arXiv subjects

Thomas Siegert

Publications and source records attributed to Thomas Siegert.

At least 19 recordsLinked to original sources

arXiv2026

A unified modeling of X-ray and gamma-ray spectro-polarimetric data: the case of Cygnus X-1

Recent X-ray and soft gamma-ray spectro-polarimetric observations of black hole X-ray binaries, such as Cygnus X-1, reveal discrepancies that challenge current models of accretion and high-energy emission. We present a general framework for broadband spectro-polarimetric modeling that simultaneously fits spectral and polarization data from independent instruments across different energy bands. The method robustly combines measurements with different energy binning and statistical significance while accounting for energy-dependent polarization mixing between model components. Applied to Cygnus X-1 over the 2 keV-2 MeV range, it provides an improved characterization of the soft gamma-ray hard-tail and constrains the optically thin synchrotron cutoff energy to $(3.9^{+0.6}_{-0.5}) \times 10^{2}$ keV. We discuss the implications for non-thermal electron acceleration in the context of Bohm diffusion and synchrotron cooling, and propose a new explanation for the observed polarization misalignment based on magnetic field helicity and Doppler boosting. The framework is broadly applicable to multi-instrument spectro-polarimetric analyses.

astro-ph.HE↗

arXiv2026

Cosmological $\gamma$-$\gamma$ 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 $\gamma$-$\gamma$ 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 $\gamma$-$\gamma$ 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

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

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↗

arXiv2026

$\gamma$-Ray Lines -- Signatures of Nucleosynthesis, Cosmic Rays, Positron Annihilation, and Fundamental Physics

The nuclear $\gamma$-ray lines in the MeV range of the electromagnetic spectrum hold a vast variety of astrophysical, particle-physical, and fundamental physical information that is otherwise extreme difficult to access. MeV $\gamma$-ray line observations provide the most direct evidence for ongoing nucleosynthesis in galaxies by measuring freshly produced radioactive isotopes from massive stars, supernovae, classical novae, or binary neutron star mergers. Their flux ratios can determine the low-energy cosmic-ray spectrum in different objects and of the Milky Way as a whole. Different phases of the interstellar medium are traced by hot nucleosynthesis ejecta, cooling positrons, or cosmic-ray interactions with molecular clouds. Positron annihilation itself can be considered as an astrophysical messenger as their production and destruction in typical space environments is inevitable. Finally, as-of-yet unknown signatures from beyond standard model physics might have their elusive imprints in $\gamma$-ray lines. This Chapter gives an overview of historical $\gamma$-ray line measurements, newest results, and open questions that may only be solved by a new generation of MeV telescopes.

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 $\gamma$-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 $g_{a\gamma}$ and $g_{ae}$ as a function of the ALP mass $m_a$. The hard X-ray and soft $\gamma$-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 $g_{a\gamma} \times g_{ae}$ is $(0.008 - 2) x 10^{-24}$ GeV$^{-1}$ (95% C.I.) while the ALP-photon coupling is constrained to $g_{a\gamma} = (0.13 - 1.26) x 10^{-11}$ 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 $\gamma$-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 ($\sim 2\sigma$), they are spatially consistent with $^{26}$Al emission from massive stars in that region, suggesting that this 511 keV emission originates from its $\beta^{+}$ 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

Updated Constraints on the Injection Energy of Positrons Generating the Galactic 511 keV $\gamma$-ray line

Even 50 years after the discovery of a positron annihilation line from the inner Galaxy, no class of astrophysical sources has emerged as a definitive explanation for both the emission morphology and flux. Positrons produced by dark matter annihilation or decay have been proposed, but the mass of any such candidate is constrained by continuum $\gamma$-ray emission at energies $>511$ keV. Earlier analyses have claimed that this emission requires that the positrons have kinetic energies less than a few MeV at injection, disfavoring both much of the dark matter parameter space and many potential compact astrophysical source classes such as pulsars. However, these constraints were not based on a full forward model of the absolute flux of the $\gamma$-ray line and continuum data, and did not marginalize over uncertainties about the relative angular distributions of the line and continuum. Here we describe an improved analysis that overcomes these limitations, and show that constraints on the injection energy are much weaker than previously claimed; even under conservative assumptions the data are consistent with initial energies up to $\sim 110$ MeV from INTEGRAL/SPI data alone, and up to $\sim 55$ MeV when including COMPTEL and EGRET data, subject to cross-normalization between them and INTEGRAL.

