arXiv subjects

F. Acero

Publications and source records attributed to F. Acero.

At least 37 records · Page 2Linked to original sources

arXiv2024

Search for Extended GeV Sources in the Inner Galactic Plane

The recent detection of extended $γ$-ray emission around middle-aged pulsars is interpreted as inverse-Compton scattering of ambient photons by electron-positron pairs escaping the pulsar wind nebula, which are confined near the system by unclear mechanisms. This emerging population of $γ$-ray sources was first discovered at TeV energies and remains underexplored in the GeV range. To address this, we conducted a systematic search for extended sources along the Galactic plane using 14 years of Fermi-LAT data above 10 GeV, aiming to identify a number of pulsar halo candidates and extend our view to lower energies. The search covered the inner Galactic plane ($\lvert l\rvert\leq$ 100$^{\circ}$, $\lvert b\rvert\leq$ 1$^{\circ}$) and the positions of known TeV sources and bright pulsars, yielding broader astrophysical interest. We found 40 such sources, forming the Second Fermi Galactic Extended Sources Catalog (2FGES), most with 68% containment radii smaller than 1.0$^{\circ}$ and relatively hard spectra with photon indices below 2.5. We assessed detection robustness using field-specific alternative interstellar emission models and by inspecting significance maps. Noting 13 sources previously known as extended in the 4FGL-DR3 catalog and five dubious sources from complex regions, we report 22 newly detected extended sources above 10 GeV. Of these, 13 coincide with H.E.S.S., HAWC, or LHAASO sources; six coincide with bright pulsars (including four also coincident with TeV sources); six are associated with 4FGL point sources only; and one has no association in the scanned catalogs. Notably, six to eight sources may be related to pulsars as classical pulsar wind nebulae or pulsar halos.

astro-ph.HE↗

arXiv2024

A new method of reconstructing images of gamma-ray telescopes applied to the LST-1 of CTAO

Imaging atmospheric Cherenkov telescopes (IACTs) are used to observe very high-energy photons from the ground. Gamma rays are indirectly detected through the Cherenkov light emitted by the air showers they induce. The new generation of experiments, in particular the Cherenkov Telescope Array Observatory (CTAO), sets ambitious goals for discoveries of new gamma-ray sources and precise measurements of the already discovered ones. To achieve these goals, both hardware and data analysis must employ cutting-edge techniques. This also applies to the LST-1, the first IACT built for the CTAO, which is currently taking data on the Canary island of La Palma. This paper introduces a new event reconstruction technique for IACT data, aiming to improve the image reconstruction quality and the discrimination between the signal and the background from misidentified hadrons and electrons. The technique models the development of the extensive air shower signal, recorded as a waveform per pixel, seen by CTAO telescopes' cameras. Model parameters are subsequently passed to random forest regressors and classifiers to extract information on the primary particle. The new reconstruction was applied to simulated data and to data from observations of the Crab Nebula performed by the LST-1. The event reconstruction method presented here shows promising performance improvements. The angular and energy resolution, and the sensitivity, are improved by 10 to 20% over most of the energy range. At low energy, improvements reach up to 22%, 47%, and 50%, respectively. A future extension of the method to stereoscopic analysis for telescope arrays will be the next important step.

astro-ph.HE↗

arXiv2024

Dark Matter Line Searches with the Cherenkov Telescope Array

Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g.~box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.

