arXiv subjects

M. Lemoine-Goumard

Publications and source records attributed to M. Lemoine-Goumard.

At least 55 records · Page 3Linked to original sources

arXiv2021

Fermi Large Area Telescope Performance After 10 Years Of Operation

The Large Area Telescope (LAT), the primary instrument for the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from 30 MeV to more than 300 GeV. We describe the performance of the instrument at the 10-year milestone. LAT performance remains well within the specifications defined during the planning phase, validating the design choices and supporting the compelling case to extend the duration of the Fermi mission. The details provided here will be useful when designing the next generation of high-energy gamma-ray observatories.

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arXiv2021

Search for dark matter annihilation signals from unidentified Fermi-LAT objects with H.E.S.S

Cosmological $N$-body simulations show that Milky Way-sized galaxies harbor a population of unmerged dark matter subhalos. These subhalos could shine in gamma-rays and be eventually detected in gamma-ray surveys as unidentified sources. We performed a thorough selection among unidentified Fermi-LAT Objects (UFOs) to identify them as possible TeV-scale dark matter subhalo candidates. We search for very-high-energy (E $\gtrsim$ 100 GeV) gamma-ray emissions using H.E.S.S. observations towards four selected UFOs. Since no significant very-high-energy gamma-ray emission is detected in any dataset of the four observed UFOs nor in the combined UFO dataset, strong constraints are derived on the product of the velocity-weighted annihilation cross section $\langle σv \rangle$ by the $J$-factor for the dark matter models. The 95% C.L. observed upper limits derived from combined H.E.S.S. observations reach $\langle σv \rangle J$ values of 3.7$\times$10$^{-5}$ and 8.1$\times$10$^{-6}$ GeV$^2$cm$^{-2}$s$^{-1}$ in the $W^+W^-$ and $τ^+τ^-$ channels, respectively, for a 1 TeV dark matter mass. Focusing on thermal WIMPs, the H.E.S.S. constraints restrict the $J$-factors to lie in the range 6.1$\times$10$^{19}$ - 2.0$\times$10$^{21}$ GeV$^2$cm$^{-5}$, and the masses to lie between 0.2 and 6 TeV in the $W^+W^-$ channel. For the $τ^+τ^-$ channel, the $J$-factors lie in the range 7.0$\times$10$^{19}$ - 7.1$\times$10$^{20}$ GeV$^2$cm$^{-5}$ and the masses lie between 0.2 and 0.5 TeV. Assuming model-dependent predictions from cosmological N-body simulations on the $J$-factor distribution for Milky Way-sized galaxies, the dark matter models with masses greater than 0.3 TeV for the UFO emissions can be ruled out at high confidence level.

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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.

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arXiv2020

Morphological and spectral study of 4FGL J1115.1-6118 in the region of the young massive stellar cluster NGC 3603

We report a detailed study of an unidentified gamma-ray source located in the region of the compact stellar cluster NGC 3603. This is a star-forming region (SFR) powered by a massive cluster of OB stars. A dedicated analysis of about 10 years of data from 10 GeV - 1 TeV, provided by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope, yields the detection of a pointlike source at a significance of 9$σ$. The source photon spectrum can be described by a power-law model with best fit spectral index of $2.35 \pm 0.03$. In addition, the analysis of a deep Chandra image in the 0.5 - 7 keV band reliably rules out an extragalactic origin for the gamma rays. We also conclude that the broadband spectral energy distribution of the point source can be explained well with both leptonic and hadronic models. No firm evidence of association with any other classes of known gamma-ray emitters is found, therefore we speculate that 4FGL J1115.1-6118 is a gamma-ray emitting SFR.

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arXiv2020

Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-Energy Emission from Prompt to Afterglow

We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to constrain the transition from internal shock to external shock dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment and find that high-energy photons observed by Fermi LAT are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process.

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arXiv2019

A very-high-energy component deep in the Gamma-ray Burst afterglow

Gamma-ray bursts (GRBs) are brief flashes of gamma rays, considered to be the most energetic explosive phenomena in the Universe. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow -- produced by the interaction between the ejected matter and the circumburst medium -- slows down, and a gradual decrease in brightness is observed. GRBs typically emit most of their energy via gamma-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elussive. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow -ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and gamma-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.

