Comparing simulated 26^{26}Al maps to gamma-ray measurements

© ESO 2019.Context. The diffuse gamma-ray emission of $^{26}{\rm Al}$ at 1.8 MeV reflects ongoing nucleosynthesis in the Milky Way, and traces massive-star feedback in the interstellar medium due to its 1 Myr radioactive lifetime. Interstellar-medium morphology and dynamics are investigated in astrophysics through 3D hydrodynamic simulations in fine detail, as only few suitable astronomical probes are available. Aims. We compare a galactic-scale hydrodynamic simulation of the Galaxy's interstellar medium, including feedback and nucleosynthesis, with gamma-ray data on $^{26}{\rm Al}$ emission in the Milky Way extracting constraints that are only weakly dependent on the particular realisation of the simulation or Galaxy structure. Methods. Due to constraints and biases in both the simulations and the gamma-ray observations, such comparisons are not straightforward. For a direct comparison, we perform maximum likelihood fits of simulated sky maps as well as observation-based maximum entropy maps to measurements with INTEGRAL/SPI. To study general morphological properties, we compare the scale heights of $^{26}{\rm Al}$ emission produced by the simulation to INTEGRAL/SPI measurements.} Results. The direct comparison shows that the simulation describes the observed inner Galaxy well, but differs significantly from the observed full-sky emission morphology. Comparing the scale height distribution, we see similarities for small scale height features and a mismatch at larger scale heights. We attribute this to the prominent foreground emission sites that are not captured by the simulation.Peer reviewedFinal Accepted Versio

Constraints on positron annihilation kinematics in the inner Galaxy

Context. The annihilation of cosmic positrons with electrons in the interstellar medium results in the strongest persistent γ-ray line signal in the sky. For the past 50 yr, this 511 keV emission - predominantly from the galactic bulge region and from a low surface-brightness disk - has puzzled observers and theoreticians. A key issue for understanding positron astrophysics is found in cosmic-ray propagation, especially at low kinetic energies (≲ 10 MeV). Aims. We want to shed light on how positrons propagate and the resulting morphology of the annihilation emission. We approach this "positron puzzle" by inferring kinematic information of the 511 keV line in the inner radian of the Galaxy. This constrains propagation scenarios and positron source populations in the Milky Way. Methods. By dissecting the positron annihilation emission as measured with INTEGRAL/SPI, we derived spectra for individual and independent regions in the sky. The centroid energies of these spectra around the 511 keV line are converted into Doppler shifts, representing the line-of-sight velocity along different galactic longitudes. This results in a longitude-velocity diagram of positron annihilation. From high-resolution spectra, we also determined Doppler-broadening from γ-ray line shape parameters to study annihilation conditions as they vary with galactic longitude. Results. We found line-of-sight velocities in the 511 keV line that are consistent with zero, as well as with galactic rotation from CO measurements (2-3 km s -1 deg -1), and measurements of radioactive 26Al (7.5-9.5 km s -1 deg -1). The velocity gradient in the inner ±30° is determined to be 4 ± 6 km s -1 deg -1. The width of the 511 keV line is constant as a function of longitude at 2.43 ± 0.14 keV, with possibly different values towards the disk. The positronium fraction is found to be 1.0 along the galactic plane. Conclusions. The weak signals in the disk leave the question open of whether positron annihilation is associated with the high velocities seen in 26Al or rather with ordinarily rotating components of the Milky Way's interstellar medium. We confirm previous results that positrons are slowed down to the 10 eV energy scale before annihilation and constrain bulk Doppler-broadening contributions to ≲ 1.25 keV in the inner radian. Consequently, the true annihilation conditions remain unclear.Peer reviewedFinal Accepted Versio

INTEGRAL/SPI γ -ray line spectroscopy : Response and background characteristics

© 2018 ESO. Reproduced with permission from Astronomy & Astrophysics. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder.Context. The space based γ-ray observatory INTEGRAL of the European Space Agency (ESA) includes the spectrometer instrument "SPI". This is a coded mask telescope featuring a 19-element Germanium detector array for high-resolution γ-ray spectroscopy, encapsulated in a scintillation detector assembly that provides a veto for background from charged particles. In space, cosmic rays irradiate spacecraft and instruments, which, in spite of the vetoing detectors, results in a large instrumental background from activation of those materials, and leads to deterioration of the charge collection properties of the Ge detectors.Aim. We aim to determine the measurement characteristics of our detectors and their evolution with time, that is, their spectral response and instrumental background. These incur systematic variations in the SPI signal from celestial photons, hence their determination from a broad empirical database enables a reduction of underlying systematics in data analysis. For this, we explore compromises balancing temporal and spectral resolution within statistical limitations. Our goal is to enable modelling of background applicable to spectroscopic studies of the sky, accounting separately for changes of the spectral response and of instrumental background.Methods. We use 13.5 years of INTEGRAL/SPI data, which consist of spectra for each detector and for each pointing of the satellite. Spectral fits to each such spectrum, with independent but coherent treatment of continuum and line backgrounds, provides us with details about separated background components. From the strongest background lines, we first determine how the spectral response changes with time. Applying symmetry and long-term stability tests, we eliminate degeneracies and reduce statistical fluctuations of background parameters, with the aim of providing a self-consistent description of the spectral response for each individual detector. Accounting for this, we then determine how the instrumental background components change in intensities and other characteristics, most-importantly their relative distribution among detectors.Results. Spectral resolution of Ge detectors in space degrades with time, up to 15% within half a year, consistently for all detectors, and across the SPI energy range. Semi-annual annealing operations recover these losses, yet there is a small long-term degradation. The intensity of instrumental background varies anti-correlated to solar activity, in general. There are significant differences among different lines and with respect to continuum. Background lines are found to have a characteristic, well-defined and long-term consistent intensity ratio among detectors. We use this to categorise lines in groups of similar behaviour. The dataset of spectral-response and background parameters as fitted across the INTEGRAL mission allows studies of SPI spectral response and background behaviour in a broad perspective, and efficiently supports precision modelling of instrumental background.Peer reviewedFinal Published versio

