Evidence for particle acceleration approaching PeV energies in the W51 complex

TL;DR

The paper demonstrates, for the first time, γ rays from the W51 complex extending to the UHE domain (up to about $E_ ext{γ}\,\approx\,200$ TeV) and reveals a significant spectral bend at tens of TeV, interpreted within a hadronic $pp$-collision framework. Joint fits with Fermi-LAT data indicate a cosmic-ray proton spectrum either as an ECPL with $\alpha\approx2.55$ and $E_{p,\rm cut}\approx385$ TeV or as a broken power law with a break at $E_{p,\rm br}\approx180$ TeV, implying acceleration of protons beyond $100$ TeV and a total proton energy around $W_p\sim1.3\times10^{50}$ erg for $n_H\approx100\,\textrm{cm}^{-3}$. The results point to SNR W51C as a potential PeVatron accelerator, likely aided by the interaction with nearby molecular clouds, while two young star clusters could also contribute to the highest-energy emission; a nearby PWN is deemed unlikely to dominate the UHE photons. These findings advance our understanding of Galactic CR origins and the role of SNRs and star clusters as extreme accelerators, with future high-resolution observations (e.g., CTAO, LACT) needed to definitively identify the dominant source and further probe the acceleration limits near the knee of the CR spectrum.

Abstract

The $γ$-ray emission from the W51 complex is widely acknowledged to be attributed to the interaction between the cosmic rays (CRs) accelerated by the shock of supernova remnant (SNR) W51C and the dense molecular clouds in the adjacent star-forming region, W51B. However, the maximum acceleration capability of W51C for CRs remains elusive. Based on observations conducted with the Large High Altitude Air Shower Observatory (LHAASO), we report a significant detection of $γ$ rays emanating from the W51 complex, with energies from 2 TeV to 200 TeV. The LHAASO measurements, for the first time, extend the $γ$-ray emission from the W51 complex beyond 100 TeV and reveal a significant spectrum bending at tens of TeV. By combining the ``$π^0$-decay bump" featured data from Fermi-LAT, the broadband $γ$-ray spectrum of the W51 region can be well-characterized by a simple pp-collision model. The observed spectral bending feature suggests an exponential cutoff at $\sim400$~TeV or a power-law break at $\sim200$~TeV in the CR proton spectrum, most likely providing the first evidence of SNRs serving as CR accelerators approaching the PeV regime. Additionally, two young star clusters within W51B could also be theoretically viable to produce the most energetic $γ$ rays observed by LHAASO. Our findings strongly support the presence of extreme CR accelerators within the W51 complex and provide new insights into the origin of Galactic CRs.

Figures (7)

• Figure 1: Gamma-ray maps in the intervals $[2-25]$(a), $[25-100]$ TeV(b) and $>100$ TeV(c) of the W51 region presented in equatorial coordinates. The color scale indicates the statistical significance of the excess $\gamma$-ray counts after subtracting the Galactic diffuse emission. Maps have been smoothed with a Gaussian kernel of $1.58$ times the PSF. The central position determined by the all-energy-range fit to the LHAASO data is marked by a red dot. The $\gamma$-ray centroids given by Fermi-LAT Fermi-LAT:2009qzy and MAGIC MAGIC:2012anb observations are marked with a magenta square and a magenta triangle, respectively. Cyan contours show the MWISP WMISP measurement of the ${}^{12}$CO emission integrated the velocity from 54 to 70 km s$^{-1}$. Black contours overlay the 1.4 GHz continuum emission as observed by the Very Large Array Stil_2006. The blue diamond marks the position of a PWN candidate CXO J192318.5$+$140305 2005ApJ...633..946K. The black cross shows where 1720 MHz OH masers are emitted from Brogan:2013jba, while six dark-green marked points localize shocked CO clumps 1997ApJ...475..194K1997ApJ...485..263K. Two white dashed rectangles define the two star-forming regions, W51A and W51B fujita2019massive. The radio shell of SNR W51C is indicated with a green dashed circle Brogan:2013jba.(Color online)
• Figure 2: Spectral energy distribution of W51C plotted as $E_\gamma^2dN_\gamma/dE_\gamma$. Top panel: Red flux points, covered an energy band from $2.24^{+3.01}_{-1.28}$ TeV to $177.83^{+46.04}_{-36.57}$ TeV, are the measured fluxes from LHAASO by performing a 3D-likelihood method assuming a PLExpCut spectrum in the simulation. The error bar for all points are given at 1 times standard deviation. The statistical significance at the highest detection point is 3.33$\sigma$. The last two flux upper limits are given at $\simeq281.84^{+72.98}_{-57.97}$ TeV and $\simeq446.68^{+102.86}_{-83.61}$ TeV, respectively. Previous observation results from MAGIC MAGIC:2012anb, H.E.S.S. HESS:2016qan, Milagro Abdo:2009ku, and HAWC HAWC:2020hrt are also depicted. The solid black line is the fitting result with a LOG function, while the dashed black line represents the fitting result with a PLExpCut function. Bottom panel: Energy-dependent $\gamma$-ray slope as derived by the LOG model. The cyan band represents the $1\sigma$ confidence interval.(Color online)
• Figure 3: Fitting results of the $\pi^0$-decay model to the $\gamma$-ray spectrum measurements of W51C. The LHAASO spectrum reported by the present work and the Fermi-LAT spectrum Jogler:2016lav are used in the fitting processes. The proton incident spectrum takes the form of ECPL (BPL) in the left (right) panel. For each panel, the main plot presents the differential energy spectrum of $\gamma$ rays, denoted by ${\rm d}N_\gamma/{\rm d}E_\gamma$. The black line is the best-fit curve, while the dark and light grey bands are the $2\sigma$ and $3\sigma$ confidence intervals of the model, respectively. Directly beneath, a subplot details the standardized residuals between the best-fit curve and the data.(Color online)
• Figure 4: Embedded YSCs in the W51 field, overlaid to LHAASO map in the $[2-25]$ TeV energy range: the dashed circle represents the WTS surrounding every cluster, while the solid circle represents the bubble radius. LHAASO PSF is also shown for comparison.(Color online)
• Figure 5: Same as Fig. \ref{['fig:2-25TeVclusters']}, but in the $[25-100]$ TeV energy range.(Color online)