arXiv subjects

Hiroki Yoneda

Publications and source records attributed to Hiroki Yoneda.

At least 19 recordsLinked to original sources

arXiv2026

Stellar flares cannot explain the Galactic 511 keV emission

The origin of the 511 keV line signal in the Milky Way remains unresolved despite decades of observations. The measured flux of $\sim 3 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ suggests a steady-state positron injection rate of $\sim 10^{43}$-$10^{44}\,\mathrm{e^+\,s^{-1}}$. One proposed contributor to this signal is stellar flaring activity since high energy Solar flares are known to produce positrons and associated annihilation radiation. We estimate the quasi-persistent 511 keV luminosity expected from flaring stellar populations and estimate the Galactic contribution. We constrain the 511 keV fluxes for different sources in the Galaxy, and in particular globular clusters. Using Solar flare observations as a calibration baseline, we construct a hierarchical Bayesian model to link flare energy to 511 keV luminosity. We further use flare frequency-energy distributions to estimate the time-averaged positron output of stellar populations. The resulting predictions are compared with INTEGRAL/SPI observations using spatial and population-based models. We find that stellar flares fall short by several orders of magnitude in explaining the Galactic positron annihilation rate. Reproducing $\sim10\%$ of the observed luminosity in the Galactic bulge would require unphysically large maximum flare energies per star reaching up to $E_\mathrm{max} \gtrsim 10^{37-39}\,\mathrm{erg}$. Spatial modeling further shows that stellar flare scenarios cannot reproduce the observed 511 keV morphology. Stellar flares cannot be the dominant source of Galactic positrons. Although previous studies have shown that the measured 511 keV morphology is broadly consistent with old stellar populations, our results exclude normal stellar flaring activity as the underlying source for this emission.

astro-ph.GA↗

arXiv2026

Cosmological $γ$-$γ$ Pair-Production Background

The origin of positrons is one of the unsolved puzzles in astrophysics as the majority of sources are still unidentified. The Cosmic Photon Background (CPB) is the isotropic radiation spanning the entire electromagnetic spectrum. Interactions of the CPB with itself may pose a promising source of positrons and secondary emission. We calculate the electron-positron pair production rate from the $γ$-$γ$ pair-production of the CPB with itself for redshifts $z \leq 10$, and determine the annihilation spectrum, Inverse Compton emission, and bremsstrahlung. The CPB is decomposed into a sum of gray body functions, of which each is being evolved according to source type luminosity functions and redshift. We compute the pair-production rate by integrating the angle- and energy-dependent cross section over the evolving CPB. The pairs produced at each redshift are then propagated towards $z=0$, taking into account a cosmological, intergalactic, energy loss function. The photon emission is calculated per redshift and then line-of-sight integrated towards a contribution of the Cosmic Gamma-Ray Background (CGB) today. The resulting pair-production emissivity increases steeply from $z=0$ of about $2 \times 10^{-36}$ to a peak of $1.8 \times 10^{-31}\,\mathrm{e^\pm\,cm^{-3}\,s^{-1}}$ at $z=2.7$, then declines again. This yields a total cosmic pair-production rate on the order of $10^{54}\,\mathrm{e^\pm\,s^{-1}}$ up to redshift $10$. The secondary emission of pairs experiencing Inverse Compton scattering off the CPB results in a sizable contribution to the CGB. The pairs from cosmological $γ$-$γ$ absorption provide a minimum level of secondary emission which needs to be taken into account for any CGB study. Especially in the range from 1 MeV to 1 GeV, this background can make up 20% of the total CGB emission and may substantially reduce the gap between MeV observations and models.

astro-ph.CO↗

arXiv2026

Low-Noise SiPM Light Readout and ASIC-Based Charge Readout of a Liquid Argon Time Projection Chamber for MeV Gamma-Ray Measurements

