arXiv subjects

Peter von Ballmoos

Publications and source records attributed to Peter von Ballmoos.

At least 19 recordsLinked to original sources

arXiv2025

Benchmarking of Geant4 simulations for the COSI Anticoincidence System

The Compton Spectrometer and Imager (COSI) is an upcoming NASA Small Explorer satellite mission, designed for all-sky observations in the soft gamma-ray domain with the use of germanium detectors (GeDs). An active Anticoincidence System (ACS) of BGO scintillators surrounds the GeDs to reduce the background and contribute to the detection of transient events. Accurately modeling the ACS performance requires simulating the intricate scintillation processes within the shields, which significantly increases the computational cost. We have encoded these effects into a correction matrix derived from dedicated Geant4 simulations with the inclusion of the optical physics. For this purpose, we use laboratory measurements for the energy and spatial response of the ACS lateral wall to benchmark the simulation and define instrument parameters, including the BGO absorption length and the electronic noise. We demonstrate that the simulations replicate the experimental energy resolution and light collection uniformity along the BGO crystal, with maximum discrepancies of 20% and 10%, respectively. The validated simulations are then used to develop the correction matrix for the lateral wall, accounting for the light collection efficiency and energy resolution based on the position within the crystal. The gamma-ray quantum detection efficiency is also position-dependent via the inclusion of the optical physics. It is enhanced by $\sim$8% close to the SiPMs and suppressed by $\sim$2% in the adjacent corners with respect to the average value. Finally, we explore the energy threshold and resolution of the bottom ACS, considering the impact of its smaller crystals compared with the lateral walls.

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arXiv2025

Detection of positron in-flight annihilation from the Galaxy

Although the annihilation of positrons towards the Galactic centre was established more than 50 years ago through the detection of a 511 keV $\gamma$-ray line, the origin of the positrons remains unknown. The $\gamma$-ray line should be accompanied by continuum emission from positron in-flight annihilation, which, until now, has not been detected. We aim to detect positron in-flight annihilation emission, as it provides information on the kinetic energy of the positrons that is key in determining the origin of Galactic positrons. We analysed archival data obtained by the COMPTEL instrument on the Compton Gamma-Ray Observatory satellite to search for positron in-flight annihilation emission in the MeV energy range. Our analysis revealed extended emission in the MeV energy range towards the bulge of the Galaxy, which we attribute to in-flight annihilation of positrons produced with kinetic energies of $\sim$2 MeV. The observed spectrum suggests that positrons are produced quasi-mono-energetically, which could occur by the annihilation of dark matter particles with masses of $\sim$3 MeV, or through bulk motion in the jet of the microquasar 1E1740.7-2942. We furthermore detected a point-like emission component in the MeV energy range towards the Galactic centre that is the plausible low-energy counterpart of the Fermi-LAT source 4FGLJ1745.6-2859. The broad band spectrum of the source may be explained by the injection of pair plasma from the supermassive black hole Sgr A$^\ast$ into the interstellar medium, which would also explain the point-like 511 keV line emission component that was discovered by INTEGRAL/SPI at the Galactic centre. The observed positron in-flight annihilation spectrum towards the Galactic bulge excludes $\beta^+$ decays from radioactive isotopes, as well as any mechanism producing highly relativistic positrons as the origin of the Galactic bulge positrons.

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arXiv2025

Bottom-up Background Simulations of the 2016 COSI Balloon Flight

The Compton Spectrometer and Imager (COSI) is a Compton telescope designed to survey the 0.2-5 MeV sky, consisting of a compact array of cross-strip germanium detectors. As part of its development, in 2016 COSI had a successful 46 day flight on board NASA's Super Pressure Balloon platform. This was a precursor to the COSI Small Explorer (COSI-SMEX) satellite mission that will launch in 2027 into a equatorial low Earth (530 km) orbit. The observation of MeV gamma-rays is dominated by background radiation, especially due to the activation of the detector materials induced by cosmic-ray interactions. Thus, background simulation and identification are crucial for the data analysis. Because the COSI-SMEX detectors will be similar to the ones used for the balloon flight, the balloon measurements provide an important tool for testing and cross-checking our background simulations for the upcoming space mission. In this work we perform Monte Carlo simulations of the background emission from the 2016 COSI balloon flight. Including a phenomenological shape correction, we obtain an agreement with the data at the 10-20% level for energies between 0.1-1.6 MeV, and we successfully reproduce most of the activation lines induced by cosmic ray interactions.

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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 ${\gamma}$-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 ${\Gamma}$ = 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 ${\Gamma}$ = 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 ${\Gamma}$ = 1.73 ${\pm}$ 0.01.