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arXiv2018

Disentangling multiple high-energy emission components in the Vela X pulsar wind nebula with the Fermi Large Area Telescope

Vela X is a pulsar wind nebula in which two relativistic particle populations with distinct spatial and spectral distributions dominate the emission at different wavelengths. An extended $2^\circ \times 3^\circ$ nebula is seen in radio and GeV gamma rays. An elongated cocoon prevails in X-rays and TeV gamma rays. We use 9.5 years of data from the Fermi Large Area Telescope (LAT) to disentangle gamma-ray emission from the two components in the energy range from 10 GeV to 2 TeV, bridging the gap between previous measurements at GeV and TeV energies. We determine the morphology of emission associated to Vela X separately at energies < 100 GeV and > 100 GeV, and compare it to the morphology seen at other wavelengths. Then, we derive the spectral energy distribution of the two gamma-ray components over the full energy range. The best fit to the LAT data is provided by the combination of the two components derived at energies < 100 GeV and > 100 GeV. The first component has a soft spectrum, spectral index $2.19\pm0.16^{+0.05}_{-0.22}$, and extends over a region of radius $1.36^\circ\pm0.04^\circ$, consistent with the radio nebula. The second component has a harder spectrum, spectral index $0.9\pm0.3^{+0.3}_{-0.1}$, and is concentrated over an area of radius $0.63^\circ\pm0.03^\circ$, coincident with the X-ray cocoon that had already been established to account for the bulk of the emission at TeV energies. The spectrum measured for the low-energy component corroborates previous evidence for a roll-over of the electron spectrum at energies of a few tens of GeV possibly due to diffusive escape. The high-energy component has a very hard spectrum: if the emission is produced by electrons with a power-law spectrum the electrons must be uncooled, and there is a hint that their spectrum may be harder than predictions by standard models of Fermi acceleration at relativistic shocks. (Abridged)

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arXiv2018

Search for extended sources in the Galactic Plane using 6 years of Fermi-Large Area Telescope Pass 8 data above 10 GeV

The spatial extension of a gamma-ray source is an essential ingredient to determine its spectral properties as well as its potential multi-wavelength counterpart. The capability to spatially resolve gamma-ray sources is greatly improved by the newly delivered Fermi-Large Area Telescope (LAT) Pass 8 event-level analysis which provides a greater acceptance and an improved point spread function, two crucial factors for the detection of extended sources. Here, we present a complete search for extended sources located within 7 degrees from the Galactic plane, using 6 years of LAT data above 10 GeV. We find 46 extended sources and provide their morphological and spectral characteristics. This constitutes the first catalog of hard LAT extended sources, named the Fermi Galactic Extended Source Catalog, which allows a thorough study of the properties of the Galactic plane in the sub-TeV domain.

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arXiv2016

Fermi Large Area Telescope Observations of the Monoceros Loop Supernova Remnant

We present an analysis of the gamma-ray measurements by the Large Area Telescope onboard the \textit{Fermi Gamma-ray Space Telescope} in the region of the supernova remnant~(SNR) Monoceros Loop~(G205.5$+$0.5). The brightest gamma-ray peak is spatially correlated with the Rosette Nebula, which is a molecular cloud complex adjacent to the southeast edge of the SNR. After subtraction of this emission by spatial modeling, the gamma-ray emission from the SNR emerges, which is extended and fit by a Gaussian spatial template. The gamma-ray spectra are significantly better reproduced by a curved shape than a simple power law. The luminosities between 0.2--300~GeV are $\sim$~$4 \times 10^{34}$~erg~s$^{-1}$ for the SNR and $\sim$~$3 \times 10^{34}$~erg~s$^{-1}$ for the Rosette Nebula, respectively. We argue that the gamma rays likely originate from the interactions of particles accelerated in the SNR. The decay of neutral pions produced in nucleon-nucleon interactions of accelerated hadrons with interstellar gas provides a reasonable explanation for the gamma-ray emission of both the Rosette Nebula and the Monoceros SNR.

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arXiv2016

Fermi LAT Discovery of Extended Gamma-Ray Emissions in the Vicinity of the HB3 Supernova Remnant

We report the discovery of extended gamma-ray emission measured by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the region of the supernova remnant (SNR) HB3 (G132.7+1.3) and the W3 HII complex adjacent to the southeast of the remnant. W3 is spatially associated with bright 12CO (J=1-0) emission. The gamma-ray emission is spatially correlated with this gas and the SNR. We discuss the possibility that gamma rays originate in interactions between particles accelerated in the SNR and interstellar gas or radiation fields. The decay of neutral pions produced in nucleon-nucleon interactions between accelerated hadrons and interstellar gas provides a reasonable explanation for the gamma-ray emission. The emission from W3 is consistent with irradiation of the CO clouds by the cosmic rays accelerated in HB3.