We have developed a compact liquid argon time projection chamber (LArTPC), NanoGRAMS, as a technology demonstrator for the Gamma-Ray and AntiMatter Survey (GRAMS). LArTPCs have the potential to enable Compton cameras with unprecedented effective area in the MeV gamma-ray band. NanoGRAMS has an active volume of $5.12 \times 5.12 \times 10~\mathrm{cm^3}$ and is equipped with a low-noise scintillation and charge readout system. The scintillation light is detected by an array of 16 SiPMs ($6 \times 6~\mathrm{mm^2}$ each), whose signals are summed and amplified by a low-noise transimpedance amplifier operable at liquid argon temperature. Ionization electrons are read out with $3.2\,\mathrm{mm}$-pitch pixels and processed by VATA-SGD ASICs, with synchronization provided by an FPGA-based data acquisition system. We irradiated the detector with a $^{60}\mathrm{Co}$ source (1173 and $1332\,\mathrm{keV}$) and successfully detected both 1-hit and 2-hit events. The collected charge was converted to deposited energy using a phenomenological recombination model, and the detector response was evaluated with a Geant4-based Monte Carlo simulation. The reconstructed energy spectrum shows Compton edges at 963 and $1118\,keV$, consistent with the expected values. For 2-hit events, the sequence of interactions was identified, and the reconstructed back-projection image agrees with the source position. These results demonstrate the feasibility of NanoGRAMS as a Compton camera for MeV gamma-ray imaging spectroscopy.

astro-ph.IM↗

arXiv2026

On the Possibility of an Extragalactic Positron Annihilation Signal

With 20 years of INTEGRAL/SPI observations, Yoneda et al. (2025) created the most detailed map of the positron annihilation line at 511keV. While central bulge and extended disk are readily recognised in this map, several hotspots at high latitude regions may either be imaging artefacts or true signals. We discuss the possibility of extragalactic positron annihilation signals from hotspots in this map. We also calculate a cosmological positron annihilation signal as a contribution to the Cosmic Gamma-ray Background (CGB). For this investigation, we compare 511 keV emission hotspots away from the Galactic plane with a high velocity cloud column density map as well as with the catalogue of Local Volume Galaxies (LVGs) up to 25 Mpc. We find that in particular the Magellanic Stream in the southern and Complex C in the northern sky matches the brightest hotspots, which may indicate a higher positron production rate inside the Milky Way than measured from the Galactic interstellar medium alone, of $10^{44}\,\mathrm{s^{-1}}$. In addition, we can explain other hotspots by the cumulative effect of LVGs in the selected regions. The CGB contribution from positron annihilation might be sub-dominant on the per-cent level. However, depending on the true intrinsic annihilation spectrum, in particular depending on the positron injection energy for in-flight annihilation and the star formation rate per galaxy, a much higher imprint beyond 10% is possible above several MeV. If these findings turn out to be true, next generation MeV telescopes will, for the first time, identify individual extragalactic 511 keV sources. In particular, several dwarf spheroidal galaxies with fluxes of up to $(1$-$2) \times 10^{-5}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, the galaxies M31 and M33, as well as some of their satellites, with potentially several $10^{-6}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, each, may be detected.

