We present an incremental version (4FGL-DR2, for Data Release 2) of the fourth Fermi-LAT catalog of gamma-ray sources. Based on the first ten years of science data in the energy range from 50 MeV to 1 TeV, it uses the same analysis methods as the 4FGL catalog did for eight years of data. The spectral parameters, spectral energy distributions and associations are updated for all sources. Light curves are rebuilt for all sources with 1-year intervals (not 2-month intervals). Among the 5064 4FGL sources, 120 are formally below the detection threshold over 10 years (but are kept in the list), while 40 are newly associated. We report 723 new sources, mostly just above the detection threshold, among which two are considered identified and 342 have a plausible counterpart at other wavelengths.

/pdf/fermi-large-area-telescope-fourth-source-catalog-data-45fjm2sqn0.pdf

Fermi Large Area Telescope Fourth Source Catalog Data Release 2

Supernova remnants (SNRs) have been considered as the dominant contributors to Galactic cosmic rays. However, the relation between high-energy particles trapped in SNRs and cosmic rays observed at the Earth remains obscure. In this paper, we fit the spectral energy distributions of 35 SNRs with a simple one-zone emission model and analyze correlations of model parameters to uncover the evolution of high-energy particle distribution in SNRs. We find that (1) the particle distribution in general can be described by a broken power-law function with a high-energy cutoff for all SNRs; (2) the low-energy spectrum becomes harder and the break energy decreases with aging of SNRs, (3) for most middle-age SNRs, the energy loss timescale of electrons at the high-energy cutoff is approximately equal to the age of the corresponding remnant implying quenching of very high-energy electron acceleration; for young SNRs, this energy loss timescale is shorter than the age of SNRs implying continuous electon acceleration at the cutoff energy; and for a few old age SNRs, the energy loss timescale is longer than the corresponding age which may suggest escaping of higher energy particles from SNRs. Finally, we comment on the implications of these results on the SNR origin of Galactic cosmic rays.

Evolution of high-energy particle distribution in Supernova Remnants

https://iopscience.iop.org/article/10.3847/1538-4357/aaf392/pdf

Evolution of High-energy Particle Distribution in Supernova Remnants

Context. There is a considerable deficiency in the number of known supernova remnants (SNRs) in the Galaxy compared to that expected. This deficiency is thought to be caused by a lack of sensitive radio continuum data. Searches for extended low-surface brightness radio sources may find new Galactic SNRs, but confusion with the much larger population of H II regions makes identifying such features challenging. SNRs can, however, be separated from H II regions using their significantly lower mid-infrared (MIR) to radio continuum intensity ratios. Aims. Our goal is to find missing SNR candidates in the Galactic disk by locating extended radio continuum sources that lack MIR counterparts. Methods. We use the combination of high-resolution 1-2 GHz continuum data from The HI, OH, Recombination line survey of the Milky Way (THOR) and lower-resolution VLA 1.4 GHz Galactic Plane Survey (VGPS) continuum data, together with MIR data from the Spitzer GLIMPSE, Spitzer MIPSGAL, and WISE surveys to identify SNR candidates. To ensure that the candidates are not being confused with H II regions, we exclude radio continuum sources from the WISE Catalog of Galactic H II Regions, which contains all known and candidate H II regions in the Galaxy. Results. We locate 76 new Galactic SNR candidates in the THOR and VGPS combined survey area of 67.4 degrees > l > 17.5 beta, |b| <= 1.25 degrees and measure the radio flux density for 52 previously-known SNRs. The candidate SNRs have a similar spatial distribution to the known SNRs, although we note a large number of new candidates near l similar or equal to 30 degrees, the tangent point of the Scutum spiral arm. The candidates are on average smaller in angle compared to the known regions, 6.4' +/- 4.7' versus 11.0' +/- 7.8', and have lower integrated flux densities. Conclusions. The THOR survey shows that sensitive radio continuum data can discover a large number of SNR candidates, and that these candidates can be efficiently identified using the combination of radio and MIR data. If the 76 candidates are confirmed as true SNRs, for example using radio polarization measurements or by deriving radio spectral indices, this would more than double the number of known Galactic SNRs in the survey area. This large increase would still, however, leave a discrepancy between the known and expected SNR populations of about a factor of two.

Galactic Supernova Remnant Candidates Discovered by THOR

HESS J1843–033 is a very high energy gamma-ray source whose origin remains unidentified. This work presents, for the first time, the energy spectrum of gamma rays beyond 100 TeV from the HESS J1843–033 region using the data recorded by the Tibet air shower array and its underground muon detector array. A gamma-ray source with an extension of 0.°34 ± 0.°12 is successfully detected above 25 TeV at (α, δ) = (281.°09 ± 0.°10, −3.°76 ± 0.°09) near HESS J1843–033 with a statistical significance of 6.2σ, and the source is named TASG J1844–038. The position of TASG J1844–038 is consistent with those of HESS J1843–033, eHWC J1842–035, and LHAASO J1843–0338. The measured gamma-ray energy spectrum in 25 TeV < E < 130 TeV is described with dN/dE=(9.70±1.89)×10−16 (E/40 TeV)−3.26±0.30 TeV−1 cm−2 s−1, and the spectral fit to the combined spectra of HESS J1843–033, LHAASO J1843–0338, and TASG J1844–038 implies the existence of a cutoff at 49.5 ± 9.0 TeV. Associations of TASG J1844–038 with SNR G28.6–0.1 and PSR J1844–0346 are also discussed in detail for the first time.

