arXiv subjects

Carolyn A. Kierans

Publications and source records attributed to Carolyn A. Kierans.

13 recordsLinked to original sources

arXiv2026

MeV dark matter decay forecasts for the COSI mission

The decay of dark matter (DM) is a generic prediction of many particle physics models, producing distinctive monochromatic signatures in environments such as the Galactic halo. Searches for decaying DM in the elusive 'MeV gap' have remained conspicuously under-explored, despite well-motivated models including axion-like particles, sterile neutrinos, and hidden-sector scenarios predicting signatures in this energy range. The upcoming Compton Spectrometer and Imager (COSI) telescope will help bridge this gap. Designed to probe this energy band with high spectral resolution, low background, and wide field of view, COSI is well-suited for DM line searches at the MeV scale. In this work, we make projections for COSI's 2-year sensitivity for DM decay in the Milky Way halo using dedicated all-sky simulations incorporating COSI's predicted instrument response. We conclude that COSI will improve the sensitivity to the DM lifetime by more than an order of magnitude over the DM mass range 0.4 to 10 MeV.

astro-ph.CO↗

arXiv2026

Predictions for Short Gamma-Ray Burst Detections by COSI

The Compton Spectrometer and Imager (COSI) is a soft gamma-ray telescope scheduled to launch in 2027. COSI will trigger onboard the spacecraft in response to short-duration transients, including gamma-ray bursts, and it will promptly disseminate alerts which include localizations to the community. This will enable rapid follow-up observations of the afterglow and associations with transients identified by other instruments. By simulating COSI observations of short gamma-ray bursts (sGRBs) and processing them with the trigger algorithm which will run onboard the spacecraft, we estimate the sensitivity of COSI to sGRBs. We then convolve this with a simulated set of sGRBs representing the intrinsic population in the universe to predict the rate that will be detected by COSI. We find that COSI is expected to trigger on $\sim$47-89 sGRBs per year with $\sim$7-10 localized to within 2.5°. With the gravitational wave sensitivity anticipated in the O5 run, we predict 0-3 joint detections with gravitational waves.

astro-ph.HE↗

arXiv2026

Testing deep learning techniques for event reconstruction in pair-production gamma-ray telescopes

In the development of next-generation tracking detectors for gamma-ray space observations, the integration of advanced deep learning techniques into event reconstruction algorithms is a promising approach for improving the performance of the mission, reducing systematic uncertainties and maximizing the scientific output. In this study, we investigate the application of deep learning techniques such as Graph Neural Networks (GNNs) for the identification and reconstruction of particle tracks resulting from pair production events within the tracker of the AMEGO-X proposed concept. The goal is improving the angular resolution and the detection efficiency of the telescope, especially in the soft energy range (below 100 MeV) where multiple Coulomb scattering significantly degrades track reconstruction and consequently limits the sensitivity in the so-called MeV gap. We use the Geant4-based MEGAlib framework and a Python-based dedicated read-out and data handler processor to format the input datasets as close as possible to the real data. The simulated datasets are then used to train and evaluate two graph neural network architectures, GraphSAGE and Interaction Networks, and to compare their performance in pair-production event reconstruction in terms of Point Spread Function 68% containment radius and effective area, also with that obtained using standard reconstruction techniques. The results of this study indicate that graph neural network-based reconstruction is a promising approach for pair-production event reconstruction below 100 MeV when compared with standard reconstruction methods, with significant potential for further optimization.

astro-ph.IM↗

arXiv2026

Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI

We investigate the detectability of polarized gamma-ray emission from blazar flares with the Compton Spectrometer and Imager (COSI). Using 17 years of Fermi Large Area Telescope observations, we analyze light curves for 1413 blazars and identify a maximum of 787 sources with flaring episodes through Bayesian block analysis. For each flare, we estimate the minimum detectable polarization MDP99 in the COSI energy band (0.2-5 MeV) using instrument response functions under a range of spectral assumptions and background conditions. Under baseline background levels (1 counts/s), and assuming that blazar flare statistics in the MeV band are comparable to those observed at GeV energies, we find that COSI can realistically detect polarization in up to ~6 flares with MDP99<50% over its two-year prime mission depending on different spectral and flare identification assumptions, with only a few most powerful ones reaching MDP99<20%. These expectations are shown to improve when shorter intervals around bright peaks within long flares are considered. We provide a ranked list of the most promising targets, finding that flat-spectrum radio quasars dominate the population of polarization-detectable events. Through its continuous all-sky monitoring in the largely unexplored MeV band, COSI will open a new observational window on blazar variability and deliver the first direct measurements of MeV polarization, offering unique insights into jet geometry and high-energy emission processes.

astro-ph.HE↗

arXiv2021

Modeling and Simulations of TXS 0506+056 Neutrino Events in the MeV Band

Neutrino detections identified with multiwavelength blazar spectra represent the first $\g$-ray-neutrino multimessenger signals. The blazar, TXS 0506+056 is also unusual in its spectral expressions over time and there are a number of models in the literature which have been proposed. In this work, we model the TXS 0506+056 data during two epochs of neutrino co-observation using a range of Fokker-Planck derived solutions, and simulate the expected response of the proposed AMEGO-X mission for a set of physically plausible scenarios.