astro-ph.HE↗

arXiv2025

Search for Axion-Like Particles from Nearby Pre-Supernova Stars

Axion-like particles (ALPs) are hypothetical pseudoscalar bosons that arise in many extensions of the Standard Model and are well-motivated dark matter candidates. Nearby massive stars in the late stages of stellar evolution provide a promising environment for enhanced ALP production due to their high core temperatures and densities. We search for a combined signal of ALP-induced hard X-ray and soft $γ$-ray emission from 18 nearby pre-supernova stars using the full public 22-year INTEGRAL/SPI dataset, construct individual stellar spectra and link them in a coherent analysis. A maximum-likelihood approach is used to extract fluxes in the 20--2000 keV energy range. Stellar evolution models are employed to obtain the expected spectral shapes of ALP production processes peaking between 50--500 keV, depending on stellar mass and evolutionary stage. We construct a joint likelihood that incorporates uncertainties in stellar parameters to derive combined constraints on the coupling constants $g_{aγ}$ and $g_{ae}$ as a function of the ALP mass $m_a$. The hard X-ray and soft $γ$-ray fluxes of all selected stars are consistent with zero within uncertainties. We provide upper limits on the continuum emission and on the 511 keV and 1809 keV line fluxes. The combined upper limit on $g_{aγ} \times g_{ae}$ is $(0.008 - 2) x 10^{-24}$ GeV$^{-1}$ (95% C.I.) while the ALP-photon coupling is constrained to $g_{aγ} = (0.13 - 1.26) x 10^{-11}$ GeV$^{-1}$ (95% C.I.) for $m_a\leqq10^{-11}$ eV, depending on the time to core collapse and magnetic field assumptions. Conservative limits of $(0.27 - 1.25) x 10^{-24}$ GeV$^{-1}$ (95% C.I.) are obtained assuming all but one star are in the early He-burning phase. These results rank among the strongest limits on ALP couplings to date and demonstrate the importance of soft $γ$-ray observations for probing ALPs and massive star evolution.

astro-ph.HE↗

arXiv2025

Imaging the positron annihilation line with 20 years of INTEGRAL/SPI observations

The e$^+$ e$^-$ annihilation line at 511 keV provides a unique probe for studying the distribution and origin of positrons in our Galaxy. The SPI spectrometer on INTEGRAL has observed this gamma-ray line for two decades. We analyze 20 years of INTEGRAL/SPI observations to produce the most sensitive all-sky map of the 511 keV line emission to date, aiming to reveal new features and provide refined measurements of known sources. We perform image deconvolution using the RL algorithm and employ bootstrap analysis to evaluate statistical uncertainties of fluxes from regions of interest. Systematic uncertainties in parameter choices are also considered. We utilize GPU acceleration to enable this computationally intensive analysis. The reconstructed image successfully recovers the basic morphological features reported in model-fitting studies: a bright central component, a broad bulge, and an elongated disk component along the Galactic plane. We also report hints of new spatial features in the reconstructed image, including an asymmetric structure in the broad bulge emission and 511 keV emission potentially associated with massive stars from the Sco-Cen and other OB associations. While the significance of these new features is marginal ($\sim 2σ$), they are spatially consistent with $^{26}$Al emission from massive stars in that region, suggesting that this 511 keV emission originates from its $β^{+}$ decay. Our 20-year dataset provides the most detailed 511 keV emission map to date, reproducing global structures suggested in model-fitting approach while revealing hints of new spatial features. These findings provide insights into the origin of Galactic positrons and propagation of low-energy positrons in the interstellar medium. Future MeV gamma-ray observations, such as COSI, are expected to confirm the reported features and shed further light on the nature of positrons in our Galaxy.

astro-ph.HE↗

arXiv2025

Signal processing and data acquisition system for the BTO detectors onboard COSI

The energy range from a few hundred keV to a few MeV includes important probes such as nuclear gamma-rays and the 511 keV annihilation line. However, compared to X-rays and GeV/TeV gamma-rays, this range suffers from lower sensitivity by orders of magnitude. The upcoming NASA SMEX satellite mission Compton Spectrometer and Imager (COSI), scheduled for launch in 2027, is expected to break through this limitation with its Compton telescope utilizing a germanium semiconductor detector, covering the 0.2-5 MeV energy range. In addition to the main instrument, two Background and Transient Observer (BTO) detectors will be installed on COSI. The detectors are NaI(Tl) scintillators coupled with SiPMs, and they are being developed as a student collaboration project. BTO aims to 1) measure background radiation in orbit to maximize COSI's sensitivity and 2) detect GRBs and other gamma-ray transients. For this purpose, it is required to cover the lower-energy range from 30 keV to 2 MeV with < 20% FWHM energy resolution. In addition, large signals and afterglow generated by heavy ions penetrating the NaI(Tl) crystal should be appropriately handled. To address these requirements, we have developed a compact signal processing and data acquisition system comprised of two main components: an analog board and a digital board. The analog board amplifies signals from the SiPM, generates triggers, and performs AD conversion. The digital board features a Microchip SAMV71 microcontroller, and we developed the software to control the analog board, read ADC data via SPI interface, add timestamps, and buffer event data. Through this development, we achieved the required wide bandpass and an energy resolution of 10% FWHM at 662 keV with a processing time of 20 us per event. We also implemented veto signal generation for large signals using a discriminator and an onboard detection algorithm for transient events.

