Ultra-high-energy gamma rays in the Cygnus Cocoon

What is the source of the highest-energy cosmic rays in our Galaxy? We report here the observation of the ultra-high-energy gamma-ray emission from a stellar superbubble in the Cygnus region, powered by the massive stellar complex called Cygnus OB2 in an attempt to answer this question.
24 μm intensity map from Spitzer’s Multiband Imaging Photometer overlaid with TeV gamma-ray  map of the Cocoon region from the HAWC observatory. Grey and white color indicate infrared emission. Green to red indicates increasing gamma-ray excess.

Galactic Cosmic rays (GCRs), charged particles traveling almost at the speed of light, are constantly bombarding our Earth. The highest-energy GCRs, which can have energy as high as a few PeV, come from the most extreme environments in our Galaxy.  The CR spectrum changes its shape around 1-3 PeV. This feature is known as the “knee” of the cosmic-ray spectrum and is likely due to the change of the source classes that dominate the CR production.  Traditionally, the explosion of a massive star event called supernova (SN) and the resulting expanding shell after the explosion called supernova remnant (SNR) were thought to be the leading sources accelerating the GCRs. To date, however, no evidence has been found that suggests CR acceleration to PeV energy in SNR environments. Thus finding CR sources other than SNRs, and identifying potential PeV CR accelerators (aka PeVatrons) are crucial to particle astrophysics. 

 Stellar superbubbles, which are formed due to the interaction of supersonic winds of massive stars, have been postulated to be GCR production factories since a few decades ago. They have been observed in many wavebands, including infrared, X-rays and GeV gamma rays, but not yet unambiguously in very-high-energy gamma rays above 10 TeV, mostly due to their extended sizes. The Cygnus region hosts one of the closest giant star-forming complexes, where the first GeV detection of gamma rays near the massive stellar complex was observed by NASA’s Fermi-Large Area Telescope (Fermi-LAT). However, we didn’t know that the stellar clusters can produce CRs at such high energy prior to our observation. Here we report evidence of CR acceleration beyond hundreds of TeV and possibly up to PeV, thus supporting the massive star clusters as an alternate source type that can explain GCR density.  

Fermi-LAT discovered a large extended region of gamma-ray emission surrounded by infrared emission.  Since the parent CRs were thought to be trapped inside this regions, the researchers dubbed it the Cygnus Cocoon. The parent CRs producing this gamma-ray emission in the Cocoon are believe to originate from the nearby stellar association Cygnus OB2, one of the most massive stellar associations in our Galaxy with 120 spectral type O stars. The stellar winds of these massive type O stars accelerate the CRs. The CRs have not travelled too far from their source of origin, which indicate that they are freshly accelerated. After detection by Fermi-LAT, the ARGO observatory reported the observation of the Cocoon up to 10 TeV. The emission also has been observed by Milagro observatory. The Fermi-LAT and other later publications hint the possibility of beyond hundreds of TeV and even PeV acceleration of CRs in the Cocoon.

To directly address whether this Cygnus superbubble produces 0.1-1 PeV CRs, we looked at gamma-ray emission at 10-100 TeV in the region. Due to the smaller effective area, space detectors like Fermi-LAT do not have the sensitivity to very-high-energy gamma rays in the TeV range. For detecting such high energy, we have the ground-based observatories. The High Altitude Water Cherenkov (HAWC) observatory is one such ground based gamma-ray observatory located in Sierra Negra, Mexico and is operated by an international collaboration with more than thirty institutions from the US, Mexico, Europe, South America, and Asia. Its main detector array consists of  300 water Cherenkov detectors spread in an area of 2200 m^2. Each tank is 7.3 m diameter by 4.5 height, holding 20,000 liters of ultra-purified water and four photomultiplier tubes. 

When the gamma rays reach the atmosphere, they produce cascades of particle in interactions with the atmospheric nuclei. When these shower particles hit the water tanks, they produce Cherenkov light as a result of traveling faster than the phase velocity of light in the water. The PMTs at the bottom of the tank detect the Cherenkov signal. We reconstruct these events to determine the direction and the energy of the parent gamma rays initiating the particle showers.  Each day HAWC records ~ 2 TB of these data. However, not all of them are triggered by gamma-ray showers. The majority are caused by the extensive air showers due to the CRs interacting with the atmospheric nuclei. CR showers are the main background in gamma-ray data analysis. 

 Using 1343 days of TeV data, we detected the Cygnus Cocoon at 14 sigma above the background emission. The 1-100 GeV Fermi-LAT observation suggests that a population of protons or electrons were accelerated to TeV energies via an efficient particle acceleration process such as the diffuse shock acceleration. The radial profile of the GeV emission suggests that most particles are confined to the center of the emission. The 1-100 TeV picture reveals additional information about the Cocoon.

In the TeV band, the photon spectrum is consistent with a power law extending beyond 100 TeV without supression. This provides direct evidence that PeV CRs are produced in this source. The flux measured by HAWC cannot be explained by inverse Compton scattering by electrons, indicating that the accelerated parent particles are protons. A study of proton energy density in the region indicates that the protons are freshly accelerated and not a part of an old GCR population. These freshly accelerated CRs interact with the nuclei of the surrounding gas producing neutral and charged pions. Since, neutral pions have very short lifetime (~ 8 x 10-17 s), they immediately decay into gamma-ray photons.

The TeV emission of the Cocoon region overlaps with the GeV emission region. Based on the age of the Cocoon, this implies that the protons are confined by the Cocoon in a much tighter way than what they would experience in the interstellar medium.Compared to GeV observation, the TeV gamma-ray spectrum softens significantly. This suggests that either the efficiency of the accelerator significantly decreases around hundreds of TeVs, or after being accelerated, the highest-energy protons escape the region. The radial profile at ~10 TeV is flatter than that at GeVs, supporting that particle transport plays a role in the gamma-ray production. 

The >10 TeV observation of the Cygnus cocoon shows that superbubbles can indeed produce cosmic rays close to the knee. It also demonstrates how cosmic-ray accelerators operate at these extreme energies and how particle transport impacts high-energy emission. 

Online link: https://www.nature.com/articles/s41550-021-01318-y

arXiv paper link: https://arxiv.org/abs/2103.06820



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