top-down models including cosmic strings and superheavy dark matter particle decay in the halo of the Milky Way.

Figure 1

Schematic representation of POEMMA (photometer and spacecraft) deployed with open shutter doors (left) and in stowed position for launch (right). Cutaways in the light shield display the internal structure of corrector plate and focal surface in the middle of the payload (blue). The spacecraft bus is shown with solar panel (blue) and communications antenna deployed in both images.

Figure 2

POEMMA’s hybrid focal surface of 1.6 m diameter. The PFC (blue), composed of 55 photodetector modules (PDMS; total 126,720 MAPMT pixels) with $1\mu \mathrm{s}$ time gates, and the PCC (red) with 28 SiPM focal surface units. The PCC observes a solid angle of 9° by 30° to monitor the Earth’s limb for up-going EASs.

Figure 3

The effective area (left) and the root mean square (RMS) spot radius (right) as a function of viewing angle for a POEMMA Schmidt telescope.

Figure 4

The QE as a function of wavelength used to model the PFC response as well as the effective QE after taking into account the transmission of the BG3 UV filter.

Figure 5

POEMMA’s simulated stereo-reconstructed angular resolution versus UHECR energy: left, azimuth angle; right, zenith angle.

Figure 6

Distribution of zenith angles of triggered events above 50 EeV.

Figure 7

A stereo reconstructed 50 EeV UHECR in the two POEMMA focal planes. The solid line denotes the simulated trajectory while the dashed line shows the reconstructed trajectory. The color map provided the photoelectron statistics in each pixel.

Figure 8

Single-photometer $X_{\max }$ resolution from photoelectron statistics.

Figure 9

The simulated UHECR aperture after event reconstruction for POEMMA for the stereo mode and tilted mode.

Figure 10

Examples of the 5-year POEMMA stereo and tilted monocular UHECR exposure in terms of Auger exposure and TA until ICRC-2017.

Figure 11

Differential exposure as a function of declination in different modes, assuming a single-mode operation for the full 5-year benchmark. Purple curves denotes the stereo (near-nadir) mode at ${10}^{19.7}$ (dashed) and ${10}^{20}\mathrm{eV}$ (solid). Red curves denote the limb-viewing mode at ${10}^{20}$ (dashed), ${10}^{20.3}$ (dash), and ${10}^{21}\mathrm{eV}$ (solid). The exposures of the surface detectors of Auger and TA (including the $\mathrm{TA}\times 4$ upgrade) assuming being in operation until 2030 are shown as green and black curves, respectively.

Figure 12

POEMMA’s UHECR sky exposures in declination versus right ascension: The color scale denoting the exposure variations in terms of the mean response taking into account the positions of the sun and the moon during the observation cycle.

Figure 13

Number of UHE events with composition information detected by POEMMA for 5 years of data taking in stereo (near-nadir) operational mode (red line). For comparison, the current event statistics collected with the fluorescence detector of the Pierre Auger Observatory is shown as solid black lines and the expected number of events from AugerPrime are indicated by dashed black lines.

Figure 14

Number of UHE events detected by POEMMA for 5 years of data taking in stereo (near-nadir) (red line) and limb-viewing (blue line) operational mode assuming the Auger energy spectrum. The expected number of events collected by Auger in case of a continued operation until 2030 is shown as dashed black line.

Figure 15

Number of UHE events detected by POEMMA for 5 years of data taking in stereo (near-nadir) (red line) and limb-viewing (blue line) operational mode assuming the TA energy spectrum. The expected number of events collected by TA (including its upgrade TAx4) in case of a continued operation until 2030 is shown as a dashed black line.

Figure 16

Exposure as a function of time collected by Auger, TA (including TAx4) and POEMMA. For Auger the exposure for two different event selections is shown. The left panel shows the exposures at 40 EeV and the right panel at 100 EeV.

Figure 17

Preliminary estimate of the $X_{\max }$ resolution of POEMMA in stereo mode. The contributions from the photoelectron statistics and angular resolution are shown in blue and gray, respectively. The total resolution, obtained by adding both contributions in quadrature, is shown in red for two cuts on the maximum zenith angle.

Figure 18

Left: Energy spectrum of UHECRs as measured by TA and Auger in the Northern and Southern hemisphere, respectively. The energy scale of the two experiments were cross-calibrated by $\pm 5.2\%$ as derived by the UHECR Spectrum Working Group at low energies. Red and blue dots with error bars illustrate the expected accuracy reached with POEMMA in stereo and limb-viewing mode within 5 years of operation. Right: Flux suppression at UHE as measured by the Pierre Auger Observatory (data points) [7]. Ninety percent confidence upper limits of the flux at UHE are shown as downward triangles (ideal limits without taking into account event migration due to the limited energy resolution of the observatories). Black, Pierre Auger Observatory 2017; red, POEMMA 5 year stereo mode; blue, POEMMA 5-year limb-viewing mode. Various model predictions for the shape of the flux suppression from [86] are superimposed as black lines.