hep-ph↗

arXiv2024

A detailed study of the very-high-energy Crab pulsar emission with the LST-1

Context: There are currently three pulsars firmly detected by imaging atmospheric Cherenkov telescopes (IACTs), two of them reaching TeV energies, challenging models of very-high-energy (VHE) emission in pulsars. More precise observations are needed to better characterize pulsar emission at these energies. The LST-1 is the prototype of the Large-Sized Telescope, that will be part of the Cherenkov Telescope Array Observatory (CTAO). Its improved performance over previous IACTs makes it well suited for studying pulsars. Aims: To study the Crab pulsar emission with the LST-1, improving and complementing the results from other telescopes. These observations can also be used to characterize the potential of the LST-1 to study other pulsars and detect new ones. Methods: We analyzed a total of $\sim$103 hours of gamma-ray observations of the Crab pulsar conducted with the LST-1 in the period from September 2020 to January 2023. The observations were carried out at zenith angles less than 50 degrees. A new analysis of the Fermi-LAT data was also performed, including $\sim$14 years of observations. Results: The Crab pulsar phaseogram, long-term light-curve, and phase-resolved spectra are reconstructed with the LST-1 from 20 GeV to 450 GeV for P1 and up to 700 GeV for P2. The pulsed emission is detected with a significance of 15.2$σ$. The two characteristic emission peaks of the Crab pulsar are clearly detected (>10$σ$), as well as the so-called bridge emission (5.7$σ$). We find that both peaks are well described by power laws, with spectral indices of $\sim$3.44 and $\sim$3.03 respectively. The joint analysis of Fermi-LAT and LST-1 data shows a good agreement between both instruments in the overlapping energy range. The detailed results obtained in the first observations of the Crab pulsar with LST-1 show the potential that CTAO will have to study this type of sources.

astro-ph.HE↗

arXiv2024

Bayesian insights in Tycho supernova remnant : a detailed mapping of ejecta properties

While Tycho's supernova remnant is one of the most studied type Ia Galactic supernova remnants, a global view of the physical properties of its ejecta is lacking, to understand its mysteries. In particular, the spatial distribution of the Si-rich ejecta line-of-sight velocity presents a large-scale unexplained asymmetry, with the north dominantly blueshifted and the south redshifted. To investigate the origin of this line-of-sight velocity asymmetry in the ejecta, we carry out a detailed X-ray spatially-resolved spectral analysis of the entire shocked ejecta in Tycho's SNR to determine the physical properties of its various components. This study is based on the archival deep X-ray observations from the Chandra space telescope. The spatially-resolved spectral analysis in 211 regions over the entire SNR is based on a tesselation method applied to the line-of-sight velocity map. A Bayesian tool is used to conduct the fitting, using a nested sampling algorithm. It allows us to obtain a complete view of the statistical landscape. We provide maps of the physical parameters of the various components across the SNR ejecta. The Doppler shift map confirms spectrally the large-scale north-south asymmetry in the line-of-sight velocity. We reveal different spatial distributions of temperature and ionization time for IMEs and for iron-rich ejecta, but none of these maps shows structure associated to the large-scale north-south asymmetry in the line-of-sight velocity distribution. The abundance maps show spatial variations, depending on the element, perhaps due to an origin in different layers during the explosion. We compare these abundances with some nucleosynthesis models. In addition, we observe for the first time an emission line at 0.654 keV possibly related to oxygen. Its spatial distribution differs from the other elements, so that this line may arise in the ambient medium.

astro-ph.HE↗

arXiv2024

Early-time gamma-ray constraints on cosmic-ray acceleration in the core-collapse SN 2023ixf with the Fermi Large Area Telescope

While SNRs have been considered the most relevant Galactic CR accelerators for decades, CCSNe could accelerate particles during the earliest stages of their evolution and hence contribute to the CR energy budget in the Galaxy. Some SNRs have indeed been associated with TeV gamma-rays, yet proton acceleration efficiency during the early stages of an SN expansion remains mostly unconstrained. The multi-wavelength observation of SN 2023ixf, a Type II SN in the nearby galaxy M101, opens the possibility to constrain CR acceleration within a few days after the collapse of the RSG stellar progenitor. With this work, we intend to provide a phenomenological, quasi-model-independent constraint on the CR acceleration efficiency during this event at photon energies above 100 MeV. We performed a maximum-likelihood analysis of gamma-ray data from the Fermi Large Area Telescope up to one month after the SN explosion. We searched for high-energy emission from its expanding shock, and estimated the underlying hadronic CR energy reservoir assuming a power-law proton distribution consistent with standard diffusive shock acceleration. We do not find significant gamma-ray emission from SN 2023ixf. Nonetheless, our non-detection provides the first limit on the energy transferred to the population of hadronic CRs during the very early expansion of a CCSN. Under reasonable assumptions, our limits would imply a maximum efficiency on the CR acceleration of as low as 1%, which is inconsistent with the common estimate of 10% in generic SNe. However, this result is highly dependent on the assumed geometry of the circumstellar medium, and could be relaxed back to 10% by challenging spherical symmetry. A more sophisticated, inhomogeneous characterisation of the shock and the progenitor's environment is required before establishing whether or not Type II SNe are indeed efficient CR accelerators at early times.