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arXiv2024

Towards a response function for the COSI anticoincidence system: preliminary results from Geant4 simulations

The Compton Spectrometer and Imager (COSI) is an upcoming NASA Small Explorer satellite mission scheduled for launch in 2027 and designed to conduct an all-sky survey in the energy range of 0.2-5 MeV. Its instrument consists of an array of germanium detectors surrounded on four sides and underneath by active shields that work as anticoincidence system (ACS) to reduce the contribution of background events in the detectors. These shields are composed of bismuth germanium oxide (BGO), a scintillator material, coupled with Silicon photomultipliers, aimed to collect optical photons produced from interaction of ionizing particles in the BGO and convert them into an electric signal. The reference simulation framework for COSI is MEGAlib, a set of software tools based on the Geant4 toolkit. The interaction point of the incoming radiation, the design of the ACS modules and the BGO surface treatment change the light collection and the overall shielding accuracy. The use of the Geant4 optical physics library, with the simulation of the scintillation process, is mandatory for a more realistic evaluation of the ACS performances. However, including the optical processes in MEGAlib would dramatically increase the computing time of the COSI simulations. We propose the use of a response function encoding the energy resolution and 3D light yield correction based on a separate Geant4 simulation of the ACS that includes the full optical interaction. We present the verification of the Geant4 optical physics library against analytical computations and available laboratory measurements obtained using PMTs as readout device, as a preparatory phase for the simulation of the COSI ACS response.

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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.

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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.

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arXiv2022

Measurement of Galactic $^{26}$Al with the Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a balloon-borne compact Compton telescope designed to survey the 0.2-5 MeV sky. COSI's energy resolution of $\sim$0.2% at 1.8 MeV, single-photon reconstruction, and wide field of view make it capable of studying astrophysical nuclear lines, particularly the 1809 keV $γ$-ray line from decaying Galactic $^{26}$Al. Most $^{26}$Al originates in massive stars and core-collapse supernova nucleosynthesis, but the path from stellar evolution models to Galaxy-wide emission remains unconstrained. In 2016, COSI had a successful 46-day flight on a NASA superpressure balloon. Here, we detail the first search for the 1809 keV $^{26}$Al line in the COSI 2016 balloon flight using a maximum likelihood analysis. We find a Galactic $^{26}$Al flux of $(8.6 \pm 2.5) \times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ within the Inner Galaxy ($|\ell| \leq 30^{\circ}$, $|b| \leq 10^{\circ}$) with 3.7$σ$ significance above background. Within uncertainties, this flux is consistent with expectations from previous measurements by SPI and COMPTEL. This analysis demonstrates COSI's powerful capabilities for studies of $γ$-ray lines and underscores the scientific potential of future compact Compton telescopes. In particular, the next iteration of COSI as a NASA Small Explorer satellite has recently been approved for launch in 2025.

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arXiv2021

The Compton Spectrometer and Imager Project for MeV Astronomy

The Compton Spectrometer and Imager (COSI) is a 0.2-5 MeV Compton telescope capable of imaging, spectroscopy, and polarimetry of astrophysical sources. Such capabilities are made possible by COSI's germanium cross-strip detectors, which provide high efficiency, high resolution spectroscopy and precise 3D positioning of photon interactions. 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, and detecting and localizing multimessenger sources. The instantaneous field of view (FOV) 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 or other gamma-ray flares over >50% of the sky. We have completed a Phase A concept study to consider COSI as a Small Explorer (SMEX) satellite mission, and here we discuss the advances COSI-SMEX provides for astrophysics in the MeV bandpass.

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arXiv2021

Constraints on the antistar fraction in the Solar system neighborhood from the 10-years Fermi Large Area Telescope gamma-ray source catalog

It is generally taken for granted that our Universe is free of antimatter objects and domains. This certitude has recently been challenged by the possible detection of anti-helium nuclei by AMS-02. Should the observation be confirmed, the existence of nearby antistars would make a plausible hypothesis to explain the origin of the antinuclei. In this paper we use the 10-years Fermi Large Area Telescope (LAT) gamma-ray source catalog to set constraints on the abundance of antistars around the Sun. We identify in the catalog 14 antistar candidates not associated with any objects belonging to established gamma-ray source classes and with a spectrum compatible with baryon-antibaryon annihilation. We use them along with an estimate of the LAT sensitivity to antistars to set upper limits on the local antistar fraction $f_{\bar{\ast}}$ with respect to normal stars. We provide parametric limits as a function of the closest antistar mass, velocity, and surrounding matter density. We also employ a novel Monte~Carlo method to set limits for a few hypotheses about the antistar population. For a population with properties equivalent to those of regular stars concentrated in the Galactic disk we obtain $f_{\bar{\ast}} < 2.5 \times 10^{-6}$ at 95\% confidence level, which is 20 times more constraining than limits previously available. For a primordial population of antistars distributed in the Galactic halo we obtain new local upper limits which decrease as a function of antistar mass $M$ from $f_{\bar{\ast}} < 0.2$ at 95\% confidence level for $M = 1 \; M_\odot$ to $f_{\bar{\ast}} < 1.6 \times 10^{-4}$ at 95\% confidence level for $M = 10 \; M_\odot$. By combining these limits with existing microlensing constraints for lighter objects in the Magellanic clouds, we infer that a primordial population of halo antistars must have a density lower than $\mathcal{O}(10^{-5}\;\text{pc}^{-3})$ to $\mathcal{O}(10^{-2}\;\text{pc}^{-3})$ depending on their masses. Our limits can constrain models for the origin and propagation of antinuclei in cosmic rays.