astro-ph.IM↗

arXiv2025

The Moon as a Cosmic-Ray Spectrometer: Prospects for MeV Gamma-Ray Observations

The Moon is the closest celestial gamma-ray emitting object. Its gamma-ray emission arises from interactions between Galactic cosmic rays (CRs) and the lunar surface. While the lunar GeV gamma-ray spectrum is dominated by a continuum from hadronic decay processes, the MeV emission exhibits both continuum and distinctive spectral lines from nuclear de-excitation and radioactive decay processes. Using Geant4 Monte Carlo particle simulations, we model the lunar gamma-ray spectrum. Our results demonstrate its consistency with Fermi-LAT observations, and predict that next-generation MeV gamma-ray instruments will detect both the lunar MeV continuum and several key spectral line features, notably the $1.779~\mathrm{MeV}$ line from $\mathrm{^{28}Si}$ de-excitation enhanced by the lunar surface composition, the $e^+e^-$ annihilation line, and radioactive decay lines from $\mathrm{^{22}Na}$ ($τ\approx3.75\,\mathrm{yr}$) and long-lived $\mathrm{^{26}Al}$ ($τ\approx1\,\mathrm{Myr}$). These gamma-ray lines are sensitive to CRs with energies $\lesssim1\,\mathrm{GeV\,nuc^{-1}}$, offering unique temporal probes of CR activity over different timescales. Observations of the lunar MeV gamma-ray spectrum will therefore open a new window to study the current irradiation of the solar-terrestrial environment by low-energy CRs and its long-term temporal evolution.

astro-ph.HE↗

arXiv2025

Light WIMPs and MeV Gamma-ray Detection with COSI

Light weakly interacting massive particles (WIMPs), whose masses are in the sub-GeV scale, have been attracting more attention due to the negative results searching for traditional WIMPs. The light WIMPs are expected to produce gamma rays from annihilation in the MeV energy region. Advancements in technology have opened up possibilities to precisely detect MeV gamma rays, leading to the upcoming space-based mission of the Compton Spectrometer and Imager (COSI). We comprehensively and quantitatively study the phenomenology of light WIMPs to determine if the COSI observations will probe their viable model parameter regions. We first construct models to describe light WIMPs based on the minimality and renormalizability of quantum field theory. Next, we impose various constraints on the models obtained from cosmological observations (CMB, BBN) and dark matter searches (accelerator, underground, astrophysical experiments, etc.). Finally, we identify viable parameter regions in each model and discuss whether or not COSI will be sensitive to the parameter regions. We find that a velocity-dependent annihilation cross-section is predicted in some regions, enabling COSI to detect the dark matter signal while avoiding severe constraints from cosmological observations.

hep-ph↗

arXiv2025

Enhancing Compton telescope imaging with maximum a posteriori estimation: a modified Richardson-Lucy algorithm for the Compton Spectrometer and Imager

We present a modified Richardson-Lucy (RL) algorithm tailored for image reconstruction in MeV gamma-ray observations, focusing on its application to the upcoming Compton Spectrometer and Imager (COSI) mission. Our method addresses key challenges in MeV gamma-ray astronomy by incorporating Bayesian priors for sparseness and smoothness while optimizing background components simultaneously. We introduce a novel sparsity term suitable for Poisson-sampled data in addition to a smoothness prior, allowing for flexible reconstruction of both point sources and extended emission. The performance of the algorithm is evaluated using simulated three-month COSI observations of gamma-ray lines of $^{44}$Ti (1.157 MeV), $^{26}$Al (1.809 MeV), and positron annihilation (0.511 MeV), respectively, representing various spatial features. Our results demonstrate significant improvements over conventional RL methods, particularly in suppressing artificial structures in point source reconstructions and retaining diffuse spatial structures. This work represents an important step towards establishing a robust data analysis for studying nucleosynthesis, positron annihilation, and other high-energy phenomena in our Galaxy.

