An Ultrahigh-energy $γ$-ray Bubble Powered by a Super PeVatron

TL;DR

The Cygnus X region hosts a colossal γ-ray bubble extending up to PeV energies, implying a Super PeVatron accelerating protons to >10 PeV. Using LHAASO's KM2A and WCDA data, the study reveals a 6°-scale, log-parabola–shaped γ-ray spectrum with eight photons above 1 PeV spread across the bubble, and a central region co-located with Cygnus OB2 and Cyg X-3. A hadronic diffusion model with slow transport inside a ~150 pc bubble successfully reproduces the SED and morphology, predicting a significant accompanying ν flux detectable by IceCube. These results bolster the case for Galactic PeVatrons and have important implications for the origin of Galactic CRs and the contribution of CR halos to the diffuse γ-ray emission.

Abstract

We report the detection of a $γ$-ray bubble spanning at least 100$\rm deg^2$ 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 $Γ(E) = (2.71 \pm 0.02) + (0.11 \pm 0.02) \times \log_{10} (E/10 \ {\rm TeV})$ 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.

Figures (13)

• Figure 1: Photon distribution in the Cygnus-X region. The size of the circle labels the point spread function of LHAASO-KM2A in the energy range above $100~\rm TeV$. The significance map of $\gamma$-rays from $2 ~\rm TeV$ to $20~\rm TeV$ of the bubble is shown by grey contours starting from 3$\sigma$ with a step of 3$\sigma$. This structure is about 20$^\circ$ in longitude and latitude. The blue diamond located at the centre of the $\gamma$-ray image marks LHAASO J2031 + 4127, which coincides with the unidentified source TeV 2031+4130 hegra_cygnus. There are 66 photon-like events within a radius of 6 degree with an estimated background of 9.5. Eight events with energy above $1 ~\rm PeV$ are marked with black circles, 12 events with energy between $600~\rm TeV$ and $1 ~\rm PeV$ are shown as pink, and the other 46 events with energy between $400~\rm TeV$ and $600~\rm TeV$ are shown with blue circles. The photons above 400 TeV extend beyond 6$^\circ$, but with a higher CR background contamination, so we do not show them individually on the map. In particular, as shown in the zoom-in figure, seven of these high energy photons, 2 of them with energy above 1 PeV, are located in the region of radius of 0.5$^\circ$ relative to the centre (red circle), which is roughly the size of the massive star association Cygnus OB2. Possible contamination of the CR background is only 0.07 events. This region contains at least three interesting objects - Cygnus OB2 (red circle), Cyg X-3 (cyan diamond) and the powerful pulsar PSR J2031+4127 (blue diamond). The larger circle in black dotted line represents the ROI used in this study, while the shaded circle within black dotted line marks the masked region near the unidentified source LHAASO J2018+3651. This source is bright and reveals a large spatial extension, thus a circular region with a radius of $2.5^{\circ}$ is masked in the analysis.
• Figure 2: The Cygnus Bubble in 3 decades of photon energy. Two-dimensional significance maps of the Cygnus Bubble in the region of interest, which are smoothed with a Gaussian kernel of $\sigma$=0.3$^{\circ}$ (upper row) and $\sigma$=1$^\circ$ (middle row). All individual sources including the SNR $\gamma$-Cygni are removed. One-dimensional angular distributions of photons in the bubble are displayed in the lower row. From left to right, the columns of panels are for photon energies in the ranges of 2-20 TeV, 25-100 TeV and above 100 TeV, respectively. The structures of the bubble at different energies are visible in the upper panels (A, B and C). Hot spots are revealed at energies above 25 TeV associated with the local molecular cloud distribution, which is indicated by the contours. The broad structure of the bubble in the middle row of panels (D, E and F) has a good association with the local HI gas distribution, which is indicated by the contours. This structure covers a very wide region i.e, 10$^\circ$ from the core. The distribution of $\gamma$ emission in Galactic longitude with a latitude range from $-2^{\circ}$-$2^{\circ}$. The two gray lines are the estimated diffuse emission from two different regions.The GDE estimation 1 and 2 are derived from inner and outer galaxy region separately.
• Figure 3: The $\gamma$-ray flux detected toward Cygnus X integrated over the region of radius of $6^{\circ}$. The spectral points (red symbols) is fitted well by a log-parabola functional form with the photon index $\Gamma=(2.71\pm 0.02)+(0.11 \pm 0.02)\times \log_{10}(E/10\rm TeV)$. The black curve corresponds the predicted flux of diffuse galactic emission calculated under the assumption that CRs are homogeneously distributed over the Glactic Plane. The fluxes of the Galactic Diffuse Emission (GDE) based on the measurements from specific parts of the inner and outer parts of Galaxy (GDE 1 and 2, respectively) but re-scaled for the column density toward the Cygnus Bubble are plotted as well.
• Figure 4: Modeling of the Cygnus Bubble that simultaneously fits the SEDs and 1-dimensional intensity profiles of $\gamma$-rays. In panel A, the measured fluxes from the entire bubble (black squares), from the $2^\circ$ Gaussian component or LHAASO J2027+4119 (red squares), and from the CO template (blue squares). Orange diamonds present the flux of the Cygnus Cocoon measured by Fermi-LAT Ackermann:2011. The proton injection luminosity is $L_p= 10^{37}\rm erg/s$ with the acceleration spectrum $E_p^{-2.25}\exp(-E_p/5\,\rm PeV)$. The diffusion coefficient is $D(E_p)=3\times 10^{26}(E_p/1\rm TeV)^{0.7} ~\rm cm^2s^{-1}$. The black solid, blue dashed, and red dot-dashed curves showcase the emission of the entire $6^\circ$ bubble (including the DGE from this region), the emission from interactions between injected protons and MCs, the emission from interactions between protons and atomic gas, respectively. Panels B, C, D show the measured surface brightness profile in the energy ranges of $2-20$ TeV, $25-100$ TeV and $>100\,$TeV (red crosses), in comparison with the model prediction (black curves). Dotted curves show the expected contribution of GDE. See Supplementary Material for details of the model.
• Figure 5: Energy density of relativistic protons as a function of radius from the centre. Solid curves show the energy densities of injected protons, whereas dotted curves mark the energy density of CR 'sea' represented by the locally measured CRs. Different colors represent different particle energies as labelled. The vertical grey dashed line marks the radius of the bubble measured by LHAASO. The inset figure shows the spectrum for the CR sea based on the measurements by DAMPE DAMPE2019_CRp and KASCADE-Grande KASCADE2013_CRp for the proton component.