44Ti^{44}\rm Ti ejecta in young supernova remnants

Context: Tracing unstable isotopes produced in supernova nucleosynthesis provides a direct diagnostic of supernova explosion physics. Theoretical models predict an extensive variety of scenarios, which can be constrained through observations of the abundant isotopes $^{56}$Ni and $^{44}$Ti. Direct evidence of the latter was previously found only in two core-collapse supernova events, and appears to be absent in thermonuclear supernovae.Aims: We aim to to constrain the supernova progenitor types of Cas A, SN 1987A, Vela Jr., G1.9+0.3, SN1572, and SN1604 through their $^{44}$Ti ejecta masses and explosion kinematics. Methods: We analyzed INTEGRAL/SPI observations of the candidate sources utilizing an empirically motivated high-precision background model. We analyzed the three dominant spectroscopically resolved de-excitation lines at 68, 78, and 1157\,keV emitted in the decay chain of $^{44}$Ti. The fluxes allow the determination of the production yields of $^{44}$Ti. Remnant kinematics were obtained from the Doppler characteristics of the lines. Results: We find a significant signal for Cas A in all three lines with a combined significance of 5.4$\sigma$. The fluxes are $(3.3 \pm 0.9) \times 10^{-5}$ ph cm$^{-2}$ s$^{-1}$, and $(4.2 \pm 1.0) \times 10^{-5}$ ph cm$^{-2}$ s$^{-1}$ for the $^{44}$Ti and $^{44}$Sc decay, respectively. We obtain higher fluxes for $^{44}$Ti with our analysis of Cas A than were obtained in previous analyses. We discuss potential differences. Conclusions: We obtain a high $^{44}$Ti ejecta mass for Cas A that is in disagreement with ejecta yields from symmetric 2D models. Upper limits for the other core-collapse supernovae are in agreement with model predictions and previous studies. The upper limits we find for the three thermonuclear supernovae consistently exclude the double detonation and pure helium deflagration models as progenitors.Comment: 15 pages, 11 figures, Accepted for publication in A&

Background modelling for γ\gamma-ray spectroscopy with INTEGRAL/SPI

The coded-mask spectrometer-telescope SPI on board the INTEGRAL observatory records photons in the energy range between 20 and 8000 keV. A robust and versatile method to model the dominating instrumental background (BG) radiation is difficult to establish for such a telescope in the rapidly changing space environment. From long-term monitoring of SPI's Germanium detectors, we built up a spectral parameter data base, which characterises the instrument response as well as the BG behaviour. We aim to build a self-consistent and broadly applicable BG model for typical science cases of INTEGRAL/SPI, based on this data base. The general analysis method for SPI relies on distinguishing between illumination patterns on the 19-element Germanium detector array from BG and sky in a maximum likelihood framework. We illustrate how the complete set of measurements, even including the exposures of the sources of interest, can be used to define a BG model. We apply our method to different science cases, including point-like, diffuse, continuum, and line emission, and evaluate the adequacy in each case. From likelihood values and the number of fitted parameters, we determine how strong the impact of the unknown BG variability is. We find that the number of fitted parameters, i.e. how often the BG has to be re-normalised, depends on the emission type (diffuse with many observations over a large sky region, or point-like with concentrated exposure around one source), and the spectral energy range and bandwidth. A unique time scale, valid for all analysis issues, is not applicable for INTEGRAL/SPI, but must and can be inferred from the chosen data set. We conclude that our BG modelling method is usable in a large variety of INTEGRAL/SPI science cases, and provides nearly systematics-free and robust results.Comment: 11 pages, 2 appendix pages, 9 figures, 4 appendix figures, 4 tables; based on the work of Diehl et al. (2018), Siegert (2017), and Siegert (2013

