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We present the high-energy emission properties of GRB 160509A, from itsprompt mission to late afterglow phase. GRB 160509A contains two emissionepisodes: 0-40s and 280-420s after the burst onset (t0). The relatively highfluence of GRB 160509A allows us to establish an evolving spectrum above 100MeV. During the first emission episode, the >100 MeV spectrum is soft with\Gamma=>3.0, which can be smoothly connected to keV energies with a Bandfunction with a high-energy cutoff. The >100 MeV spectrum rapidly changes to ahard spectrum with \Gamma<=1.5 after t0+40s. The existence of very energeticphotons, e.g., a 52 GeV that arrives t0+77 seconds, and a 29 GeV that arrivest0+70 ks, is hard to reconcile by the synchrotron emission from forward-shockelectrons, but likely due to inverse Compton mechanism (e.g., synchrotronself-Compton emission). A soft spectrum (\Gamma~2) between 300s and 1000s afterthe burst onset is also found at a significance of about 2 standard deviations,which suggests a different emission mechanism at work for this short period oftime. GRB 160509A represents the latest example where inverse Compton emissionhas to be taken into account in explaining the afterglow GeV emission, whichhad been suggested long before the launch of Fermi LAT.

An evolving GeV spectrum from prompt to afterglow: the case of GRB 160509A