Ultrahigh-Energy Gamma-Ray Sources Need Not Be Hadronic PeVatrons

Abstract

Ultrahigh-energy gamma rays ($E_γ>100 \, {\rm TeV}$) have been detected from a handful of astrophysical sources. Due to the Klein-Nishina suppression of inverse Compton scattering at such high energies, it has sometimes been argued that these sources must be accelerators of PeV-scale protons, making them the long-sought-after Galactic ''PeVatrons.'' Here, we challenge this conclusion, demonstrating that these sources can be straightforwardly explained by simple leptonic models. In this context, we consider the microquasar SS 433, the Galactic Center, and TeV halos, showing in each case that the observation of PeV-scale gamma rays from these sources does not indicate that they are accelerators of hadronic cosmic rays. We also note that the measured angular extension of SS 433 is in good agreement with the predictions of our model, favoring a leptonic origin for the gamma-ray emission from this source. A definitive identification of a PeVatron would require additional information, such as the combined observation of the pion bump and synchrotron peak, the spatial correlation of gamma-ray emission with gas, or the detection of neutrinos with $E_ν \gtrsim 100 \, {\rm TeV}$.

Figures (8)

• Figure 1: The spectrum of the gamma-ray emission from the microquasar SS 433. The flux attributed to the central extended source is shown in green, while the central source upper limits from H.E.S.S. are shown in orange. Based on the spectrum of the central source alone, we obtain the best fit for an injected electron spectral index of $\alpha=2.98$ and a cutoff energy of $E_c=0.50 \, {\rm PeV}$, as shown in solid blue. This model, however, is inconsistent with LHAASO observations at lower energies, which do not reveal any appreciable emission from the central region of this source. For $\alpha \sim 1.8-2.2$ and $E_c \sim 1 {\rm PeV}$, our leptonic model predicts a spectrum (shown as dash-dotted curves) that is broadly consistent with LHAASO's observations, both at relatively low energies and above 150 TeV.
• Figure 2: The gamma-ray flux from the central source of the microquasar SS 433, as predicted by our leptonic model (with $\alpha=2.0$ and $E_c=1 \, {\rm PeV}$). LHASSO is not expected to be sensitive to the emission at $1-25$ or $25-100 \, {\rm TeV}$. In the lower frame, 39% of the flux at $>100 \, {\rm TeV}$ is predicted to come from within a radius of $r_{39} = 0.36^{\circ}$, in good agreement with LHAASO's measurement of $r_{39} = 0.32^{\circ} \pm 0.04^{\circ}$. This concordance favors a leptonic origin for the ultrahigh-energy emission from this source. LHAASO's point spread function is taken from LHAASO:2024psv.
• Figure 3: The spectrum of the Galactic Center point source, HESS J1745-290 (upper frames), and of the surrounding diffuse emission (lower frames), compared to the predictions of our leptonic model. We have adopted a magnetic field strength of either $50\, \mu{\rm G}$ (left) or $100 \, \mu{\rm G}$ (right), chosen an electron cutoff energy of $E_c= 0.1 \, {\rm PeV}$ or 1 PeV, and adjusted the spectral index, $\alpha$, to obtain the best fit. Our simple leptonic model can achieve broad agreement with the spectrum observed by H.E.S.S.
• Figure 4: The angular distribution of the gamma-ray flux, integrated above $1 \, {\rm TeV}$, predicted from electrons emitted from the Galactic Center. We show results for magnetic field strengths of $50 \, \mu{\rm G}$ (left) or $100 \, \mu{\rm G}$ (right), an electron cutoff energy of $E_c= 0.1 \, {\rm PeV}$, and have adjusted the electron spectral index to obtain the best fit to the observed spectrum of HESS J1745-290. In orange, we show the radius within which 39% of the total flux is predicted to be emitted, and in red we show the approximate corresponding value of the H.E.S.S. point spread function at $10 \, {\rm TeV}$. This comparison demonstrates that the point-like nature of HESS J1745-290 is consistent with our leptonic model, as long as $B $>$ $\sim$ ~ 50 \, \mu{\rm G}$ in the innermost $\sim 10 \, {\rm pc}$ of the Milky Way (or the diffusion coefficient is substantially smaller than that found in the ISM).
• Figure 5: The gamma-ray spectrum from a generic leptonic source, adopting an injected electron spectrum with a power-law index of 2.2 and an exponential cutoff at 0.5 PeV. The results are shown for several values of the magnetic field strength, and are compared to the spectrum of SS 433 (summing the central source and both lobes), as reported by the LHAASO Collaboration LHAASO:2024psv.