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arXiv2021

Gamma-ray Astrophysics in the MeV Range: the ASTROGAM Concept and Beyond

The energy range between about 100 keV and 1 GeV is of interest for a vast class of astrophysical topics. In particular, (1) it is the missing ingredient for understanding extreme processes in the multi-messenger era; (2) it allows localizing cosmic-ray interactions with background material and radiation in the Universe, and spotting the reprocessing of these particles; (3) last but not least, gamma-ray emission lines trace the formation of elements in the Galaxy and beyond. In addition, studying the still largely unexplored MeV domain of astronomy would provide for a rich observatory science, including the study of compact objects, solar- and Earth-science, as well as fundamental physics. The technological development of silicon microstrip detectors makes it possible now to detect MeV photons in space with high efficiency and low background. During the last decade, a concept of detector ("ASTROGAM") has been proposed to fulfil these goals, based on a silicon hodoscope, a 3D position-sensitive calorimeter, and an anticoincidence detector. In this paper we stress the importance of a medium size (M-class) space mission, dubbed "ASTROMEV", to fulfil these objectives.

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arXiv2020

Imaging the 511 keV positron annihilation sky with COSI

The balloon-borne Compton Spectrometer and Imager (COSI) had a successful 46-day flight in 2016. The instrument is sensitive to photons in the energy range $0.2$-$5$ MeV. Compton telescopes have the advantage of a unique imaging response and provide the possibility of strong background suppression. With its high-purity germanium detectors, COSI can precisely map $γ$-ray line emission. The strongest persistent and diffuse $γ$-ray line signal is the 511 keV emission line from the annihilation of electrons with positrons from the direction of the Galactic centre. While many sources have been proposed to explain the amount of positrons, $\dot{N}_{\mathrm{e^+}} \sim 10^{50}\,\mathrm{e^+\,yr^{-1}}$, the true contributions remain unsolved. In this study, we aim at imaging the 511 keV sky with COSI and pursue a full-forward modelling approach, using a simulated and binned imaging response. For the strong instrumental background, we describe an empirical approach to take the balloon environment into account. We perform two alternative methods to describe the signal: Richardson-Lucy deconvolution, an iterative method towards the maximum likelihood solution, and model fitting with pre-defined emission templates. Consistently with both methods, we find a 511 keV bulge signal with a flux between $0.9$ and $3.1 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, confirming earlier measurements, and also indications of more extended emission. The upper limit we find for the 511 keV disk, $< 4.3 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, is consistent with previous detections. For large-scale emission with weak gradients, coded aperture mask instruments suffer from their inability to distinguish isotropic emission from instrumental background, while Compton-telescopes provide a clear imaging response, independent of the true emission.

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arXiv2020

Detection of the 511 keV Galactic positron annihilation line with COSI

The signature of positron annihilation, namely the 511 keV $γ$-ray line, was first detected coming from the direction of the Galactic center in the 1970's, but the source of Galactic positrons still remains a puzzle. The measured flux of the annihilation corresponds to an intense steady source of positron production, with an annihilation rate on the order of $\sim10^{43}$~e$^{+}$/s. The 511 keV emission is the strongest persistent Galactic $γ$-ray line signal and it shows a concentration towards the Galactic center region. An additional low-surface brightness component is aligned with the Galactic disk; however, the morphology of the latter is not well constrained. The Compton Spectrometer and Imager (COSI) is a balloon-borne soft $γ$-ray (0.2--5 MeV) telescope designed to perform wide-field imaging and high-resolution spectroscopy. One of its major goals is to further our understanding of Galactic positrons. COSI had a 46-day balloon flight in May--July 2016 from Wanaka, New Zealand, and here we report on the detection and spectral and spatial analyses of the 511 keV emission from those observations. To isolate the Galactic positron annihilation emission from instrumental background, we have developed a technique to separate celestial signals utilizing the COMPTEL Data Space. With this method, we find a 7.2$σ$ detection of the 511 keV line. We find that the spatial distribution is not consistent with a single point source, and it appears to be broader than what has been previously reported.

