We consider a Higgs portal ( vector ) dark matter ( DM ) model where the hidden scalar , nearly degenerate with DM in mass , mediates the interaction of the secluded DM with Standard Model ( SM ) due to its mixing with the SM Higgs .
We find that the parameter region m _ { X } \in [ 60 , 132 ] ~ { } \text { GeV } can provide a good fit to the Fermi Galactic center gamma-ray excess spectrum , appearing a prominent gamma-ray line with the energy \in [ 30 , 66 ] GeV .
The best fit gives m _ { X } \simeq m _ { S } \simeq 86 GeV with a p -value = 0.42 , so that the resultant gamma-ray line , arising from the decay of the scalar mediator into \gamma \gamma , peaks at 43 GeV .
We derive constraints on the annihilation cross section from the Fermi-LAT gamma-ray line search , gamma-ray observations of the Fermi-LAT dwarf spheroidal galaxies , and Planck cosmic microwave background measurement .
For the secluded vector DM model , the parameter space constrained by the current XENON1T and future LUX-ZEPLIN is shown .
Finally , for the mixing angle between the Higgs sectors , we discuss its lower bound , which is required by the big bang nucleosynthesis constraint and relevant to the hidden sector decoupling temperature .