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↗

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.

astro-ph.HE↗

arXiv2024

Using $^{26}$Al to detect ongoing self-enrichment in young massive star clusters

Self-enrichment is one of the leading explanations for chemical anomalies in globular clusters. In this scenario, various candidate polluter stars have been proposed to eject gas with altered chemical composition during the self-enrichment process. Most of the proposed polluters will also eject radioactive $^{26}$Al into the surroundings. Hence, any detection of $^{26}$Al in young massive star clusters (YMCs) would support the self-enrichment scenario if YMCs were indeed the progenitors of globular clusters. Observations of gamma-ray data from COMPTEL and INTEGRAL, as well as detections of $^{26}$AlF molecules by the Atacama Large Millimeter-submillimeter Array (ALMA), indicate the maturing of $^{26}$Al detection methods. Detection possibilities will be enhanced in the short- to mid-term by the upcoming launch of the Compton Spectrometer and Imager (COSI). The Square Kilometer Array (SKA) could in principle also detect radio recombination lines of the positronium formed from the decay products of $^{26}$Al. Here, we show for a sample of YMCs in the nearby Universe, where self-enrichment could plausibly take place. For some nearby galaxies, this could enhance $^{26}$Al by an order of one magnitude. Detecting $^{26}$AlF with ALMA appears feasible for many candidate self-enrichment clusters, although significant challenges remain with other detection methods. The Large Magellanic Cloud, with its YMC R136, stands out as the most promising candidate. Detecting a 1.8~MeV radioactive decay line of $^{26}$Al here would require at least 15 months of targeted observation with COSI, assuming ongoing self-enrichment in R136.

astro-ph.GA↗

arXiv2024

Atmospheric Response for MeV Gamma Rays Observed with Balloon-Borne Detectors

The atmospheric response for MeV gamma rays (~ 0.1 - 10 MeV) can be characterized in terms of two observed components. The first component is due to photons that reach the detector without scattering. The second component is due to photons that reach the detector after scattering one or more times. While the former can be determined in a straightforward manner, the latter is much more complex to quantify, as it requires tracking the transport of all source photons that are incident on Earth's atmosphere. The scattered component can cause a significant energy-dependent distortion in the measured spectrum, which is important to account for when making balloon-borne observations. In this work we simulate the full response for gamma-ray transport in the atmosphere. We find that the scattered component becomes increasingly more significant towards lower energies, and at 0.1 MeV it may increase the measured flux by as much as a factor of ~2-4, depending on the photon index and off-axis angle of the source. This is particularly important for diffuse sources, whereas the effect from scattering can be significantly reduced for point sources observed with an imaging telescope.

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 $\gamma$-rays at 1332 and 1809 keV, respectively. These $\gamma$-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 $\gamma$-ray lines, as well as the resulting 511 keV line from positron annihilation due to the $^{26}\mathrm{Al}$ $\beta^+$-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 $\gamma$-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 $\gamma$-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↗

arXiv2024

Sub-GeV Dark Matter Annihilation: Limits from Milky Way observations with INTEGRAL

From 16 years of INTEGRAL/SPI $\gamma$-ray observations, we derive bounds on annihilating light dark matter particles in the halo of the Milky Way up to masses of about 300 MeV. We test four different spatial templates for the dark matter halo, including a Navarro-Frenk-White (NFW), Einasto, Burkert, and isothermal sphere profile, as well as three different models for the underlying diffuse Inverse Compton emission. We find that the bounds on the s-wave velocity-averaged annihilation cross sections for both the electron-positron and the photon-photon final states are the strongest to date from $\gamma$-ray observations alone in the mass range $\lesssim 6$ MeV. We provide fitting formulae for the upper limits and discuss their dependences on the halo profile. The bounds on the two-photon final state are superseding the limits from the Cosmic Microwave Background in the range of 50 keV up to $\sim 3$ MeV, showing the great potential future MeV mission will have in probing light dark matter.

