Acceleration of cosmic rays at supernova remnant shocks: constraints from gamma-ray observations

In the past few years, gamma-ray astronomy has entered a golden age. At TeV energies, only a handful of sources were known a decade ago, but the current generation of ground-based imaging atmospheric Cherenkov telescopes has increased this number to more than one hundred. At GeV energies, the Fermi Gamma-ray Space Telescope has increased the number of known sources by nearly an order of magnitude in its first 2 years of operation. The recent detection and unprecedented morphological studies of gamma-ray emission from shell-type supernova remnants is of great interest, as these analyses are directly linked to the long standing issue of the origin of the cosmic-rays. However, these detections still do not constitute a conclusive proof that supernova remnants accelerate the bulk of Galactic cosmic-rays, mainly due to the difficulty of disentangling the hadronic and leptonic contributions to the observed gamma-ray emission. In this talk, I will review the most relevant cosmic ray related results of gamma ray astronomy concerning supernova remnants.Comment: Proceedings of the ICATPP conference 2001, Villa Olmo (Italy), 9 pages, 2 figure

Evidence for proton acceleration and escape from the Puppis A SNR using Fermi-LAT observations

Supernova remnants (SNRs) are the best candidates for galactic cosmic ray acceleration to relativistic energies via diffusive shock acceleration. The gamma-ray emission of SNRs can provide direct evidence of leptonic (inverse Compton and bremsstrahlung) and hadronic (proton-proton interaction and subsequently pion decay) processes. Puppis A is a ~ 4 kyr old SNR interacting with interstellar clouds which has been observed in a broad energy band, from radio to gamma-ray. We performed a morphological and spectral analysis of 14 years of observations with Fermi-LAT telescope in order to study its gamma-ray emission. We found a clear asymmetry in high-energy brightness between the eastern and western sides of the remnant, reminiscent to that observed in the X-ray emission. The eastern side, interacting with a molecular cloud, shows a spectrum which can be reproduced by a pion decay model. Moreover, we analyzed two gamma-ray sources located close to the remnant. The hardness of their spectra suggests that the gamma-ray emission can be due to particles escaping from the shock of Puppis A.Comment: Presented at the 38th International Cosmic Ray Conference (ICRC 2023), 8 pages, 2 figures, 3 table

A Broadband Study of the Emission from the Composite Supernova Remnant MSH 11-62

MSH 11-62 (G291.1-0.9) is a composite supernova remnant for which radio and X-ray observations have identified the remnant shell as well as its central pulsar wind nebula. The observations suggest a relatively young system expanding into a low density region. Here we present a study of MSH 11-62 using observations with the Chandra, XMM-Newton, and Fermi observatories, along with radio observations from the Australia Telescope Compact Array (ATCA). We identify a compact X-ray source that appears to be the putative pulsar that powers the nebula, and show that the X-ray spectrum of the nebula bears the signature of synchrotron losses as particles diffuse into the outer nebula. Using data from the Fermi LAT, we identify gamma-ray emission originating from MSH 11-62. With density constraints from the new X-ray measurements of the remnant, we model the evolution of the composite system in order to constrain the properties of the underlying pulsar and the origin of the gamma-ray emission.Comment: 12 Pages, 12 figures. Accepted for publication in the Astrophysical Journa

Gammapy: A Python package for gamma-ray astronomy

In this article, we present Gammapy, an open-source Python package for the analysis of astronomical $\gamma$-ray data, and illustrate the functionalities of its first long-term-support release, version 1.0. Built on the modern Python scientific ecosystem, Gammapy provides a uniform platform for reducing and modeling data from different $\gamma$-ray instruments for many analysis scenarios. Gammapy complies with several well-established data conventions in high-energy astrophysics, providing serialized data products that are interoperable with other software packages. Starting from event lists and instrument response functions, Gammapy provides functionalities to reduce these data by binning them in energy and sky coordinates. Several techniques for background estimation are implemented in the package to handle the residual hadronic background affecting $\gamma$-ray instruments. After the data are binned, the flux and morphology of one or more $\gamma$-ray sources can be estimated using Poisson maximum likelihood fitting and assuming a variety of spectral, temporal, and spatial models. Estimation of flux points, likelihood profiles, and light curves is also supported. After describing the structure of the package, we show, using publicly available $\gamma$-ray data, the capabilities of Gammapy in multiple traditional and novel $\gamma$-ray analysis scenarios, such as spectral and spectro-morphological modeling and estimations of a spectral energy distribution and a light curve. Its flexibility and power are displayed in a final multi-instrument example, where datasets from different instruments, at different stages of data reduction, are simultaneously fitted with an astrophysical flux model.Comment: 26 pages, 16 figure

A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope

Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices $(0.7 < \Gamma <1.4)$ and clear evidence for an exponential cut-off in the range 1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices $(1.0 < \Gamma < 1.7)$, however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters, commensurate with previous estimates. The observation of high-energy gamma-ray emission from a globular cluster thus provides a reliable independent method to assess their millisecond pulsar populations that can be used to make constraints on the original neutron star X-ray binary population, essential for understanding the importance of binary systems in slowing the inevitable core collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J. Kn\"odlseder, N. Webb, B. Pancraz

Gamma-ray and radio properties of six pulsars detected by the fermi large area telescope

