The counterjet dominates the production of PeV photons from Cyg X-3

Abstract

We study the physical mechanisms underlying the production of orbitally-modulated PeV photons from Cyg X-3, recently discovered by the LHAASO collaboration. Our key findings are as follows. Helium nuclei are accelerated in a compact and strongly magnetized region within the jet, but they then quickly advect downstream to regions with a weaker field, allowing them to diffuse out of the jet, where they produce pions in hadronic collisions with both the stellar photons and the stellar wind of the Wolf-Rayet donor. The optical depths across the binary are $\lesssim$1 for both types of interactions, implying that their rates are proportional to the column densities along the particle paths. Given the low viewing angle of Cyg X-3 ($i\approx26^\circ$--$28^\circ$), most of the observed photons are produced by the relativistic hadrons accelerated in the counterjet (for which the column densities toward the observer are much longer than for the jet). This also explains the peak of the phase-folded PeV photon flux to be on the opposite side of the superior conjunction than that for the (also orbitally-modulated) GeV photons, which are produced by collisions of relativistic electrons with stellar photons in the optically thick regime. This then implies that the GeV emission is produced in the approaching jet.

Figures (8)

• Figure 1: The broad-band spectrum of Cyg X-3 in its $\gamma$-ray flaring state, which corresponds to the soft state in X-rays. The LHAASO PeV data corrected for the CMB absorption (red) are from LHAASO25. We show three example soft-state spectra in the 3--$10^2$ keV range from SZM08, the GeV spectrum (magenta) from Dmytriiev24, and the TeV upper limits (blue) from Aleksic10.
• Figure 2: The orbital-phase folded and normalized light curve of the photon flux from Cyg X-3 in its $\gamma$-ray flaring state for photon energies $E\geq$0.1 PeV as observed (not corrected for the CMB absorption. Adapted from LHAASO25. The normalization factor is $1.05\times 10^{-15}$ cm$^{-2}$ s$^{-1}$ (Cong Li on behalf of the LHAASO team, private communication).
• Figure 3: The $p$-$\gamma$ cross sections in mb as functions of the $\gamma$-ray energy in GeV in the proton rest frame, $E'$. That for all inelastic $p$-$\gamma$ interactions is shown by the solid black curve. Those for the production of single $\pi^0$ and $\pi^+$ pions are shown by the dashed blue and dotted red curves, respectively. The green dot-dashed curve shows the difference between the total cross section and those for single neutral and charged pion production (which includes a minor contribution from diffractive scattering; see figure 3 of Mucke00). Adapted from Kelner08 and Mucke00.
• Figure 4: The IR-to-UV spectrum of Cyg X-3 as seen at 9 kpc. The cyan symbols show the adopted model spectrum with the hydrostatic core at $R_*=1\times 10^{11}$ cm and $T_*=1\times 10^5$ K, see Section \ref{['photon']}. We see that this spectrum is significantly softer than the blackbody at these $R_*$ and $T_*$, shown by the red dashed curve. The IR measurements from UKIRT and ISO are shown as magenta dots, with blue dots showing the measurement errors. The black solid curve shows our phenomenological approximation to the spectrum, Equation (\ref{['efe']}).
• Figure 5: The radial dependence of the wind temperature between $R_*$ and $R_{2/3}$ for the model used here, see footnote \ref{['powr']}.