Francesca Calore

Context. The origin of the 511 keV line signal in the Milky Way remains unresolved despite decades of observations. The measured flux of ∼3 × 10 ⁻³ ph cm ⁻² s ⁻¹ suggests a steady-state positron injection rate of ∼10 ⁴³ -10 ⁴⁴ 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. Aims. We estimated the quasi-persistent 511 keV luminosity expected from flaring stellar populations and estimated the Galactic contribution. We constrained the 511 keV fluxes for different sources in the Galaxy, in particular for globular clusters. Methods. Using solar flare observations as a calibration baseline, we constructed a hierarchical Bayesian model to link flare energy to 511 keV luminosity. We further used flare frequency-energy distributions to estimate the time-averaged positron output of stellar populations. We compared the resulting predictions with INTEGRAL/SPI observations using spatial and population-based models. Results. We find that stellar flares fall short by several orders of magnitude in explaining the Galactic positron annihilation rate. Reproducing ∼10% of the observed luminosity in the Galactic bulge would require unphysically large maximum flare energies per star reaching up to E _max ≳ 10 ³⁷⁻³⁹ erg. Spatial modeling further shows that stellar flare scenarios cannot reproduce the observed 511 keV morphology. Conclusions. 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.

The Galactic Center is potentially hosting the largest indirect signal from particle dark matter (DM), which in many well-motivated models would produce gamma rays as their final states. However, this region has often been dismissed for DM studies because of the evidence for an unexpected gamma-ray component over astrophysical backgrounds at GeV energies, firstly discovered in the data of the Large Area Telescope (LAT), the so-called Galactic Center excess (GCE). While this was initially considered to hint at GeV thermal relics, recent work supports a GCE interpretation in terms of a stellar population of millisecond pulsarlike sources in the Galactic bulge. Building on this preference, we reevaluate the Galactic Center as a powerful target for indirect DM searches via gamma rays. This is achieved by combining adaptive template fitting and photon-count statistical methods to assess the role of subthreshold point sources in the observed -LAT gamma-ray counts while minimizing the mismodeling of Galactic diffuse-emission backgrounds. In a fully self-consistent way, the gamma-ray data are fitted with a mixed model comprising a DM signal and a stellar bulge, both potentially contributing to the GCE. The space left for signals from weak-scale DM particle annihilations is quantified by extracting 95% confidence level upper limits on the annihilation cross section, which, depending on the DM density profile, result in stringent limits for masses ≲ 300 GeV . The robustness of our results is supported by tests on simulated data.

Aims . The mysterious Galactic center (GC) gamma-ray ( γ -ray) excess (GCE) could be explained by a large population of millisecond pulsars (MSPs) hiding in the Galactic bulge, too faint to be detected as individual high-energy point sources by the Fermi Large Area Telescope and too fast and dispersed to be detected in shallow radio pulsation surveys. Methods . Motivated by an innovative candidate selection method, we aim to detect millisecond pulsars associated with the GCE by carrying out deep radio pulsation searches toward promising candidates detected in the inner Galaxy in X rays by Chandra and in radio or γ rays by the Very Large Array or Fermi . Results . We conducted deep radio observation and follow-up campaigns with MeerKAT, as well as the Murriyang and Green Bank telescopes toward nine X-ray candidate sources. Conclusions . We report the detection of two new MSPs, including a black widow candidate, toward the Galactic bulge: PSRs J1740-2805 and J1740-28. These discoveries double the number of MSPs discovered within the innermost 2° from the GC.

Ground-based radio telescopes are routinely used to search for light dark matter (DM) candidates such as axionlike particles or dark photons. However, these instruments face inherent limitations to push the searches to masses below 10 − 7 eV , due to the effect of the Earth’s ionosphere. The extant and planned space- or Moon-based radio telescopes motivate this study: We systematically investigate their sensitivity to resonant conversion of light DM into radio signals from three solar system targets: the Sun, the Earth, and Jupiter. The perspectives are especially encouraging for dark photon searches using the Sun as a target, and for axionlike particles conversion in Jupiter’s magnetosphere.