astro-ph.IM↗

arXiv2025

A Study of Afterglow Signatures in NaI and CsI Scintillator Modules for the Background and Transient Observer Instrument on COSI

We present measurements of the afterglow signatures in NaI(Tl) and CsI(Tl) detector modules as part of the Background and Transient Observer (BTO) mission detector trade-study. BTO is a NASA Student Collaboration Project flying on the Compton Spectrometer and Imager (COSI) Small Explorer mission in 2027. The detectors utilized in this study are cylindrical in shape with a height and diameter of 5.1 cm and were read out by silicon photomultipliers (SiPMs). We conducted a radiation campaign at the HIMAC accelerator in Japan where the scintillators were irradiated with a 230 MeV/u helium beam (He beam) and 350 MeV/u carbon beam (C beam). We find that both the CsI and NaI scintillators exhibit afterglow signatures when irradiated with the C and He beams. The CsI crystal exhibits a stronger afterglow intensity with afterglow pulses occurring for an average 2.40 ms for C and 0.9 ms for He after the initial particle pulse. The duration of afterglow pulses in CsI is 8.6x and 5.6x the afterglow signal duration in NaI for C and He (0.28 ms and 0.16 ms, respectively). Although CsI has advantages such as a higher light yield and radiation hardness, the stronger afterglows in the CsI detector increase the complexity of the electronics and lead to a ~7x larger dead time per afterglow event or a ~3x higher energy threshold value. We use the measured dead times to predict the amount of observing time lost to afterglow-inducing events for an instrument like BTO in low Earth orbit. We simulate the background rates in a BTO-like orbit and find a total value of 114 counts/s for the full two-detector system. Based on the particle energies in the HIMAC experiment, we then determine that an event with sufficient energy to produce an afterglow signal occurs once every ~70 s and ~1.4 s in NaI and CsI detectors, respectively. Thus, we conclude that NaI is the better choice for the BTO mission.

astro-ph.IM↗

arXiv2024

Imaging and Spectral Fitting of Bright Gamma-ray Sources with the COSI Balloon Payload

The Compton Spectrometer and Imager balloon payload (COSI-Balloon) is a wide-field-of-view Compton $γ$-ray telescope that operates in the 0.2 - 5 MeV bandpass. COSI-Balloon had a successful 46-day flight in 2016 during which the instrument observed the Crab Nebula, Cygnus X-1, and Centaurus A. Using the data collected by the COSI-Balloon instrument during this flight, we present the source flux extraction of signals from the variable balloon background environment and produce images of these background-dominated sources by performing Richardson-Lucy deconvolutions. We also present the spectra measured by the COSI-Balloon instrument, compare and combine them with measurements from other instruments, and fit the data. The Crab Nebula was observed by COSI-Balloon and we obtain a measured flux in the energy band 325 - 480 keV of (4.5 ${\pm}$ 1.6) ${\times}$ 10$^{-3}$ ph cm$^{-2}$ s$^{-1}$. The model that best fits the COSI-Balloon data combined with measurements from NuSTAR and Swift-BAT is a broken power law with a measured photon index $Γ$ = 2.20 ${\pm}$ 0.02 above the 43 keV break. Cygnus X-1 was also observed during this flight, and we obtain a measured flux of (1.4 ${\pm}$ 0.2) ${\times}$ 10$^{-3}$ ph cm$^{-2}$ s$^{-1}$ in the same energy band and a best-fit result (including data from NuSTAR, Swift-BAT, and INTEGRAL/ IBIS) was to a cutoff power law with a high-energy cutoff energy of 138.3 ${\pm}$ 1.0 keV and a photon index of $Γ$ = 1.358 ${\pm}$ 0.002. Lastly, we present the measured spectrum of Centaurus A and our best model fit to a power law with a photon index of $Γ$ = 1.73 ${\pm}$ 0.01.

