Evidence for protons accelerated and escaped from the Puppis A region using Fermi-LAT observations

¹ Dipartimento di Fisica e Chimica E. Segrè, Università degli Studi di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif sur Yvette, France
⁴ Universitè Bordeaux, CNRS, LP2I Bordeaux, UMR 5797, 33170 Gradignan, France
⁵ Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁶ Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
⁷ Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu, Gifu 501-1193, Japan
⁸ Dr. Karl Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany
⁹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
¹⁰ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

^★ Corresponding author.

Received: 11 April 2025
Accepted: 15 July 2025

Abstract

Supernova remnants (SNRs) interacting with molecular clouds are interesting laboratories for studying the acceleration of cosmic rays and their propagation in the dense ambient medium. We analyzed 14 years of Fermi-LAT observations of the supernova remnant Puppis A to investigate its asymmetric γ-ray morphology and spectral properties. This middle-aged remnant (~4 kyr) is evolving in an inhomogeneous environment, interacting with a dense molecular cloud in the northeast and a lower-density medium in the southwest. We find clear differences in both γ-ray luminosity and spectral energy distribution between these two regions. The emission from both sides is consistent with a hadronic origin. However, whereas the southwestern emission can be explained by standard diffusive shock acceleration (DSA), the northeastern side may involve reacceleration of preexisting cosmic rays or acceleration via reflected shocks in the dense cloud environment. Additionally, we identify two significant γ-ray excesses outside the remnant, including a previously unreported source to the south. These features are likely produced by cosmic rays that have escaped Puppis A and are interacting with nearby dense molecular material. From this extended emission, we estimate the total energy in escaping cosmic rays to be W_CR ~ 1.5 × 10⁴⁹ erg, providing important constraints on cosmic-ray propagation around the remnant.