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arXiv2015

Study of TeV shell supernova remnants at gamma-ray energies

The breakthrough developments of Cherenkov telescopes in the last decade have led to angular resolution of 0.1° and an unprecedented sensitivity. This has allowed the current generation of Cherenkov telescopes to discover a population of supernova remnants (SNRs) radiating in very-high-energy (VHE, E>100 GeV) gamma-rays. A number of those VHE SNRs exhibit a shell-type morphology spatially coincident with the shock front of the SNR. The members of this VHE shell SNR club are RX J1713.7-3946, Vela Jr, RCW 86, SN 1006, and HESS J1731-347. The latter two objects have been poorly studied in high-energy (HE, 0.1 5 sigma. With this Fermi analysis, we now have a complete view of the HE to VHE gamma-ray emission of TeV shell SNRs. All five sources have a hard HE photon index (<1.8) suggesting a common scenario where the bulk of the emission is produced by accelerated electrons radiating from radio to VHE gamma-rays through synchrotron and inverse Compton processes. In addition when correcting for the distance, all SNRs show a surprisingly similar gamma-ray luminosity supporting the idea of a common emission mechanism. While the gamma-ray emission is likely to be leptonic dominated, this does not rule out efficient hadron acceleration in those SNRs.

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arXiv2014

HESS J1640-465 and HESS J1641-463: two intriguing TeV sources in the light of new Fermi LAT observations

We report on gamma-ray analysis of the region containing the bright TeV source HESS J1640-465 and the close-by TeV source HESS J1641-463 using 64 months of observations with the Fermi Large Area Telescope (LAT). Previously only one GeV source was reported in this region and was associated with HESS J1640-465. With an increased dataset and the improved sensitivity afforded by the reprocessed data (P7REP) of the LAT, we now report the detection, morphological study and spectral analysis of two distinct sources above 100 MeV. The softest emission in this region comes from the TeV source HESS J1641-463 which is well fitted with a power law of index Gamma = 2.47 +/- 0.05 +/- 0.06 and presents no significant gamma-ray signal above 10 GeV, which contrasts with its hard spectrum at TeV energies. The Fermi-LAT spectrum of the second TeV source, HESS J1640-465 is well described by a power-law shape of index Gamma = 1.99 +/- 0.04 +/- 0.07 that links up naturally with the spectral data points obtained by the High Energy Stereoscopic System (H.E.S.S.). These new results provide new constraints concerning the identification of these two puzzling gamma-ray sources.

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arXiv2013

The Vela-X Pulsar Wind Nebula revisited with 4 years of Fermi-Large Area Telescope observations

The Vela supernova remnant is the closest supernova remnant to Earth containing an active pulsar, the Vela pulsar (PSR B0833-45). This pulsar is the archetype of the middle-aged pulsar class and powers a bright pulsar wind nebula (PWN), Vela-X, spanning a region of 2° x 3° south of the pulsar and observed in the radio, X-ray and very high energy gamma-ray domains. The detection of the Vela-X PWN by the Fermi Large Area Telescope (LAT) was reported in the first year of the mission. Subsequently, we have re-investigated this complex region and performed a detailed morphological and spectral analysis of this source using 4 years of Fermi-LAT observations. This study lowers the threshold for morphological analysis of the nebula from 0.8 GeV to 0.3 GeV, allowing inspection of distinct energy bands by the LAT for the first time. We describe the recent results obtained on this PWN and discuss the origin of the newly detected spatial features.

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arXiv2013

Constraints on the Galactic Population of TEV Pulsar Wind Nebulae Using Fermi Large Area Telescope Observations

Pulsar wind nebulae (PWNe) have been established as the most populous class of TeV gamma-ray emitters. Since launch, the Fermi Large Area Telescope (LAT)identified five high-energy (100MeV <E< 100 GeV) gamma-ray sources as PWNe, and detected a large number of PWNe candidates, all powered by young and energetic pulsars. The wealth of multi-wavelength data available and the new results provided by Fermi-LAT give us an opportunity to find new PWNe and to explore the radiative processes taking place in known ones. The TeV gamma-ray unidentifiedsources (UNIDs) are the best candidates for finding new PWNe. Using 45 months of Fermi-LAT data for energies above 10 GeV, an analysis was performed near the position of 58TeV PWNe and UNIDs within 5deg of the Galactic Plane to establish new constraints on PWNe properties and find new clues on the nature of UNIDs. Of the 58 sources, 30 were detected, and this work provides their gamma-rayfluxes for energies above 10 GeV. The spectral energy distributions (SED) andupper limits, in the multi-wavelength context, also provide new information on the source nature and can help distinguish between emission scenarios, i.e. between classification as a pulsar candidate or as a PWN candidate. Six new GeV PWNe candidates are described in detail and compared with existing models. A population study of GeV PWNe candidates as a function of the pulsar/PWN system characteristics is presented.

