A Large Population of Sub-threshold Gamma-ray Pulsars

Identifying gamma-ray sources is one of the main scientific goals of the Fermi Large Area Telescope. Of the ~3000 pulsars reported in the Australia Telescope National Facility (ATNF) pulsar catalog, only 279 have been detected in gamma-rays. In this work, we search for gamma-ray pulsars at 525 high latitude locations that coincide with ATNF pulsar positions with stack methods. We report 8 newly detected pulsar candidates and 36 sub-threshold pulsar candidates. Stacking their likelihood profiles in the spectral parameter space implies a pulsar-like spectral index, cutoff energy and a flux an order of magnitude below the Fermi sensitivity. The same procedures performed on empty control fields limit the false association rate. This study also probes a unique region of parameter space populated by older, transitional, and recycled (millisecond) pulsars. Many of these sources have lower rotational energy loss rates implying that the empirical gamma-ray “death line” could be predominantly a sensitivity limit. If confirmed, these results will expand the number of gamma-ray pulsars by 16%. Furthermore, the improved MSP luminosity function with these new sources can help better characterize their possible contribution to the Galactic Center GeV excess.