astro-ph.HE↗

arXiv2023

Prospects for $γ$-ray observations of the Perseus galaxy cluster with the Cherenkov Telescope Array

Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster medium. We estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse gamma-ray emission from the Perseus galaxy cluster. We perform a detailed spatial and spectral modelling of the expected signal for the DM and the CRp components. For each, we compute the expected CTA sensitivity. The observing strategy of Perseus is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio within the radius $R_{500}$ down to about $X_{500}<3\times 10^{-3}$, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index $α_{\rm CRp}=2.3$. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure $α_{\rm CRp}$ down to about $Δα_{\rm CRp}\simeq 0.1$ and the CRp spatial distribution with 10% precision. Regarding DM, CTA should improve the current ground-based gamma-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to $\sim 5$, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with $τ_χ>10^{27}$s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.

astro-ph.HE↗

arXiv2023

Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnants

The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at energies around 3~PeV. Sources which are capable of accelerating hadrons to such energies are called hadronic PeVatrons. However, hadronic PeVatrons have not yet been firmly identified within the Galaxy. Several source classes, including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron candidates. The potential to search for hadronic PeVatrons with the Cherenkov Telescope Array (CTA) is assessed. The focus is on the usage of very high energy $γ$-ray spectral signatures for the identification of PeVatrons. Assuming that SNRs can accelerate CRs up to knee energies, the number of Galactic SNRs which can be identified as PeVatrons with CTA is estimated within a model for the evolution of SNRs. Additionally, the potential of a follow-up observation strategy under moonlight conditions for PeVatron searches is investigated. Statistical methods for the identification of PeVatrons are introduced, and realistic Monte--Carlo simulations of the response of the CTA observatory to the emission spectra from hadronic PeVatrons are performed. Based on simulations of a simplified model for the evolution for SNRs, the detection of a $γ$-ray signal from in average 9 Galactic PeVatron SNRs is expected to result from the scan of the Galactic plane with CTA after 10 hours of exposure. CTA is also shown to have excellent potential to confirm these sources as PeVatrons in deep observations with $\mathcal{O}(100)$ hours of exposure per source.

astro-ph.HE↗

arXiv2022

Constraining leptonic emission scenarios for the PeVatron candidate HESS J1702-420 with deep XMM-Newton observations

The unidentified TeV source HESS J1702-420 has recently been proposed as a new hadronic PeVatron candidate, based on the discovery of a small-scale emission sub-region with extremely hard gamma-ray spectrum up to 100 TeV (named HESS J1702-420A). Given the difficulty to discriminate between a hadronic or leptonic origin of the TeV emission, based on the H.E.S.S. measurement alone, we opted for a multi-wavelength approach. A deep X-ray observation was carried out using the XMM-Newton satellite, with the goal of probing a possible association with a hidden leptonic accelerator. No evidence of a clear counterpart for HESS J1702-420A was found in the X-ray data. After excluding an association with all nearby X-ray point sources, we derived strict upper limits on the diffuse X-ray emission and average magnetic field in the HESS J1702-420A region. We additionally report the serendipitous discovery of a new extended X-ray source, whose association with HESS J1702-420A is not obvious but cannot be ruled out either. A set of scripts dedicated to the multi-wavelength modeling of X-ray and gamma-ray data, based on Gammapy, Naima and Xspec, was developed in the context of this work and is made publicly available along with this paper.