Figure 19

Capability of POEMMA to measure $⟨X_{\max }⟩$ and $\sigma (X_{\max })$ for composition studies at UHE. The width of the blue band illustrates the expected statistical uncertainties given the number of events per 0.1 in logarithm of energy in 5-year stereo operation, the $X_{\max }$ resolution and efficiency from Fig. 17 for $\theta <70°$, and intrinsic shower-to-shower fluctuations of $40\mathrm{g}/{\mathrm{cm}}^2$. The band spans the energy range for which more than ten events are within a 0.1 dex bin.

Figure 20

Circles representing the composition-layered structure of hot spots at different energies, for proton sources (top) and nuclei sources (bottom). The radii of the circles respect the proportions of the angular sizes given by (2), for protons (black), helium (magenta), nitrogen (yellow), silicon (green), and iron (red), as well as for 40 (left), 70, (center) and 100 EeV (right).

Figure 21

Power of the statistical test for different alternative hypotheses, that is, different nuclei and number of events per hot spot. The horizontal axis on the top indicates the projected timescale for POEMMA.

Figure 22

Cumulative distributions for the dipole and quadrupole moments for isotropically distributed skies with $N=1400$ events.

Figure 23

Left: Skymap of nearby starburst galaxies from Refs. [35, 103] weighted by radio flux at 1.4 GHz, the attenuation factor accounting for energy losses incurred by UHECRs through propagation, and the exposure of POEMMA. The map has been smoothed using a von Miser-Fisher distribution with concentration parameter corresponding to a search radius of $15.0°$ as found in Ref. [35]. The color scale indicates ${\mathcal{F}}_{\mathrm{src}}$, the probability density of the source sky map, as a function of position on the sky. The white dot-dashed line indicates the supergalactic plane. Right: Same as at left for nearby galaxies from the 2MRS catalog [105] and weighting by K-band flux corrected for Galactic extinction.

Figure 24

TS profile for 1400 events for a particular scenario using the starburst source sky map in Fig. 23. In the scenario pictured here, the fraction of events drawn from the source sky map is $f=10\%$ (left) and 20% (right), and the angular spread is $\mathrm{\Theta }=15°$.

Figure 25

Composition scenarios with UHE protons from [86] at ${10}^{19.1}$ (left) and ${10}^{19.6}\mathrm{eV}$ (right). There is no helium left above 40 EeV and the mass distribution is broad with a peak at around $A\sim 40$.

Figure 26

Potential of a measurement of the UHE proton-air cross section with POEMMA. Shown are also current model predictions and a complete compilation of accelerator data converted to a proton-air cross section using the Glauber formalism. The expected uncertainties for two composition scenarios (left, $p:N=1:9$; right, $p:Si=1:3$) are shown as red markers with error bars. The two points are slightly displaced in energy for better visibility.

Figure 27

Lower limit on the lifetime of SHDM particles together with the sensitivity (defined as the observation of one photon event above ${10}^{11.3}\mathrm{GeV}$ in 5 years of data collection) of POEMMA operating in stereo mode [122].

Figure 28

POEMMA sensitivity to EAS showers produced by neutrinos interacting in the atmosphere. Left: Black curves are POEMMA sensitivity using [143] (solid) and [144] (dashed) cross sections: Predictions for strongly coupled string moduli (orange range) [138], LORD sensitivity (blue band) [141], ANITA I-IV limits (purple) [91], and WSRT [142]. Right: POEMMA neutrino aperture from fluorescence with [143] (solid) and [144] (dashed) cross sections of neutrino-nucleon scattering for CC interactions of ${\nu }_e$ (blue), ${\nu }_{\mu }$, and ${\nu }_{\tau }$ (red), NC interactions (pink),and total (black).

Figure 29

EAS fluorescence image on one POEMMA PDM for a 100 EeV, 60° inclined shower. Background is not shown.

Figure 30

Exposure curve of POEMMA monocular observation. Different curves correspond to different tilting angles.

Figure 31

Example of reconstructed events. The event on the left ($E=1.7\times {10}^{20}\mathrm{eV}$, $\theta =49.6°$ and $\varphi =9.2°$) has been reconstructed with the Cherenkov method (it is possible to recognize the Cherenkov peak—blue dotted line). The right one ($E=2.5\times {10}^{20}\mathrm{eV}$, $\theta =56.3°$ and $\varphi =344.0°$) has been reconstructed by the method based on the direction reconstruction (slant depth method). The red fitted lines are the GIL functions (an analytical approximation of the EAS longitudinal development—details can be found in [156]). The vertical black lines identify the fitting interval and the horizontal one the background level.

Figure 32

Left: Comparison between the exposures obtained from POEMMA simulations (triggered events, blue; events reconstructed by Cherenkov method, green; events reconstructed by the slant depth method, red). Right: Comparison between the triggered spectra obtained from POEMMA simulations (triggered events, blue; events reconstructed by Cherenkov method, green; events reconstructed by the slant depth method, red).