Galactic Population Synthesis of Radioactive Nucleosynthesis Ejecta

© The Authors 2023. This is an Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0).Diffuse gamma-ray line emission traces freshly produced radioisotopes in the interstellar gas, providing a unique perspective on the entire Galactic cycle of matter from nucleosynthesis in massive stars to their ejection and mixing in the interstellar medium. We aim at constructing a model of nucleosynthesis ejecta on galactic scale which is specifically tailored to complement the physically most important and empirically accessible features of gamma-ray measurements in the MeV range, in particular for decay gamma-rays such as $^{26}$Al, $^{60}$Fe or $^{44}$Ti. Based on properties of massive star groups, we developed a Population Synthesis Code which can instantiate galaxy models quickly and based on many different parameter configurations, such as the star formation rate, density profiles, or stellar evolution models. As a result, we obtain model maps of nucleosynthesis ejecta in the Galaxy which incorporate the population synthesis calculations of individual massive star groups. Based on a variety of stellar evolution models, supernova explodabilities, and density distributions, we find that the measured $^{26}$Al distribution from INTEGRAL/SPI can be explained by a Galaxy-wide population synthesis model with a star formation rate of $4$-$8\,\mathrm{M_{\odot}\,yr^{-1}}$ and a spiral-arm dominated density profile with a scale height of at least 700 pc. Our model requires that most massive stars indeed undergo a supernova explosion. This corresponds to a supernova rate in the Milky Way of $1.8$-$2.8$ per century, with quasi-persistent $^{26}$Al and $^{60}$Fe masses of $1.2$-$2.4\,\mathrm{M_{\odot}}$ and $1$-$6\,\mathrm{M_{\odot}}$, respectively. Comparing the simulated morphologies to SPI data suggests that a frequent merging of superbubbles may take place in the Galaxy, and that an unknown but strong foreground emission at 1.8 MeV could be present.Peer reviewe

The Physics of Star Cluster Formation and Evolution

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe

26^{26}Al gamma rays from the Galaxy with INTEGRAL/SPI

© 2023 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/Context. The presence of radioactive 26Al at 1.8 MeV reveals an ongoing process of nucleosynthesis in the Milky Way. Diffuse emission from its decay can be measured with gamma-ray telescopes in space. The intensity, line shape, and spatial distribution of the 26Al emission allow for studies of these nucleosynthesis sources. The line parameters trace massive-star feedback in the interstellar medium thanks to its 1 My lifetime. Aims. We aim to expand upon previous studies of the 26Al emission in the Milky Way, using all available gamma-ray data, including single and double events collected with SPI on INTEGRAL from 2003 until 2020. Methods. We applied improved spectral response and background as evaluated from tracing spectral details over the entire mission. The exposure for the Galactic 26Al emission was enhanced using all event types measured within SPI. We redetermined the intensity of Galactic 26Al emission across the entire sky, through maximum likelihood fits of simulated and model-built sky distributions to SPI spectra for single and for double detector hits. Results. We found an all-sky flux of (1.84±0.03)×10−3 ph cm−2 s−1 in the 1.809 MeV line from 26Al, determined via fitting to sky distributions from previous observations with COMPTEL. Significant emission from higher latitudes indicates an origin from nearby massive-star groups and superbubbles, which is also supported by a bottom-up population synthesis model. The line centroid is found at (1809.83±0.04 keV), while the line broadening from source kinematics integrated over the sky is (0.62±0.3) keV (FWHM).Peer reviewe

Gamma-Ray Emission of 60Fe and 26Al Radioactivity in Our Galaxy

© 2020. The American Astronomical Society. All rights reserved.The isotopes 60Fe and 26Al originate from massive stars and their supernovae, reflecting ongoing nucleosynthesis in the Galaxy. We studied the gamma-ray emission from these isotopes at characteristic energies 1173, 1332, and 1809 keV with over 15 yr of SPI data, finding a line flux in 60Fe combined lines of (0.31\pm 0.06)\× 10-3\,\&ph;\,cm-2\,s-1 and the Al line flux of (16.8\pm 0.7)\× 10-4\,\&ph;\,-2\,s-1 above the background and continuum emission for the whole sky. Based on the exponential disk grid maps, we characterize the emission extent of 26Al to find scale parameters R0=7.0-1.0+1.5 and z 0=0.8-0.2+0.3 kpc; however, the 60Fe lines are too weak to spatially constrain the emission. Based on a point-source model test across the Galactic plane, the 60Fe emission would not be consistent with a single strong point source in the Galactic center or somewhere else, providing a hint of a diffuse nature. We carried out comparisons of emission morphology maps using different candidate source tracers for both 26Al and 60Fe emissions and suggest that the 60Fe emission is more likely to be concentrated toward the Galactic plane. We determine the 60Fe/ 26Al γ-ray flux ratio at 18.4% ± 4.2% when using a parameterized spatial morphology model. Across the range of plausible morphologies, it appears possible that 26Al and 60Fe are distributed differently in the Galaxy. Using the best-fitting maps for each of the elements, we constrain flux ratios in the range 0.2-0.4. We discuss the implications for massive star models and their nucleosynthesis.Peer reviewedFinal Accepted Versio