FERMI OBSERVATION OF THE TRANSITIONAL PULSAR BINARY XSS J12270-4859

Because of the disappearance of its accretion disk during the time period of 2012 November-December, XSS J12270-4859 has recently been identified as a. transitional millisecond pulsar binary, joining. PSR J1023+0038. We have carried out a detailed analysis of the. Fermi. Large Area Telescope data for this. binary. While both spectra. are well-described by an exponentially cut-off power law before and after the disk-disappearance transition, which is typical for pulsars' emissions in Fermi's 0.2-300 GeV band, we have detected a factor of 2 flux decrease related to the transition. A weak orbital modulation is possibly seen, but is only detectable in the after-transition data, making it the same as. orbital modulations found in X-rays. In the long-term light curve of the source before the transition, a factor of 3 flux variations are seen. Compared. to the properties of J1023+0038, we discuss the implications from these results. We suggest that since the modulation is aligned with the modulations in X-rays in the orbital phase, it possibly arises due to the occultation of the gamma-ray emitting region by the companion. The origin of the variations in the long-term light curve is not clear. because the source field also contains unidentified radio or X-ray sources and their contamination cannot be excluded. Multi-wavelength observations of the source field will help identify the origin of the variations by detecting any related flux changes from the in-field sources