astro-ph.HE↗

arXiv2024

Gamma-ray line emission from the Local Bubble

Deep-sea archives that include intermediate-lived radioactive $^{60}\mathrm{Fe}$ particles suggest the occurrence of several recent supernovae inside the present-day volume of the Local Bubble during the last $\sim 10$ Myr. The isotope $^{60}\mathrm{Fe}$ is mainly produced in massive stars and ejected in supernova explosions, which should always result in a sizeable yield of $^{26}\mathrm{Al}$ from the same objects. $^{60}\mathrm{Fe}$ and $^{26}\mathrm{Al}$ decay with lifetimes of 3.82 and 1.05 Myr, and emit $γ$-rays at 1332 and 1809 keV, respectively. These $γ$-rays have been measured as diffuse glow of the Milky Way, and would also be expected from inside the Local Bubble as foreground emission. Based on two scenarios, one employing a geometrical model and the other state-of-the-art hydrodynamics simulations, we estimate the expected fluxes of the 1332 and 1809 keV $γ$-ray lines, as well as the resulting 511 keV line from positron annihilation due to the $^{26}\mathrm{Al}$ $β^+$-decay. We find fluxes in the range of $10^{-6}$-$10^{-5}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ for all three lines with isotropic contributions of 10-50%. We show that these fluxes are within reach for the upcoming COSI-SMEX $γ$-ray telescope over its nominal satellite mission duration of 2 yr. Given the Local Bubble models considered, we conclude that in the case of 10-20 Myr-old superbubbles, the distributions of $^{60}\mathrm{Fe}$ and $^{26}\mathrm{Al}$ are not co-spatial - an assumption usually made in $γ$-ray data analyses. In fact, this should be taken into account however when analysing individual nearby targets for their $^{60}\mathrm{Fe}$ to $^{26}\mathrm{Al}$ flux ratio as this gauges the stellar evolution models and the age of the superbubbles. A flux ratio measured for the Local Bubble could further constrain models of $^{60}\mathrm{Fe}$ deposition on Earth and its moon.

astro-ph.HE↗

arXiv2023

Further Evidence for the ~ 9 s Pulsation in LS 5039 from NuSTAR and ASCA

The present study aims to reinforce the evidence for the ~9 s pulsation in the gamma-ray binary LS 5039, derived with a Suzaku observation in 2007 and that with NuSTAR in 2016 (Yoneda et al 2000). Through a reanalysis of the NuSTAR data incorporating the orbital Doppler correction, the 9.0538 s pulsation was confirmed successfully even in the 3--10 keV range, where it was undetectable previously. This was attained by perceiving an energy-dependent drift in the pulse phase below 10 keV, and correcting the pulse timing of individual photons for that effect. Similarly, an archival 0.7--12 keV data set of LS 5039, taken with the ASCA GIS in 1999 October, was analyzed. The data showed possible periodicity at about 8.882 s, but again the energy-dependent phase drift was noticed below 10 keV. By correcting for this effect, and for the orbital Doppler delays in the LS 5039 system, the 2.8--12 keV periodicity became statistically significant at 8.891+- 0.001 s. The periods measured with ASCA, Suzaku, and NuSTAR approximately follow an average period derivative of dP/dt = 3.0 e-10 s/s. These results provide further evidence for the pulsation in this object, and strengthen the scenario by (Yoneda et al 2000), that the compact object in LS 5039 is a strongly magnetized neutron star.

