The Compton Spectrometer and Imager

Tomsick, John A.; Zoglauer, Andreas; Sleator, Clio; Lazar, Hadar; Beechert, Jacqueline; Boggs, Steven E.; Roberts, Jarred; Siegert, Thomas; Lowell, A.; Wulf, E.; Grove, Eric; Phlips, Bernard F.; Brandt, T. J.; Smale, A. P.; Kierans, Carolyn; Burns, E.; Hartmann, D. H.; Leising, M. D.; Ajello, M.; Fryer, Chris L.; Amman, M.; Chang, Hsiang-Kuang; Jean, P.; Ballmoos, P. von

doi:10.48550/arxiv.1908.04334

Cited by 29 publications

(29 citation statements)

References 20 publications

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“…Theoretical modelling of nucleosynthesis yields predict values close to our upper bounds, which suggests a bright future for the next generation of soft γ-ray telescopes, such as COSI (Tomsick et al 2019), in both, nuclear decay as well as the 511 keV positron annihilation line.…”

Section: Positrons From Classical Novaesupporting

confidence: 72%

Nucleosynthesis constraints through $γ$-ray line measurements from classical novae

Siegert,

Ghosh,

Mathur

et al. 2021

Preprint

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Context. Classical novae are among the most frequent transient events in the Milky Way, and key agents of ongoing nucleosynthesis. Despite their large numbers, they have never been observed in soft γ-ray emission. Measurements of their γ-ray signatures would provide both, insights on explosion mechanism as well as nucleosynthesis products. Aims. Our goal is to constrain the ejecta masses of 7 Be and 22 Na from classical novae through their γ-ray line emissions at 478 and 1275 keV. Methods. We extract posterior distributions on the line fluxes from archival data of the INTEGRAL/SPI spectrometer telescope. We then use a Bayesian hierarchical model to link individual objects and diffuse emission and infer ejecta masses from the whole population of classical novae in the Galaxy. Results. Individual novae are too dim to be detectable in soft γ-rays, and the upper bounds on their flux and ejecta mass uncertainties cover several orders of magnitude. Within the framework of our hierarchical model, we can, nevertheless, infer tight upper bounds on the 22 Na ejecta masses, given all uncertainties from individual objects as well as diffuse emission, of < 2.0 × 10 −7 M (99.85th percentile). Conclusions. In the context of ONe nucleosynthesis, the 22 Na bounds are consistent with theoretical expectations, and exclude that most ONe novae happen on white dwarfs with masses around 1.35 M . The upper bounds from 7 Be are uninformative. From the combined ejecta mass estimate of 22 Na and its β + -decay, we infer a positron production rate of < 5.5 × 10 42 e + s −1 , which would make at most 10 % of the total annihilation rate in the Milky Way.

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Section: Positrons From Classical Novaesupporting

confidence: 72%

Nucleosynthesis constraints through $γ$-ray line measurements from classical novae

Siegert,

Ghosh,

Mathur

et al. 2021

Preprint

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“…Nonetheless, a better sensitivity in the MeV range, and therefore a future mission covering the MeV sensitivity gap, such as the recently selected small explorer mission COSI, the Compton Spectrometer and Imager 2 (Tomsick et al 2019)…”

Section: Summary Discussion and Conclusionmentioning

confidence: 99%

Diffuse Galactic emission spectrum between 0.5 and 8.0 MeV

Siegert,

Berteaud,

Calore

et al. 2022

Preprint

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The last measurement of the diffuse emission spectrum of the Milky Way in the megaelectronvolt (MeV) photon energy range was performed by CGRO/COMPTEL more than 20 years ago. We report a new analysis with the spectrometer SPI aboard INTEGRAL in the band 0.5-8.0 MeV, finally superseding the signal-to-noise ratio of the historic observations. This is possible thanks to an elaborate instrumental background model and careful considerations of the selected data, which are strongly affected by solar activity. We base our analysis on energy-dependent spatial template fitting in a region of ∆l × ∆b = 95 • × 95 • around the Galactic centre. Our flux estimates are consistent with COMPTEL measurements and show no 'MeV bump'. The spectrum follows a power-law shape with index −1.39 ± 0.09 stat ± 0.10 syst and an integrated flux of (5.7 ± 0.8 stat ± 1.7 syst ) × 10 −8 erg cm −2 s −1 between 0.5 and 8.0 MeV. We find that cosmic-ray electrons and propagation models consistent with the latest Fermi/LAT, Voyager 1, and AMS-02 data are broadly in agreement with the inferred inverse Compton spectral shape. However, a mismatch of a factor of 2-3 in normalisation with respect to baseline expectations may point to enhanced target photon densities and/or electron source spectra in the inner Galaxy, slightly modified diffusion properties, or the presence of an unresolved population of MeV γ-ray sources.

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“…This is a remarkable result, both in terms of accuracy and precision: current soft γ-ray telescopes suffer from the 'MeV gap' in sensivity (e.g. De Angelis et al 2018;Timmes et al 2019;Tomsick et al 2019), which most of the time allows them to only study the sources inside, or the Milky Way itself. The statistical uncertainties on z 0 we estimate from only using INTEGRAL/SPI data and a simple first-order analytic geometric model are only a factor of a few away from the most precise measurements using star counts or H II regions (17 ± 2 pc), which show a systematic spread of ≈ 12 pc (Karim & Mamajek 2016), however.…”

Section: Discussionmentioning

confidence: 99%

Vertical position of the Sun with γ-rays

Siegert

2019

A&A

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We illustrate a method for estimating the vertical position of the Sun above the Galactic plane by γ-ray observations. Photons of γ-ray wavelengths are particularly well suited for geometrical and kinematic studies of the Milky Way because they are not subject to extinction by interstellar gas or dust. Here, we use the radioactive decay line of 26 Al at 1.809 MeV to perform maximum likelihood fits to data from the spectrometer SPI on board the INTEGRAL satellite as a proof-of-concept study. Our simple analytic 3D emissivity models are line-of-sight integrated, and varied as a function of the Sun's vertical position, given a known distance to the Galactic centre. We find a vertical position of the Sun of z 0 = 15 ± 17 pc above the Galactic plane, consistent with previous studies, finding z 0 in a range between 5 and 29 pc. Even though the sensitivity of current MeV instruments is several orders of magnitude below that of telescopes for other wavelengths, this result reveals once more the disregarded capability of soft γ-ray telescopes. We further investigate possible biases in estimating the vertical extent of γ-ray emission if the Sun's position is set incorrectly, and find that the larger the true extent, the less is it affected by the observer position. In the case of 26 Al with an exponential scale height of 150 pc (700 pc) in the inner (full) Galaxy, this may lead to misestimates of up to 25 %.

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The Compton Spectrometer and Imager

Cited by 29 publications

References 20 publications

Nucleosynthesis constraints through $γ$-ray line measurements from classical novae

Nucleosynthesis constraints through $γ$-ray line measurements from classical novae

Diffuse Galactic emission spectrum between 0.5 and 8.0 MeV

Vertical position of the Sun with γ-rays

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