astro-ph.HE↗

arXiv2023

Probing the Galactic Diffuse Continuum Emission with COSI

In 2016 the Compton Spectrometer and Imager (COSI) had a successful 46-day flight onboard NASA's Super Pressure Balloon platform. In this work we report measurements of the Galactic diffuse continuum emission (GDCE) observed towards the inner Galaxy during the flight, which in the COSI energy band (0.2 - 5 MeV) is primarily generated from inverse Compton radiation. Within uncertainties we find overall good agreement with previous measurements from INTEGRAL/SPI and COMPTEL. Based on these initial findings, we discuss the potential for further probing the GDCE with the 2016 COSI balloon data, as well as prospects for the upcoming satellite mission.

astro-ph.HE↗

arXiv2023

MHz to TeV expectations from scotogenic WIMP dark matter

The indirect search for dark matter is typically restricted to individual photon bands and instruments. In the context of multiwavelength observations, finding a weak signal in large fore- and backgrounds at only one wavelength band is hampered by systematic uncertainties dominating the signal strength. Dark matter particle annihilation is producing Standard Model particles of which the prompt photon emission is searched for in many studies. However, also the secondary emission of charged particles from dark matter annihilation in the TeV range results in comparable or even stronger fluxes in the GHz-GeV range. In this study, we calculate the prompt and secondary emission of a scotogenic WIMP with a mass of $1\,\mathrm{TeV}$ in 27 dwarf galaxies of the Milky Way. For the secondary emission, we include Inverse Compton scattering, bremsstrahlung, and synchrotron radiation, which results in a "triple hump" structure characteristic for only dark matter and no other astrophysical source. In order to determine the best candidates for multi-instrument analyses, we estimate the diffuse emission component of the Milky Way itself, including its own dark matter halo from the same scotogenic WIMP model. We find signal-to-background ratios of individual sources on the order of $10^{-3}$-$10^{-2}$ across X- to $\gamma$-rays assuming $J$-factors for the cold dark matter distribution inferred from observations and no additional boosting due to small-scale clumping. We argue that a joint multi-wavelength analysis of all nearby galaxies as well as the extension towards the Cosmic Gamma-ray Background is required to disentangle possible dark matter signals from astrophysical back- and foregrounds.

astro-ph.HE↗

arXiv2023

Time-Variable Diffuse $\gamma$-ray Foreground

While the data analysis of $\gamma$-ray telescopes has now become more robust, some signals may be misinterpretations of a time-variable foreground emission from the Solar System, induced by low-energy cosmic-ray interactions with asteroids. Our goal is to provide emission templates for this time-variable diffuse $\gamma$-ray foreground by considering the populations of Main Belt Asteroids, Jovian and Neptunian Trojans, Kuiper Belt Objects, and the Oort Cloud. We model the spatial distribution of all known asteroids by performing 3D-fits to determine their density profiles and calculate their appearances by line-of-sight integrations. Because Earth and the asteroids are moving with respect to each other, we obtain diffuse emission templates varying on timescales of days to decades. We find that the temporal variability can lead to flux enhancements which may mimic emission features unless properly taken into account. This variation is further enhanced by the Solar cycle as the cosmic-ray spectrum is attenuated by the Solar modulation potential, leading to a relative flux increase of the outer asteroids. The cumulative effect of the time-dependent emission is illustrated for the case of the 511 keV OSSE fountain, and for emission features near the Galactic Centre, both being possible misinterpretations of the Solar System albedo. We recommend that $\gamma$-ray data analyses should always take into account the possibility of a time-variable foreground. Due to the ecliptic overlap with the Galactic plane, the Galactic emission is expected to be weaker by 0.1-20%, depending on time (relative planetary motion), energy, and Solar cycle, which has immense consequences for the interpretation of dark matter annihilation cross sections, cosmic-ray spectra and amplitudes, as well as nucleosynthesis yields and related parameters.

astro-ph.HE↗