We report the detection of pulsed γ-rays for PSRs J0631+1036, J0659+1414, J0742-2822, J1420-6048, J1509-5850, and J1718-3825 using the Large Area Telescope on board the Fermi Gamma-ray Space Telescope (formerly known as GLAST). Although these six pulsars are diverse in terms of their spin parameters, they share an important feature: their γ-ray light curves are (at least given the current count statistics) single peaked. For two pulsars, there are hints for a double-peaked structure in the light curves. The shapes of the observed light curves of this group of pulsars are discussed in the light of models for which the emission originates from high up in the magnetosphere. The observed phases of the γ-ray light curves are, in general, consistent with those predicted by high-altitude models, although we speculate that the γ-ray emission of PSR J0659+1414, possibly featuring the softest spectrum of all Fermi pulsars coupled with a very low efficiency, arises from relatively low down in the magnetosphere. High-quality radio polarization data are available showing that all but one have a high degree of linear polarization. This allows us to place some constraints on the viewing geometry and aids the comparison of the γ-ray light curves with high-energy beam models

Ground-based gamma-ray astronomy

This article is the write-up of a rapporteur talk given at the 34th ICRC in The Hague, Netherlands. It attempts to review the results and developments presented at the conference and associated to the vibrant field of ground-based gamma-ray astronomy. In total, it aims to give an overview of the 19 gamma-ray sessions, 84 talks and 176 posters presented at the 34th ICRC on this topic. New technical advances and projects will be described with an emphasis given on the cosmic-ray related studies of the Universe

Unveiling Galactic cosmic-ray accelerators with gamma-ray observations: their relation to supernova remnants

Depuis leur découverte, un débat sans fin s'anime autour de l'origine des rayons cosmiques, les vestiges de supernovae étant souvent cités comme étant les meilleurs candidats pour leur accélération dans la Galaxie. Après un chapitre résumant mon parcours puis une introduction au domaine, les deux chapitres suivants de ce mémoire résument les observations en rayons gamma du Large Area Telescope (LAT) à bord du satellite Fermi et du High Energy Stereoscopic System (H.E.S.S.) pour deux types de vestiges de supernovae: les vestiges de supernovae jeunes supposés être des accélérateurs efficaces, et les vestiges évolués pour lesquels des signes clairs d'accélération de protons ont été obtenus. Ces chapitres démontrent que les vestiges de supernovae sont capables d'accélérer des particules (électrons et protons) à très haute énergie par accélération diffusive par onde de choc. Des preuves de l'accélération de protons sont visibles dans plusieurs cas: les vestiges de supernovae historiques Cassiopeia A et Tycho, de même que Puppis A et les vestiges évolués W44, IC 443, W51C et W49B. Dans tous les cas étudiés, l'environnement dans lequel le vestige évolue est l'élément clé gouvernant le spectre gamma observé ainsi que la fraction de particules accélérées. En parallèle, la sensibilité accrue des détecteurs actuels nous a permis d'entrer dans une ère de catalogues, révélant ainsi différentes classes de sources astrophysiques contribuant potentiellement au spectre du rayonnement cosmique, telles que les nébuleuses de pulsar, les nuages moléculaires, les régions de formation d'étoiles et même un premier PeVatron au centre Galactique. C'est l'objet du cinquième chapitre qui décrit également les recherches récentes de nébuleuses de pulsars et de sources étendues dans la Galaxie avec le LAT. Quelques perspectives instrumentales et observationnelles sont enfin proposées dans le dernier chapitre. Ce manuscrit n'est ni une liste exhaustive de tous les résultats du domaine, ni une liste complète des travaux et publications que j'ai menés mais plutôt un mélange entre les deux.Since their discovery, there has been a long standing debate concerning the origin of cosmic rays, supernova remnants (SNR) being often cited as the best candidate for their acceleration in the Galaxy. After a chapter relating my career path and an introduction to the subject, the following two chapters of this manuscript summarize the gamma-ray observations acquired by the Large Area Telescope (LAT) aboard Fermi and by the High Energy Stereoscopic System (H.E.S.S.) on two different types of supernova remnants: young supernova remnants thought to be the most efficient accelerators and middle-aged remnants that show clear signs of proton acceleration. These chapters demonstrate that SNRs are able to accelerate particles (electrons and protons) to very high energies through diffusive shock acceleration. Evidences in favor of proton acceleration are visible in several cases: the historical SNRs Cassiopeia A and Tycho as well as the transition case Puppis A and the middle-aged SNRs W44, IC 443, W51C and W49B. In all cases, the environment in which the SNR is evolving is a key characteristic controlling the gamma-ray spectrum as well as fraction of accelerated particules. In the meantime, the increased sensitivity of the current detectors allowed us to enter an era of catalogs, revealing different classes of astrophysical sources potentially contributing to the cosmic-ray spectrum, such as pulsar wind nebulae, molecular clouds, star forming regions and even a first PeVatron at the Galactic center. This is described in the fifth chapter that also summarizes the recent searches for pulsar wind nebulae and extended sources in the Galaxy with the LAT. Finally, instrumental and observational perspectives are proposed in the last chapter. This manuscript is neither an exhaustive list of all results in the field, nor a complete list of the works and publications that I carried but rather a mix between the two