[Paper Review] An Ultrahigh-energy $γ$-ray Bubble Powered by a Super PeVatron

The LHAASO Collaboration, Cao, Z.+279 more|arXiv (Cornell University)|Oct 16, 2023

Astrophysics and Cosmic PhenomenaPhysics and Astronomy3 citations

TL;DR

LHAASO detects an ultrahigh-energy (UHE) gamma-ray bubble spanning over 100 deg² in Cygnus X, powered by a super PeVatron accelerating protons to at least 10 PeV. The bubble's morphology, energy spectrum (log-parabola with Γ(E) = 2.71 ± 0.02 + 0.11 ± 0.02 × log₁₀(E/10 TeV)), and PeV photon concentration near Cygnus OB2 and Cygnus X-3 are best explained by continuous injection of protons into ambient interstellar gas, with a predicted neutrino flux detectable by IceCube over ten years.

ABSTRACT

We report the detection of a $γ$-ray bubble spanning at least 100$ m deg^2$ in ultra high energy (UHE) up to a few PeV in the direction of the star-forming region Cygnus X, implying the presence Super PeVatron(s) accelerating protons to at least 10 PeV. A log-parabola form with the photon index $Γ(E) = (2.71 \pm 0.02) + (0.11 \pm 0.02) imes \log_{10} (E/10 \ { m TeV})$ is found fitting the gamma-ray energy spectrum of the bubble well. UHE sources, `hot spots' correlated with very massive molecular clouds, and a quasi-spherical amorphous $γ$-ray emitter with a sharp central brightening are observed in the bubble. In the core of $\sim 0.5^{\circ}$, spatially associating with a region containing massive OB association (Cygnus OB2) and a microquasar (Cygnus X-3), as well as previously reported multi-TeV sources, an enhanced concentration of UHE $γ$-rays are observed with 2 photons at energies above 1 PeV. The general feature of the bubble, the morphology and the energy spectrum, are reasonably reproduced by the assumption of a particle accelerator in the core, continuously injecting protons into the ambient medium.

Motivation & Objective

• To identify and characterize ultrahigh-energy gamma-ray emission from the Cygnus X star-forming region.
• To determine the origin of the extended UHE gamma-ray emission and its association with massive stellar structures.
• To test the hypothesis that a super PeVatron in the core of the Cygnus X region is responsible for accelerating protons to at least 10 PeV.
• To model the gamma-ray and neutrino emission from hadronic interactions in the bubble and predict detectable neutrino signals.

Proposed method

• LHAASO's KM2A and WCDA arrays collected 3,200 photon-like events above 100 TeV with 5× higher statistics than prior data.
• The energy spectrum was fitted with a log-parabola function: Γ(E) = (2.71 ± 0.02) + (0.11 ± 0.02) × log₁₀(E/10 TeV).
• A 3D particle transport model simulated proton injection from a central accelerator into ambient HI gas, with diffusion coefficient D(E) ∝ E¹ᐟ³.
• The model assumed a broken power-law proton injection spectrum: N(Eₚ) = N₀Eₚ⁻².²⁵(1 + Eₚ/30 TeV)⁻⁰·⁵exp(−Eₚ/10 PeV).
• Neutrino fluxes were estimated using the Kelner et al. semi-analytical method, assuming 1:1:1 flavor ratio after oscillation.
• IceCube sensitivity was calculated via effective area integration to predict detectable muon neutrino events over 10 years.

Experimental results

Research questions

• RQ1 What is the spatial and spectral morphology of the ultrahigh-energy gamma-ray emission in the Cygnus X region?
• RQ2 Can the observed UHE gamma-ray bubble be explained by hadronic interactions from a central particle accelerator?
• RQ3 Where is the location of the putative super PeVatron responsible for proton acceleration?
• RQ4 What is the expected neutrino flux from the same hadronic interactions, and is it detectable by IceCube?

Key findings

• A gamma-ray bubble extending over 100 deg² was detected, with a log-parabola energy spectrum fitting the data with a photon index of 2.71 ± 0.02 at 10 TeV and a curvature of 0.11 ± 0.02.
• Two photons above 1 PeV were detected within a 0.5° radius centered on Cygnus OB2, with a background contamination of only 0.07 events.
• The central region of the bubble, coinciding with Cygnus OB2 and Cygnus X-3, shows a 90% intensity peak within a 1° radius, consistent with a central accelerator.
• The model constrains the particle accelerator to within ~1° of the KM2A center, supporting a core location in the starburst region.
• For a 10-year IceCube operation, the model predicts 29 muon neutrino events above 1 TeV from the entire bubble, with 39% originating from the inner 3° region.
• The predicted (anti-)muon neutrino flux from the bubble exceeds the cosmic-ray-induced background flux by a factor of ~2 at 10 TeV.