Search for dark matter signals with Fermi-LAT observation of globular clusters NGC 6388 and M15

The globular clusters are probably good targets for dark matter (DM) searches in gamma-rays due to the possible adiabatic contraction of DM by baryons. In this work we analyse the three-year data collected by Fermi Large Area Telescope of globular clusters NGC 6388 and M 15 to search for possible DM signals. For NGC 6388 the detection of gamma-ray emission was reported by Fermi collaboration, which is consistent with the emission of a population of millisecond pulsars. The spectral shape of NGC 6388 is also shown to be consistent with a DM contribution if assuming the annihilation final state is b (b) over bar. No significant gamma-ray emission from M 15 is observed. We give the upper limits of DM contribution to gamma-ray emission in both NGC 6388 and M 15, for annihilation final states b (b) over bar, W+W-, mu(+)mu(-), tau(+)tau(-) and monochromatic line. The constraints are stronger than that derived from observation of dwarf galaxies by Fermi.