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arXiv2019

The Compton Spectrometer and Imager

In this Astro2020 APC White Paper, we describe a Small Explorer (SMEX) mission concept called the Compton Spectrometer and Imager. COSI is a Compton telescope that covers the bandpass often referred to as the "MeV Gap" because it is the least explored region of the whole electromagnetic spectrum. COSI provides a significant improvement in sensitivity along with high-resolution spectroscopy, enabling studies of 511 keV electron-positron annihilation emission and measurements of several radioactive elements that trace the Galactic history of supernovae. COSI also measures polarization of gamma-ray bursts (GRBs), accreting black holes, and pulsars as well as detecting and localizing multimessenger sources. In the following, we describe the COSI science, the instrument, and its capabilities. We highlight many Astro2020 science WPs that describe the COSI science in depth.

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arXiv2017

Polarimetric Analysis of the Long Duration Gamma Ray Burst GRB 160530A With the Balloon Borne Compton Spectrometer and Imager

A long duration gamma-ray burst, GRB 160530A, was detected by the Compton Spectrometer and Imager (COSI) during the 2016 COSI Super Pressure Balloon campaign. As a Compton telescope, COSI is inherently sensitive to the polarization of gamma-ray sources in the energy range 0.2-5.0 MeV. We measured the polarization of GRB 160530A using 1) a standard method (SM) based on fitting the distribution of azimuthal scattering angles with a modulation curve, and 2) an unbinned, maximum likelihood method (MLM). In both cases, the measured polarization level was below the $99\%$ confidence minimum detectable polarization levels of $72.3 \pm 0.8\%$ (SM) and $57.5 \pm 0.8\%$ (MLM). Therefore, COSI did not detect polarized gamma-ray emission from this burst. Our most constraining $90\%$ confidence upper limit on the polarization level was $46\%$ (MLM).

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arXiv2017

Maximum Likelihood Compton Polarimetry with the Compton Spectrometer and Imager

Astrophysical polarization measurements in the soft gamma-ray band are becoming more feasible as detectors with high position and energy resolution are deployed. Previous work has shown that the minimum detectable polarization (MDP) of an ideal Compton polarimeter can be improved by $\sim 21\%$ when an unbinned, maximum likelihood method is used instead of the standard approach of fitting a sinusoid to a histogram of azimuthal scattering angles. Here we outline a procedure for implementing this maximum likelihood approach for real, non-ideal polarimeters. As an example, we use the recent observation of GRB 160530A with the Compton Spectrometer and Imager. We find that the MDP for this observation is reduced by $20\%$ when the maximum likelihood method is used instead of the standard method.

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arXiv2017

The e-ASTROGAM mission (exploring the extreme Universe with gamma rays in the MeV-GeV range)

e-ASTROGAM (`enhanced ASTROGAM') is a breakthrough Observatory mission dedicated to the study of the non-thermal Universe in the photon energy range from 0.3 MeV to 3 GeV. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with polarimetric capability. In the largely unexplored MeV-GeV domain, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on Galactic ecosystems. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous generation instruments, will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LIGO-Virgo-GEO600-KAGRA, SKA, ALMA, E-ELT, TMT, LSST, JWST, Athena, CTA, IceCube, KM3NeT, and the promise of eLISA. Keywords: High-energy gamma-ray astronomy, High-energy astrophysics, Nuclear Astrophysics, Compton and Pair creation telescope, Gamma-ray bursts, Active Galactic Nuclei, Jets, Outflows, Multiwavelength observations of the Universe, Counterparts of gravitational waves, Fermi, Dark Matter, Nucleosynthesis, Early Universe, Supernovae, Cosmic Rays, Cosmic antimatter.

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arXiv2017

The 2016 Super Pressure Balloon flight of the Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a balloon-borne, soft-gamma ray imager, spectrometer, and polarimeter with sensitivity from 0.2 to 5 MeV. Utilizing a compact Compton telescope design with twelve cross-strip, high-purity germanium detectors, COSI has three main science goals: study the 511 keV positron annihilation line from the Galactic plane, image diffuse emission from stellar nuclear lines, and perform polarization studies of gamma-ray bursts and other extreme astrophysical environments. COSI has just completed a successful 46-day flight on NASA's new Super Pressure Balloon, launched from Wanaka, New Zealand, in May 2016. We present an overview of the instrument and the 2016 flight, and discuss COSI's main science goals, predicted performance, and preliminary results.

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