A redback system is a binary system composed of a pulsar and a main sequence star. The inverse Compton (IC) scattering between the stellar soft photons and the relativistic pulsar wind will generate orbital-modulating GeV photons. We look for these IC emissions from redback systems. A multi-wavelength observation of an unassociated gamma-ray source, 3FGL J2039.6-5618, by Salvetti et al. (2015) detected an orbital modulation with a period of 0.2 days in both X-ray and optical cases. They suggested 3FGL J2039.6-5618 to be a new redback candidate. We analyzed the gamma-ray emission of 3FGL J2039.6-5618 using the data from the Fermi large area telescope (Fermi-LAT) and obtained the spectrum in different orbital phases. We propose that the spectrum has orbital dependency and estimate the characteristic energy of the IC emission from the stellar-pulsar wind interaction.

1. INTRODUCTION
 2. OBSERVATIONS
  2.1 Orbital-Phase-Dependent Test-Statistics Maps
  2.2 Orbital-Phase-Dependent Emission Spectra
 3. DISCUSSION AND CONCLUSION
 ACKNOWLEDGMENTS
 REFERENCES

• Cho-Wing Ng(The University of Hong Kong, Pokfulam, Hong Kong) Corresponding Author
• Kwong-Sang Cheng(The University of Hong Kong, Pokfulam, Hong Kong)
• Jumpei Takata(Huazhong University of Science and Technology, Wuhan 430074, China)