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Abstract

The 1.69 ms spin period of PSR J1227-4853 was recently discovered in radio
observations of the low-mass X-ray binary XSS J12270-4859 following the
announcement of a possible transition to a rotation-powered millisecond pulsar
state, inferred from decreases in optical, X-ray, and gamma-ray flux from the
source. We report the detection of significant (5$\sigma$) gamma-ray pulsations
after the transition, at the known spin period, using ~1 year of data from the
Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The
gamma-ray light curve of PSR J1227-4853 can be fit by one broad peak, which
occurs at nearly the same phase as the main peak in the 1.4 GHz radio profile.
The partial alignment of light-curve peaks in different wavebands suggests that
at least some of the radio emission may originate at high altitude in the
pulsar magnetosphere, in extended regions co-located with the gamma-ray
emission site. We folded the LAT data at the orbital period, both pre- and
post-transition, but find no evidence for significant modulation of the
gamma-ray flux. Analysis of the gamma-ray flux over the mission suggests an
approximate transition time of 2012 November 30. Continued study of the pulsed
emission and monitoring of PSR J1227-4853, and other known redback systems, for
subsequent flux changes will increase our knowledge of the pulsar emission
mechanism and transitioning systems.