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A Large Population Of Sub-threshold Gamma-ray Pulsars

Presentation #128.02 in the session “Neutron Stars, Symbiotic Stars, & GRBs”.

Published onJun 18, 2021
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 (LAT). Of the 2892 pulsars reported to date in the Australia Telescope National Facility (ATNF) pulsar catalog, only 269 have been detected in gamma-rays. In this work, we search for gamma-ray pulsars at 560 high latitude locations that coincide with ATNF pulsar positions with stack methods. We report 18 newly detected pulsar candidates (Test Statistic, TS > 25) and 66 sub-threshold (10 < TS < 25) pulsar candidates. Summing these 66 TS maps, which yields a spatial distribution of relative likelihood, reveals a centrally peaked source with a high significance (ΔTS = 654). Stacking their likelihood profiles in the spectral parameter space implies a soft pulsar-like spectral index and a flux two orders of magnitude below the Fermi sensitivity. The same procedures performed on empty control fields yield null results. This study also probes a unique region of parameter space populated by older, transitional, and recycled (millisecond) pulsars (MSPs). Many of these sources have lower rotational energy loss rates implying that the empirical γ-ray “death line” could be predominantly a sensitivity limit. If confirmed, these results will expand the number of gamma-ray pulsars by 31%. Furthermore, the improved MSP luminosity function with these new sources can help better characterize their possible contribution to Galactic Center GeV excess.

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