A Stacking Survey of Gamma-ray Pulsars

Yuzhe Song, Timothy A. D. Paglione, Joshua Tan, Charles Lee-Georgescu

Published 2021-12-19, updated 2022-03-07Version 2

We report on a likelihood stacking search for gamma-ray pulsars at 525 high latitude locations that coincide with known radio pulsar positions. We significantly detect a stacked signal over the background. Stacking their likelihood profiles in spectral parameter space implies a pulsar-like spectral index and a characteristic flux one order of magnitude below the Fermi-LAT sensitivity. The same procedures performed on empty control fields imply a false detection rate as much as 50\%, although the stacked spectra of the control fields are distinctly softer than those of the pulsars. 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. These new stacking results sensitively probe an unexplored population of low luminosity and low spin-down power pulsars. The millisecond pulsar luminosity function measured for the stack of sub-threshold millisecond pulsars constrains their contribution to the Galactic center GeV excess.