We report the first high-significance GeV γ-ray detections of supernova remnants HESS J1731−347 and SN 1006, both of which have been previously detected by imaging atmospheric Cherenkov Telescopes ...above 1 TeV. Using 8 years of Fermi-LAT Pass 8 data at energies between 1 GeV and 2 TeV, we detect emission at the position of HESS J1731−347 with a significance of and a spectral index of . The hardness of the index and the good connection with the TeV spectrum of HESS J1731−347 support an association between the two sources. We also confirm the detection of SN 1006 at with a spectral index of . The northeast (NE) and southwest (SW) limbs of SN 1006 were also fit separately, resulting in the detection of the NE region ( ) and the non-detection of the SW region. The significance of different spectral components for the two limbs is , providing first indications of an asymmetry in the GeV γ-ray emission.
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 yr of data from 10 GeV to 1 TeV, provided by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, yields the detection of a pointlike source at a significance of 9 . The source photon spectrum can be described by a power-law model with a best-fit spectral index of 2.35 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.
Context. The breakthrough developments of Cherenkov telescopes in the past decade have led to angular resolution of 0.1° and an unprecedented sensitivity. This has allowed the current generation of ...Cherenkov telescopes (H.E.S.S., MAGIC, and VERITAS) to discover a population of supernova remnants (SNRs) radiating in very-high-energy (VHE; E > 100 GeV) γ-rays. A number of those VHE SNRs exhibit a shell-type morphology that is spatially coincident with the shock front of the SNR. Aims. The members of this VHE shell SNR club are RX J1713.7−3946, RX J0852.0−4622, RCW 86, SN 1006, and HESS J1731−347. The last two objects have been poorly studied in high-energy (HE; 0.1 < E < 100 GeV) γ-rays and need to be investigated in order to draw the overall picture of this class of SNRs and to constrain the characteristics of the underlying population of accelerated particles. Methods. Using 6 years of Fermi-LAT P7 reprocessed data, we studied the GeV counterpart of the SNRs HESS J1731−347 and SN 1006. The two SNRs are not detected in the data set, and given that there is no hint of detection, we do not expect any detection in coming years from the SNRs. However in both cases, we derived upper limits that significantly constrain the γ-ray emission mechanism and can rule out a standard hadronic scenario with a confidence level >5σ. Results. With this Fermi analysis, we now have a complete view of the HE to VHE γ-ray emission of TeV shell SNRs. All five sources have a hard HE photon index (Γ < 1.8), which suggests a common scenario where the bulk of the emission is produced by accelerated electrons radiating from radio to VHE γ-rays through synchrotron and inverse Compton processes. In addition when correcting for the distance, all SNRs show a surprisingly similar γ-ray luminosity supporting the idea of a common emission mechanism. While the γ-ray emission is likely to be leptonic-dominated at the scale of the whole SNR, this does not rule out efficient hadron acceleration in those objects.
Aims.
The supernova remnant (SNR) G150.3+4.5 was recently discovered in the radio band; it exhibits a shell-like morphology with an angular size of ~ 3°, suggesting either an old or a nearby SNR. ...Extended
γ
-ray emission spatially coincident with the SNR was reported in the
Fermi
Galactic Extended Source Catalog, with a power-law spectral index of Γ = 1.91 ± 0.09. Studying particle acceleration in SNRs through their
γ
-ray emission is of primary concern to assess the nature of accelerated particles and the maximum energy they can reach.
Methods.
Using more than ten years of
Fermi
-LAT data, we investigate the morphological and spectral properties of the SNR G150.3+4.5 from 300 MeV to 3 TeV. We use the latest releases of the
Fermi
-LAT catalog, the instrument response functions and the Galactic and isotropic diffuse emissions. We use ROSAT all-sky survey data to assess any thermal and nonthermal X-ray emission, and we derive minimum and maximum distance to G150.3+4.5.
Results.