astro-ph.HE↗

arXiv2021

AMEGO: Exploring the Extreme Multimessenger Universe

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a Probe-class mission concept that will provide essential contributions to multimessenger astrophysics in the next decade. AMEGO operates both as a Compton and pair telescope to achieve unprecedented sensitivity between 200 keV and $>$5 GeV. The instrument consists of four subsystems. A double-sided strip silicon Tracker gives a precise measure of the first Compton scatter interaction and tracks of pair-conversion products. A novel CdZnTe Low Energy Calorimeter with excellent position and energy resolution surrounds the bottom and sides of the Tracker to detect the Compton-scattered photons which enhances the polarization and narrow-line sensitivity. A thick CsI High Energy Calorimeter contains the high-energy Compton and pair events. The instrument is surrounded by a plastic anti-coincidence detector to veto the cosmic-ray background. We have performed detailed simulations to predict the telescope performance and are currently building a prototype instrument. The AMEGO prototype, known as ComPair, will be tested at the High Intensity Gamma-Ray Source in 2021, followed by a balloon flight in Fall of 2022. In this presentation we will give an overview of the science motivation, a description of the observatory, and an update of the prototype instrument development.

astro-ph.IM↗

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.

astro-ph.HE↗

arXiv2019

Benchmarking simulations of the Compton Spectrometer and Imager with calibrations

The Compton Spectrometer and Imager (COSI) is a balloon-borne gamma-ray (0.2-5 MeV) telescope designed to study astrophysical sources. COSI employs a compact Compton telescope design utilizing 12 high-purity germanium double-sided strip detectors and is inherently sensitive to polarization. In 2016, COSI was launched from Wanaka, New Zealand and completed a successful 46-day flight on NASA's new Super Pressure Balloon. In order to perform imaging, spectral, and polarization analysis of the sources observed during the 2016 flight, we compute the detector response from well-benchmarked simulations. As required for accurate simulations of the instrument, we have built a comprehensive mass model of the instrument and developed a detailed detector effects engine which applies the intrinsic detector performance to Monte Carlo simulations. The simulated detector effects include energy, position, and timing resolution, thresholds, dead strips, charge sharing, charge loss, crosstalk, dead time, and detector trigger conditions. After including these effects, the simulations closely resemble the measurements, the standard analysis pipeline used for measurements can also be applied to the simulations, and the responses computed from the simulations are accurate. We have computed the systematic error that we must apply to measured fluxes at certain energies, which is 6.3% on average. Here we describe the detector effects engine and the benchmarking tests performed with calibrations.

astro-ph.IM↗

arXiv2019

Positron Annihilation in the Galaxy

The 511 keV line from positron annihilation in the Galaxy was the first $γ$-ray line detected to originate from outside our solar system. Going into the fifth decade since the discovery, the source of positrons is still unconfirmed and remains one of the enduring mysteries in $γ$-ray astronomy. With a large flux of $\sim$10$^{-3}$ $γ$/cm$^{2}$/s, after 15 years in operation INTEGRAL/SPI has detected the 511 keV line at $>50σ$ and has performed high-resolution spectral studies which conclude that Galactic positrons predominantly annihilate at low energies in warm phases of the interstellar medium. The results from imaging are less certain, but show a spatial distribution with a strong concentration in the center of the Galaxy. The observed emission from the Galactic disk has low surface brightness and the scale height is poorly constrained, therefore, the shear number of annihilating positrons in our Galaxy is still not well know. Positrons produced in $β^+$-decay of nucleosynthesis products, such as $^{26}$Al, can account for some of the annihilation emission in the disk, but the observed spatial distribution, in particular the excess in the Galactic bulge, remains difficult to explain. Additionally, one of the largest uncertainties in these studies is the unknown distance that positrons propagate before annihilation. In this paper, we will summarize the current knowledge base of Galactic positrons, and discuss how next-generation instruments could finally provide the answers.

astro-ph.HE↗

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).

astro-ph.HE↗

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.

astro-ph.IM↗

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.

astro-ph.IM↗

arXiv2017

Benchmarking COSI's detector effects engine

The Compton Spectrometer and Imager (COSI) is a balloon-borne gamma-ray (0.2-5 MeV) telescope with inherent sensitivity to polarization. COSI's main goal is to study astrophysical sources such as $γ$-ray bursts, positron annihilation, Galactic nucleosynthesis, and compact objects. COSI employs a compact Compton telescope design utilizing 12 high-purity cross strip germanium detectors (size: $8\times8\times1.5$ cm$^3$, 2 mm strip pitch). We require well-benchmarked simulations to simulate the full instrument response used for data analysis, to optimize our analysis algorithms, and to better understand our instrument and the in-flight performance. In order to achieve a reasonable agreement, we have built a comprehensive mass model of the instrument and developed a detailed detector effects engine, which takes into account the individual performance of each strip as well as the characteristics of the overall detector system. We performed detailed Monte-Carlo simulations with Cosima/Geant4, applied the detector effects engine, and benchmarked the results with pre-flight calibrations using radioactive sources. After applying the detector effects engine, the simulations closely resemble the measurements, and the standard calibration, event reconstruction, and imaging pipeline used for measurements can also be applied to the simulations. In this manuscript, we will describe the detector effects engine, the benchmarking tests with calibrations, and the application to preliminary results from COSI's 46-day balloon flight in 2016.

astro-ph.IM↗