Karim Sabri | Searcharxiv

arXiv2025

HESS J1831$-$098 -- Exploring a pulsar halo scenario with H.E.S.S. data

Pulsar halos are a class of extended very-high-energy (VHE) sources highlighted by the HAWC observatory towards the Geminga pulsar and PSR B0656$+$14. These VHE sources are interpreted as the inverse Compton emission from electrons and positrons diffusing in the interstellar medium at an inhibited rate, having escaped the pulsar wind nebula. Our aim is to search for new pulsar halos using H.E.S.S. data and to constrain their physical properties. Using a physically-motivated model of pulsar halos, we created template-based models of the spatial and energetic distributions of the expected gamma-ray emission using the Gammapy library. A promising candidate source to which this model can be effectively applied is HESS J1831$-$098, an extended VHE source spatially coincident with two energetic pulsars, which also exhibits spectral continuity and morphological compatibility with the ultra-high-energy source 1LHAASO J1831$-$1007u*. It could be powered by the radio pulsar PSR J1831$-$0952 with a characteristic age of 128 kyr. We present a spectro-morphological analysis of this source with H.E.S.S. data, revealing that the emission is well described with a pulsar halo model, although we cannot reject a simple 2D Gaussian morphology. We discuss the implication of the derived physical parameters of the model.

astro-ph.HE↗

arXiv2025

Detection of positron in-flight annihilation from the Galaxy

Although the annihilation of positrons towards the Galactic centre was established more than 50 years ago through the detection of a 511 keV $\gamma$-ray line, the origin of the positrons remains unknown. The $\gamma$-ray line should be accompanied by continuum emission from positron in-flight annihilation, which, until now, has not been detected. We aim to detect positron in-flight annihilation emission, as it provides information on the kinetic energy of the positrons that is key in determining the origin of Galactic positrons. We analysed archival data obtained by the COMPTEL instrument on the Compton Gamma-Ray Observatory satellite to search for positron in-flight annihilation emission in the MeV energy range. Our analysis revealed extended emission in the MeV energy range towards the bulge of the Galaxy, which we attribute to in-flight annihilation of positrons produced with kinetic energies of $\sim$2 MeV. The observed spectrum suggests that positrons are produced quasi-mono-energetically, which could occur by the annihilation of dark matter particles with masses of $\sim$3 MeV, or through bulk motion in the jet of the microquasar 1E1740.7-2942. We furthermore detected a point-like emission component in the MeV energy range towards the Galactic centre that is the plausible low-energy counterpart of the Fermi-LAT source 4FGLJ1745.6-2859. The broad band spectrum of the source may be explained by the injection of pair plasma from the supermassive black hole Sgr A$^\ast$ into the interstellar medium, which would also explain the point-like 511 keV line emission component that was discovered by INTEGRAL/SPI at the Galactic centre. The observed positron in-flight annihilation spectrum towards the Galactic bulge excludes $\beta^+$ decays from radioactive isotopes, as well as any mechanism producing highly relativistic positrons as the origin of the Galactic bulge positrons.

astro-ph.HE↗