Neutrino observations of LHAASO sources: Present constraints and future prospects

The Large High Altitude Air Shower Observatory (LHAASO) observed a dozen gamma-ray sources with significant emission above 100 TeV that may be strong candidates for PeVatrons. Neutrino observations are crucial for diagnosing whether the gamma-ray radiative process is hadronic or leptonic. We use the Bayesian method to analyse the 10-yr (2008-2018) IceCube data, and hence constrain the hadronic gamma-ray emission in the LHAASO sources. The present neutrino data show that the hadronic gamma-ray flux from the Crab Nebula is lower than the observed gamma-ray flux at the 90 per cent confidence level and contributes less than 86 per cent, which disfavours the hadronic origin of gamma-rays below tens of TeV. For the other LHAASO sources, the present neutrino observations cannot put useful constraints on the gamma-ray radiative process. We consider the uncertainty of the source extension: the upper limits on the hadronic gamma-ray flux tend to increase with the extension; and some sources, namely LHAASO J2032+4102, LHAASO J1929+1745, and LHAASO J1908+0621, show a relatively high statistical significance of neutrino signals if the extension is less than or similar to 0.degrees 6. Finally, we estimate the future observational results of LHAASO sources by proposed neutrino telescopes. If the LHAASO-observed PeV gamma-rays are of hadronic origin, the Crab Nebula may be detected at >100 TeV at the 3 sigma confidence level within 20 years by a neutrino detector with an effective area 30 times that of IceCube.

Automated summary

The Large High Altitude Air Shower Observatory (LHAASO) has observed a number of gamma-ray sources that may be strong candidates for PeVatrons. Neutrino observations are crucial for determining the nature of the gamma-ray radiative process. Using Bayesian analysis of IceCube data, we constrain the hadronic gamma-ray emission in the LHAASO sources. The present neutrino data suggest that the hadronic origin of gamma-rays below tens of TeV is unlikely.