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Gamma-ray orbital modulation of the transitioning millisecond pulsar binary XSS J12270-4859

We report on gamma-ray orbital modulation of the transitioning millisecond pulsar binary XSS J12270-4859 detected in the Fermi Large Area Telescope (LAT) data. We use long-term optical data taken with the XMM-Newton Optical Monitor and the Swift UltraViolet Optical Telescope to inspect radio timing solutions that are limited to relatively short time intervals, and find that extrapolation of the solutions aligns well the phasing of the optical data over 15 years. The Fermi-LAT data folded on the timing solutions exhibit significant modulation (p = 5x10^-6) with a gamma-ray minimum at the inferior conjunction of the pulsar. Intriguingly, the source seems to show similar modulation in both the low-mass X-ray binary (LMXB) and the millisecond pulsar (MSP) states, implying that mechanisms for gamma-ray emission in the two states are similar. We discuss these findings and their implications using an intra-binary shock scenario.