astro-ph.HE↗

arXiv2023

Line profile of nuclear de-excitation gamma-ray emission from very hot plasma

De-excitation gamma-ray lines, produced by nuclei colliding with protons, provide information about astrophysical environments where particles have kinetic energies of $10-100$ MeV per nucleon. In general, such environments can be categorized into two types: the interaction between non-thermal MeV cosmic rays and ambient gas, and the other is thermal plasma with a temperature above a few MeV. In this paper, we focus on the latter type and investigate the production of de-excitation gamma-ray lines in very hot thermal plasma, especially the dependence of the line profile on the plasma temperature. We have calculated the line profile of prompt gamma rays from $^{12}$C and $^{16}$O and found that when nuclei have a higher temperature than protons, gamma-ray line profiles can have a complex shape unique to each nucleus species. This is caused by anisotropic gamma-ray emission in the nucleus rest frame. We propose that the spectroscopy of nuclear de-excitation gamma-ray lines may enable to probe energy distribution in very hot astrophysical plasmas. This diagnostics can be a new and powerful technique to investigate the physical state of a two-temperature accretion flows onto a black hole, especially the energy distributions of the protons and nuclei, which are difficult to access for any other diagnostics.

astro-ph.HE↗

arXiv2023

The Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX) satellite mission in development with a planned launch in 2027. COSI is a wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV. It provides imaging, spectroscopy, and polarimetry of astrophysical sources, and its germanium detectors provide excellent energy resolution for emission line measurements. Science goals for COSI include studies of 0.511 MeV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization measurements, and detecting and localizing multimessenger sources. The instantaneous field of view for the germanium detectors is >25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts and other gamma-ray flares over most of the sky. In the following, we provide an overview of the COSI mission, including the science, the technical design, and the project status.

astro-ph.HE↗

arXiv2023

The cosipy library: COSI's high-level analysis software

The Compton Spectrometer and Imager (COSI) is a selected Small Explorer (SMEX) mission launching in 2027. It consists of a large field-of-view Compton telescope that will probe with increased sensitivity the under-explored MeV gamma-ray sky (0.2-5 MeV). We will present the current status of cosipy, a Python library that will perform spectral and polarization fits, image deconvolution, and all high-level analysis tasks required by COSI's broad science goals: uncovering the origin of the Galactic positrons, mapping the sites of Galactic nucleosynthesis, improving our models of the jet and emission mechanism of gamma-ray bursts (GRBs) and active galactic nuclei (AGNs), and detecting and localizing gravitational wave and neutrino sources. The cosipy library builds on the experience gained during the COSI balloon campaigns and will bring the analysis of data in the Compton regime to a modern open-source likelihood-based code, capable of performing coherent joint fits with other instruments using the Multi-Mission Maximum Likelihood framework (3ML). In this contribution, we will also discuss our plans to receive feedback from the community by having yearly software releases accompanied by publicly-available data challenges.

astro-ph.HE↗

arXiv2023

The origin of MeV gamma-ray diffuse emission from the inner Galactic region

The origin of the inner Galactic emission, measured by COMPTEL with a flux of $\sim ~ 10^{-2}$ MeV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ in the 1-30 MeV range, has remained unsettled since its discovery in 1994. We investigate the origin of this emission by taking into account individual sources which are not resolved by COMPTEL and the Galactic diffuse emission. The source contribution is estimated for sources crossmatched between the Swift-BAT and Fermi-LAT catalogs by interpolating the energy spectra in the hard X-ray and GeV gamma-ray ranges, as well as unmatched sources. This results in a flux of $\sim$20% of the COMPTEL excess. The Galactic diffuse emission is calculated by GALPROP to reconcile the cosmic-ray and gamma-ray spectra with observations by AMS-02, Voyager, and Fermi-LAT, resulting in a flux of $\sim$30-80% of the COMPTEL emission. Thus, we show that the COMPTEL emission could be roughly reproduced by a combination of the sources and the Galactic diffuse emission. Furthermore, combined with the extragalactic emission, we construct all-sky images in the MeV gamma-ray range to pinpoint some potential interesting targets for future missions, which would be critical for bridging the MeV gap in the spectra of gamma-ray sources.

astro-ph.HE↗