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Register a new userDeep morphological and spectral study of the SNR RCW 86 with Fermi-LAT
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RCW 86 is a young supernova remnant (SNR) showing a shell-type structure at several wavelengths and is thought to be an efficient cosmic-ray (CR) accelerator. Earlier textit{Fermi} Large Area Telescope results reported the detection of $gamma$-ray emission coincident with the position of RCW 86 but its origin (leptonic or hadronic) remained unclear due to the poor statistics. Thanks to 6.5 years of data acquired by the textit{Fermi}-LAT and the new event reconstruction Pass 8, we report the significant detection of spatially extended emission coming from RCW 86. The spectrum is described by a power-law function with a very hard photon index ($Gamma = 1.42 pm 0.1_{rm stat} pm 0.06_{rm syst}$) in the 0.1--500 GeV range and an energy flux above 100 MeV of ($2.91$ $pm$ $0.8_{rm stat}$ $pm$ $0.12_{rm syst}$) $times$ $10^{-11}$ erg cm$^{-2}$ s$^{-1}$. Gathering all the available multiwavelength (MWL) data, we perform a broadband modeling of the nonthermal emission of RCW 86 to constrain parameters of the nearby medium and bring new hints about the origin of the $gamma$-ray emission. For the whole SNR, the modeling favors a leptonic scenario in the framework of a two-zone model with an average magnetic field of 10.2 $pm$ 0.7 $mu$G and a limit on the maximum energy injected into protons of 2 $times$ 10$^{49}$ erg for a density of 1 cm$^{-3}$. In addition, parameter values are derived for the North-East (NE) and South-West (SW) regions of RCW 86, providing the first indication of a higher magnetic field in the SW region.
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Aims: We aim for an understanding of the morphological and spectral properties of the supernova remnant RCW~86 and for insights into the production mechanism leading to the RCW~86 very high-energy gamma-ray emission. Methods: We analyzed High Energy Spectroscopic System data that had increased sensitivity compared to the observations presented in the RCW~86 H.E.S.S. discovery publication. Studies of the morphological correlation between the 0.5-1~keV X-ray band, the 2-5~keV X-ray band, radio, and gamma-ray emissions have been performed as well as broadband modeling of the spectral energy distribution with two different emission models. Results:We present the first conclusive evidence that the TeV gamma-ray emission region is shell-like based on our morphological studies. The comparison with 2-5~keV X-ray data reveals a correlation with the 0.4-50~TeV gamma-ray emission.The spectrum of RCW~86 is best described by a power law with an exponential cutoff at $E_{cut}=(3.5pm 1.2_{stat})$ TeV and a spectral index of $Gamma$~$1.6pm 0.2$. A static leptonic one-zone model adequately describes the measured spectral energy distribution of RCW~86, with the resultant total kinetic energy of the electrons above 1 GeV being equivalent to $sim$0.1% of the initial kinetic energy of a Type I a supernova explosion. When using a hadronic model, a magnetic field of $B$~100$mu$G is needed to represent the measured data. Although this is comparable to formerly published estimates, a standard E$^{-2}$ spectrum for the proton distribution cannot describe the gamma-ray data. Instead, a spectral index of $Gamma_p$~1.7 would be required, which implies that ~$7times 10^{49}/n_{cm^{-3}}$erg has been transferred into high-energy protons with the effective density $n_{cm^{-3}}=n/ 1$ cm^-3. This is about 10% of the kinetic energy of a typical Type Ia supernova under the assumption of a density of 1~cm^-3.
Galactic cosmic ray (CRs) sources, classically proposed to be Supernova Remnants (SNRs), must meet the energetic particle content required by direct measurements of high energy CRs. Indirect gamma-ray measurements of SNRs with the Fermi Large Area Telescope (LAT) have now shown directly that at least three SNRs accelerate protons. With the first Fermi LAT SNR Catalog, we have systematically characterized the GeV gamma-rays emitted by 279 SNRs known primarily from radio surveys. We present these sources in a multiwavelength context, including studies of correlations between GeV and radio size, flux, and index, TeV index, and age and environment tracers, in order to better understand effects of evolution and environment on the GeV emission. We show that previously sufficient models of SNRs GeV emission no longer adequately describe the data. To address the question of CR origins, we also examine the SNRs maximal CR contribution assuming the GeV emission arises solely from proton interactions. Improved breadth and quality of multiwavelength data, including distances and local densities, and more, higher resolution gamma-ray data with correspondingly improved Galactic diffuse models will strengthen this constraint.
