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Register a new userSupernova remnants interacting with molecular clouds as seen with H.E.S.S
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About 30 Galactic supernova remnants (SNRs) are thought to be physically associated with molecular clouds (MCs). These systems are prime g-ray source candidates as the accelerated particles from shock fronts collide with the surrounding high-density medium thus emitting gamma-rays through hadronic interactions. However only a handful of such interacting SNRs are detected at TeV energies. We report the current status of the High Energy Stereoscopic System (H.E.S.S.) observations towards these SNR-MC systems, with a particular emphasis on the latest results.
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Very-high-energy (VHE, E > 100 GeV) gamma radiation has already been detected from several supernova remnants (SNRs). These objects, which are well-studied in radio, optical and X-ray wavelengths, constitute one of the most intriguing source classes in VHE astronomy. H.E.S.S., an array of four imaging atmospheric Cherenkov telescopes in Namibia, has recorded an extensive dataset of VHE gamma-ray observations covering the central region of the Milky Way, both from pointed observations as well as from the Galactic Plane Survey conducted in the inner region of the Galaxy. From radio observations, several hundred SNRs are known in the Milky Way, but until now only few of them have been identified as VHE gamma-ray emitters. Using the H.E.S.S. dataset and a large ensemble of radio SNRs localized in the inner region of the Galaxy, the standard framework that links the origin of cosmic rays to the gamma-ray visibility of SNRs can now be tested. Here we present the ensemble of investigated SNRs and discuss constraints on the parameter space used within a theoretical model of hadronic VHE gamma-ray production.
We report the detection of gamma-ray emission coincident with four supernova remnants (SNRs) using data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. G349.7+0.2, CTB 37A, 3C 391 and G8.7-0.1 are supernova remnants known to be interacting with molecular clouds, as evidenced by observations of hydroxyl (OH) maser emission at 1720 MHz in their directions. SNR shocks are expected to be sites of cosmic rays acceleration, and clouds of dense material can provide effective targets for production of gamma-rays from pion-decay. The observations reveal unresolved sources in the direction of G349.7+0.2, CTB 37A and 3C 391, and a possibly extended source coincident with G8.7-0.1, all with significance levels greater than 10 sigma.
The giant molecular clouds (MCs) found in the Milky Way and similar galaxies play a crucial role in the evolution of these systems. The supernova explosions that mark the death of massive stars in these regions often lead to interactions between the supernova remnants (SNRs) and the clouds. These interactions have a profound effect on our understanding of SNRs. Shocks in SNRs should be capable of accelerating particles to cosmic ray (CR) energies with efficiencies high enough to power Galactic CRs. X-ray and gamma-ray studies have established the presence of relativistic electrons and protons is some SNRs and provided strong evidence for diffusive shock acceleration as the primary acceleration mechanism, including strongly amplified magnetic fields, temperature and ionization effects on the shock-heated plasmas, and modifications to the dynamical evolution of some systems. Because protons dominate the overall energetics of the CRs, it is crucial to understand this hadronic component even though electrons are much more efficient radiators and it can be difficult to identify the hadronic component. However, near MCs the densities are sufficiently high to allow the gamma-ray emission to be dominated by protons. Thus, these interaction sites provide some of our best opportunities to constrain the overall energetics of these particle accelerators. Here we summarize some key properties of interactions between SNRs and MCs, with an emphasis on recent X-ray and gamma-ray studies that are providing important constraints on our understanding of cosmic rays in our Galaxy.
The supernova remnant (SNR) W49B originated from a core-collapse supernova that occurred between one and four thousand years ago, and subsequently evolved into a mixed-morphology remnant, which is interacting with molecular clouds (MC). $gamma$-ray observations of SNR/MC associations are a powerful tool to constrain the origin of Galactic cosmic-rays, as they can probe the acceleration of hadrons through their interaction with the surrounding medium and subsequent emission of non-thermal photons. The detection of a $gamma$-ray source coincident with W49B at very high energies (VHE; E > 100 GeV) with the H.E.S.S. Cherenkov telescopes is reported together with a study of the source with 5 years of Fermi-LAT high energy $gamma$-ray (0.06 - 300 GeV) data. The smoothly-connected combined source spectrum, measured from 60 MeV to multi-TeV energies, shows two significant spectral breaks at $304pm20$ MeV and $8.4_{-2.5}^{+2.2}$ GeV, the latter being constrained by the joint fit from the two instruments. The detected spectral features are similar to those observed in several other SNR/MC associations and are found to be indicative of $gamma$-ray emission produced through neutral-pion decay.
Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray accelerators. When low-energy cosmic-ray protons (LECRp) collide with interstellar gas, they ionize neutral iron atoms and emit the neutral iron line (Fe I K$alpha$) at 6.40keV. We search for the iron K-shell line in seven SNRs from the Suzaku archive data of the Galactic plane in the $6^{circ} lesssim l lesssim 40^{circ}, |b| < 1^{circ}$ region. All these SNRs interact with molecular clouds. We discover Fe I K$alpha$ line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe I K$alpha$ line is produced by interactions between LECRp and the adjacent cold gas. The proton energy density is estimated to be $gtrsim$ 10-100 eV cm$^{-3}$, which is more than 10 times higher than that in the ambient interstellar medium.
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