astro-ph.HE↗

arXiv2022

Search for new cosmic-ray acceleration sites within the 4FGL catalog Galactic plane sources

Cosmic rays are mostly composed of protons accelerated to relativistic speeds. When those protons encounter interstellar material, they produce neutral pions which in turn decay into gamma rays. This offers a compelling way to identify the acceleration sites of protons. A characteristic hadronic spectrum, with a low-energy break around 200 MeV, was detected in the gamma-ray spectra of four Supernova Remnants (SNRs), IC 443, W44, W49B and W51C, with the Fermi Large Area Telescope. This detection provided direct evidence that cosmic-ray protons are (re-)accelerated in SNRs. Here, we present a comprehensive search for low-energy spectral breaks among 311 4FGL catalog sources located within 5 degrees from the Galactic plane. Using 8 years of data from the Fermi Large Area Telescope between 50 MeV and 1 GeV, we find and present the spectral characteristics of 56 sources with a spectral break confirmed by a thorough study of systematic uncertainty. Our population of sources includes 13 SNRs for which the proton-proton interaction is enhanced by the dense target material; the high-mass gamma-ray binary LS~I +61 303; the colliding wind binary eta Carinae; and the Cygnus star-forming region. This analysis better constrains the origin of the gamma-ray emission and enlarges our view to potential new cosmic-ray acceleration sites.

astro-ph.HE↗

arXiv2021

Multiple accelerated particle populations in the Cygnus Loop with Fermi-LAT

The Cygnus Loop (G74.0-8.5) is a very well-known nearby supernova remnant (SNR) in our Galaxy. Thanks to its large size, brightness, and angular offset from the Galactic plane, it has been studied in detail from radio to $γ$-ray emission. The $γ$ -rays probe the populations of energetic particles and their acceleration mechanisms at low shock speeds. We present an analysis of the $γ$-ray emission detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope over 11 years in the region of the Cygnus Loop. We performed detailed morphological and spectral studies of the $γ$-ray emission toward the remnant from 100 MeV to 100 GeV and compared it with X-ray, UV, optical, and radio images. The higher statistics with respect to the previous studies enabled us to decompose the emission from the remnant into two morphological components to model its nonthermal multiwavelength emission. The extended $γ$-ray emission is well correlated with the thermal X-ray and UV emission of the SNR. Our morphological analysis reveals that a model considering two contributions from the X-ray and the UV emission regions is the best description of the $γ$-ray data. Both components show a curved spectrum, but the X-ray component is softer and more curved than the UV component, suggesting a different physical origin. The multiwavelength modeling of emission toward the SNR suggests that the nonthermal radio and $γ$-ray emission associated with the UV component is mostly due to the reacceleration of preexisting cosmic rays by radiative shocks in the adjacent clouds, while the nonthermal emission associated with the X-ray component arises from freshly accelerated cosmic rays.

astro-ph.HE↗

arXiv2021

Survey of the Galactic Plane with the CherenkovTelescope Array

Observations with the current generation of very-high-energy gamma-ray telescopes have revealed an astonishing variety of particle accelerators in the Milky Way, such as supernova remnants, pulsar wind nebulae, and binary systems. The upcoming Cherenkov Telescope Array (CTA) will be the first instrument to enable a survey of the entire Galactic plane in the energy range from a few tens of GeV to 300 TeV with unprecedented sensitivity and improved angular resolution. In this contribution we will revisit the scientific motivations for the survey, proposed as a Key ScienceProject for CTA. We will highlight recent progress, including improved physically-motivated models for Galactic source populations and interstellar emission, advance on the optimization of the survey strategy, and the development of pipelines to derive source catalogues tested on simulated data. Based on this, we will provide a new forecast on the properties of the sources thatCTA will detect and discuss the expected scientific return from the study of gamma-ray source populations.