https://iopscience.iop.org/article/10.3847/1538-4357/ac6ef4/pdf

Measurement of the Gamma-Ray Energy Spectrum beyond 100 TeV from the HESS J1843–033 Region

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 a SNR shell and a pulsar wind nebula (PWN) which has been crushed by the reverse shock. With the recent Fermi-LAT data release Pass 8 providing increased acceptance and angular resolution, we investigate the morphology of this SNR to disentangle the PWN from the SNR contributions and understand the nature of the γ-ray emission. We thus perform a morphological and spectral analysis from 300 MeV to 300 GeV which highlights the contributions from these two components. The simplest interpretation is hadronic emission from the SNR and harder leptonic emission from the PWN.

https://hal.archives-ouvertes.fr/hal-01815168/document

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 $\gamma$-ray studies of SNR G326.3$-$1.8 revealed bright and extended emission with uncertain origin. Understanding the nature of the $\gamma$-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 $\gamma$-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 $\gamma$-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 ($\Gamma$ = 2.17 $\pm$ 0.06) and a harder spectrum for the PWN ($\Gamma$ = 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.

/pdf/disentangling-hadronic-from-leptonic-emission-in-the-1r8uyjmfdy.pdf

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

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 $\gamma$-ray emission. HESS J1640$-$465 and the PeVatron candidate HESS J1641$-$463 are two neighboring (\ang[astroang]{0.25}) $\gamma$-ray sources, spatially coincident with the radio supernova remnants (SNRs) G338.3$-$0.0 and G338.5+0.1. Detected both by H.E.S.S. and \textit{Fermi}-LAT, we present here a morphological and spectral analysis of these two sources using 8 years of \textit{Fermi}-LAT data between 200 \si{\mega\electronvolt} and 1 \si{\tera\electronvolt} with multi-wavelength observations to assess their nature. The morphology of HESS J1640$-$465 is described by a 2D Gaussian ($\sigma=$ \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 $\Gamma = 1.8\pm0.1_{\rm stat}\pm0.2_{\rm syst}$. HESS J1641$-$463 is detected as a point-like source and its GeV emission is described by a logarithmic-parabola spectrum with $\alpha = 2.7 \pm 0.1_ {\rm stat} \pm 0.2_ {\rm syst} $ and significant curvature of $\beta = 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.

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

Abstract 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 a SNR shell and a pulsar wind nebula (PWN) which has been crushed by the reverse shock. With the recent Fermi-LAT data release Pass 8 providing increased acceptance and angular resolution, we investigate the morphology of this SNR to disentangle the PWN from the SNR contributions and understand the nature of the γ-ray emission. We thus perform a morphological and spectral analysis from 300 MeV to 300 GeV which highlights the contributions from these two components. The simplest interpretation is hadronic emission from the SNR and harder leptonic emission from the PWN.

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

Aims. Among the γ-ray sources discovered at high and very-high energies, a large fraction still lack a clear identification. In particular, the H.E.S.S. Galactic Plane Survey (HGPS) revealed 78 TeV sources among which 47 are not clearly associated with a known object. Multiwavelength data can help identify the origin of the very-high energy γ-ray emission, although some bright TeV sources have been detected without clear counterparts. We present a multiwavelength approach to constrain the origin of the emission from unidentified HGPS sources.Methods. We present a generic pipeline that explores a large database of multiwavelength archival data toward any region in the Galactic plane. Along with a visual inspection of the retrieved multiwavelength observations to search for faint and uncataloged counterparts, we derive a radio spectral index that helps disentangle thermal from nonthermal emission and a mean magnetic field through X-ray and TeV data in case of a leptonic scenario. We also search for a spectral connection between the GeV and the TeV regimes with the Fermi-LAT cataloged sources that may be associated with the unidentified HGPS source. We complete the association procedure with catalogs of known objects (supernova remnants, pulsar wind nebulae, H II regions, etc.) and with the source catalogs from instruments whose data are retrieved.Results. The method is applied on two unidentified sources, namely HESS J1427−608 and HESS J1458−608, for which the multiwavelength constraints favor the pulsar wind nebula (PWN) scenario. We model their broadband nonthermal spectra in a leptonic scenario with a magnetic field B ≲ 10 μG, which is consistent with that obtained from ancient PWNe. We place both sources within the context of the TeV PWN population to estimate the spin-down power and the characteristic age of the putative pulsar. We also shed light on two possibly significant γ-ray excesses in the HGPS: the first is located in the south of the unidentified source HESS J1632−478 and the second is spatially coincident with the synchrotron-emitting supernova remnant G28.6−0.1. The multiwavelength counterparts found toward both γ-ray excesses make these promising candidates for being new very-high energy γ-ray sources.Key words: gamma rays: ISM / ISM: individual objects: HESS J1427-608 / cosmic rays / radio continuum: ISM / ISM: individual objects: HESS J1458-608 / ISM: supernova remnants

/pdf/multiwavelength-constraints-on-the-unidentified-galactic-tev-3oiqk4oz5g.pdf

Multiwavelength constraints on the unidentified Galactic TeV sources HESS J1427−608, HESS J1458−608, and new VHE γ-ray source candidates

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