Context. Diffusive shock acceleration (DSA) is the most promising mechanism to accelerate Galactic cosmic rays (CRs) in the shocks of supernova remnants (SNRs). The turbulence upstream is supposedly generated by the CRs, but this process is not well understood. The dominant mechanism may depend on the evolutionary state of the shock and can be studied via the CRs escaping upstream into the interstellar medium (ISM). Aims. Previous observations of the $gamma$-Cygni SNR showed a difference in morphology between GeV and TeV energies. Since this SNR has the right age and is at the evolutionary stage for a significant fraction of CRs to escape, we aim to understand $gamma$-ray emission in the vicinity of the $gamma$-Cygni SNR. Methods. We observed the region of the $gamma$-Cygni SNR with the MAGIC Imaging Atmospheric Cherenkov telescopes between May 2015 and September 2017 recording 87 h of good-quality data. Additionally we analysed Fermi-LAT data to study the energy dependence of the morphology as well as the energy spectrum in the GeV to TeV range. The energy spectra and morphology were compared against theoretical predictions, which include a detailed derivation of the CR escape process and their $gamma$-ray generation. Results. The MAGIC and Fermi-LAT data allowed us to identify three emission regions, which can be associated with the SNR and dominate at different energies. Our hadronic emission model accounts well for the morphology and energy spectrum of all source components. It constrains the time-dependence of the maximum energy of the CRs at the shock, the time-dependence of the level of turbulence, and the diffusion coefficient immediately outside the SNR shock. While in agreement with the standard picture of DSA, the time-dependence of the maximum energy was found to be steeper than predicted and the level of turbulence was found to change over the lifetime of the SNR.
Diffusive shock acceleration by the shockwaves in supernova remnants (SNRs) is widely accepted as the dominant source for Galactic cosmic rays. However, it is unknown what determines the maximum energy of accelerated particles. The surrounding environment could be one of the key parameters. The SNR RCW 86 shows both thermal and non-thermal X-ray emission with different spatial morphologies. These emission originate from the shock-heated plasma and accelerated electrons respectively, and their intensities reflect their density distributions. Thus, the remnant provides a suitable laboratory to test possible association between the acceleration efficiency and the environment. In this paper, we present results of spatially resolved spectroscopy of the entire remnant with Suzaku. The spacially-resolved spectra are well reproduced with a combination of a power-law for synchrotron emission and a two-component optically thin thermal plasma, corresponding to the shocked interstellar medium (ISM) with kT of 0.3-0.6 keV and Fe-dominated ejecta. It is discovered that the photon index of the nonthermal component becomes smaller with decreasing the emission measure of the shocked ISM, where the shock speed has remained high. This result implies that the maximum energy of accelerated electrons in RCW 86 is higher in the low-density and higher shock speed regions.
The middle-aged supernova remnant (SNR) CTB 37A is known to interact with several dense molecular clouds through the detection of shocked ${rm H_{2}}$ and OH 1720 MHz maser emission. In the present work, we use eight years of $textit Fermi$-LAT Pass 8 data, with an improved point-spread function and an increased acceptance, to perform detailed morphological and spectral studies of the $gamma$-ray emission toward CTB 37A from 200 MeV to 200 GeV. The best fit of the source extension is obtained for a very compact Gaussian model with a significance of 5.75$sigma$ and a 68% containment radius of $0.116^{circ}$ $pm$ $0.014^{circ}_{rm stat}$ $pm$ $0.017^{circ}_{rm sys}$ above 1 GeV, which is larger than the TeV emission size. The energy spectrum is modeled as a LogParabola, resulting in a spectral index $alpha$ = 1.92 $pm$ 0.19 at 1 GeV and a curvature $beta$ = 0.18 $pm$ 0.05, which becomes softer than the TeV spectrum above 10 GeV. The SNR properties, including a dynamical age of 6000 yr, are derived assuming the Sedov phase. From the multiwavelength modeling of emission toward the remnant, we conclude that the nonthermal radio and GeV emission is mostly due to the reacceleration of preexisting cosmic rays (CRs) by radiative shocks in the adjacent clouds. Furthermore, the observational data allow us to constrain the total kinetic energy transferred to the trapped CRs in the clouds. Based on these facts, we infer a composite nature for CTB 37A to explain the broadband spectrum and to elucidate the nature of the observed $gamma$-ray emission.
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