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arXiv2013

The Hot and Energetic Universe: The astrophysics of supernova remnants and the interstellar medium

The study of both supernova remnants and the hot and cold phases of the interstellar medium are essential for understanding the final stages of stellar evolution and their feedback on the evolution of galaxies through injection of energy and heavy elements. These studies are also crucial for understanding the physics of supernovae, their cosmological implication, and the origin of galactic cosmic rays. The unique capabilities of Athena+ will allow us to explore a new parameter space. Spatially-resolved high-resolution spectroscopy using Athena+ X-IFU of young remnants will allow to characterize individual parcels of ejected material in the line of sight in terms of kinematics, ionization and composition, providing access to the three dimensional geometry of the explosion. Athena+ will also allow studying shock physics and particle acceleration in supernova remnants, as well as their interaction with their environment. Athena+ X-IFU will also characterize the ionization mechanisms competing in forming the complex structures of the hot interstellar medium, likely to keep the echo of past star formation activity, both in our Galaxy and nearby ones. For the first time the dust and gas of the densest cold medium, like in the Galactic Centre environment, will be studied. Athena+ X-IFU will observe, along with the Mg K and Si K edges, which are the main tracers of the silicates content of the ISM, the Fe K edge with unprecedented sensitivity and energy-resolution. This will allow us to study for the first time the nature of Fe-bearing dust in such regions.

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arXiv2013

Detection of the Characteristic Pion-Decay Signature in Supernova Remnants

Cosmic rays are particles (mostly protons) accelerated to relativistic speeds. Despite wide agreement that supernova remnants (SNRs) are the sources of galactic cosmic rays, unequivocal evidence for the acceleration of protons in these objects is still lacking. When accelerated protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma rays. This offers a compelling way to detect the acceleration sites of protons. The identification of pion-decay gamma rays has been difficult because high-energy electrons also produce gamma rays via bremsstrahlung and inverse Compton scattering. We detected the characteristic pion-decay feature in the gamma-ray spectra of two SNRs, IC 443 and W44, with the Fermi Large Area Telescope. This detection provides direct evidence that cosmic-ray protons are accelerated in SNRs.

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arXiv2012

Binary Millisecond Pulsar Discovery via Gamma-Ray Pulsations

Millisecond pulsars, old neutron stars spun-up by accreting matter from a companion star, can reach high rotation rates of hundreds of revolutions per second. Until now, all such "recycled" rotation-powered pulsars have been detected by their spin-modulated radio emission. In a computing-intensive blind search of gamma-ray data from the Fermi Large Area Telescope (with partial constraints from optical data), we detected a 2.5-millisecond pulsar, PSR J1311-3430. This unambiguously explains a formerly unidentified gamma-ray source that had been a decade-long enigma, confirming previous conjectures. The pulsar is in a circular orbit with an orbital period of only 93 minutes, the shortest of any spin-powered pulsar binary ever found.

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arXiv2012

Fermi-LAT and WMAP observations of the Puppis A Supernova Remnant

We report the detection of GeV γ-ray emission from the supernova remnant Puppis A with the Fermi Gamma-Ray Space Telescope. Puppis A is among the faintest supernova remnants yet detected at GeV energies, with a luminosity of only 2.7x10^34 (D/2.2 kpc)^2 erg/s between 1 and 100 GeV. The γ-ray emission from the remnant is spatially extended, with a morphology matching that of the radio and X-ray emission, and is well described by a simple power law with an index of 2.1. We attempt to model the broadband spectral energy distribution, from radio to γ-rays, using standard nonthermal emission mechanisms. To constrain the relativistic electron population we use 7 years of WMAP data to extend the radio spectrum up to 93 GHz. Both leptonic and hadronic dominated models can reproduce the nonthermal spectral energy distribution, requiring a total content of cosmic ray (CR) electrons and protons accelerated in Puppis A of at least (1-5)x10^49 erg.

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