We describe the
γ
-ray emission of G150.3+4.5 by an extended component which is found to be spatially coincident with the radio SNR. The spectrum is hard and the detection of photons up to hundreds of GeV points towards an emission from a dynamically young SNR. The lack of X-ray emission gives a tight constraint on the ambient density
n
0
≤ 3.6 × 10
−3
cm
−3
. Since G150.3+4.5 is not reported as a historical SNR, we impose a lower limit on its age of
t
= 1 kyr. We estimate its distance to be between 0.7 and 4.5 kpc. We find that G150.3+4.5 is spectrally similar to other dynamically young and shell-type SNRs, such as RX J1713.7−3946 or Vela Junior. The broadband nonthermal emission is explained with a leptonic scenario, implying a downstream magnetic field of
B
= 5
μ
G and acceleration of particles up to few TeV energies.
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
μ
G, 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.
Spatial extension is an important characteristic for correctly associating gamma-ray-emitting sources with their counterparts at other wavelengths and for obtaining an unbiased model of their ...spectra. We present a new method for quantifying the spatial extension of sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi). We perform a series of Monte Carlo simulations to validate this tool and calculate the LAT threshold for detecting the spatial extension of sources. We then test all sources in the second Fermi-LAT catalog for extension. We report the detection of seven new spatially extended sources.
ABSTRACT We present an analysis of the gamma-ray measurements by the Large Area Telescope on board the 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 and 300 GeV are erg s−1 for the SNR and 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.
Context. 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° ...× 3° nebula is seen in radio and GeV gamma rays. An elongated cocoon prevails in X-rays and TeV gamma rays. Aims. We use ~9.5 yr 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. Methods. We determine the morphology of emission associated to Vela X separately at energies <100 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. Results. The best overall fit to the LAT data is provided by the combination of the two components derived at energies <100 and >100 GeV. The first component has a soft spectrum, spectral index 2.19 ± 0.16−0.22+0.05 $2.19\pm0.16^{+0.05}_{-0.22}$ 2.19±0.16−0.22+0.05 , and extends over a region of radius 1.°36±0.°04, consistent with the size of the radio nebula. The second component has a harder spectrum, spectral index 0.9 ± 0.3−0.1+0.3 $0.9\pm0.3^{+0.3}_{-0.1}$ 0.9±0.3−0.1+0.3 , and is concentrated over an area of radius 0.°63±0.°03, coincident with the X-ray cocoon that had already been established as accounting for the bulk of the emission at TeV energies. Conclusions. The spectrum measured for the low-energy component corroborates previous evidence for a roll-over of the electron spectrum in the extended radio nebula 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.
Abstract
There are only a few very-high-energy sources in our Galaxy that might accelerate particles up to the knee of the cosmic-ray spectrum. To understand the mechanisms of particle acceleration ...in these PeVatron candidates, Fermi-Large Area Telescope (LAT) and High-Energy Stereoscopic System (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 (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 the Fermi-LAT, we present here a morphological and spectral analysis of these two sources using 8 yr of Fermi-LAT data between 200 MeV and 1 TeV with multiwavelength observations to assess their nature. The morphology of HESS J1640–465 is described by a 2D Gaussian (
σ
= 0.053° ± 0.011°
stat
± 0.03°
syst
) and its spectrum is modeled by a power law with a spectral index Γ = 1.8 ± 0.1
stat
± 0.2
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 ± 0.1
stat
± 0.2
syst
and significant curvature of
β
= 0.11 ± 0.03
stat
± 0.05
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/pulsar wind nebula origin. Our new Fermi-LAT results and the radio and flux X-ray upper limits pose severe constraints on some of these models.
The Vela supernova remnant (SNR) is the closest SNR to Earth containing an active pulsar, the Vela pulsar (PSR B0833-45). This pulsar is an archetype of the middle-aged pulsar class and powers a ...bright pulsar wind nebula (PWN), Vela-X, spanning a region of 2degrees x 3degrees 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 reinvestigated this complex region and performed a detailed morphological and spectral analysis of this source using 4 yr of Fermi-LAT observations. This study lowers the threshold for morphological analysis of the nebula from 0.8 GeV to 0.3 GeV, allowing for the 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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