Search for PeV Gamma-Ray Emission from the Southern Hemisphere with 5 Yr of Data from the IceCube Observatory

Abstract

The measurement of diffuse PeV gamma-ray emission from the Galactic plane would provide information about the energy spectrum and propagation of Galactic cosmic rays, and the detection of a pointlike source of PeV gamma-rays would be strong evidence for a Galactic source capable of accelerating cosmic rays up to at least a few PeV. This paper presents several unbinned maximum-likelihood searches for PeV gamma-rays in the Southern Hemisphere using 5 yr of data from the IceTop air shower surface detector and the in-ice array of the IceCube Observatory. The combination of both detectors takes advantage of the low muon content and deep shower maximum of gamma-ray air showers and provides excellent sensitivity to gamma-rays between ∼0.6 and 100 PeV. Our measurements of pointlike and diffuse Galactic emission of PeV gamma-rays are consistent with the background, so we constrain the angle-integrated diffuse gamma-ray flux from the Galactic plane at 2 PeV to 2.61×10 −19 cm −2 s −1 TeV −1 at 90% confidence, assuming an E −3 spectrum, and we estimate 90% upper limits on pointlike emission at 2 PeV between 10 −21 and 10 −20 cm −2 s −1 TeV −1 for an E −2 spectrum, depending on decl. Furthermore, we exclude unbroken power-law emission up to 2 PeV for several TeV gamma-ray sources observed by the High Energy Spectroscopic System and calculate upper limits on the energy cutoffs of these sources at 90% confidence. We also find no PeV gamma-rays correlated with neutrinos from IceCube's high-energy starting event sample. These are currently the strongest constraints on PeV gamma-ray emission.

Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers

“…Using its unique combination of the surface and in-ice array, IceCube provides a high mass-separation power that has facilitated the first measurement of individual spectra from four elemental mass groups between 2.5 PeV and 1 EeV [82]. Moreover, such hybrid observations have also allowed for searches for PeV gamma-ray emission from the southern hemisphere [84,85]. Although the sensitivity achieved with the data and analysis methods available now has not lead to a discovery, by improving analysis techniques and continued operation, IceCube may eventually discover PeV photon sources, in particular, since PeV photon sources are meanwhile known to exist and be observable with a square-kilometer size array [86,87].…”

High-Energy and Ultra-High-Energy Neutrinos

“…In addition to cross-correlation with gamma-ray sources [414][415][416], analyses have searched for neutrino emission from known sources across the electromagnetic spectrum, including X-ray, UV/optical/IR, and radio sources. These include blazars as well as the cores of active galactic nuclei (AGN), ultra luminous infrared galaxies (ULIRGs), and radio galaxies [417].…”

Cosmic ray spectrum from 250 TeV to 10 PeV using IceTop

“…Reference [19] describes an analysis using 3 years of data (henceforth referred to as "the 3-year IceTop analysis"). IceTop has also been used in searches for PeV gamma rays [27,28], solar ground level enhancements [29], and solar flares [29]. Another analysis [30] includes single tank hits to identify the component of ∼GeV muons in large showers for studies of primary composition.…”