Figure 33

Energy resolution $(E_{\mathrm{reco}}-E_{\mathrm{real}})/E_{\mathrm{real}}$ for events belonging to two different energy intervals (low energy on the left panels and high energy on the right panels). The top plots refer to the performance of the Cherenkov method while the bottom ones refer to the slant depth method.

Figure 34

Efficiency in the angular reconstruction for POEMMA as a function of zenith angle for different EAS energies.

Figure 35

Left: Angular resolution for POEMMA defined through the ${\gamma }_{68\%}$ parameter. The circled point indicates that a limited bias exists for these points; see Table 5 for details. Right: Distribution of the ${\gamma }_{68\%}$ parameter for $E={10}^{20}\mathrm{eV}$ and $\theta =60°$.

Figure 36

ESAF simulated energy resolution assuming 1° zenith and azimuth angular resolution: Left, 50 EeV results; right, 100 EeV results.

Figure 37

Normalized source sky maps for nearby galaxies from the 2MRS catalog [105] weighted by K-band flux corrected for Galactic extinction, the attenuation factor accounting for energy losses incurred by UHECRs through propagation, and the exposure of POEMMA. The maps have been smoothed using a von Miser-Fisher distribution with concentration parameters corresponding to angular spreads of $\mathrm{\Theta }=5°$ (top left), 10° (top right), 20° (bottom left), and 25° (bottom right). The color scales indicate ${\mathcal{F}}_{\mathrm{src}}$ as a function of position on the sky. The white dot-dashed line indicates the supergalactic plane.

Figure 38

Probability density sky maps corresponding to the select astrophysical scenarios with parameters selected based on the best-fit parameters from Ref. [103]. Top left: 2MRS galaxies with $f_{\mathrm{sig}}=19\%$ and $\mathrm{\Theta }=15°$. Top right: Starburst galaxies with $f_{\mathrm{sig}}=11\%$ and $\mathrm{\Theta }=15°$. Bottom: Swift-BAT AGNs with $f_{\mathrm{sig}}=8\%$ and $\mathrm{\Theta }=15°$. The color scales indicate ${\mathcal{F}}_{\mathrm{sky}}$ as a function of position on the sky. The blue dot-dashed line indicates the supergalactic plane.

Figure 39

Values of ${\mathrm{\Lambda }}_{\eta }^{\mathrm{opt}}$ and the number of events for $E_{\mathrm{CR}}>40\mathrm{EeV}$ and $X_{\max }>X_{\max }^{\text{start}}$ for $p:N=1:9$, $\eta =0.02$ with EPOS-LHC (left) and $p:\mathrm{Si}=1:3$, $\eta =0.13$ with QGSJET (right). The blue histograms show the frequencies of ${\mathrm{\Lambda }}_{\eta }^{\mathrm{opt}}$ and $N_{\mathrm{evts}}(X_{\max }>X_{\max }^{\text{start}})$. The vertical and horizontal dotted lines show the mean values of ${\mathrm{\Lambda }}_{\eta }^{\mathrm{opt}}$ and $N_{\mathrm{evts}}(X_{\max }>X_{\max }^{\text{start}})$, respectively.

Figure 40

The energy and $X_{\max }$ smeared distribution $\mathrm{d}N/\mathrm{d}{X}_{\max }$ for qgs jetii.04 (upper panels) and EPOS-LHC (lower panels) from the parametrization of Ref. [158] for $E_{\mathrm{CR}}>40\mathrm{EeV}$ and $p:N=1:9$, $\eta =0.02$ (upper) and $p:\mathrm{Si}=1:3$, $\eta =0.13$ (lower). The blue histograms show the full $X_{\max }$ distributions, while the green histograms show the proton components. The data points, with error bars, show the distribution of events for one sample of $N_{\mathrm{evts}}=1$, 400 events with error bars according to $\sqrt{N_i}/N_{\mathrm{evts}}$, for $N_i$ being the number of events in the bin $i$. The red data points show $X_{\max }$ bins above $X_{\max }^{\text{start}}$. In the right panels, the shaded red band shows the slope of the tail determined by ${\mathrm{\Lambda }}_{\eta }^{\mathrm{opt}}$ with $1\sigma$ statistical errors on ${\mathrm{\Lambda }}_{\eta }^{\mathrm{opt}}$ and the number of events above $X_{\max }^{\text{start}}$.

Figure 41

The $X_{\text{Start}}$ distributions defined by the observed starting point of proton EASs and a parametric fit using two Gaussian and an exponential distribution. Top left: 40 EeV (fit reduced ${\chi }^2=1.13$); Top right: 60 EeV (fit reduced ${\chi }^2=1.34$); bottom left: 100 EeV (fit reduced ${\chi }^2=0.95$); bottom right: (fit reduced ${\chi }^2=0.85$).

Figure 42

Comparison of the instantaneous electron neutrino apertures based on stereo air fluorescence measurements. Upper points and curve are for $X_{\text{Start}}\ge 1500{\mathrm{g}/\mathrm{cm}}^2$ while the lower points and curve are for $X_{\text{Start}}\ge 2000{\mathrm{g}/\mathrm{cm}}^2$. The lower curve is 85% of the upper curve over the energy band.