astro-ph.IM↗

arXiv2021

Origin and role of relativistic cosmic particles

This white paper briefly summarizes the importance of the study of relativistic cosmic rays, both as a constituent of our Universe, and through their impact on stellar and galactic evolution. The focus is on what can be learned over the coming decade through ground-based gamma-ray observations over the 20 GeV to 300 TeV range. The majority of the material is drawn directly from "Science with the Cherenkov Telescope Array", which describes the overall science case for CTA. We request that authors wishing to cite results contained in this white paper cite the original work.

astro-ph.HE↗

arXiv2021

Constraining the origin of the puzzling source HESS J1640-465 and the PeVatron candidate HESS J1641-463 using Fermi-LAT observations

There are only few very-high-energy sources in our Galaxy which might accelerate particles up to the knee of the cosmic-ray spectrum. To understand the mechanisms of particle acceleration in these PeVatron candidates, \textit{Fermi}-LAT and H.E.S.S. observations are essential to characterize their $γ$-ray emission. HESS J1640$-$465 and the PeVatron candidate HESS J1641$-$463 are two neighboring (\ang[astroang]{0.25}) $γ$-ray sources, spatially coincident with the radio supernova remnants (SNRs) G338.3$-$0.0 and G338.5+0.1. Detected both by H.E.S.S. and \textit{Fermi}-LAT, we present here a morphological and spectral analysis of these two sources using 8 years of \textit{Fermi}-LAT data between 200 \si{\mega\electronvolt} and 1 \si{\tera\electronvolt} with multi-wavelength observations to assess their nature. The morphology of HESS J1640$-$465 is described by a 2D Gaussian ($σ=$ \ang[astroang]{0.053} $\pm$ \ang[astroang]{0.011}$_{stat}$ $ \pm$ \ang[astroang]{0.03}$_{syst}$) and its spectrum is modeled by a power-law with a spectral index $Γ= 1.8\pm0.1_{\rm stat}\pm0.2_{\rm syst}$. HESS J1641$-$463 is detected as a point-like source and its GeV emission is described by a logarithmic-parabola spectrum with $α= 2.7 \pm 0.1_ {\rm stat} \pm 0.2_ {\rm syst} $ and significant curvature of $β= 0.11 \pm 0.03_ {\rm stat} \pm 0.05_ {\rm syst} $. Radio and X-ray flux upper limits were derived. We investigated scenarios to explain their emission, namely the emission from accelerated particles within the SNRs spatially coincident with each source, molecular clouds illuminated by cosmic rays from the close-by SNRs, and a pulsar/PWN origin. Our new \emph{Fermi}-LAT results and the radio and flux X-ray upper limits pose severe constraints on some of these models.

astro-ph.HE↗

arXiv2021

Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation

The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for $γ$-ray astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of $γ$-ray cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of $γ$-ray absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift $z=2$ and to constrain or detect $γ$-ray halos up to intergalactic-magnetic-field strengths of at least 0.3pG. Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from $γ$-ray astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of $γ$-ray cosmology.

astro-ph.HE↗

arXiv2020

ALMA CO Observations of Gamma-Ray Supernova Remnant N132D in the Large Magellanic Cloud: Possible Evidence for Shocked Molecular Clouds Illuminated by Cosmic-Ray Protons

N132D is the brightest gamma-ray supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out $^{12}$CO($J$ = 1-0, 3-2) observations toward the SNR using the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find diffuse CO emission not only at the southern edge of the SNR as previously known, but also inside the X-ray shell. We spatially resolved nine molecular clouds using ALMA with an angular resolution of $5''$, corresponding to a spatial resolution of $\sim$1 pc at the distance of the LMC. Typical cloud sizes and masses are $\sim$2.0 pc and $\sim$100 $M_\odot$, respectively. High-intensity ratios of CO $J$ = 3-2 / 1-0 $> 1.5$ are seen toward the molecular clouds, indicating that shock-heating has occurred. Spatially resolved X-ray spectroscopy reveals that thermal X-rays in the center of N132D are produced not only behind a molecular cloud, but also in front of it. Considering the absence of a thermal component associated with the forward shock towards one molecular cloud located along the line of sight to the center of the remnant, this suggests that this particular cloud is engulfed by shock waves and is positioned on the near side of remnant. If the hadronic process is the dominant contributor to the gamma-ray emission, the shock-engulfed clouds play a role as targets for cosmic-rays. We estimate the total energy of cosmic-ray protons accelerated in N132D to be $\sim$0.5-$3.8 \times 10^{49}$ erg as a conservative lower limit, which is similar to that observed in Galactic gamma-ray SNRs.

