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Search for gamma-ray emission from four accreting millisecond pulsars with Fermi/LAT

We report our search for γ-ray emission in the energy range from 100 MeV to 300 GeV from four Accreting Millisecond Pulsars (AMPs), SAX J1808.4-3658, IGR J00291+5934, XTE J1814-338, and XTE J0929-314. The data are from four-year observations carried out by Large Area Telescope (LAT) onboard the Fermi γ-ray Space Telescope. The AMPs were not detected, and their γ-ray luminosity upper limits we obtain are 5.1*10^33 ergs/s for SAX J1808.4-3658, 2.1*10^33 ergs/s for IGR J00291+5934, 1.2*10^34 ergs/s for XTE J1814-338, and 2.2*10^33 ergs/s for XTE J0929-314. We compare our results with γ-ray irradiation luminosities required for producing optical modulations seen from the companions in the AMPs, which has been suggested by Takata et al. (2012), and our upper limits have excluded γ-ray emission as the heating source in these systems except XTE J0929-314, the upper limit of which is not deep enough. Our results also do not support the model proposed by Takata et al. (2012) that relatively strong γ-ray emission could arise from the outer gap of a high-mass neutron star controlled by the photon-photon pair-creation for the AMPs. Two AMPs, SAX J1808.4-3658 and IGR J00291+5934, have the measurements of their spin-down rates, and we derive the upper limits of their γ-ray conversion efficiencies, which are 57% and 3%, respectively. We discuss the implications to the AMP systems by comparing the efficiency upper limit values with that of 20 γ-ray millisecond pulsars (MSP) detected by Fermi and the newly discovered transitional MSP binary J1023+0038.