The HAWC Observatory peering into the Extreme Universe with LS 5039

The research presents **new observations of the gamma-ray binary system LS 5039 using the High Altitude Water Cherenkov (HAWC) observatory**, revealing significant insights into the nature of this high-energy source.

One of the most striking findings is that **HAWC detected gamma rays from LS 5039 extending up to 200 TeV with no apparent spectral cutoff**. This is a crucial extension of previous observations by the High Energy Stereoscopic System (H.E.S.S.), which had observed the system up to TeV energies. The spectral energy distribution (SED) presented in Figure 2 shows this extension, particularly during the inferior conjunction (INFC). The lower limit on the maximum energy measured by HAWC for LS 5039 is 208 TeV at a 68% confidence level during INFC.

Furthermore, the HAWC data **confirms with a 4.7σ confidence level that the gamma-ray emission between 2 TeV and 118 TeV is modulated by the orbital motion of the binary system**. This modulation, where the emission is more significant during the inferior conjunction (INFC) compared to the superior conjunction (SUPC), strongly suggests that these high-energy photons are produced within or very near the binary orbit. The study notes that despite a longer phase interval for the SUPC data, LS 5039 was more significantly detected during INFC due to a higher flux. This modulation up to 100 TeV provides strong evidence for gamma-ray production inside the binary.

These high-energy observations pose a challenge to purely **leptonic scenarios** for gamma-ray production. In a leptonic scenario, the highest energy photons would be produced by electrons inverse Compton scattering stellar photons. The detection of photons up to 200 TeV would require electrons to be accelerated to at least this energy, demanding an extremely efficient acceleration mechanism within LS 5039, especially given the dense radiation and potentially high magnetic fields within the binary system. The study suggests that achieving such high electron energies within the stellar photosphere would require an acceleration efficiency η close to 1 and a magnetic field not significantly larger than 0.1 Gauss to avoid substantial synchrotron losses.

Alternatively, the HAWC radiation can be interpreted through a **hadronic scenario**. In this case, protons are accelerated to peta-electronvolt (PeV) energies and then produce gamma rays through interactions with either the dense gas (stellar winds) or the intense radiation fields within and close to the binary orbit. The timescale for proton-proton collisions and subsequent pion decay is remarkably close to the binary period, making this a viable explanation. If the gamma rays are of hadronic origin, LS 5039 would be an astronomical accelerator capable of producing PeV-scale hadrons. The required jet power to produce the observed gamma-ray luminosity through proton-proton interactions is estimated, and the study suggests that binary jets powered by either Bondi-type accretion or colliding winds could potentially provide the necessary luminosity.

In conclusion, the HAWC observations provide compelling evidence for **gamma-ray emission beyond 100 TeV from LS 5039 and confirm the orbital modulation of this emission**, suggesting that the production of these very high-energy photons occurs within the binary system. These findings have significant implications for our understanding of particle acceleration and radiation processes in gamma-ray binaries, potentially hinting at a hadronic origin for the highest energy emission and establishing LS 5039 as a candidate PeVatron. Future observations at even higher energies could provide crucial evidence to further elucidate the underlying mechanisms at play.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: J. Goodman