Context. Axion-like particles (ALPs) are hypothetical pseudoscalar bosons that arise in many extensions of the Standard Model and can be 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. Aims. In this work, we aim to search for a combined signal of ALP-induced hard X-ray and soft γ -ray emission from 18 nearby pre-supernova stars. We intend to use the full public INTEGRAL/SPI 22-year database to create individual datasets and link the resulting spectra for a coherent analysis. Methods. We used a maximum-likelihood approach to extract the fluxes of the selected nearby stars from 20–2000 keV. From stellar-evolution models, we obtain the expected spectral shapes of ALPs producing processes peaking in the 50–500 keV range, depending on the age and mass of the star. We then constructed a joint likelihood that acknowledges the uncertainties in individual stellar parameters toward a combined estimate for the coupling constants g _aγ and g _ae as a function of the ALP mass m _a . Results. We find that the hard X-ray and soft γ -ray fluxes of all selected stars are consistent with zero within uncertainties. We provide upper limits on the continuum flux as well as the 511 keV and 1809 keV lines from these sources. The combined estimate of the upper limit of the product g _aγ × g _ae is (0.008 − 2)×10 ⁻²⁴ GeV ⁻¹ (95% C.I.) and the ALP-photon coupling g _aγ = (0.13 − 1.26)×10 ⁻¹¹ GeV ⁻¹ (95% C.I.) up to a mass of m _a ≦ 10 ⁻¹¹ eV for different times to core-collapse and different magnetic-field models. Conclusions. Our results are among the strongest limits on the ALP coupling constants in the literature. We also provide conservative limits on the coupling constants, g _aγ × g _ae , of (0.27 − 1.25)×10 ⁻²⁴ GeV ⁻¹ (95% C.I.) by assuming all stars but one to be in the early He-burning phase. This work shows that soft γ -ray observations are required to efficiently probe the ALP parameter space, as well as massive-star evolution models in general.

The Fermi Large Area Telescope (LAT) provides advantages for radio pulsar searches by enabling efficient target selection. We can confidently point radio telescopes to the positions of Fermi unidentified gamma-ray sources that have a high probability of hosting a pulsar. As part of Transients and Pulsars with MeerKAT (TRAPUM), we conducted a survey of Fermi-LAT sources using the Ultra High Frequency (UHF; 544–1088 MHz) receiver of the MeerKAT radio telescope. We observed 79 sources that were identified as pulsar-like candidates using a random forest technique from the Fermi-LAT Fourth Source Catalogue. We observed each target for 10 minutes at two separate epochs. As a result, we discovered nine new millisecond pulsars (MSPs) and six slow pulsars. Based on the radio discoveries, we also searched for gamma-ray pulsations, confirming that seven of the newly discovered MSPs are associated with Fermi-LAT sources, and performed joint radio and gamma-ray pulsar timing. Companion mass estimates and evidence of radio eclipses indicate that among the nine MSPs there are three black widows and three redbacks. Lastly, we compared the discovered pulsars in the MeerKAT UHF survey against the previous Fermi sources TRAPUM survey at L band, concluding the superiority of UHF observations in sensitivity to fainter pulsars and in detection rate than L band for finding new gamma-ray MSPs.

The nuclear $γ$-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 $γ$-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 $γ$-ray lines. This Chapter gives an overview of historical $γ$-ray line measurements, newest results, and open questions that may only be solved by a new generation of MeV telescopes.

Astrophysical jets from powerful active galactic nuclei (AGN) have recently been proposed as promising probes of dark matter (DM) in the sub-GeV mass range. AGN launch relativistic jets that accelerate cosmic rays (CRs) to very high energies, which can then interact with their surroundings and produce multiwavelength (MW) emission spanning from radio frequencies to TeV γ rays. If DM consists of light particles, their interactions with CRs could lead to an additional cooling mechanism that modifies the expected MW emission. In this work, we analyze the MW spectrum of Markarian 421, a well-studied AGN, using a multizone leptonic jet model that includes the interactions between CR electrons and DM particles. For the first time, we account for the uncertainties in the astrophysical jet dynamics, which have been previously neglected when constraining the CR-DM interactions. By fitting simultaneously jet parameters and DM-electrons interactions, we use the MW data from Markarian 421 to set constraints on the DM-induced CR cooling. We obtain 5 σ upper limit σ DM − e ≲ 1 × 10 − 34 cm 2 for a DM mass of 1 MeV. We demonstrate that this is about a factor of 2–10 stronger than traditional approaches depending on DM mass. This improvement originates from having indeed considered the full multiwavelength emission from the source, instead if a simplified approach. Properly accounting for degeneracies between jet dynamics and DM interactions is also key to deriving robust constraints on DM interactions.