astro-ph.GA↗

arXiv2019

PHEMTO : Polarimetric High Energy Modular Telescope Observatory

With the opening of the X and gamma--ray windows in the sixties, thanks to to sounding rockets and satellite-borne instruments, extremely energetic and violent phenomena were discovered and subsequently found to be ubiquitous in the Universe. Observations in the high energy domain are fundamental for understanding how matter is organized and behaves around black holes; unravelling how these extreme objects influence their environments on a very large scale; and finding the still elusive obscured massive objects in the centre of galaxies. Other major problems in contemporary astrophysics, such as the understanding of acceleration processes at shocks of all sizes (those of pulsar wind nebulae, supernova remnants, but also at larger scales those of Active Galactic Nuclei radio lobes) in relation to the origin of cosmic-rays, or the definitive characterization of the debated non-thermal X-ray energy content of clusters of galaxies, also requires observations at very high energies. An observatory type medium mission operating from around 1 keV to about 600 keV can provide direct insights into these major questions. The essential characteristics will be coverage of the full energy range by telescopes featuring a large throughput and arc-second resolution optics, coupled to a compact focal plane assembly, with excellent imaging resolution and spectroscopy. In addition, the mission will provide unique polarimetry measurements in the hard X-ray domain, an important new diagnostic tool at energies for which the non-thermal processes dominate. The Polarimetric High-Energy Modular Telescope Observatory (PHEMTO) is designed to have performance several orders of magnitude better than the present hard X-ray instruments. This gives to PHEMTO the improvements in scientific performance needed for a mission in the 2050 era.

astro-ph.HE↗

arXiv2018

Disentangling hadronic from leptonic emission in the composite SNR G326.3$-$1.8

G326.3$-$1.8 (also known as MSH 15$-$56) has been detected in radio as a middle-aged composite supernova remnant (SNR) consisting of an SNR shell and a pulsar wind nebula (PWN), which has been crushed by the SNR's reverse shock. Previous $γ$-ray studies of SNR G326.3$-$1.8 revealed bright and extended emission with uncertain origin. Understanding the nature of the $γ$-ray emission allows probing the population of high-energy particles (leptons or hadrons) but can be challenging for sources of small angular extent. With the recent $\textit{Fermi}$ Large Area Telescope data release Pass 8, we investigate the morphology of this SNR to disentangle the PWN from the SNR contribution. We perform a morphological and spectral analysis from 300 MeV to 300 GeV. We use the reconstructed events with the best angular resolution to separately investigate the PWN and the SNR emissions, which is crucial to accurately determine the spectral properties of G326.3$-$1.8 and understand its nature. The centroid of the $γ$-ray emission evolves with energy and is spatially coincident with the radio PWN at high energies (E $>$ 3 GeV). The morphological analysis reveals that a model considering two contributions from the SNR and the PWN reproduces the $γ$-ray data better than a single-component model. The associated spectral analysis using power laws shows two distinct spectral features, a softer spectrum for the remnant ($Γ$ = 2.17 $\pm$ 0.06) and a harder spectrum for the PWN ($Γ$ = 1.79 $\pm$ 0.12), consistent with hadronic and leptonic origin for the SNR and the PWN respectively. Focusing on the SNR spectrum, we use one-zone models to derive some physical properties and, in particular, we find that the emission is best explained with a hadronic scenario in which the large target density is provided by radiative shocks in HI clouds struck by the SNR.

astro-ph.HE↗